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12v Fluorescent lamp drivers

posted Oct 7, 2011, 2:28 PM by Jake Vosloo   [ updated Jan 5, 2014, 1:35 PM ]
12v Fluoro Inverter
This is a low-cost project for 20 or 40 watt fluorescent tubes. However the most efficient is to use a 40 watt tube (or two 20 watt tubes in series).

The circuit doesn't require many components but its operation is quite complex. The clever component is the transformer. It performs 3 functions.
Firstly it is acting as a feedback component for the transistor to create an oscillator circuit. Secondly it is providing a high voltage (over 1 000 volts) to strike the tube and keep it struck and thirdly it is supplying spikes of energy to illuminate the tube.
The circuit is shown in figure 1 and we will take a detailed look at how the transformer carries out the three functions.
The transformer in this project is not a lethal device as the output wattage is slightly below the value that produces electrocution. However the output is in excess of 1,000v and the ends of the secondary winding should not be touched when the transformer is operating.
To get a shock you must touch both
ends at the same time - it is not sufficient to touch one end and any other part of the circuit as the secondary is an isolated winding.
Even a simple transformer such as the one we are winding in this project will demonstrate a number of interesting features. One of them is the ability to step-up a voltage. This is the main purpose in this project as we require a voltage of approximately 1,000v to strike the tube.
Another interesting feature is the availability to get positive or negative voltage (phase) from a separate winding on the transformer, simply by connecting the winding around one way or the other. In this project we connect the winding to get positive feedback so that a single transistor will drive the circuit.

The oscillator works on positive feedback. This positive feedback comes from a separate 13 turn winding on the transformer called the feedback winding.
The cycle starts by turning on the transistor a fair bit via the 180R resistor on the base and this causes current to flow in the primary winding. The flow of current causes magnetic flux to be produced by the winding and this passes through the ferrite core. The feedback winding is also wound around the core and the magnetic lines pass through this winding and produce a voltage.
The winding is connected to the transistor so that the voltage from the winding ASSISTS the voltage from the 180R resistor and causes the transistor to turn on harder.
Thus more current flows through the primary winding and the magnetic flux increases. This causes more voltage to be produced in the feedback winding and the transistor turns on even harder.
This continues until the transistor is fully turned ON and maximum current is flowing in the primary winding.
Now comes the important part.
Even though maximum current is flowing in the primary winding and maximum flux is produced in the core, this flux is a steady flux and not an increasing flux.
The only time a voltage is produced in a secondary (or feedback) winding is when the flux is INCREASING (or decreasing). When the flux is stationary, the voltage in any of these windings ceases to be produced.
Thus we come to a point in the cycle where the current in the primary is a maximum but the voltage in the feedback winding is zero.
The only current flowing into the base of the transistor comes from the turn on resistor but this is note enough to fully turn the transistor ON and so the transistor turns off a small amount.
This has the effect of reducing the flux in the ferrite rod and we now have a situation where the flux is DECREASING. This changes the situation in the feedback winding. The voltage in the feedback winding is now produced in the OPPOSITE DIRECTION and the transistor begins to turn off even more.
The magnetic flux begins to collapse very quickly and this produces a very high reverse voltage on the base of the transistor (up to about 25v) to turn the transistor off completely.
This is how we get a positive and negative voltage for the transistor.
This is quite an amazing achievement as the voltage through the primary doesn't change direction - it merely increases and decreases in value - but the voltage from any of the other windings changes direction!
The collapsing magnetic flux cuts the turns of the secondary winding and produces a voltage of about 2.5v per turn in the 450 turns, making a total of about 1 ,000 appearing across the ends of the tube. This is sufficient to strike the tube and as we mentioned above, the resistance (or impedance) of the tube reduces as more current flows. In our case the voltage across the tube is about 400v (this voltage depends on how hard the tube is driven and is riot a fixed value).
When the magnetic flux has almost fully been converted to electrical energy, the 180R turn-on resistor on the base of the transistor starts the cycle over again.
The voltages produced by the transformer are very spiky and the gas in a fluorescent tube is very quick to react to these spikes. The gas produces ultraviolet light that strikes the fluorescent material on the inside of the tube and causes it to produce visible light.
The tube forms part of the load for the transformer and has an effect on quenching the spikes to the transistor so it is not advisable to operate the transformer without the tube connected.


1 – 2R2 1/4watt (for testing)
1 - 47R 1/4watt
1 - 180R 1watt
1 - 47k
1 - 100k mini rim pot
1 - 100ngreencap
1 - 100u 16v electrolytic
1 - BC338 transistor
1 - TIP 3055 transistor
1 - on/off switch
1 - 12mm bolt and nut for transistor
1 - heat-sink 5cm x 10cm
1 - ferrite rod 10mm dia x 8m long
1 - 30m winding wire .28mm dia
1 - 4m winding wire .61 mm dia
(wire diameters are NOT critical)
1 - insulation tape either sticky tape
or masking tape
1 - interlayer insulation - paper


2 - 20 watt fluorescent tubes
1 - box to house project
1 - 6m of figure-8 flex to go between inverter and tube(s)

8 Watt driver

The schematic for the 8 Watt driver will show you that this design is quite different from both previous ones. It uses capacitive ballasting like the 2 Watt driver, and a two-transistor saturation-limited oscillator like the 20 Watt design. But note an important difference: This circuit has a choke added in the DC supply, that produces more effects than you may think: Thanks to this choke, mutual conduction between the two transistors is no longer a problem, allowing for a very simple drive scheme and almost lossless operation. The input current becomes almost clean DC, minimizing further filtering requirements. And the waveform becomes a quite clean sine wave, which gives a tremendous advantage in terms of radiated noise! This lamp can be used in a radio station without causing any interference.


The circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light. The circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp starting method is adcrude but the circuit does work.

12VDC Fluorescent Lamp Driver


40W Fluorescent Lamp Inverter


Part Total Qty. Description
R1 1 180 Ohm 1W Resistor
R2 1 47 Ohm 1/4W Resistor
R3 1 2.2 Ohm 1W Resistor (only needed once)
C1, C2 2 100uF 16V Electrolytic Capacitor
C3 1 100nF Ceramic Disc Capacitor
Q1 1 TIP 3055 or 2N3055 or equivalent
L1 1 See "Notes"
T1 1 See "Notes"