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Re: [TCML] Determining a transformer's voltage and current

```Hello Brian,

In the past, I have used a variac to send a low voltage through the primary
and used a multimeter to measure the output voltage. You should not need a
resister since is already one built into the meter. If you send 10 V in and
read 400 V out, you can solve for the voltage out on an input of 120 V.  It
would be 5000 V. Be sure to use an AC meter that can handle at least 1000 V
AC.  Also, be careful not to send to high of a voltage in or you could break

As for the current, you send the full 120 or 220 into the transformer and
measure the current on the primary side. If you read 5 A at 12 V and you
already know the secondary voltage of 5000 V, you can calculate the
secondary output voltage.  In this case, it would be 120 mA.  This would be
an extremely powerful bug zapper, but I wanted to use some number that would
be easy.

Good luck,
JP

-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Brian Hall
Sent: Friday, April 15, 2016 2:44 PM
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Subject: [TCML] Determining a transformer's voltage and current

I posted a question similar to this a couple weeks ago, but it was a bit
cluttered or may have just gotten lost in the shuffle - there were no
replies.

To simplify:
Given a 120v 60Hz primary high voltage transformer (looks like it could have
come from a bug zapper or similar device), but there is no plate or markings
indicating the HV output voltage, current, or manufacturer:

1) How could I determine the output voltage?
I got a cord and wire nuts, and hooked it up to 120v 60Hz AC to get a max
spark of 1.8mm between the sharp ends of two bolts.  What is the scale in mm
for room temperature (20 degrees C) and still air on that max spark gap?

2) How could I determine the output current?

I have a variac and some decent power resistors, if putting in say 10 or 20
volts on the primary side, then a resistive load on the hv side would help
along with Ohms Law for finding the current on the HV output.
Just wondering if anyone else has encountered this and already knows how to
find such answers, and has advice on how to determine with some degree of
accuracy the HV side voltage and current values of an unmarked transformer.

----------------------------------
Brian Hall

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```