Dealing with Bipolarity

September 5, 2011 12:13 PM

I am in need of bipolar power supply that is +/-5vdc to work with an electronic device that is not important (thus I'm not worried about frying it). I was wondering if it would be possible to get two 5 volt adapters and wire one into the +5v circuit and the other into -5v circuit with the corresponding -/+ wires going to ground?

Sometimes I think I know stuff about electronics, but for the most part I'm clueless.

posted by cloeburner to technology (14 answers total)

Yes, this is possible in principle. There are some caveats.

I think it's mostly that the outputs of the 5V supplies must not be connected to input ground (or have any other galvanic reference to a common ground). If they are floating, then the supplies will cheerfully accept the configuration, and won't know the difference. If an output pin *is* connected to input ground, then you'll create a short circuit.

BUT! I'm not sure, nor do I have the complete picture.
posted by krilli at 12:26 PM on September 5, 2011

It might be possible, but only if the adapters don't have the (-) side of the output tied to ground or neutral internally. I've never tested that.

I think most standard two-prong "wall warts" probably won't, because they lack a ground and tying to neutral would probably be a Bad Thing in terms of electrical codes. But you would want to do a smoke test and see what happens if you ground the (+) side and then put some load across from the (-) side to ground (keeping in mind the adapter's rated capacity and giving a healthy safety margin).

Some very old ATX (but not newer "ATX 12V") PSUs have both +5 and -5 rails; it might be worth taking a look at any that you have sitting around to see if you have a solution without a whole lot of hacking.
posted by Kadin2048 at 12:29 PM on September 5, 2011

This is the unit. It features a proprietary power supply that is hard to obtain. There are several different input grounds on the board, they do not appear to be connected, I believe all I have to do is connect to the different grounds.
posted by cloeburner at 12:31 PM on September 5, 2011

FWIW, there are a lot of +/-5VDC power supplies available that won't require any potentially dodgy hack (though I LOVE DODGY HAX)

1
2
3
4

etc.
posted by fake at 12:44 PM on September 5, 2011

Is the power supply just MIA or is it dead?

Building a power supply that does exactly this thing would not be that difficult. You'd basically need to build this only replace the 7812 and 7912 rectifiers with a 7805 and a 7905 (because you want 5 volts not 12 volts), and you'll want a transformer that takes you from 110 to somewhere in the 8 - 20 volts range rather than what they describe in the picture (or you could run an extension cord to the UK).

The 7805 and 7905, diodes and capacitors are likely to cost you two or three bucks. The difficult (but hardly impossible) task will be finding a center tap transformer that is exactly what you want.

On preview, one of the supplies fake links to will probably get you going faster, but it won't be an adventure!
posted by Kid Charlemagne at 12:51 PM on September 5, 2011

Take notice, the first 2 of fake's links are supplies that take 18v AC. You would need a transformer to convert from your household AC supply to 18 volts. I'm curious to know where an 18v AC source would come from.
posted by scose at 1:37 PM on September 5, 2011

Great catch scose. Of course all you'd need is a transformer - like the second one on this page:

http://www.minute-man.com/acatalog/18V_AC_Power_Transformers.html

Just put 120VAC in, get 18VAC out.
posted by fake at 2:28 PM on September 5, 2011

More power supplies
posted by buzzv at 3:08 PM on September 5, 2011

To answer your question: Its possible depending on the supply, the supplies I use in the lab are built for such a task, but standard wall warts are not and you risk the negative being grounded so you would have to test the supplies to see if that's the case.

I am Nthing make your own out of an ATX power supply. Here is an instructable. If your comfortable with electronics it should only take an hour or two to put together, the parts needed are cheap and available at radio shack. I have done this multiple times to build power supplies for my projects. Just make sure the supply has a -5V rail, should tell your the rails and rail current ratings on the side of the power supply.

If you do decide to go this route mefi mail me with any questions.
posted by token-ring at 5:30 PM on September 5, 2011

Thanks, I think I will build it, this schematic looks pretty solid. Google Docs Link. Where would I go about finding the transformer? I've got a few laying around, but there's no information on them. I'm sure with research I'll answer this question soon. Thanks for all the help.

Radio Hut has the 7805 regulator but they don't have the 7905, I might order online and just wait on this project.
posted by cloeburner at 7:11 PM on September 5, 2011

Oops, PDF of the broken Google Docs link.
posted by cloeburner at 7:14 PM on September 5, 2011

If it is true, as you say, that you are clueless about electronics, I would not recommend building your own power supply from AC mains, for safety reasons.

Your original plan of connecting two 5-volt supplies back to back for your +5 and -5 is the simplest and safest. You don't indicate the current requirements, but you may find that you can use a couple of wall warts that you have laying around. As long as your supplies only have two AC prongs, without a ground, you should be okay to connect them as you planned.
posted by JackFlash at 9:08 PM on September 5, 2011

Lotsa transformers available- here's one and this outfit has all the other parts you'll need. This has enough current to power that puppy; the label in your link says it requires +5v at 500ma, -5v at 600ma.
One caution- the 7805 - 7905 regulators are rated at 1.5 amps, but for any current over about 100 ma they DO NEED HEATSINKS. They'll get hot. An aluminum project box ought to be enough area to keep em cool; you will also need to insulate the tabs from the box, use thermal compound and this.

I would not recommend using two random wall-warts without knowing exactly what's inside 'em. For one thing, if they contain only transformer/rectifier, the output voltage is not regulated and may be high enough at low loads to damage the device. (if it sez 5v at 350ma, it may output, say, 10v at a 50ma load. I've seen that.)
posted by drhydro at 9:47 PM on September 5, 2011

Every 2-prong wallwart I've seen has been floating/isolated, by the way— presumably they have to be, or else if you plug it in backwards (which you will 50% of the time if they don't have a ground pin or polarized plug) you'll end up with a "ground" at 120VAC.

Random transformers, BTW, can still be had from Radio Shack, or from hardware stores as doorbell transformers. Do not omit the fuse on the mains side!
posted by hattifattener at 10:28 PM on September 5, 2011