Small variable power supply

I was wiring up my bench power supply and got a nifty idea of a circuit for small loads. Small loads as in 1 volt to maybe 12 volts with a maximum of say 500mA. With 1A it would be pushing it.

varable regulator

The circuit is simple. You can use a 7812 or a 7805 regulator and then you have the ability to adjust it. You could slap on a wall-wart and have a universal adjustable unit or even for something crazy such as a USB voltage adjustment. This would also work great for variable fan control for a PC. Not to mention a small battery powered variable supply.

While playing with this circuit you need the 1K ohm resistor on the output or it will be unstable for the control. I tried other resistor tastes and 1K seems to do it just fine. The capacitor on the output isn’t really needed but if you plan to use a 78xx regulator then I would advise to use it. With my test with a 7805 regulator I was able to get a maximum voltage of 4.25 volts and 210mA. When I tried a 7812 I got 11.14 volts at 350mA.With a un-regulated wall-wart with a output of 14.75 volts and 1.2A I can get around 12.42 volts and 310mA. However that was too much punishment for the small 2n3904 BJT.


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