Tuesday, August 26, 2014

Transformerless Power Supply Unit

Good afternoon everyone, hope you guys are doing great. Right now I am going to write about something which can be very simple to make but very useful and it is a transformer-less power supply circuit diagram.


We all have used power supply unit that uses transformer to step down High voltage AC to a Low voltage AC which then be rectified to be used with our devices. That diagram is pretty good for using, with almost any application, but The major problem with that diagram is the fact that transformers are bulky, even if we want a very small amount of current you will be needing a large transformer for doing that. Just omitting the transformer will give us a smaller,lighter and compact power supply unit but how can you do that? Let's dive deeper.

In this post I will be posting mainly two different diagrams in four images.Starting out with the first diagram of-course.

Basic Idea:

So, Basic idea for this diagram is to use a high voltage capacitor's Capacitive Reactance to lower the voltage. You can think of this Reactance as resistance so there will be a voltage drop across the capacitor and we will be able to use the rest. The reason for not using a resistor is the because it will burns a lot of power comparing to capacitor as capacitors current and voltage do not stay in phase.


How it works:

Simple and straightforward circuit diagram. In this diagram The 1uF 400V capacitor is the main capacitor that will drop most of the voltage across it. 1Megaohm resistor is the bleeding resistor. We know capacitor can hold charge, and as this is a high voltage application the voltage stored by the capacitor will be sufficient to give a powerful electric shock. So the resistor will bleed the power stored in it when the AC supply is off.

Next the bridge rectification is done by 4 general purpose rectifier diodes. Hope you know how to do it. If not then follow this link.

next the 100Ohm resistor will drop some more voltage and you have to use a high watt capacitor to make sure it won't get burnt.

The 10uF 50V capacitor will smooth the output somewhat. As the output contains huge amount of ripple, it needs to be smoothed.

The Zener Diode will regulate the voltage to a certain level. See how it is done, click here.

This diagram can be used for providing about 100mA of current. If you need more than that use a 2.2uF capacitor and a high watt Zener diode.

Now lets modify this diagram slightly.

This diagram is also same as the previous one. Just added a couple of extra items.

Like the 0.1uF capacitor which will filter out AC noises partially. And a larger smoothing capacitor will give better voltage with lower ripple.

Next a 14V Zener is used also a general purpose diode is used. Both the elements will drop certain amount of voltage, after that we will get around 12V.

And as we have used capacitor rated at around 3uF in the input section, we will get slightly higher current.

Next I will show another diagram which is slightly different from this one but can perform the same.

This diagram uses a transistor as its active component. Lets take a look at the diagram.

This is also a very straightforward diagram. The main line AC is fed into the circuit the first diode rectifies the voltage and make it DC.

The 4.7uF capacitor smooths out the voltage somewhat. Remember if you use bridge rectifier you will need a higher rated capacitor, like 400V or above.

100Kohm resistor drops the current to a very lower level to be regulated and used on the Base of the transistor. 10uF capacitor smooths it slightly.

Then we get the output voltage which has moderate amount of current for running small stuffs like couple of LEDs, ICs or even charge batteries.

If we want a variable power supply we can just modify it like this.

This diagram is pretty much the same as the earlier diagram. Just like the earlier diagrams, a couple of things are added. Like a 0.1uF capacitor which will lower the high voltage noises, somewhat.

Next the variable resistor let's you choose the voltage from 0V to 24V. The Zener regulates the voltage to 24V as a 24V is used. The rest of the part is same as the earlier diagram.

The reason for using a MJE13005 transistor is its very high collector emitter voltage.

What if we want a dual polarity power supply?

Then this simple diagram can be used. Here we can change the value of Zener diode and the resistor to get different voltages. Lowering the value of resistor will allow us to get more voltage and with more voltage we have to use larger capacitors too. This circuit will provide around 30mA or so.


First advantage of this type of power supply diagram is the size of it, its very small so can be implemented anywhere.

This circuits can provide a maximum amount of 300mA which is enough for powering small integrated circuits, light emitting diodes or even charge batteries. You can build one of it right into the wall socket and get desired output from it.

Another advantage of this diagram is, its cheap.

Does not produce much heat.


It can't provide high amount of current.

Shock Hazard! As this circuit is not isolated from the main AC line it will surely give you lethal shock. Don't make this circuit if you don't know clearly about these things. Even if the output is siting at 12V or something if you touch it you will get hurt. So, don't touch the running circuit. And make sure the whole circuit is housed in a insulated case or box.

It won't give your device protection against surges.

So, experiment safely. That's all.

Transistor MJE13005 datasheet here.

My other posts here.