Triac light bulb flasher

This 800 W light bulb flasher operates directly off the line and needs no transformer. Power for the timer circuit is derived by limiting the current using a 330 nF capacitor (acts like a 9.6 k resistor at 50 Hz), rectifying with a full-wave rectifier composed of four diodes (you may also use a pre-made bridge rectifier instead of the diodes, of course, but make sure the voltage rating is 400 V, or 250 V RMS). Then the voltage is limited with a 9 V zener diode (almost any of this voltage will work), a 1 W type. The 100 µF capacitor filters the power, a 16 V rating may be a bit safer. Remember: if the zener diode fails, the capacitor will blow because it gets peaks of up to 330 V, although current-limited). In this configuration, the timer gives long pulses at 1.3 Hz.

Now there's one problem: we can't drive the triac directly, because the controlling voltage is not isolated from the line since there is no transformer. The easiest way to drive it is thus by using a triac optocoupler. The K3021 or MOC3021 is well suited for this purpose, as it works like a small triac and thus allows it to directly drive the gate of the larger triac. The coupler is connected to turn on when the timer outputs a low, so we get short pulses.

Please note that this only works with resistive loads like incandescent light bulbs or heaters. It does not work with fluorescent lamps (need a snubber network to do that).


Electronic Parts:

R1 470R
R2 100K
R3 1M5
R4 1K
C1 330nF/400V
C2 100µF/25V
C3,C5 10nF/63V

C4 10µF /25V
D1,D2 1N4007
D3 BZX79C10
D4 TIC206M
Q1 BC557
IC1 7555 or TS555CN CMos Timer IC
SW1 SPST Mains suited Switch

How the circuit work:
Due to the low current drawing, the circuit can be supplied from 220Vac mains without a transformer. Supply voltage is reduced to 10Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. IC1 is a CMos 555 timer wired as a monostable, providing 15 seconds on-time set by R3 & C4. When SW1 is closed, IC1 output (pin 3) is permanently on, driving Triac D4 which in turn feeds the lamp. Opening SW1 operates the monostable and, after 15 seconds, pin 3 of IC1 goes low switching off the lamp.


This circuit using high voltage source. Please Make sure you use a safety device before trying to activate this circuit.

Source: redcircuit.com




We use these types of circuits in most ceremonies (wesak festival, christmas, wedding, …). This 5 WAY AC FLASHER circuit running system base on cd4017 ic, timer base on ne555, output base on the component called “Triac” and power supply base on the capacitor. we don’t use steps down transformer and can be given directly 230V to this circuit .







































1MΩ o.5W




0.68µF 400V mylar capacitor


220µF 25V


2.2µF 25V


4× 1N4004


























NE 555


MN 4017




Use heat sink when more lights are switched  on by the triac.

Mica sheets use between Triac and heat sink when using a single heat sink

use mylar type 0.68µF 400V capacitor to the power supply (C1) .

Don’t touch any component, when circuit is connected to the 230V AC power.



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How to build Car-Bulb Flasher

September 26, 2010 - category: Automotive light


This astonishingly simple circuit allows one or two powerful 12V 21W car bulbs to be driven in flashing mode by means of a power MosFet. Devices of this kind are particularly suited for road, traffic and yard alerts and in all cases where mains supply are not available but a powerful flashing light are yet necessary.

Circuit Diagram:




. Thyristors, triacs, diacs

There are several thyristors displayed on 6.1. Triacs look the same, while diacs look like small power rectifying diodes. Their symbols, and pin-out is found in figure 6.2.

Fig. 6.1: Several thyristors and triacs

A thyristor is an improved diode. Besides anode (A) and cathode (k) it has another lead which is commonly described as a gate (G), as found on picture 6.2a. The same way a diode does, a thyristor conducts current when the anode is positive compared to the cathode, but only if the voltage on the gate is positive and sufficient current is flowing into the gate to turn on the device. When a thyristor starts conducting current into the gate is of no importance and thyristor can only be switched off by removing the current between anode and cathode. For example, see figure 6.3. If S1 is closed, the thyristor will not conduct, and the globe will not light. If S2 is closed for a very short time, the globe will illuminate. To turn off the globe, S1 must be opened. Thyristors are marked in some circuits as SCR, which is an acronym for Silicon Controlled Rectifier.
A triac is very similar to a thyristor, with the difference that it can conduct in both directions. It has three electrodes, called anode 1 (A1), anode 2 (A2), and gate (G). It is used for regulation of alternating current circuits. Devices such as hand drills or globes can be controlled with a triac.

Thyristors and triacs are marked alphanumerically, KT430, for example.
Low power thyristors and triacs are packed in same housings as transistors, but high power devices have a completely different housing. These are shown in figure 6.1. Pin-outs of some common thyristors and triacs are shown in 6.2 a and b.
Diacs (6.2c), or two-way diodes as they are often referred to, are used together with thyristors and triacs. Their main property is that their resistance is very large until voltage on their ends exceeds some predefined value. When the voltage is under this value, a diac responds as a large value resistor, and when voltage rises it acts as a low value resistor.

Fig. 6.2: Symbols and pin placements for: a - thyristor, b - triac, c - diac

Fig. 6.3: Thyristor principle of work

6.1 Practical examples

Picture 6.5 detects when light is present in a room.  With no light, the photo-transistor does not conduct. When light is present, the photo-transistor conducts and the bell is activated. Turning off the light will not stop the alarm. The alarm is turned off via S1.

Fig. 6.5: Alarm device using a thyristor and a photo-transistor

A circuit to flash a globe is shown in figure 6.6  This circuit flashes a 40w globe several times per second. Mains voltage is regulated using the 1N4004 diode. The 220u capacitor charges and its voltage rises. When this voltage reaches the design-voltage of the the diac (20v), the capacitor discharges through the diac and into the triac. This switches the triac on and lights the bulb for a very short period of time, after a period of time (set by the 100k pot), the capacitor is charged again, and the whole cycle repeats. The 1k trim pot sets the current level which is needed to trigger the triac.

Fig. 6.6: Flasher

A circuit to control the brightness of a globe or the speed of a motor is shown in figure 6.7

Fig. 6.7: Light bulb intensity or motor speed controller

If the main use for this circuit is to control the brightness of a light bulb, RS and CS are not necessary.


This is the circuit diagram of the simplest lamp dimmer or fan regulator.The circuit is based on the principle of power control using a Triac.The circuit works by varying the firing angle of the Triac . Resistors R1 ,R2 and capacitor C2 are associated with this.The firing angle can be varied by varying the value of any of these components.Here R1 is selected as the variable element .By varying the value of R1 the firing angle of Triac changes (in simple words, how much time should Triac conduct) changes.This directly varies the load power, since load is driven by Triac.The firing pulses are given to the gate of Triac T1 using Diac D1.


Assemble the circuit on a good quality PCB or common board.The load whether lamp ,fan or any thing ,should be less than 200 Watts.To connect higher loads replace the Triac BT 136 with a higher Watt capacity Triac . All parts of the circuit are active with potential shock hazard.So be careful.

I advice to test the circuit with a low voltage supply (say 12V or 24V AC) and a small load (a same volt bulb) ,before connecting the circuit to mains.

Parts List

R1 1o K 1 Watt Resistor

R2 1o0 K Potentiometer (Variable Resistance)

C1 0.1 uF (500V or above ) Polyester Capacitor

T1 BT 136 Triac

D1 DB2 Diac

Circuit Diagram                                        BT 136Triac Necessary Data.


The diacs, because of their symmetrical bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits em­ployed for lamp dimmer, heat control, universal motor speed control etc.

