Hardly an auction goes by without a box of one or more power transformers
crossing the auction block. Whether you’re a repairman, experimenter,
or “tinkerer”; you know how much new or rewound power transformers can cost.
Transformers that appear in our auctions or show up at swap meets are usually
salvaged from junked equipment and may or may not be defective. Fortunately,
I have yet to buy one in an assortment that is defective or burned out.
Usually a tell tale burned transformer odor will linger for years and just
the smell or the sight of melted insulation run down will steer an experienced
person away from a purchase. Unfortunately, many of the older transformers,
especially those with woven cloth insulation on the wires, lack any kind
of color coding for winding identification. Many years ago when we
used to meet in the museum basement, one of our HVRA members described how
he identified unknown windings using a volt ohmmeter and a 25 Watt light
bulb.
The procedure goes as follows:
Assuming the transformer is good, identify windings with an ohmmeter
and mark the wires with small adhesive paper labels (masking tape works great).
Make certain none of the wires touch each other or can short out during testing.
High voltage windings may read in the hundreds of Ohms. The high voltage
center tap can easily be identified when the resistance between it and the
two outside leads reads approximately the same resistance. For example,
by holding one ohmmeter probe on the center tap and touching each of the
other two wires, one might read 200 Ohms both ways. Across those two
wires, one would read around 400 Ohms. In order to avoid a potentially dangerous
shock, it is best to tape off the ends of the high voltage wires or use plastic
twist wire nuts to isolate them and prevent accidental contact or shorting
before continuing.
The primary winding and low voltage secondary windings are sometimes
difficult to distinguish. Connect a 25 Watt incandescent light bulb
in series with each winding and test each winding individually (see diagram
below). The light bulb will glow dimly or may not glow at all when connected
in series with the primary winding (assuming of course the primary is designed
for 110 to 125 volt operation). The light bulb will glow brightly across
the low voltage windings (i.e. 5, 6.3, or 12.6 etc.).
To identify the various output voltages, simply connect the primary
winding to an outlet (use an isolation transformer for safety) and measure
the various windings output with your AC voltmeter. Record the voltages
on the paper tape labels.
Power transformer wiring diagram showing 25 Watt light bulb test.
