Re: find mA rating for unknown power transformer?
- From: "Phil S." <psymonds_no_spam@xxxxxxxxxxx>
- Date: Sun, 28 Oct 2007 17:36:06 -0400
"Chronic Philharmonic" <karl.uppiano@xxxxxxxxxxx> wrote in message
news:p73Vi.698$TO4.187@xxxxxxxxxxx
"Phil S." <psymonds_no_spam@xxxxxxxxxxx> wrote in message
news:_d6dnSkHOsanNrnanZ2dnUVZ_hOdnZ2d@xxxxxxxxxxxxxx
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?
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.
.
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