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

```If you believe the transformer is a current-limited type (e.g. bug
zapper, OBIT, NST but not pole pig and mostly-not MOT) you can measure
the output short-circuit current by simply hooking up an ordinary
ammeter (i.e. multimeter) on a suitable mA scale to the secondary.
The ammeter will present a short circuit to the transformer and will
not see a high voltage.  Be sure the fuse in the multimeter is intact
before you do this, and obviously keep a safe distance.

To get the turns ratio (and thus output voltage), you can turn the
transformer around, put 120V into the secondary and measure the
primary voltage.

Henry

On Fri, Apr 15, 2016 at 11:44 AM, Brian Hall <brianh4242@xxxxxxxxxxx> wrote:
>
> 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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