find mA rating for unknown power transformer?

[Phil S.]

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?

Regards,
Phil

 

[Chronic Philharmonic]

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.

 

[Phil S.]

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.

 

[Chronic Philharmonic]

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.

 

[Phil S.]

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.

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.

Yup, I get it. This temperature thing is clever! Thanks.

 

[Chronic Philharmonic]

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.

Those infared thermometers are pretty slick.
http://www.amazon.com/Professional-Contact-Digital-Infrared-Thermometer/dp/B000GYN95S

Or you might be able to use a stick-on LCD thermometer.
http://www.thermometersdirect.co.uk...ct__Liquid_Crystal_Thermometers__LCD__17.html
They're pretty inexpensive, if you can find one with enough range.

 

[Xenu]

www.dynaload.com

 

[neon]

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.