Although a triac may be fired into the conducting state by a simple resistive triggering circuit, but triggering devices are typically placed in series with the gates of SCRs and triacs as they give reliable and fast triggering. Diac is the most popular triggering device for the triac. This is illustrated in the following applications.

triac lamp dimmer circuit

1. Triac Lamp Dimmer Circuit.

The circuit for a triac controlled by an R-C phase-shift network and a diac is given in figure. This circuit is an example of a simple lamp dimmer. The triac conduction angle is adjusted by adjusting the potentiometer R. The longer the triac conducts, the brighter the lamp will be. The diac acts like an open-circuit until the voltage across the capacitor exceeds its breakover or switching voltage (and the triac’s required gate trigger voltage).

Diac Heat Control Circuit

2. Heat Control Circuit.

A typical diac-triac circuit used for smooth control of ac power to a heater is shown in figure. The capacitor C1 in series with choke L across the triac slows-up the voltage rise across the device during off-state. The resistor R4 across the diac ensures smooth control at all positions of potentiometer R2. The triac conduction angle is adjusted by adjusting the potentiometer R2. The longer the triac conducts, the larger the output will be from the heater. Thus a smooth control of the heat output from the heater is obtained.

Read more:
Under Creative Commons License:


Basic Triacs and SCR Projects

by Lewis Loflin

This page will discuss basic triacs and SCRs. A triac is a bidirectional, three-terminal dual, back-to-back thyristor (SCR) switch. This device can switch the current in either direction by applying a small current of either polarity between the gate and one of the two main terminals. The triac is fabricated by integrating two thyristors in an inverse parallel connection. It is used in AC applications such as light dimming, motor-speed control, etc. Triacs can also be used in microcontroller power control.

If one is not familiar with diodes and AC rectification see the following:

Turning a Diode On/Off

Pictured above is a silicon controlled rectifier (SCR) or thyrister. It's a diode with a "gate." An SCR not only conducts in one direction like any other diode, but the gate allows the conduction itself to be cut on and off. When the ON switch is pressed, the SCR is turned on, and current flows from negative to positive through the SCR and load to the positive terminal. Once turned on, the SCR will remain on until the Off switch is pressed, breaking the current path.

Note that the ON switch is referred to as 'normally open' (N.O.) and makes (closes) a connection when pressed. The OFF switch is referred to as 'normally closed' (N.C.) breaks (opens) the connection when pressed. Both of these are push button switches. In electrical terms an 'open' is an undesired broken connection while a 'short' is an undesired connection.

In the circuit above the Load is a DC motor. Press the ON switch the motor runs and will run until the Off switch is pressed. Note the direction of a DC motor depends on polarity. Reverse the leads on the motor, it will run in the opposite direction.

In this example we have placed a diode in series with the gate on/off switch. When one presses the ON switch, the motor will run, the light will come on, etc. When the switch is released, the power is killed without use of an OFF switch. This is because the AC input goes back to zero volts at 180 and 360 degrees shutting off the SCR. And as a diode, the SCR only conducts one-half the cycle.

In this circuit example we have placed variable resistor (potentiometer) in series with the gate diode. (This was also known as an old style volume control knob.) By "turning the knob" we in alter the trip point in turning on the SCR only part of the half-cycle or if enough resistance, turn the SCR off. For a picture of a variable resistors click here.

Full wave pulsating DC.

In another note we can use pulsating, unfiltered DC with a thyristor. When we use AC in the above circuit examples we get only half-wave rectification. Also see Basic AC Rectification and Filtering

Here we use full-wave pulsating DC with a SCR.

This illustrates to process with full-wave unfiltered D.C.

In the above circuit we are using as LASCR output (H11C6) opto-coupler to switch off via diode bridge D1 a triac. The output of the H11C6 is a light activated silicon controlled rectifier or LASCR. The opto-coupler could also be a NTE3046.

For more see
What is a Light Activated Silicon Controlled Rectifier? (LASCR)
H11C6 SCR opto-Coupler data sheet (PDF file)

Above is a practical SCR test circuit. The lamp will come on only when SW3 is pressed. The lamp will be at half brightness because the SCR acts as a half-wave rectifier. R4 can be in the range of 100 to 470 ohms. The lamp should be completely off unless the switch is pressed or the device is defective. (Fully or partially shorted.)

This circuit is also good for comparing different SCRs of the same part number. For example I once had a problem circuit board with six SCRs, but one SCR of the six when working switched on at a very different trigger voltage than the other five. The lamp was a different brightness level than the other five. Replacing that one SCR fixed that very expensive circuit board.


A triac is a solid state AC switch. A small current on the gate terminal can switch very large AC currents. Think of a triac as two back-to-back SCRs where the cathode of one SCR is connected to the anode of the other and vise-versa. The gates are connected together. Because we have the two SCRs type configuration allows the switching of both half-cycles.

Closing the switch will cut on the triac. The idea is to use a small low-power switch to control high power devices such as motors or heaters. The danger here is the high voltage AC is on the switch.

Above is a practical TRIAC test circuit. Press either switch and the lamp will come on at half brightness. Press both together full brightness. This allows testing of both SCR sides individually. The brightness should be the same for both sides or the TRIAC is defective. With no switch pressed, lamp should be totally off. R1 and R2 should be in the range of 100 to 470 ohms.

Basic triac circuit with potentiometer.

Better triac circuit with capacitor.

Best response triac circuit with a diac.

The key to successfully triggering a TRIAC is to make sure the gate receives its triggering current from the main terminal 2 side of the circuit (the main terminal on the opposite side of the TRIAC symbol from the gate terminal). Identification of the Mt1 and Mt2 terminals must be done via the TRIAC's part number with reference to a data sheet or book.

The DIAC, or 'diode for alternating current', is a trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the DIAC abruptly decreases, leading to a sharp decrease in the voltage drop across the DIAC itself thus producing a sharp increase in current flow through the triac gate. This assures a fast, clean cut on of the TRIAC. The DIAC remains in its conduction mode until the voltage drops to a very low value far below the trigger voltage. This is called the holding current. Below this value, the diac switches back to its high-resistance (off) state. This behavior is bidirectional, meaning typically the same for both the positive and negative half cycles.

Most DIACs have a breakdown voltage around 30 V. In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.

DIACs have no gate electrode, unlike some other thyristors. Some TRIACs contain a built-in DIAC in series (I've never seen one in the field) with the TRIAC's "gate" terminal for this purpose. DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2. Most specification sheets don't bother to label A1/A2 or MT1/MT2.

Note: a DIAC can be used with an SCR.

Commercial lamp dimmer in 220 volt countries. Br100 is a diac.

A DIAC provides cleaner switching for the triac. DIACS are specialized Shockley diodes connected back-to-back.


A snubber circuit (usually of the RC type) is often used between Mt1 and Mt2. Snubber circuits are used to prevent premature triggering caused for example by voltage spikes in the AC supply or those produced by inductive loads such as motors. Also, a gate resistor or capacitor (or both in parallel) may be connected between gate and Mt1 to further prevent false triggering. That could increase the required trigger current and perhaps a delay in turnoff as the capacitor discharges.

In this circuit above the "hot" side of the line is switched and the load connected to the cold or ground side. The 39-ohm resistor and 0.01µF capacitor are for snubbing of the triac, and the 470 ohm resistor and 0.05 µF capacitor are for snubbing the coupler. These components may or may not be necessary depending upon the particular load used.

For more on the above opto-coupler see
moc30xx series opto-isolator (pdf file)

Read this safety warning and Disclaimer

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Posted May 6, 2010:

Transformers and misc. topics:

ATMEGA168 Arduino Micro Controller

For more technical details on this see ATMEGA168 Arduino Micro Controller Projects

Triac/SCR based projects:

PDF files and spec sheets

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Diac controlled LED flasher

January 6, 2010 D.Mohankumar 3 comments

This is probably the simplest idea to generate flashing light from an LED using AC. The circuit is relatively the simple way of flashing one or more LEDs from a high voltage DC obtained from Mains. This can be used as a Mains indicator or Mock flasher.

