# How can I make this variable power supply go from 1.25V and -1.25V to 0?

I'm trying to learn and in this case I have a power supply circuit that has a fixed 5V, a 1.25 to 12V and a -12V to -1.25. I understand that the regulators I'm using have a minimum limit output of 1.25V, but I'm trying to figure out if there is a way to make that value go to 0V. R3 is to control 1.25 to 12V and R2 for -12 to -1.25.

From what I've been reading since I already have positive and negative rails available I could use a shunt regulator or zeners, but I can't make it work.

Sorry in advance if the design burns your eyes I'm trying to learn the basics.

• Why the 30R to ground? You realize that this resistor will constantly burn away a significant amount of the current that the 7805 can deliver, yeah? These shouldn't need a minimum load to work. May 11, 2022 at 6:24

Since you have supplies of both polarities, you could make a regulated -1.25 V supply and connect R3 to that instead of to Ground (and do likewise for the negative supply). These +/-1.25 V supplies would only have to provide 10 mA or less each.

• Yes, that was my original idea on how to solve it, but I have to be getting something wrong since I ran into some problems and I can never make it work properly. Do you mind explaining a little bit further how would it work in this case? May 11, 2022 at 3:33
• I was able to get it down to 52.8 mV, no more. May 11, 2022 at 3:50
• Doing what I suggest will move the "zero volt" reference for the positive regulator below ground by 1.25 volts (or whatever voltage you provide). If you can only get the output down to 52 mV, you need to get the "-1.25 V' point a little more negative. May 11, 2022 at 4:04

How much current do you need? A buffered op-amp could source or sink between +12 and -12.

This answer has some circuit ideas that could do the job: How to increase the output current of a dual supply (virtual ground) circuit

You could modify any of these to manipulate the 'virtual ground' reference to be your desired voltage range.

I did some hacking on the Sziklai pair version and came up with 0 to 12V and 0 to -12V versions (simulate them here):

Design notes:

• These are biased to supply up to about 4A max
• The op-amps can be an ordinary ones like a uA741 or LM358