Friday, April 26, 2019

Simple Voltage Doubler Circuit

Let's make a voltage doubler circuit with minimum component and try to understand how it works. Well that might sound like it's a theoretical circuit but for small amount of current it can be a practical as well.

Components required:

1. Transformer - 220V to 12V step-down line-frequency.
2. Diode - 2 1N4007.
3. Capacitor - 2 4700μF 25V, 1 4700μF 50V. 


How does it work? 

It might not look easy the first time but actually it is a simple circuit. During the Positive half cycle Capacitor C1 charges through Diode D1, and it charges to the peak incoming voltage which let's for argument's sake say that is the same on the AC side. Then on the negative half cycle Capacitor C2 charges via Diode D2 to the peak value as well.

Now on the load side it will see these two capacitors in series and the way they are charged their voltage will add up. So, load will actually see double the peak voltage as each capacitor was charged at the peak voltage.

So, if we assume that we have 12V AC, C1 will charge at 12V and C2 will also charge at 12V(DC of-course as they are charging via Diode). Together they will produce 24V. You can use KVL on this side to confirm the voltage as well.  C3 is there just to smooth out the output voltage. 

Some of you might be wondering about capacitor voltage rating right now. C1 and C2 are rated at 25V because they will only be charged up to 12V each, so no need for going above 25V but C3 is actually seeing the voltage that is combined of those two capacitors. So for C3 we need higher rating. 35V would do here but 50V is fine as well.


1. This will only work with AC input, not on DC.
2. Different input voltage will yield different doubled output voltage but make sure to use properly rated capacitors and diodes. 
3. Not suitable for very high current application. 

Monday, April 1, 2019

Simple NiCd Battery Charger

With the advancement of battery technology Nickel Cadmium cells are kind of obsolete now mostly replaced by Nickel Metal Hydride or Lithium ion. Yet if you have some old NiCd cells that work, you can definitely use them. However one must be cautious because NiCd cells contains Cadmium which is not good for environment and human. If these type of cells are to be used they need to be charged so let's look at a simple circuit that will serve this purpose.

Component list:

1. Transistor - 1 BD140 with heat sink and thermal compound.
2. Diode - 2 1N4148.
3. Capacitor - 1 1000μF, 1 0.1μF.
4. Resistor - 1 220Ω 1W, 1 10kΩ.
5. Switch - 1 SPST.
6. Power supply - Ready-made 12V DC supply or a 12V Center Tapped Transformer with two 4007 diode.

If the power supply in use already has an output capacitor the 1000μF one can be omitted.


How does it Work?

In this diagram Transistor is biased using two 4148 diodes and provides a constant current. SPST switch controls the rate of charge.

For 1.2V NiCd cell charge rate is around 50mA and 100mA respectively in low and high position.

This circuit can also charge 9V(8.4V) NiCd batteries as well although charge rate will go down to 15mA or 30mA.


This circuit doesn't have any overcharge protection so you kind of have to guess the amount of time required for charging and remove battery after that time. Approximately for a 500mAh cell you will need around 12 hours on low settings and 6 hours on high settings.