The circuit uses a diac for the alternate switching of LED. The diac is usually used in pulse generator circuits to trigger SCR and Triac. If a low voltage passes through a diac, it simply behaves like an open circuit and only very low current passes through it. But if the voltage increases to the breakdown threshold of the diac, it will pass heavy current. Usually 35 volt DC is required to attain the threshold level of diac. Unlike SCR, diac conduct in both the directions. In the circuit, a commonly available DB3 diac is used. Diode D1 rectifies AC and generates a high volt DC. Resistor R1 safely controls the DC to operate diac and LED.

Normally LED will be OFF. When the capacitor charges fully, diac gets the threshold voltage and fires. This provides current to LED and it lights. Resistor R2 makes the LED current to a safer value of 30 mA. When the diac conducts, C1 discharges and again the breakdown voltage of diac decreases and LED turns off. Thus the charging/discharging cycles of C1 makes the LED flashing. The value of C1 determines the flash rate. Higher values give slow flash rate and vice versa. If the threshold level of diac is not obtained using the given value of R1, reduce it to 10K, but its wattage should be increased to 5 watts.

Caution: The circuit is directly connected to high volt AC and there is no galvanic isolation. Take utmost care while handling the circuit. Enclose it in a shock proof case. Do not touch any points when it is connected to Mains.

Diac controlled Flasher Circuit

Designed by D.Mohankumar


The Triac

Figure 1 Triac Circuit Symbol

The triac is similar in operation to two thyristors connected in reverse parallel but using a common gate connection. This gives the triac the ability to be triggered into conduction while having a voltage of either polarity across it. In fact it acts rather like a "full wave" thyristor. Either positive or negative gate pulses may be used. The circuit symbol for the triac is shown in figure 1.

Triacs are mainly used in power control to give full wave control. This enables the voltage to be controlled between zero and full power. With simple "half wave" thyristor circuits the controlled voltage may only be varied between zero and half power as the thyristor only conducts during one half cycle. The triac thus provides a wider range of control in AC circuits without the need for additional components, e.g. bridge rectifiers, a second thyristor etc normally found in practical full wave thyristor circuits. The triggering of the triac is also simpler than that required by thyristors in AC circuits, and can normally be achieved using a simple DIAC circuit. A simplified triac control circuit is shown in fig Figure 2. The operation will be explained after introducing the Diac.

Figure 2 Simplified AC Power Control Circuit using a Triac

The Diac

This is a bi-directional trigger diode used mainly in firing Triacs and Thyristors in AC control circuits. Its circuit symbol (shown in figure 3a) is similar to that of a Triac, but without the gate terminal; in fact it is a simpler device and consists of a PNP structure (like a transistor without a base) and acts basically as two diodes connected cathode to cathode as shown in figure 3b.

Figure 3 The Diac Circuit symbol and an equivalent diagram using diodes.

The DIAC is designed to have a particular break over voltage, typically about 30 volts, and when a voltage less than this is applied in either polarity the device remains in a high resistance state with only a small leakage current flowing.

Once the break over voltage is reached however, in either polarity, the device exhibits a negative resistance as can be seen from the characteristic curve in figure 4.

Figure 4 Typical Diac Characteristics.

When the voltage across the diac exceeds about 30 volts (a typical break-over voltage) current flows and an increase current is accompanied by a drop in the voltage across the Diac. Normally Ohm's law states that an increase in current through a component causes an increase in voltage across that component; however the opposite effect is happening here, that's why we say "The Diac exhibits negative resistance at break-over"

In the simple power control circuit in Figure 2 the Diac is used to trigger a Triac by the "Phase Control" method. The AC mains waveform is phase shifted by the RC circuit so that a reduced amplitude, phase delayed version of the mains waveform appears across C. As this wave reaches the break over voltage of the Diac, it conducts and discharges C into the gate of the Triac, so triggering the Triac into conduction. The Triac then conducts for the remainder of the mains half cycle, and when the mains voltage passes through zero it turns off. The Triac then conducts for the remainder of the mains half cycle, and when the mains voltage passes through zero it turns off. Some time into the next (negative) half cycle, the voltage on C reaches break over voltage in the other polarity and the Diac again conducts, providing an appropriate trigger pulse to turn on the Triac.

By making R a variable value, the amount of phase delay of the waveform across C can be varied, allowing the time during each half cycle at which the Triac fires to be controlled. Thus the amount of power delivered to the load can be varied.

Note that in practical control circuits using Thyristors, Triacs and Diacs large voltages are switched very rapidly. This can give rise to serious RF interference, and steps must be taken in circuit design to minimise this. Also as Mains is present in the circuit there must be some form of safe isolation between the low voltage control components (e.g. the Diac and phase shift circuits) and the mains "live" components, e.g. the Triac and load. This can easily be achieved by "Opto-coupling" the low voltage control circuit to the high voltage power control (Triac or SCR) part of the circuit.

Figure 5 The Opto Triac

The materials used in the manufacture of Triacs and SCRs, like any semiconductor device, are light sensitive. Their conduction is changed by the presence of light; that's why they are normally packaged in little chunks of black plastic. However, if we include an LED within the package we can turn on the device output in response to a very small input current through the LED. This is the principle used in Opto-Triacs and opto-SCRs, which are readily available in Integrated circuit (I.C.) form and do not need very complex circuitry to make them work. Simply provide a small pulse at the right time and the power is switched on. The main advantage of these optically activated devices is the excellent insulation between the low power and high power circuits, (typically several thousand volts) that provides safe isolation between the low voltage input and high voltage output.

Testing Thyristors, Traics and Diacs

Resistance tests on these devices are of limited use; SCRs and Triacs often operate at mains voltage and when they fail the results can be dramatic. At least the violent blowing of a fuse will be the usual result of a short circuit. Such a fault can be confirmed by measuring the resistance between the two main current carrying terminals of a SCR or Triac; a short circuit in both directions means a faulty component. It is quite possible however for these devices to be faulty and not show any fault symptoms on an ohmmeter test. They may seem OK at the low voltages used in such meters, but still fail under mains voltage conditions.

The normal method of testing would be the checking of voltages and waveforms if the circuit was operating; or substitution of a suspect part when damage (e.g. blown fuses) is apparent. In many cases these components will be designated "safety critical components" and must only be replaced using manufacturers recommended methods and components. It is common for manufacturers to supply complete "service kits" of several semiconductor devices and possible other associated components, all of which must be replaced, since the failure of one power control device can easily damage other components in a way that is not always obvious at the time of repair.


If you have not been trained in the safe working practices that are essential for work on these types of circuit DON'T DO IT! These circuits can kill!


This is a standard text-book circuit. A triac may be considered as two SCR's (Silicon Controlled Rectifiers) connected in opposite directions. A diac is a gate trigger device. Triacs, diacs & SCR's are different types of Thyristors.

A triac is a 3 terminal AC semiconductor switch which is triggered ON when a low energy signal is applied to its Gate. Switching is fast. The low energy of switching means that a wide range of low cost control circuits can be used. Since the triac is bilateral (2 SCR's connected in opposite directions) the terms anode and cathode have no meaning. So the terms Main Terminal 1 and 2 (MT1, MT2) are used. It is standard to use MT1 as a reference point.

The circuit controls the average power to a load through the triac by phase control. The AC supply is applied to the load for only a controlled fraction of each cycle. The triac is held in an OFF condition for a portion of its cycle then is triggered ON at a time determined by the circuit.

