Re: find mA rating for unknown power transformer?
- From: "Chronic Philharmonic" <karl.uppiano@xxxxxxxxxxx>
- Date: Sun, 28 Oct 2007 16:52:37 GMT
"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.
.
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