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Measuring HV Transformers
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Thursday, October 23, 1997 10:33 AM
To: tesla-at-pupman-dot-com
Subject: Measuring HV Transformers
Fellow Coilers,
Here is a simple way to measure High Voltage Transformers:
Connect the sucker backwards and measure the primary voltage with 120 VAC (or
whatever) applied to the HV secondary.
CAVEATS: If the transformer has any part of the windings connected to the case,
make sure that the case is fully isolated when conducting this test. Neons and
microwave transformers both have windings connected to the core. Just make sure
the transformer is sitting on a piece of plastic or other insulating material,
and do NOT touch the transformer case while measurements are being made.
OK, here's an example. You have a microwave transformer of unknown voltage. You
place it on an insulating material and wire the HV secondary to 120VAC. Plug it
in, and actually MEASURE the supposed 120 VAC (it may be anywhere from 105 to
125). Write the value down. Now measure the output at the primary. Write it
down. Let's say you put in 120 and got out 5 VAC.
Esecondary=120 * (120/5)
So Esecondary=2,880 VAC
NOTE: Some kinds of AC meters have a large error when reading voltages below 5
volts. Digital meters do not have this problem, but many of the older analog
meters do, since the diode bridge typically used has a diode drop. For example,
a silicon diode will have a diode drop between .6 and .8 volts. That will
produce a significant error on the 10 volt scale!
Connect transformer, make sure it is isolated, plug in, THEN attach meter
and make measurements of high and low sides. When measuring an unknown voltage,
start out at the highest scale first. Once you know the approximate voltage,
then detach the meter,
switch to the proper scale, and measure again for maximum accuracy. It is NOT a
good idea to switch a meter while it is attached to a live circuit.
Hope this helps.
Fr. Tom McGahee