Each time the triac is turned on, the load current changes very quickly - a few micro seconds - from zero to a value determined by the lamp resistance and the value of the mains voltage at that instant in time. This transition generates Radio Frequency Interference. It is greatest when the triac is triggered at 90 degree and least when it is triggered at close to zero or 180 degree of the mains AC waveform. L-C suppression network is thus used to suppress these electrical noise.

Light Dimmer Circuit Parts List

The parts list of the project is as shown below.




SCRs are unidirectional (one-way) current devices, making them useful for controlling DC only. If two SCRs are joined in back-to-back parallel fashion just like two Shockley diodes were joined together to form a DIAC, we have a new device known as the TRIAC: (Figure below)

The TRIAC SCR equivalent and, TRIAC schematic symbol

Because individual SCRs are more flexible to use in advanced control systems, these are more commonly seen in circuits like motor drives; TRIACs are usually seen in simple, low-power applications like household dimmer switches. A simple lamp dimmer circuit is shown in Figure below, complete with the phase-shifting resistor-capacitor network necessary for after-peak firing.

TRIAC phase-control of power

TRIACs are notorious for not firing symmetrically. This means these usually won't trigger at the exact same gate voltage level for one polarity as for the other. Generally speaking, this is undesirable, because unsymmetrical firing results in a current waveform with a greater variety of harmonic frequencies. Waveforms that are symmetrical above and below their average centerlines are comprised of only odd-numbered harmonics. Unsymmetrical waveforms, on the other hand, contain even-numbered harmonics (which may or may not be accompanied by odd-numbered harmonics as well).

In the interest of reducing total harmonic content in power systems, the fewer and less diverse the harmonics, the better -- one more reason individual SCRs are favored over TRIACs for complex, high-power control circuits. One way to make the TRIAC's current waveform more symmetrical is to use a device external to the TRIAC to time the triggering pulse. A DIAC placed in series with the gate does a fair job of this: (Figure below)

DIAC improves symmetry of control

DIAC breakover voltages tend to be much more symmetrical (the same in one polarity as the other) than TRIAC triggering voltage thresholds. Since the DIAC prevents any gate current until the triggering voltage has reached a certain, repeatable level in either direction, the firing point of the TRIAC from one half-cycle to the next tends to be more consistent, and the waveform more symmetrical above and below its centerline.

Practically all the characteristics and ratings of SCRs apply equally to TRIACs, except that TRIACs of course are bidirectional (can handle current in both directions). Not much more needs to be said about this device except for an important caveat concerning its terminal designations.

From the equivalent circuit diagram shown earlier, one might think that main terminals 1 and 2 were interchangeable. These are not! Although it is helpful to imagine the TRIAC as being composed of two SCRs joined together, it in fact is constructed from a single piece of semiconducting material, appropriately doped and layered. The actual operating characteristics may differ slightly from that of the equivalent model.

This is made most evident by contrasting two simple circuit designs, one that works and one that doesn't. The following two circuits are a variation of the lamp dimmer circuit shown earlier, the phase-shifting capacitor and DIAC removed for simplicity's sake. Although the resulting circuit lacks the fine control ability of the more complex version (with capacitor and DIAC), it does function: (Figure below)

This circuit with the gate to MT2 does function.

Suppose we were to swap the two main terminals of the TRIAC around. According to the equivalent circuit diagram shown earlier in this section, the swap should make no difference. The circuit ought to work: (Figure below)

With the gate swapped to MT1, this circuit does not function.

However, if this circuit is built, it will be found that it does not work! The load will receive no power, the TRIAC refusing to fire at all, no matter how low or high a resistance value the control resistor is set to. The key to successfully triggering a TRIAC is to make sure the gate receives its triggering current from the main terminal 2 side of the circuit (the main terminal on the opposite side of the TRIAC symbol from the gate terminal). Identification of the MT1 and MT2 terminals must be done via the TRIAC's part number with reference to a data sheet or book.


Light dimmer circuits



I disclaim everything. The contents of the articles below might be totally inaccurate, inappropriate, or misguided. There is no guarantee as to the suitability of said circuits and information for any purpose whatsoever other than as a self-training aid.

How light dimmers work

Light dimmers work by chopping the AC voltage. When only part of the AC waveform is passed through the bulb it gets less power. Typical light dimmers are built using thyristors and the exact time when the thyristor is triggered relative to the zero crossings of the AC power is used to determine the power level. When the the thyristor is triggered it keeps conducting until the current passing though it goes to zero (exactly at the next zero crossing if the load is purely resistive, like light bulb). By changing the phase at which you trigger the triac you change the duty cycle and therefore the brightness of the light.

Here is an example of normal AC power you get from the receptable (the picture should look like sine wave):

... ...

. . . .

. . . .


. . . .

. . . .

... ...

And here is what gets to the light bulb when the dimmer fires the triac on in the middle of AC phase:

... ...

| . | .

| . | .


| . | .

| . | .

... ...

As you can see, by varying the turn-on point, the amount of power getting to the bulb is adjustable, and hence the light output can be controlled.

The advantage of thyristors over simple variable resistors is that they (ideally) dissipate very little power as they are either fully on or fully off. Typically thyristor causes voltage drop of 1-1.5 V when it passes the load current.

What are thyristors and triacs

A Silicon Controlled Rectifier is one type of thyrister used where the power to be controlled is unidirectional. The Triac is a thyrister used where AC power is to be controlled.

Both types are normally off but may be triggered on by a low current pulse to an input called the gate. Once triggered on, they remain on until the current flowing through the main terminals of the device goes to zero.

Both SCRs and Triacs are 4 layer PNPN structures. The usual way an SCR is described is with an analogy to a pair of cross connected transistors - one is NPN and the other is PNP.


+ >------------+ LOAD +----------------+

+------+ |


E \|

PNP |---+-------< IG(-)

C /| |

| |

| |/ C

Gate IG(+) >-----+---| NPN

|\ E




If we connect the positive terminal of a supply to say, a light bulb, and then to the emitter of the PNP transistor and its return to the emitter of the NPN transistor, no current will flow as long as the breakdown voltage ratings of the transistor are not exceeded because there is no base current to either.

However, if we provide some current to the base of the NPN (IG(+)) transistor, it will turn on and provide current to the base of the PNP transistor which will turn on providing more current to the NPN transistor. The entire structure is now in the on state and will stay that way even when the input to the NPN's base is removed until the power supply goes to 0 and the load current goes to 0.

The same scenario is true if we reverse the power supply and use the IG(-) input for the trigger.

A Triac works basically in a similar manner but the polarity of the Gate can be either + or - during either half cycle of an AC source. Typically the trigger signals used for triggering triacs are short pulses.

Incandescent lamp physics

A typical incandescent lamp take power and uses it to heat up a filament until it will start to radiate light. In the process about 10% of the energy is converted to visible light. When the lamp is first turned on, the resistance of the cold filament can be 29 times lower than it's warm resistance. This characteristic is good in terms of quick warmup times, but it means that 20 times the steady-state current will be drawn for the first few milliseconds of operation. The semiconductors, wiring, and fusing of the dimmer must be designed with this inrush current in mind.

Because lamp filament has a finite mass, it take some time (depending on lamp size) to reach the operating temperature and give full light output. This delay is perceived as a "lag", and limtis how quicly effect lighting can be dimmed up. In theatrical application those problems are reduced using preheat (small current flows through lamp to keep it warm when it is dimmed out).

The ideal lamp would produce 50% light output at 50% power input. Unfortunately, incandescents aren't even close that. Most require at least 15% power to come on at all, and afterwards increase in intensity at an exponential rate.

To make thing even more complicated, the human eye perceives light intensity as a sort of inverse-log curve. The relation of the the phase control value (triac turn on delay after zero cross) and the power applied to the light bulb is very non-linear. To get around those problems, most theatrical light dimmer manufacturers incorporate proprietary intensity curves in their control circuits to attempt to make selected intensity more closely approximate perceived intensity.

