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Simple Variac Question

Discussion in 'Electronic Repair' started by Matthew Rossiter, Mar 2, 2007.

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  1. I built a test fixture for doing all kinds of different things. One of the
    ideas I had in mind was using a Variac to adjust AC voltage for certain
    types of monitors, one which requires two 45VAC inputs.

    Would I be able to use a VARIAC to set the voltage to 90 VAC and have both
    terminals deliver equal 45VAC in reference to ground?

    Thanks

    Matt
     
  2. Arfa Daily

    Arfa Daily Guest

    No, because there is no centre tap on a variac. Electrically, it is just a
    variable auto transformer. One output leg is common with one of the line in
    legs.

    Arfa
     
  3. What if you used a secondary transformer with a Center Tap and used the
    VARIAC to control the AC input? I guess that's a pretty simple question,
    but just want to make sure I'm not missing something.
     
  4. Arfa Daily

    Arfa Daily Guest

    Yes, that would work. You would need to find a transformer with a 50v - 0 -
    50v secondary ( or two 0 - 50v windings that you could hook in series ) that
    could supply the current demand of the monitors that you want to power.
    Then, as you say, you would connect the variac to the primary. This scheme
    will have the added advantage that it will provide safety isolation that the
    variac on its own, won't.

    If you couldn't find such a transformer, you could use two single-secondary
    transformers, with the secondaries hooked up in series, and the primaries in
    parallel, to arrive at the same end result.

    Arfa
     
  5. Meat Plow

    Meat Plow Guest

    The output of a variable AC supply is almost always isolated from any
    reference to any kind of "ground" you might be thinking of.
     
  6. Meat Plow

    Meat Plow Guest

    If you used a 1:1 transformer rated at 110 volts or better with a center
    tapped secondary you should be able to obtain the desired result.
     
  7. I might as well ask. Can you recommend a good place to find a 1:1 Center
    Tapped "High Current" Transformer?

    Thanks.

    Matt
     
  8. Arfa Daily

    Arfa Daily Guest

    I would have to disagree there. A variable AC source in the form of a
    traditional variac, is an adjustable autotransformer, which means that one
    of the line legs - lets call it "neutral" or "cold" - is common to both the
    input and output, which means that the variable output is referenced to the
    incoming supply, which is itself tied to ground at the local substation in
    most countries, as far as I know. A variac does not provide any form of
    electrical isolation, so the output from it *must not* be treated as
    'bench-safe', unless its input is plugged into a proper isolation
    transformer.

    Arfa
     
  9. Guest

    ABSOLUTELY CORRECT.

    It is NOT a transformer. = NO isolation.
     
  10. Meat Plow

    Meat Plow Guest

    Was thinking of my Sencore PR57 which is a VARIAC as in (variable AC
    supply) and it does have an isolated output, no big fucking deal.
     
  11. Meat Plow

    Meat Plow Guest


    www.google.com.
     
  12. James Sweet

    James Sweet Guest


    Well it is a transformer, it's just one that has one side of the
    secondary tied directly to one side of the primary, hence no isolation.

    You can get cheap 1:1 isolation transformers, they're sold for use with
    older video arcade monitors. Any place that sells amusement supplies
    should have them. I get most of my parts here
    http://homearcade.org/BBBB/ Not sure offhand what current the
    transformers are good for, but usually a few amps.
     
  13. Guest

    Yours then is exceptional. To assume a variac has an iso output is
    asking for a nasty accident, as most have outputs connected direct to
    line.


    NT
     
  14. Arfa Daily

    Arfa Daily Guest

    Sorry old boy - wasn't meaning to dis you ... I just wanted to make sure
    that no one had a safety issue by not understanding the difference between a
    double wound and an auto tranny. Thanks for the support in that other thread
    further up though. That Aussie twat pops up on other groups as well. I think
    it must be every time he runs out of money to buy his meds ...

    Arfa
     
  15. I'm having a little hard time finding a 1:1 Center tapped transformer with
    some good power.

    Hammond makes an 80CT transformer @ 4 amps
    http://www.hammondmfg.com/165.htm - 165N80
    but that would only give me 40VAC each tap, but pleny of current. Maybe
    that's all I'd need, I don't know.

    I think their 300 series would be a little under-powered.
    http://www.hammondmfg.com/300series.htm - or am I reading the specs wrong?

    I've been doing searches on google (and on ebay) but it seems finding a 1:1
    center tapped transformer with plenty of power is hard. unless maybe I'm
    not looking in the right place.

    Just looking for a little help.

    Thanks!

    Matt
     
  16. James Sweet

    James Sweet Guest


    You can use a pair of 1:1 transformers in series to achieve that.
    Primaries in series and then secondaries in series. That will give you
    center tapped 1:1.

    What sort of motor is this that requires two transformer windings?
     
  17. Doesn't the Electrohome G08 monitor require two 45Volt AC inputs?
     
  18. Guest

    Actually, it is not a transformer.

    It is ONE coil of wire connected across the AC supply with a variable
    tap. Output is tied between tap and (Hopefully) neutral of the AC
    input.
     
  19. James Sweet

    James Sweet Guest


    I think you should read up on autotransformers. Variac is a trade name
    for a variable autotransformer, and they are in fact transformers. The
    output of most of them can be adjusted *above* the incoming line
    voltage, standard is about 10%. The one I have sitting here accepts 120V
    input and outputs 0-140V so it is clearly a transformer.

    For a bit more detailed info on the subject here's an article.

    http://en.wikipedia.org/wiki/Variac
     
  20. Meat Plow

    Meat Plow Guest

    Understood. I've got to remember to always consider the rookies
    who read SER. As far as Phil goes, he's got issues with those whose
    knowledge far surpasses his own.
     
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