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Dumb Transformer question

  1. Nov 12, 2009 #1
    I've been running some tests on some simple AC step up transformers, and I can't seem to justify something to myself:

    I hook up 12 VAC to the primary coil, but the voltage on the coil is actually more like 1 VAC, when measured with a voltmeter. It's not an issue with the supply--it puts out 12 VAC just fine when not hooked up to my transformer.

    What simple concept am I not remembering? Something to do with the impedance of the inductor, maybe?
  2. jcsd
  3. Nov 12, 2009 #2


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    Sounds like the supply is being loaded down by the transformer windings.

    Welcome to PF
  4. Nov 12, 2009 #3
    If the secondary is open then it could mean that you have a bad transformer. If the secondary has a load on it then it could mean that the power requirement of the load exceeds the power capability of the power supply. Many power supplies will foldback when overloaded.
  5. Nov 12, 2009 #4
    The secondary had a 1 megaohm resistor for the load, since that's the closest resistor I had laying around. So, there wasn't much power being drawn.
  6. Nov 12, 2009 #5
    Are you sure your power supply is AC only? If it is AC only then it sounds like you have a bad transformer.
  7. Nov 12, 2009 #6


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    How and where are you taking the measurement?

  8. Nov 12, 2009 #7
    Turtle: I trust that the power supply is AC only, as that's never been an issue before. I could hook it up to an oscilloscope next time I'm in the lab to make sure. I didn't take DC readings, so I don't know what I'd read there.

    Also, it would really surprise me if it was a bad transformer. All the transformers I tested acted this way, and the voltage ratios were all to spec, even at the lower-than-expected voltage.

    Stewart: I've got the 12VAC leads hooked up to the primary side from an AC power supply, and a 1 megaohm resistor hooked up to the secondary side. I'm using a pair of multimeters and measuring VAC at the legs of primary and secondary side of the transformer.
  9. Nov 12, 2009 #8


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    I mean where are you reading the 1 VAC...across the primary terminals or the secondary terminals?

  10. Nov 12, 2009 #9
    The primary is roughly 1 VAC. It's not exactly that, just ball park.
  11. Nov 12, 2009 #10


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    Have you performed an open circuit and short circuit test? If so, what did you get?

  12. Nov 12, 2009 #11


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    If you have a transformer that normally converts 110 volts to 12 volts, then you can use that transformer backwards by feeding it with 12 volts AC to get 110 volts out.

    But, if it previously gave 4 volts out then you can't put 12 volts on it and expect to get 330 volts out. The problem is that the smaller winding won't have enough inductance and will draw too much current from the 12 volt supply.

    Do you know what the rated voltage is for each of the windings on this transformer?
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