Re: find mA rating for unknown power transformer?
- From: "Phil S." <psymonds_no_spam@xxxxxxxxxxx>
- Date: Sun, 28 Oct 2007 19:32:01 -0400
"Chronic Philharmonic" <karl.uppiano@xxxxxxxxxxx> wrote in message
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"Phil S." <psymonds_no_spam@xxxxxxxxxxx> wrote in message
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"Chronic Philharmonic" <karl.uppiano@xxxxxxxxxxx> wrote in message
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"Phil S." <psymonds_no_spam@xxxxxxxxxxx> wrote in message
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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?
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
resistors.
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.
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.
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.
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.
.
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