# Get +12/-12VDC out of 24VDC Supply

I'm trying to use a single 24VDC power supply but rig it to give a heating coil the equivalent of 24Volts but at +12/-12 in reference to ground for safety reasons. Is this as simple as a voltage divider with a high value resistor to ground? Would that work or just kind of short out the supply? Is the negative terminal of a power supply normally connected to ground or is it floating? Let me know if there's a cheap, simple way of accomplishing this aside from buying two 12V power supplies or one that outputs +/-12VDC. Thanks.

• "For safety reasons"? You still have a 24V difference. – Ignacio Vazquez-Abrams Dec 28 '14 at 20:13
• But if it shorts to something that is grounded you have a max of 12 volts difference – user2152277 Dec 28 '14 at 20:15
• Have you verified whether or not the supply is isolated? Because if it is then your whole "safety issue" goes right out the window. – Ignacio Vazquez-Abrams Dec 28 '14 at 20:18
• Yes I believe it is isolated, it's a desktop power adapter from 110 VAC so there's a transformer. Does that mean the 24vdc is floating with respect to ground? Would that mean it would be safe for a grounded person to grab an exposed 24vdc lead with wet hands? The circuit will be operating around people with the potential for water to be spilt on it. – user2152277 Dec 28 '14 at 20:29
• Just having a transformer isn't enough for it to be isolated; power supply return could be connected to earth ground regardless. – Ignacio Vazquez-Abrams Dec 28 '14 at 20:35

It depends on your power requirement. This question is a very classic one, a quick search on Google with the right keywords gave me this first result which explains why a simple voltage divider is not enough in most cases: the more you try to minimise the quiescent current of the divider, the less stiff the symmetry of the rails is (there is an offset in the virtual ground), as the offsets will increase with current draw.

Below are two solutions suggested by the author.

Even there the voltage divider should be replaced with a voltage reference (e.g. TLE2426) if the power draw is asymmetric (but it sounds like it won't).

You'll certainly recognise a push pull structure here, common in audio.

However this will NOT make the system safer, as there is still 24V across the heater. But 24V is still safe-ish in a dry environment.

• Power is pretty high: 160-200W for a heating element. What were the key words you used for the search? – user2152277 Dec 28 '14 at 20:34
• Split power rails I think, though virtual ground would have certainly been a better choice. You definitely won't be able to use a voltage divider alone with that kind of power, unless you're ok with a kilowatt of quiescent power... – Mister Mystère Dec 28 '14 at 20:35
• It's unlikely that you'll find an integrated opamp with this kind of power output though (solution 1). Is it worth it to split the supply, that's the question. – Mister Mystère Dec 28 '14 at 22:04