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

Question

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.

Answer 1

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.

Answer 2

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