Power Supplies

All electronic equipment runs off electrons, but where can you get them from? A good, in deed the only source, is a power supply but what sort?


Power Supplies

Multiple Power Supplies

Having a electronic circuit requires by definition some sort of power supply to make it work, there are many options from the simple single battery cell to a fully regulated, multi rail current regulated, overload protection bench supply. This page offers a quick round up of power supply ratings and how to use two supplies in a system.


The simplest for of power supply is one you tap from somewhere else. With the Arduino being connected to your computer using a USB connection it is your first stop. The USB port should be able to supply you with 500 mA of current at 5 volts, although some lap tops limit you to less than this. This is fine for most initial applications but what happens if you need more current, have to have a bigger voltage or want to make your project independent from your computer. Then you have to bring another power supply into the mix.


Every power supply has a rating it describes how much voltage it will give out and how much current IT IS CAPABLE of giving. So don’t think because it says it will supply 3 Amps it will always supply 3 Amps. It will only supply that much if it has a load with a low enough resistance. If the load has an even lower resistance it will want to take more current than the power supply can give, we say it is OVER LOADED. Therefore if your system takes only 1 Amp then you can use any supply that provides anything greater than 1 Amp.


Power supplies come in two types, regulated and un-regulated. A regulated supply has electronic components in it to ensure that the voltage output is always the same (or within a tight limit) what ever current is being drawn from it. Un-regulated supplies have no such electronics and the voltage they give will depend on the current that they are supplying. Generally as the current increases the voltage drops or sags. Normally you will prefer a regulated supply but note that a battery is an example of an un-regulated power supply.

Normally an un-regulated supply will be regulated by some electronics in the project so it is better to use regulated supplies. However, even regulated supplies might need regulating down to the correct voltage for a particular part or whole of your circuit. This can be done in two ways, with a liner regulator or a switch regulator.

You don’t need to go into details at the moment of exactly how they work, in fact a switch regulator is often one of the more complex circuits and there are specialists who do nothing else but design them. What is important to know is what difference it makes to you. Basically it is a matter of efficiency, in converting the input voltage to the output voltage. The difference between the input and the output voltages is known as the voltage drop, with a liner regulator the voltage drop times the current flowing through it is all converted into heat. We say the excess energy is burnt off. This heat is not only wasteful but it has to be got out of the circuit using large heat sinks. This is fine for small voltage drops and medium to low currents but it becomes impractical for large currents and large to medium voltage drops.

A switch mode regulator on the other hand has a conversion efficiency usually between 80 to 90% so with a regulator of 90% efficiency only 10% the power in the whole circuit is burnt off as heat generated. However they are more expensive and harder to make. Fortunately there are many ready made switch mode regulators nowadays.

Two Supplies

One common situation is where you need a voltage bigger than the 5V you get from a micro controller. This might be say 12v to drive a solenoid or relay. You need another supply and some way of boosting your 5V signal like a transistor or FET. Most beginners do something like this:-


The reason it will not work is that you have two supplies one for the Arduino and one for the solenoid. In order for the transistor to turn on current has to flow through the base to the emitter. But there is no path for the current to flow round from the Arduino through the transistor and BACK into the Arduino. There is a break in the path for that. Therefore no current flows because there is no path for it to flow round. And so the transistor and hence the solenoid is never turned on.

What you need to do is to provide a path that will allow the current to flow through the transistor so that it is turned on.

When using an extra power supply:-

Always connect the negative terminal to the ground

Unless, of course, you know what you are doing, and you know you don’t want to do that. If you don’t know what you are doing you will never want to do any different.