Where do you get those voltages?

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I see so many circuits that want +/- 10 volts or 12 or 15. But when I look at transformers, they're always around 6 or 12 volts. If I want a

+/- 15 volt supply, I'd think I'd want a 36 volt transformer, to put three volts higher than the output on the regulator. But I rarely see transformers so beastly, unless they're for high-power applications.

So where do these +/- 15 volt levels usually come from?

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Transformers are rated in RMS; the peak voltage is about 1.4 times more than that. So, a 24 VAC transformer has a peak of 34 volts. If you use a center tapped transformer, that's about 17 volts per bus, which is about right for a +- 12 VDC regulated supply. Or, you can use a bigger filter cap and a low dropout regulator for +- 15 VDC, but input ripply might be a problem for higher currents.

Of course, if you want an unregulated +-15 VDC, a 24 VAC transformer is just about right.

Another option is to use a boost regulator, which provides +15v from, say, +12v or less.

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They come from a centertapped transformer of approximately 30 volts. The tap becomes GROUND so with respect to ground, one side is +15VAC and the other is minus 15VAC.

These voltages are then rectified, filtered and regulated to achieve the desired output.

Here is an example:

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Hi, Greg. Newer analog electronics generally work on lower supply voltages. In the '70s, analog op amps always used dual +/-12V or

+/-15V supplies. But since then, advances in electronics have led to single supply op amps, "rail-to-rail" op amps (where the inputs can extend all the way to the + and - rails of the power supply, and the ouputs can go almost all the way to the supply rails), and special low voltage op amps that will work with single supplies well below 5V. You can do just about anything you would have done with a +/-15V supply in 1980 with a single +5V supply now, by choosing ICs that work on those supply voltages and using single supply design techniques.

Most manufacturers of transformers will catalog their product by VA rating, and you can usually find the 28VCT or 36VCT you'll need for a small +/-12V or +/-15V supply if you look. They're not beastly -- they just have smaller current ratings for a given size of transformer, and they're not as commonly made as they used to be.

But you're right -- wall warts aren't commonly available in the higher voltages. But they do exist, and you can take advantage of the ones that are out there. You can also use an AC output wall wart to get the split supplies, use a small AC-to-DC tabletop linear power supply to give you canned, regualted +/-12V outputs, or use a small DC-to-DC converter to get the higher analog split supply from a lower digital level one.

Good luck Chris

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When you rectify and filter a sine wave, you get the peak voltage (minus whatever ripple), which, as has been said, is 1.4 times the RMS voltage. You have to subtract the diode drops, and figure out how much ripple you can put up with to specify a filter cap.

So, technically, for a raw (unregulated) +15/-15 supply, you'd use something like a 22- or 23-volt center-tapped transformer and a bridge, and ground the center-tap.

Of course, the amount of ripple will depend on the size of the capacitor.

Usually, when you want a specified voltage, you'll design a supply that creates enough headroom for a regulator, where you can get any arbitrary voltage out (depending on the regulator design, of course.)

Good Luck! Rich

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I hadn't actually thought of P-P versus RMS voltages. Thanks for the heads-up.

But I don't see 24 VAC transformers in stores or catalogs. I see 6 volt and 12 volt transformers./

A voltage multiplier consisting of diodes and capacitors?

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On Fri, 04 Aug 2006 22:41:32 -0500, in message , Greg Hansen scribed:

Take two 120/12VAC transformers and - taking care to observe polarities - wire the primaries in parallel and the secondaries in series, and viola, you have a center-tapped 24VAC transformer (you'll need that center-tap for your return reference).

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Oh, great. Just as I'm trying to get back into this sort of thing, I find out that the rules have changed. I'm thinking of applications like a lock-in amplifier, where the central theme is the amplifier that switches between positive and negative gain. Do they do that with single supply devices, too?

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A lock-in amplifier sounds like it would need a split power supply, at least for the multiplier/demodulator.

Single supply is becoming more common for convenience and cost. But for high-end stuff, split supplies are still used. Every time I've needed a multiplier, I've used split supplies.

The rules haven't changed -- it's just gotten a lot easier and less expensive.

Cheers Chris

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Well, I appreciate the friendly help I've gotten, and I'll be playing with this stuff RealSoonNow. The scope I thought I had doesn't give a trace, I'm waiting on a function generator...

It was so much easier to dink around when I was at a lab that had tools, parts, test equipment, a machine shop, and workspace all available. Even shelves and bins to put stuff-- didn't think much about that until I had stuff to put. Maybe I got spoiled, but I feel like I can't do anything really meaningful unless I at least have a function generator and a scope.

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It is more difficult working from your basement, and you can't do anything meaningful without the tools.

Try tramping around and looking for business and side jobs. There's nothing like having a little paying work and a check at the other end of the job to help along the endless quest for some decent lab equipment, stock parts and a few tools of your own.

You also might end up working for a scope, function generator or lathe instead of a check. You'd be amazed how many places have the equipment sitting in a corner taking up space, but the shop manager is suffering because the bean counters say they won't pay to hire anyone to work in front of it.

Hmmn. "Junk" taking up space. Product to repair. Intrepid technically inclined soul needs lab equipment and cash. You could solve three problems at once (two of theirs and one of yours) by negotiating. It works.

Good luck Chris

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Transformers are available in a wide range of voltages and currents - have a look around

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