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find mA rating for unknown power transformer?

Discussion in 'General Electronics' started by Phil S., Oct 28, 2007.

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  1. Phil S.

    Phil S. Guest

    I've got an old power transformer that is meant for a tube amplifier. It
    has 3 filament windings and one HT secondary. Running the primary direct
    from the wall supply, 120v, the HT secondary with out a load is 655vac
    across the outer legs. It has a center tap that I ignored for this purpose.
    So, I can guess that 327-0-327 is probably 300-0-300 give or take 10-15v.
    The hard part is figuring out how much current capacity exists without
    killing the transformer.

    I have tested with 10W sandblock resistors (what I have on-hand) across the
    HT secondary and have these results. 14K7 = 643vac, 9K8=640vac, 5K8=634vac,
    and 1K5 smoked & toasted at 590vac. Now, I realize that 1K5/590v is 390mA
    and 230W. This seems well beyond what is appropriate for this transformer.
    I am guessing it is capable of something around 150-180mA. But all this is
    trial and error.

    Is there a more definitive approach to uncovering the required information
    and properly back-solving for an answer? To repeat, the question is how
    many mA capacity is there?

  2. You have enough data to establish a load line (plot output voltage vs.
    current and extend the line to zero volts and maximum current - a short
    circuit). Most transformers are rated for a particular voltage at a
    particular current, and the voltage will drop as you load it more. That
    doesn't mean it isn't capable of sourcing more current; only that it won't
    deliver a particular rated voltage anymore.

    The more heavily you load it, the hotter it will become. I would pick a
    temperature above which it should not go (Fahrenheit 451? - no probably less
    than that :) and see how much load it can handle before it reaches your
    selected cutoff temperature. This is a steady-state temperature. It should
    be able to source considerably higher current without overheating if the
    duty cycle is short.

    I would start by measuring the temperature with no load after several hours.
    Then measure the temperature with moderate load after several hours. Plot
    those two lines on a graph, and extend the line to the short-circuit current
    (obtained from the load line), and see where the temperature line crosses
    your cutoff temperature. Assuming the output voltage is still high enough,
    that's your maximum steady state-load.
  3. Phil S.

    Phil S. Guest

    Thanks. I recognize that a transformer is a passive thing that will
    continute to provide what current is demanded until is burns up. Curiously
    enough, I didn't consider that temperature is an indicator that could be
    used in working the problem. It seems I'll need to get a heftier (able to
    handle more watts) load and something that can be scaled, like a bank of 25W
    wirewound resistors. Then I get to plot both temperature and voltage drop.
    Between the two, I ought to be able to get a decent idea of a reasonable and
    safe limit. I'm thinking a drop of 5% is safe and 10% might be too much.
    This is just based on my concept of what a manufacturer would likely have
    allowed. Given the age of the transformer, I'd guess 40 years old, I'd
    expect it to be a little overbuilt, but even in those days, the
    manufacturers were watching cost. So it seems, the idea is to get it
    running at maybe 120 F and certainly no more than 180F. 120 is too hot to
    touch comfortably, and is hot enough for my taste.
  4. Plotting this on graphs, you might be able to use your existing loads if
    they can withstand continuous operation (given the need for extended
    temperature tests). If the temperature measurement is precise enough and the
    data points not too close together, you should be able to plot a couple of
    points, and simply extrapolate the line. It's probably accurate enough.
  5. Phil S.

    Phil S. Guest

    10W wirewound resistors are not beefy enough for sustained operation. The
    test will let the smoke out of those in short order. I need to get the
    whole thing onto a fireproof surface and build a ladder with 25W rated

    What would I use to measure temperature? Your run of the mill kitchen
    thermometer, like that all metal one I stick in a turkey? I'm not looking
    to buy something I'll use only one time, though it would be a perfectly good
    excuse to buy a gadget.
    Yup, I get it. This temperature thing is clever! Thanks.
  6. Those infared thermometers are pretty slick.

    Or you might be able to use a stick-on LCD thermometer.
    They're pretty inexpensive, if you can find one with enough range.
  7. neon


    Oct 21, 2006
    an old power transformer usualy meant 6.3v ac for filament at hi amperage. your primary should read ~15 ohms. since a transformer is passive and predictable ratio from the size of it and ohmic resitance you should be able to determine the total power. expect a 20% efficiency drop of transfer. checking temperature is not a safe method of finding the threshold not unless you can control the temperature and air flow across it. face it at 100 degree room temperature and 60 degrees the results can be wild. and it takes a long time to stabilize the bigger it is the longer it will take also the hotter the more I it will output. the size is the only indication of power.
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