Typical 120V AC dimmer circuit

The following circuit is based on information from Repair FAQs: http://www.paranoia.com/~filipg/REPAIR/Repair.html

This is the type of common light dimmer widely available at hardware stores and home centers. The circuit is a basic model for light dimmer for 120V AC voltages.

While designed for incandescent or heating loads only, these will generally work to some extent with universal motors as well as fluorescent lamps down to about 30 to 50 percent brightness. Long term reliability is unknown for these non-supported applications.


| | |

| R1 \ |

| 220 K /<-+ |

| \ | |

| | | |

| +--+ |

| | |

| R2 / |

C1 _|_ 47 K \ |

.047 uF --- / __|__ TH1

| | _\/\_ SC141B

| +---|>| / | 200 V

| | |<|--- |

| C2 _|_ D1 |

| .062 uF --- Diac |

| | |

Red o-----------------+---CCCCCC---+-----------+


40 T #18, 2 layers

1/4" x 1" ferrite core

The purpose of the pot P1 and capacitor C2 in a diac/triac combination is just to delay the firing point of the diac from the zero crossing. The larger the resistance (P1+R2) feeding the capacitor C2, the longer it takes for the voltage across the capacitor to rise to the point where the diac D1 fires turning on the triac TH1. Capacitor C1 and inductor L1 make a simple radio frequency interference filter. Without it the circuit would generate quite much interference because firing of the triac in the middle of the AC phase causes fast rising current surges.

I also saw a quite similar dimmer circuit posted to sci.electronics.design newsgroup one day (posted by Sam Goldwasser). This is the type of common light dimmer (e.g., replacements for standard wall switches) widely available at hardware stores and home centers. This circuit uses slightly different component values than the previous one and does not have any radio frequency interference filtering. This one contains just about the minimal number of components to work at all!


| |

| | |

R1 \ | |

185 K /<-+ |

\ v CW |

| __|__ TH1

| _\/\_ Q2008LT

+---|>| / | 600 V

| |<|--' |

C1 _|_ Diac |

.1 uF --- (part of |

S1 | TH1) |


S1 is part of the control assembly which includes R1. The reostat, R1, varies the amount of resistance in the RC trigger circuit. The enables the firing angle of the triac to be adjusted throughout nearly the entire length of each half cycle of the power line AC waveform. When fired early in the cycle, the light is bright; when fired late in the cycle, the light is dimmed.

Due to some unavoidable (at least for these cheap dimmers) interaction between the load and the line, there is some hysteresis with respect to the dimmest setting: It will be necessary to turn up the control a little beyond the point where it turns fully off to get the light to come back on again.

1 kW 230V AC light dimmer circuit

The following circuit is HELVAR 1 kW light dimmer dimmer circuit published at Bebek Electronics magazine. The circuit is a quite typical TRIAC based dimmer circuit with no fancy special features. The triggering circuit is a little bit improded compared to the 120V AC design. This circuit is only designed to opertate with non-inductive loads like standard light bulbs.


| | | | | |

| P1 \ | P2 \ | |

| 500 K /<-+ 1M / <-+ |

| LIN \ \ |

| | | |

230V | +---------+ |

AC IN | | |

| R1 / |

C1 _|_ 2k2 \ | A2

150 nF --- / R2 __|__ TH1

400V | | 6k8 _\/\_ TIC226D

| +-/\/\/---+---|>| G / | A1

| | | |<|---- |

| C2 _|_ C3 _|_ D1 |

| 150nF --- 33nF --- ER900/ |

| 400V | | BR100-03 |

| | | |



40..100 uH

Safety issues on building the circuits

Because light dimmers are directly connected to mains you must make sure that no part of the circuit can be touched when it is operating. This can be best dealt by buildign the dimmer circuit to small plastic box. Remeber to use potentiometer with plastic shaft and install it so that no potentiometer metal parts are exposed to user.

Remeber to make circuit board so that the traces have enough current carrying capacity for the maximum load. Make sure that you have enough separation between PCB traces to widthstand mains voltage. Remeber to install correct size fuse for the circuit (fast acting if you want to give any protection to TRIAC). Make sure that all components can handle the voltages they face in the circuit. For 230V operation use at least 400V triac (600V better). The capacitor which is connected between the dimmer circuit mains wires should be a capacitor which is rated for this kind of applications (those are marked with letter X on the case).

Remeber to use coil type which can handle the full load current without overheating or saturating. Use capacitors with enough high voltage rating. Make sure that the TRIAC has enough ventilation so that it does not overheat at full load.

Radio frequency interference details

Triac based light dimmer circuits the mains sine wave is chopped, which causes fast voltage and current changes. Thost fast voltage and current changes cause high frequency interference going to mains wiring unless there are suitable reafio frequency interference (RFI) filter built into the circuit. The corners in th waveform effectively consist of 50/60Hz plus varying amounts of other frequencies that are multiples of 50/60Hz. In some cases the interference goes up to 1..10Mhz frequencies and even higher. The wiring in your house acts as an antenna and essentially broadcasts it into the air. Cheap bad quality light dimmers don't have adequate filtering and they cause easily lots of radio interference.

Dimmer circuits typically use coils that limit limit the rate of rise of current to that value which would result in acceptable EMI. The coil itself does not solve the problem because of the self-capacitance of the inductor: they typically resonate below 200 kHz and look like capacitors to disturbances above the resonance frequency. That's why therte must be also capacitors to suppress the interference at higher frequencies.

If your dimmer circuit cause interference, you can try to filter out the interference by adding a small capacitor (typically 22nF to 47 nF) in parallel with the dimmer circuit as near as possible to the electronics inside the circuit as possible. Keep in mind to use a capacitor which is rated for this kind of applications (use capacitors marked with X). Keep in mind that the filter capacitor and it's wiring make a resonance circuit with certain resonance frequency (typically around 3.6 MHz with 0.1 uF capacitor). The capacitor does not work well as filter with the frequencies higher than the resonance frequency of the circuit.

Buzzing problems with dimmers

Each good dimmer has a filter choke inside. Those chokes hekp to filter out electrical noise that often causeshum to be picked up in sound system and musical instrument pick-ups. The chokes also help to eliminate 'lamp singing' that can cause audible noise to come from the lighting fixtures. In providing those filtering functions, the chokes themselves can generate a slight buzz. Fast current changes in the coil can make the coil wiring and core material easily vibrate which causes buzzing noise. A little bit of puzzing is normal with filtered dimmers. If the buzz from dimmer can be a problem it is recommended that the dimmer is placed in the area where this buzz will not be a problem.

As far as the 'bulb singing' concerned, a bulb consists of a series of supports and, essentially, fine coils of wire. When the amount of current flow abruptly changes the magnetism change can be much stronger than it is on a simple sine wave. Hence, the filaments of the bulb will tend to vibrate more with a dimmer chopping up the wave form, and when the filaments vibrate against their support posts, you will get a buzz. If you have buzzing, it's always worth trying to replace the bulb with a different brand. Some cheap bulb brands have inadequate filament support, and simply changing to a different brand may help.

Buzzing bulbs are usually a sign of a "cheap" dimmer. Dimmers are supposed to have filters in them. The filter's job is to "round off" the sharp corners in the chopped waveform, thereby reducing EMI, and the abrupt current jumps that can cause buzzing. In cheap dimmers, they've economized on the manufacturing costs by cost-reducing the filtering, making it less effective.

In very high power dimming systems the wiring going to lighting can also cause buzzing. The fast current makes the electrical wiring to vibrate a little bit and if the wire is installed so that the vibration can be transferred to some other materialk then the buzzing could be heard. The bussing caused by the vibration of the wiring is only problem in very high power systems like theatrical lighting with few kW of lights connected to the same cable. Better filtered dimmers can reduce the problem because the filter makes the current changes slower so the wires make less noise.

