Re: Tranny tests
- From: Gary Gerhart <Gary@xxxxxxxxxxxxxxx>
- Date: Sun, 28 Oct 2007 12:25:14 -0500
J.P. wrote:
Still a little short on info but might have some ideas..I assume that
some variable resistance or voltage divider with light bulbs in the
circuit where more could be in or unscrewed out of circuit with a
milliamp meter in the cricuit with a bypass cap might could be used to
test PT output using temperature checked every 15 minutes or so for a
couple of hours could measure approximate output ma...
http://www.ozvalveamps.elands.com/repairs/trannytest.htm
Actually, that's a pretty good explanation.
I'm guessing here, but I think the confusion, or lack of
clarity may be because of this:
The current rating of a power transformer is, necessarily, based
upon criteria that must be established first. They are the percent
regulation at load and/or the temperature rise. If you have an
unknown transformer and you want to determine its current capacity
then *you* must determine the parameters for the test.
A transformer is a passive device. It will continue to work, will
deliver higher and higher current until it self-destructs. Once the
current draw reaches a point, the voltage will drop and the temperature
of the transformer will rise due to internal resistances. As the
current draw continues to rise, the voltage will continue to drop
and the temperature will continue to rise.
You, as the tester, need to establish limits prior to testing.
5% regulation is a very good, and usually completely safe, parameter.
25degC rise is also a pretty standard limit.
You also *must* determine the estimated power requirements for the load
you choose to use for the test. Volts X Amps = Watts.
Since the discussion was originally based upon the B+ ratings, I prefer
to test the transformer for DC ratings. This requires rectification and
filtering between the transformer and the load. Simple enough.
If you want to determine the transformer's current capacity, without
a specific application in mind, then you need to have a variable load
connected to the output terminals. Unless you have a really big
rheostat on hand (in the neighborhood of 10K 500watt) you must make
some semi-educated guesses as to the *expected* performance of the
transformer.
Let's assume you have the 10K 500W rheostat.
Let's assume you've set 5% regulation as your limit.
Set the rheostat to max resistance (10K). This will create the minimum
current draw.
Measure the DC voltage across the rheostat. Let's say it's 500vDC.
V/R=I ... 500/10000 = .05 or 50mA. If you have ammeter in series
with the load it should confirm this number.
Turn the rheostat to lower the resistance of the load until the voltage
reads 475vDC. This is your 5% regulation limit. If you have an ammeter
in series before the load, read the current directly. You are done. If
not, power down the test rig, wait until the voltage has completely
bled down to zero, and measure the resistance of the rheostat. Let's
say it measures 790 ohms. V/R=I ... 475/790 = .601 or 601mA. That's
300W! Good thing you used that 500W rheostat...
In this case, you have a transformer rated:
500vDC @ 601mA with 5% regulation.
If you decided that 10% regulation is good enough, then continue the
process until the voltage reads 450vDC. This will net a higher current
rating.
In either case, run the power supply at full load for an hour and
monitor the temperature rise of the transformer. You'll find that,
with 5% regulation the transformer stays cooler than at 10%.
It is completely up to you to determine what temperature rise is
acceptable. Generally, the lower the temperature rise, the longer
the life of the transformer.
If you use 5% regulation as your limit, you'll not go far wrong.
Gary Gerhart
Gerhart Amplification
.
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