Re: Finding current of xfmr w/o meter

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Speck by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dspeck-at-relex-dot-com>

Matt,

It's not entirely clear which sort of current you are trying to measure.

If you are looking to determine how much current a particular circuit is
drawing,
you can measure the voltage across a low value precision resistor, say a
0.1 Ohm,
which will show a drop of 0.1 volts for each amp through it.  A 10 watt
precision
resistor would allow you to measure up to 100 amps without burning up, but the
measurements will be more accurate if you run the resistor at a small
fraction of
its rating, to minimize heating of the resistor and a consequent rise in its
resistance.

If you are trying to determine how much current the transformer is rated
for, then
there is no easy way to determine it by measurement.  Any transformer will
try to
put out as much current as possible, even to the point of burning itself up, if
presented with too small a load.  (I know, as I smoked a lab meter system
in high
school trying to determine exactly that parameter.  I got a very loud lesson in
transformer theory that day!  :o))   I believe that there are tables of
transformer design that can tell you how much power (watts) you can expect
to put
through a core of a particular volume.   From the wattage, knowing the output
voltage, you can determine the maximum allowable output current with the
equation
I = W/E.   The larger the core, the greater the power handling capacity.
The size
of the primary windings and the presence of magnetic shunts, as in a microwave
oven or neon sign transformer may also affect the results.

HTH,
Dave

Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Fucian-at-aol-dot-com>
>
> Hi,
>
> I have a 30volt power xfmr and no way to measure current. Could I connect say
> a 2.2 ohm resistor and measure voltage off the xfmr and then divide voltage
> by 2.2 ohms?
>
> Matt