Why does dimmed lighting sometimes hum, and how can it be corrected?

Because of the way all dimmers deliver power at settings other than full brightness, the filaments inside a light bulb may vibrate when lighting is dimmed. This filament vibration causes the hum. To silence the fixture, a slight change in the brightness setting will usually eliminate bulb noise. The most effective way to quiet the fixture is to replace the light bulb.

How can I avoid the buzzing the dimmers cause to my sound system ?

There are numerous ways that dimmer noise can get into audio systems and it's largely trial and error in determining what in particular is causing your problem and hence how to fix it. The principle ways are either back up the mains or induced into your audio equipment or cables.

What you hear typically in audio system is common mode noise on the hot and neutral, the spike of turn-on of the scr. The higher the rise time of the current in the dimmer, more noise is sent to the mains wiring. So well filtered dimmer will generate less noise problems.

Reduce the possibility of it coming up the mains by taking a totally separate mains supply from the lighting, if possible get a totally separate power socket (or sockets) run in for sound from wherever the electricity board intake is. If this is not possible, then an isolation transformer stops quite much of the noise on the secondary side (better with shield between coils). So put the sound system on the isolation transformer and tie to earth (ground) almost no problems. This assume that sound wiring is correct, especially shielding is done well and ground loop are avoided.

To reduce the possibility of interference induced to the audio cables, run all non speaker level audio cables as balanced lines (or certainly all of any length). You might have to buy balancing transformers if your kit isn't balanced already. Also keep them as far away physically from any lighting cable runs as you can. Make sure that your system does hot have any harmbul ground loops. Make sure none of your audio kit is anywhere near the dimmer racks.

Now can I dim up the lights smoothly ?

With many cheap dimmers, the lights "Pop On" rather than dim up smoothly. This problem is usually related to the construction of the dimmer electronics. One technique used in some cheap dimmers to allow dimming up smoothly is to place another potentiometer (trimmer) across the control potentiometer. That trimmer potentiometer is set so that the dimmer works smoothly:

Continue until minimum voltage/current is supplied to lamps (filaments do not seem to glow at all). When everything is properly adjusted, the dimmer circuit will nicely dim up from the lowest setting up to maximum brightness.

Dimming inductive loads

Normal light dimmers are designed to only dim non-lunductive loads like light bulbs and electric heaters. Normal light dimmers are not suitable to dim inductive loads like transformers, fluorescent lamps, neon lamps, halogen lamps with transformers and electric motors. There are special dimmers available for those applications.

If you connect inductive loads to the dimmer the dimmer might not work as expected (for example does not dim that load properly) and can even be damaged by the voltage surges generated by the inductive load when current changed radiply. Another problem is the phase shift between the voltage and current cause by the inductance. If you use a normal simple light dimmer which is just in series with the wire going to the load, this will cause that the dimmer circuit will not wirk properly with highly inductive loads. Special dimmers which have a separate controlling electronics connected to both live and neutral wire and then the triac which controls the current to the load usually work much bettter with inductive loads.

Often when inductive loads cause problems on normal dimmers, you can eliminate said problems by patching an incandescent "ballast" load in parallel with the inductive load. Usually 100W is enough for many inductive loads. Remeber that indictive loads can hum quite noticably when dimmed and the transformers can heat more because of increased harmonics content in the power coming to them.

Dimming lights with built-in transformers

Fully loaded halogen transformers usually dim quite well. If you are planning to dim halogen light transformers, try only dim traditional transformes, because toroidal core transformer do not usully dim well. Most of the cheap halogen light transformers belong to this category as well as the transformer in for example PAR36 pinspot lights. When dimming transformers it is a good idea to put a fuse in sereis with the transformer primary so that it will blow when transfromer tries to get too much power from the line. This will protect the transformer from overhating which might be caused because of transformer core saturation (which might be caused by small DC bias caused by not very well operating dimmer). A proper fuse will save transformers from burning out.

If your halogen light system uses an electronic transformer then you must very carefully check if it can be dimmed. Some of the electronic transformers are made dimmable and work well with traditional light dimmers. The ones which are not ment to be dimmed can be damaged by the dimming.

Dimming fluorescent lights

If you try to dim fluorescent light on normal dimmer you have to turn the dimmer full on to make the light to turn on and you can only dim it down only down to 30-50% brightness. For anythign less than this you will need a special dimmers and special fluorescent fitting.

Dimming electric motors

Typical dimmer packs will supply power to motors and make them run, but the dimmers aren't designed for it. Some dimmers can be damaged by connecting inductive loads to them. And when the triac fails half-wave it takes the motor out too. A good idea to protect motor failures is to use a fure sized for the motor load in series with the motor. This fuse will propably burn before motor is damaged if it is sized correctly.

How touch dimmers work ?

The basic dimmer operation principle is the same as in dimmers above. The only difference is how the dimemr is controlled. The rouch controlling is done using a special control IC and touchable metal plate. The dimmer usually has a metal plate which is coupled to the circuit via a high value resistor (>1Meg Ohm). Your body acts a little like an antenna and couples 50Hz mains signal (or 60 Hz depending on country) into the circuitry. The AC signal is fed to a shaping circuit(converted to a square wave) and then usually into a dimmer IC.

A typical touch dimmer has following circuit parts:

Touch dimmers which typically control the TRIAC in a 45�C to 152�C conductivity region of the mains half period while the IC draws its power from the remaining power up to the 180�C of the half period.

Siemens is one of the companies who supply these IC's (for example SLB-0586). The IC itself will function differently depending how long you touch the plate for.

Advanced dimming systems

Lighting dimmers use phase-control - you switch on at a point on the supply voltage waveform after the zero-crossing, so that the total energy input to the lamp is reduced. The time between zero crossing and switching is controlled by external control interface which is most often 0-10V DC control voltage or digital DMX512 interface.

Simple voltage controlled dimmer

230V AC o---FUSE----LAMP--------------+-----------+---------------+

INPUT 2A | | |

\ R2 | |

/ 2.2K | |

R1 \ | R4 |

2.2 kohm / | 220 ohm /

+ o--/\/\------+ | | 1W \

CONTROL __|_ ----> / R3 | /

VOLTAGE LED _\/_ ----> \ LDR | |

| / __|__ TH1 |

- o------------+ | _\/\_ BTA04/600T |

+---|>| / | |

| |<|--' | |

C1 _|_ Diac | C2 _|_

100 nF --- | 100 nF ---

| | 250VAC |


This circuit can control loads up to 2A (460VA). The circuit is basically a normal light dimmer circuit, but the potentiometer is replaced with LDR resistor which changes it's resistance depending on the light level. In this circuit a LED powerred form control voltage source is used for shining variable intensity light to the LDR, so you must make sure that LDR does not receive light from other sources.

This circuit is basically very simple and not very sensitive on what LDR is used as R2. The disadvantage of this circuit is that the control is not very linear and the different dimmers built around this circuit can have quite varying characteristics (depending mainly on the LED and LDR characteristics). The control voltage is optically isolated from the dimmer circuit connected to mains. If you need a safety solation then remeber to have enough distace between the LED and LDR or use a transparent isolator between them to guarantee good electrical isolation. If the dimmer sensitivity is not suitable with the circuit described above, then you can adjust the value of R1 to get the control voltage range you want.

This circuit is a part of an automatic light dimmer circuit published in Elektor Electronics Magazine July/August 1998 issue pages 75-76.

