# Where do you get those voltages?

Discussion in 'Electronic Basics' started by Greg Hansen, Aug 3, 2006.

1. ### Greg HansenGuest

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?

2. ### DJ DelorieGuest

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

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

3. ### Lord GarthGuest

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:
http://www.epanorama.net/pdf_redirect.php?url=http://www.psyber.com/~dibsed/CIRCATS/opamp-ps.pdf

4. ### ChrisGuest

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

5. ### Rich GriseGuest

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

6. ### Greg HansenGuest

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

7. ### Alan BGuest

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

8. ### Greg HansenGuest

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?

9. ### ChrisGuest

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

10. ### Greg HansenGuest

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.

11. ### ChrisGuest

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

12. ### Peter BennettGuest

Transformers are available in a wide range of voltages and currents -
have a look around http://www.hammondmfg.com/5cpwr.htm, for example.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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