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Old 7th May 2012, 5:42 PM   #1
Thalyn Thread Starter
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Default +/- 5VDC from USB

As people have probably noticed on this forum,over the past few weeks I've been trying to solve a lot of problems caused by using an op-amp with a single supply. Thus far, my attempts have ranged from "almost" successful to not even close. Since I can't seem to solve the problem at the other end, it stands to reason the best way to solve it is at the beginning and try to get a bipolar power supply, thus avoiding all of the earlier problems entirely.

But I have no idea how!

So I ask the good folk of the E&E OCAU forum: how does one create a bipolar power supply from a single DC rail? I've heard all about charge pumps and bipolar DC-DC converters, but I'm having trouble finding out whether such things would be suitable for my needs - or even possible to obtain as individual parts.

Ideally I'm trying to keep cost and size down. Since I'm indending to power a BB OPA2134PA from it, absolute maximum current draw should be no more than 90mA (2x5mA Iq and 2x35mA Io) - and I suspect it will often be significantly less, likely never even reaching 20mA. As referenced in the thread title, the power source will be a standard USB port, meaning a total of 5VDC and 500mA will be available to me (though I should try to keep it under 100mA for compatability).

PS I promise this will be my last silly question on the subject of my side-project. If there's a solution to this part, the rest of it should pretty easily fall into place.

Last edited by Thalyn; 7th May 2012 at 5:45 PM.
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Old 7th May 2012, 5:58 PM   #2
paulvk
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Yes Jaycar have them on special 5volt in and 5volt out isolated.
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Old 7th May 2012, 7:35 PM   #3
Thalyn Thread Starter
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I'm assuming that's just a straight inverter, which would then be run in tandem with the original +5VDC line. Are you able to give me a product code so I can look it up?

That sounds like it could be exactly what I'm after.

Ed: Or perhaps not. Were you referring to MP-3200? If so, that appears to be just an regulator/isolator - not an inverter as well. Admittedly I could likely still make use of it, but it means I'd still need to use a floating ground. Unless there's another way to apply it to get the desired result and I'm just not quite clear on how exactly...

Last edited by Thalyn; 7th May 2012 at 7:44 PM.
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Old 7th May 2012, 8:24 PM   #4
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Quote:
Originally Posted by Thalyn View Post
that appears to be just an regulator/isolator - not an inverter as well.
You've missed the point of an isolated output.
Think of the low side as 5V below your reference and it will begin to make sense.
(or you could use two of them and have a true floating ground - maybe a benefit to that)
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Old 7th May 2012, 10:08 PM   #5
Thalyn Thread Starter
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I seem to be particularly slow at the moment (at the moment, he says - ha!), so forgive the following. I do need some confirmation because my brain is telling me that the following is both right and wrong - it just refuses to tell me which parts are which.

I imagine using it this way would involve having the -Vin and +Vout hooked into each other, creating "ground". Then +Vin and -Vout become +/- 5VDC respectively.

To my mind, this suggests putting a +5V potential onto -Vin, causing the same sort of problems as my earlier Vgnd efforts. However, the VGnd I was making before was with direct reference to -Vin, rather than being isolated from it. So does the isolation make this a non issue, in the same way that two cells/batteries in series don't cause similar problems? This would mean, with two of them (which would eliminate most, if not all of the "dirt" in the USB supply), the +Vout1 and -Vout2 would be linked together to create the floating ground, with the remaining +Vout2 and -Vout1 again being +/- 5VDC respectively - right?

I'd rather ask this now and get it right than guess and get it wrong. Guessing has already cost me $100 for a new soundcard, so I'm perhaps a little overly cautious.

Last edited by Thalyn; 7th May 2012 at 10:37 PM.
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Old 8th May 2012, 2:54 AM   #6
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An isolated supply's outputs have no reference to the input voltage and ground. It's just a voltage potential across those two output terminals.

If you connect the isolated "positive" terminal to the input ground, then the negative terminal is -5V referenced to the ground.

You can think of isolated outputs as the two terminals of a battery. Touching the positive terminal of a battery to another circuits ground doesn't cause any current to flow if the negative terminal isn't connected


(if it's not an isolated supply, and is just a voltage regulator, it will be referenced to input ground (because the output ground is connected to it), and you'll short circuit the supply)
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Old 8th May 2012, 12:10 PM   #7
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There's a couple of ways I can think of that might work

You can use one of these things . It's a single package that accpets +5v and gives you a dual rail supply. I don't know of a local source for these, you'd probably have to order them from the USA. Farnell may also carry them. (There's lots of similar parts, so search digikey for other options, I just picked the first one I found.). This is probably the simplest option.

Another way of doing it is by using a Maxim MAX743. It's a switchmode voltage regulator that accepts 5v and supplies +/- 12v or 15v. I've used these before, however you need inductors, low esr capacitors and diodes to make it work, so PCB space may be a problem. You can get the MAX743 from futurlec or digikey. You'll also have to search about for the other parts to make it work.

Alternatively, if an external power supply is an option you could use an AC plugpack and a voltage doubler type circut to derive a dual rail supply. (Although that would require bulky capacitors etc)
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Old 8th May 2012, 2:07 PM   #8
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Quote:
Originally Posted by Thunder View Post
I don't know of a local source for these,
This one
+5Vin : +/- 15Vout

Any of these DC-DC converters may be too noisy for audio, but a good proof of concept.

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Old 8th May 2012, 7:43 PM   #9
Thalyn Thread Starter
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I went ahead and picked up a pair of the Jaycar jobs. I'll definitely keep the links you guys have provided on-hand in case I go ahead and make another variation - which is seeming more and more likely as I learn more about these things.

Before I go hooking anything to anything else, though, I'll give these modules a run down with a multimeter. Never hurts to do a quick continuity check to be sure!
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