Professional voltage controlled dimmers

Remotely controlled light dimmers in theatrical and architechtural applications typically use 0-10V control signal for controlling the lamp brightness. In this case 0V means that the lamp is on and 10V signal means that the lamp in fully on. A voltage between those values adjust the phase when the TRIAC will fire. Here is a typical control circuit schematic:


| \ Resistor

0-10V input >-------------|+ \

| >-----/\/\/\------+

+---|- / |

| | / optocoupler to TRIAC circuit

| |

Ramp signal Ground

goes from 10V to 0V

in one mains half cycle

(10 ms at 50 Hz mains frequnecy)

The circuit works so that the comparator output in low when the input voltage is higher than the ramp voltage. When the ramp signal voltage gets lower than the input voltage the comparator output goes high which causes that curresnbt sarts to flow through resistor to optocoupler which causes the triac to connect. Because the ramp signal starts at every zero crossing from 10V and goes linearly to 0V at the time of one half cycle the input voltage controls the time when the triac is triggered after every zero crossing (so the voltage controls the ignition phase. The necessary linear ramp signal can be generated by a circuit which discharges a capacitor at constant current and charger it quickly at every zero crossing of mains voltage.

You can use your own circuit for triggering the TRIAC or you can use a ready made semicondictor relay for this (it comes in compact package and provides optoisolation in same package with TRIAC). If you plan to usre ready made solid state relay you need an SSR WITHOUT zero-crossing switching. You need an inductor in series with the SSR to prevent di/dt problems and help to cut down emission of r.f. noise. Values vary typicallt from 2 to 6 mH: they are usually specified in terms of the rise-time of the switch-on edge, but it is only rough because the inductors used are non- linear: the inductance varies with load current.

The optocoupled TRIAC triggering circuit can be for example constructed using MOC3020 optodiac and some other component. Here is one example circuit (part of dimmer circuit from Elektor Electronics 302 circuits book):

R1 R2

180 1K

+---/\/\/\----------+ +----/\/\/-------------+------------+-----------> 230V

1| |6 | | Hot

+=====+ IC1 | MT1 |

| MOC | TRIAC +-+ |

| 3020| Driver G | | TRIAC |

+=====+ /| | TIC226D |

2| |4 / +-+ |



| | |

\ | |

R4 / | | C1

1K \ | --- 100 nF

/ | --- 400V

| | |

| ) |

| ( L1 |

| ) 50..100 |

| ( uH |

| | | Neutral

+--+------------+----o o--> 230V


Most professional stage-ligting dimmers do use solid state relays. They have more in them than you would expect, usually including opto-isolation of the control input. The exact contents are commercially confidential but the operation of voltage controlled version is very similar to the idea described above.

Phase controlling using microprocessor

If you want a digital control of light dimmer you can use a simple microcontroller to do the phase controlling. The microcontroller has to first read the dimmer setting value through some interface (commercial digital dimmers use DMX512 interface). typically the control value is 8 bit number where 0 means light off and 255 that light is fully on.

The microcontroller can easily generate the necessary trigger signal using following algorith:

Software loop is quite simple method and useful when you know how long time it takes to execute each microprocessor command. Another possibility is to utilize microcontroller timers:

Reverse phase control

Reverse phase control is a new way to do light dimming. The idea in reverse phase controlling is to turn on then switching component to conduct at at every zero crossing point and turn it off at the adjustable position in the middle of the AC current phase. Tming of the turn-off point then controls the power to the load. The waveform is exact reverse of that is used in traditional light dimmers.

... ...

. | . |

. | . |


. | . |

. | . |

... ...

Because the switching component must be turned off at the middle of the AC phase, traditional thyristors and TRIACs are not suitable components. Possible components for this kind of controlling would be transistors, FETs and GTO-thyristors. Power MOSFETs are quite auitable components for this and they have been used in some example dimmer circuits.

Reverse phase controlling has some advantages over traditional drimmers in many dimmer applications. Because turning on point is always exact at the zero phase there are no huge current spikes and EMI caused by turn on. Using power MOSFETs it is possible to make the turn-off rate relatively slot to achieve quite operations in terms of EMI and acoustical or incandescent lamp filament noise.

Variable transformer as dimmer

One old approach for dimming of lights is do it by using variable transformer (Variac or similar brand) as a dimmer. Some of these are made specifically for this application - they'll fit into a double-size wall box (maybe even into a single-size wall box if you get a small one) and will handle several hundred watts. They're heavy and mechanically "stiff" (compared to a triac dimmer) and not cheap - but they put out a nice, clean 60 Hz sinewave (or very near to it) at all voltages, and don't add switching noise.

Other not so good ideas for dimming

Zero cross switching will minimize noise in switchign and dimming. Unfortunately that appriach is not very practical for lampi dimming. At 60 Hz line frequency, you'd be limited to turning the lamp on and off at discrete 120 Hz intervals. You'd easily end up with a rather nasty 15-20 Hz flickering, unless the dimmer-driver can do some sort of dithering to spread out the flicker spectrum. I've never seen a dimmer of this sort being used.

In some occasions a single diode can be to dim a light bulb when wired in series with the lamp. The diode then passes only the positive or negative half of the mains voltage to the light bulb. If you put a witch in parallel with the diode, you end up having a dimmer wich has two settings: full on and dimmed. Diode will indeed work on small loads, but with larger loads the DC component this diode causes is not good for the distribution transformers in the electrical distribution system (will cause them them to heat up more than in normal use).


An Experimental Lamp Dimmer and Motor Speed Control Using A Simulated Diac

The diac is a common component used to trigger triacs. The construction of a diac is similar to that with a transistor, but with both junctions doped to similar levels so that the two junctions' revers break down characteristics are similar. This is a fun project to demonstrate the use of a pair of bipolar transistors operating in their negative resistance region to simulate a diac triggering a triac in an incandescent  lamp dimmer.  The circuit is not optimized for performance -it is only intended to demonstrate the principle.

Find updates at www.projects.cappels.org


This project uses lethal voltages.  If you are not experienced in working with lethal voltages, read this project, but don't build it. You only have one life, and AC power can take it from you very quickly. Or leave you with terrible injuries.

When working on high voltages, remember:

Keep one hand in you back pocket. Don't provide a path through your heart.
• Use and insulated mat or proper insulated footwear (bare feet on a tile or concrete floor doesn't make it.)
• Make sure your equipment is in good repair and is properly grounded.
• Never
assume a conductor is safe to touch.
• Don't work when alone.
• When working with AC line voltages, use a ground fault interrupter.
• Never work on high voltages while under the influence of alcohol or drugs.

Do not build this project if you are not experienced in working with lethal voltages.


The Principle of Operation

The waveforms above should look like the last 90 degrees of the positive and negative half cycles of the AC line, but this is what the hot AC line actually looks like in my neighborhood. No wonder some jurisdictions require appliances to meet power factor requirements.  By the way, be careful to consider your scope probe's voltage rating when probing the AC mains, even through an isolation transformer, since the peak voltage may be higher than the probe or scope's rated voltage. I used a X100 probe rated at 1kV.

I'm not going to spend much space on this because this is not a beginners' project and the operation of this kind of lamp dimmer is covered elsewhere on the web.

The photograph above shows the voltage across an incandescent light bulb that is being driven by the triac dimmer circuit. Notice that the lamp is only being driven 1/2 the time of each half of the power line cycle. That means that the lamp only gets half the average power. The fraction of each power line half cycle applied to the lamp is determined by the time within the half cycle that the triac, which switches the voltage across the lamp, turns on. When the voltage goes to zero, the triac turns off and waits for the next trigger pulse. That's how this kind of dimmer works. But varying the phase within each half cycle at which the triac turns on.

The Dimmer Circuit

In the classic lamp dimmer circuit, a diac would be
used in place of the two parallel 2N2222 transistors.

