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Where do you get those voltages?

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

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  1. Greg Hansen

    Greg Hansen Guest

    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 Delorie

    DJ Delorie Guest

    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.
  3. Lord Garth

    Lord Garth Guest

    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

    Here is an example:
  4. Chris

    Chris Guest

    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
  5. Rich Grise

    Rich Grise Guest

    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!
  6. Greg Hansen

    Greg Hansen Guest

    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 B

    Alan B Guest

    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).
  8. Greg Hansen

    Greg Hansen Guest

    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. Chris

    Chris Guest

    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

  10. Greg Hansen

    Greg Hansen Guest

    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. Chris

    Chris Guest

    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
  12. Transformers are available in a wide range of voltages and currents -
    have a look around, for example.

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  13. Guest

    Wow. All the transformers I could want. I didn't know about those
    people. Thanks.
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