A good way of testing a transformer for suitability is to start by measuring
the secondary resistance as Rich says. Then measure the primary resistance,
and convert that to an equivalent series secondary resistance: sum it with
the measured secondary resistance. You can now use the rectifier resistance
and the load resistance to come up with the ratio of RL/RS, the load to
series resistance ratio. From the graphs in the books, you figure the output
voltage for various values of RL for the capacitor. You can use standard
formulas to figure the RMS current in the xfmr secondary. Now use a Variac
on the primary, an AC ammeter across the secondary, and wind up the primary
volts until you reach the calculated rms secondary current. Leave it to
cook, while keeping a eye on the xfmr to see how much it heats up. If it
doesn't get too hot (smells too much!) and is OK after say, an hour (for
RTTY or contesting), remove the supply and measure the secondary ohms again.
Apply the formula for the increase of resistance with temperature, and you
have the winding temp of the xfmr.
NOTE: THIS IS AN ABBREVIATED PROCEDURE AND YOU MUST TAKE SUITABLE SAFETY
PRECAUTIONS. If the ammeter opens for any reason, you can have a surprising
amount of volts floating around - depending on the leakage reactance and the
winding resistance.
73
Peter G3RZP
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