Everything that happens within this circuit happens within one half of a power line cycle. Generally every half cycle looks about the same, at least they will if everything is working correctly.

Within each half cycle, the power line voltage charges the .068 uf capacitor through the 250K pot. The voltage across the .068 uf capacitor further charges the .047 uf capacitor through the 68k resistor, until the voltage across the .047 uf capacitor reaches about 10 volts. When the voltage across the .047 uf capacitor reaches about 10 volts, one of the two 2N2222 transistors abruptly switches from a non conducting state to a very low resistance. Being a low resistance, it places the .047 uf capacitor across the trigger terminal and main terminal 1 of the triac, turning the triac on for the rest of the half power line cycle. The lower the resistance of the 250k pot (used as a rheostat), the earlier in the cycle the voltage across the .047 uf capacitor reaches the triggering voltage. Varying the resistance of the 250k pot varies the phase of the trigger pulse with respect to the phase of the power line, thus achieving a variation in the average on time of the triac, which results in a varying duty cycle across the lamp.

Often, the function of the trigger, that provided by the 2N2222 transistors, is provided by a diac, a specially designed semiconductor with a structure similar to that of a transistor, that abruptly changes from not conducting to conducting when the voltage across it exceeds some specific value. I wanted to do this with transistors to show that, in a pinch at least, transistors can be used for this function.

Two transistors were used in place of the one diac so that one transistor would break down on one half cycle and the other transistor would break down on the other half cycle. The break-downs occurs in the reverse biased emitter-base junction, which is a much lower voltage than the reverse biased collector to base junction.

In this circuit, the 2N2222's provided pulses of about 1 microsecond with amplitudes of 100 to 200 milliamps to the gate of the triac.

Since the behavior of transistors in their negative resistance region is not specified by transistor manufacturers, at least as far as I know, it is not wise to design a product around this concept.

This circuit does not have a large dynamic range. Most likely this range can be extended by increasing the value of the .068 capacitor. This circuit can also stand some refinements, such as the addition of a resistor between the gate and main terminal 1 to reduce the chances of spurious turnoff, and possibly some transient protection.


The circuit was assembled on a piece of pre-punched vector board -yes, real Vector board made by Vector corporation back in the 1980's. You can use other kinds of board as long as they can stand off the high peak voltages.

The circuit side of the dimmer circuit - not many connections.
The plastic knob is a very good idea from a safety standpoint. I would not risk
my life on the bet that a cheap pot made for use as a tone control has good
enough insulation at 340 peak volts to prevent me from getting a shock.

  This is the component side of the dimmer circuitversion . The potentiometer is a dual potentiometer,
only because that was the only 250K Ohm pot that I had on hand. The 68k 1 watt resistors was made up of a series-parallel arrangement of four 68k 1/4 watt resistors.

The .o47 uf capacitor is a polyester film capacitor, but that's only because I have a lot of them. Its not really critical. The .068 uf capacitor is critical. Since it has hundreds of volts across it with some settings of the 250k pot, it is rated at 275 volts AC. It is also an "X" capacitor - one this was designed for use as a filter capacitors across the power line. These capacitors are designed to survive high voltages and even transients that cause temporary shorts, without catching fire. I suggest only using capacitors marked for use as "X" capacitors or "Y" capacitors -even better because they are designed to go from line to ground - in this sort of application.

The Motor Speed Contrl Circuit

As with some of my other projects, this one started out as an experiment to prove something to myself or just for fun. As it turnede out, bought a new weed trimmer - the kind that spins a length of plastic fishing line at high speed, and this cuts right through most weeds and grasses. The motor turns at such a high rate of speed, it makes a whole lot of noise. I had leared many years ago, to rapidly pulse the motor on and off so it spun at a lower rate of speed. This had a lot of advatages. For one, it was not so noisey, and it was much less likely to disturb my neighbors. Another advantage was that the plastic fishing line didn't break nearly as often, making the job go a whole lot faster. When cutting heavy growth, I needed to hold the switch closed so the motor could run at its maximum power and speed.

Sometime after finishing the lamp dinner experiment, I realized that if I could also use this as a motor speed control, I might be able to run the motor on my weed trimmer a little slower and get the benefits of less noise and longer finsihg line life at the same time, just as when I pulsed the power switched on and off. After this idea floating into and out of my conciousness over a period of months, I finally got around to trying it. It worked fine.

The only difference between the lamp dimmer circuit and the motor speed control version is thte .01 uf capacitor and the 1.2k resistor I added as a snubber circuit. The snubber limits how quickly the voltage changes across the triac's main termainals when the triac switches off. If the voltage were to rise too quickly, it could result in destruction of the triac.

One hitcht that I started with a single 820 Ohm 1/2 watt resistor in the snubber circuit. After a short test run, I opened the plastic enclsoure and smelled what we called "ode to Allen Bradley," the smell of burned resistor. The snubber resistor had become noticeably more brown. I changed it to the six resitor comibation shown in the schematic, and the resistor seems to be fine. The value of the damping components depend on the needs of the triac and the motor charactaristics. In this case, I just picked some good sounding numbers and gave it a try. After all, I have several spare triacs.

The remaining hitch was that after a short test run, the triac had become quite warm. I know that semiconductors can run with pretty high junction tempeartures, but I always liked it when it wasn't painful to touch a running semiconductor. In truth, I don't know how hot it got because I unplugged the circuit before opening the case and feeliing the triac.

I used some scap alminum to make a quick heatsink. After edging 20 meters of lawn, the triac seemd to be just fine. 

If you decide to try to use this as a motor speed control, be prepared to make adjustments to the snubber circuit, and maybe add a heatsink to the triac itself. By the way, the triac that I used is one of those with a completely insulated tab, making an insulated washer unnecessary. I like those.

The motor speed control version includes a resistor and capacitor
that are absent in the lamp dimmer version.

The Test Setup

I really dislike projects that involve AC line voltage. They are dangerous, difficult to troubleshoot, and sometimes painful. Here is the setup I used on this project (image above). The ground fault interrupter is there in case of accidental contact with the power line or in case of a failure of the insulation in the isolation transformer. The isolation transformer allows me to ground one side of the circuit safely and observe the circuit with an oscilloscope. For the motor speed control, the circuit was first tested with an incandescant lamp and this isolation circuit.  After the circuit worked, I plugged in the weed trimmer without the isolation transformer,and I did not probe the circuit without the isolation transformer.

In my test setup, I am first and foremost, careful to not get shocked. Decades of being shocked, ever since I was a young child, have conditioned me to dislike the sensation immensely. I use an ground fault interrupter (Also called a "GFIC"), to shut off the current within a few tens of milliseconds, just in case I come in contact with the mains voltage. Since the house I live in does not have Ground Fault Interrupter  on all outlets, I bought a Ground Fault Interrupter circuit breaker at the hardware store and but it in a plastic box, along with male and female AC power connectors. Ground Fault Interrupters are sold in this configuration in some places. I could not find one already assembled in Thailand, so I had to build my own.

The Ground Fault Interrupter is also a circuit breaker, so it offers some protection against gross overloads.

By the way, I have a second ground fault interrupter like the own shown above. I use it outdoors with the electric lawnmower, the weed trimmer, and my electric drill.


I made my own isolation transformer by connecting two rectifier transformers back-to-back, as shown in the schematic above. These were 24 volt center tapped transformers that I picked  up a Amorn, a surplus dealer that has outlets around Thailand. Since I don't have a lot of faith in the integrity of the insulation of surplus store transformers bought in Thailand, the addition of the ground fault interrupter gives me some peace of mind.

The transformers are pretty small, only about 1 VA, and this is a good thing, because it limits power to the circuit under test.

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Contents ©2007 Richard Cappels All Rights Reserved. Find updates at

First posted in September, 2007. Revised May, 2008.

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