# Use a single-output floating supply as a dual relative to ground

I have a single output 15V supply that is isolated with respect to any ground. I'd like to use it to produce something like +12V and -2V with respect to the ground of my circuit. The current required is less than 500 mA.

Is this possible, and if so, what's the best way to tie it to my circuit ground?

I'd like to use it to produce something like +12V and -2V with respect to the ground of my circuit.

Depending on how much current you might be expecting to take from the 12V (or the -2V) to 0V (ground) a beafier version of the following may be needed.

From the 15VDC uses a voltage regulator to produce 2V relative to the most negative lead on the 15V DC supply. Connect this output to 0V on the rest of the system.

You now have a -2V rail (formerly the most negative lead on the isolated 15V supply). By inference you also have a +13V rail that can be regulated down to +12V with a low drop-out regulator.

Or maybe you use one of these: -

And then a negative voltage regulator to get -2V

• Bear in mind that LDOs etc won't work for this - they can source current but not sink it, so it won't work if the load on the '12V' rail exceeds the load on the '2V' rail. A power opamp may be a better choice. Jun 5 '15 at 10:19
• @NickJohnson yes I know so maybe the OP can give some idea. If the 2V rail is low power then a simple zener regulator may do the job or even a synchronous buck regulator - this operates in push-pull so would handle asymmetrical currents OK. Jun 5 '15 at 10:24
• Yes, the 12V line will be higher power than the -2V. Can I instead use a 12V LDO from the V+ output of the power supply? Jun 5 '15 at 10:47
• It's not the power, it's the current - will the current demand on the 12V always be greater than the current demand on the -2V? If so then an LM7905 type negative 12V regulator would work - the output of the regulator would connect to ground on the main system and your former +15V would be regarded as +12V. The most negative lead of the 15V supply would become -3V (requires one more regulator to drop to -2V). Any doubts about this and you'll probably need to go to synchronous buck regulators or power op-amps. Jun 5 '15 at 10:53
• The current demand on the 12V is also higher than on the -2V. This is pretty much for sure, but I'll probably make a proto circuit to test Jun 5 '15 at 10:55

Although the schematic above is nice, I think it's too complicated. Also there's a transformer and I only use those when I have to ;-)

I would be looking at something like this:

simulate this circuit – Schematic created using CircuitLab

But this simple configuration would only work if the current in and out of the ground on the right was a few mA. If you need to be able to supply more current through the ground one or more transistors will need to be added ending up in something like:

BTW, in case you have an old fashioned audio power amplifier IC lying around (TDA 2003 etc) then you could use that instead of the opamps + push-pull stage. As a bonus you will get short circuit and temperature protection :-)

• If you needed more current from the top schematic, why would a buffer op-amp not work? Jun 5 '15 at 15:09
• Only if the buffer opamp can deliver a higher output current than the opamp that is used now. And then I would just replace the opamp bij the buffer opamp. It's not that the opamp needs extra buffering. It needs a more powerful output stage. Jun 5 '15 at 15:13
• Thank you for that! Would something like a OPA544 work? I have a ton of those lying around. Jun 5 '15 at 15:19
• @Mr Mascaro Looking at the datasheet for the OPA544 I see one big limitation and that is the input and output voltage range. It can only work between positive supply - 4V and negative supply + 4V. So for opening poster's design it would be unable to make +12.5V, gnd, -2.5 V. It might be able to do +7.5V, gnd, -7.5V though ! The OPA544 is intended for high supply voltages and then such limited range is acceptable. So if you'd have 30 V and wanted to make +15V, gnd, -15V I think the OPA544 would be an excellent choice ! Also temp. protection etc. Nice :-) Jun 5 '15 at 21:09

Assuming most of the current flows from the +12V to GND, you could use an LM7912 to give you the GND rail and then the minus input would be at -3V, which you could regulate to -2V with another negative regulator (perhaps a TLV431 shunt regulator if you only need a few tens of mA on the -2V rail). Of course the LM7905 will dissipate about 1.5W worst-case, so a smallish heat sink would be required.