Virtual ground for multirail power supply - Page 1
Forum section: electronics / projects

author: hlavac
author_rank: Frequent Contributor
timestamp: December 14, 2012, 07:41:42 pm
content: My potential OSHW universal programmer project needs a programmable power supply that can do two 0-25.6V +-0.1V rails, and +5V to power the microcontroller. I want the variable supplies to be able to both source and sink current. I came upon a nice power opamp that could do it -L165V from ST.However, it needs power supply at least 3V above and below of what it will work with. That means +29, -3V.I would have to go for a 30V AC transformer to supply the power to this. I can't get hold of anything other than 2x15V one for a reasonable price.Now to the poblem: All I have is 30V AC, rectified to about 42V DC. I need a virtual ground, and not in the middle even! (Lets ignore the +5V rail for the moment)I would need two caps in series, with ground in the middle, and make some balancing circuit to split the ~42V rectified to charge the upper one to ~+33V and the lower one to ~-9V... then put normal linear regulators against the virtual ground to turn it into stabilized +29, -5 rails - right?

author: hlavac
author_rank: Frequent Contributor
timestamp: December 14, 2012, 08:21:39 pm
content: Alright, I came up withthis thing. Not much margin for regulation, hardly the 3V needed, will probably need LDO linear regulators....

author: chrome
author_rank: Regular Contributor
timestamp: December 14, 2012, 09:08:50 pm
content: I would go for a switched PSU for something like this, you hardly need a lot of current to program most devices.That can also be totally floating.

author: hlavac
author_rank: Frequent Contributor
timestamp: December 14, 2012, 09:24:26 pm
content: A littleimproved version.I'm shooting for extra cheap here, not sure switched supplies would be possible...

author: mamalala
author_rank: Unknown
timestamp: December 15, 2012, 02:27:55 am
content: Check the ICL7660. +5V in gives you -5V out. It's a really cheap chip.Also, if you have a µC anyways on the board, you can build yourself a simple chargepump with two Schottky diodes and small caps, using a PWM output of the µC:http://elm-chan.org/docs/lcd/lcd3v.htmlShould be enough to just supply a few mA for the negative rail of the OpAmp.Greetings,Chris

author: LEECH666
author_rank: Frequent Contributor
timestamp: December 16, 2012, 11:52:58 am
content: Not sure if this exactly meets your requirements, but I recently build up a small virtual ground circuit that should* be able to sources(/sink?) a couple of 100mA.*(claimed by the the webpage I found this circuit on, not tested yet)You can adjust the voltage level of the VGND with the input resistor divider, but I am not sure if that would cause any issues with that circuit.https://www.eevblog.com/forum/beginners/trouble-adjusting-op-offset-voltage/Post #6 and #7 in that thread.You can also take a look here for some virtual ground circuits:http://tangentsoft.net/elec/vgrounds.htmlHope that helps.

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2012, 12:11:09 pm
content: If your transformer secondary is floating just use a 3V9 1W zener diode to give the required negative rail. A 220uF capacitor across it and you will have a floating ground capable of handling around 200mA total. If the whole circuit draws more either use a 5W zener or add an external NPN or PNP pass transistor to boost the capacity of the zener diode. Simple, cheap and reasonably robust.