Re: [TCML] Identifying an unknown current transformer

[Ed Phillips <evp@xxxxxxxxxxx>]

David Speck wrote:

> List,
> I recently scored a set of unmarked Amprobe clamp-on current 
> transformers.  I'd like to be able to calibrate and use them.  If I 
> just measure their DC resistance, they come out to about 20 ohms each.
> I know that if current is passed through a CT without a burden 
> resistor into a high impedance voltmeter, that very high voltages can 
> be developed at even low source current levels.  I guess that I could 
> start with an external burden resistor of, say, 1 ohm, and work 
> upward, and see what kind of voltages I get for a known currents  
> through the detection loop.
> I just wondered if there was a more elegant way to determine 
> electrically whether I have a plain  inductor, or a parallel inductor 
> / resistor pair inside the "black box", and, if there is indeed a 
> burden resistor present, what is its value.
> Thanks in advance,
>
> Dave 

   Those transformers are intended to have a relatively low-impedance 
ammeter as the secondary load.  If you don't have one you can use your 
external resistor of 1 ohm and measure the ratio of the voltage across 
it to the current through the wire it is clamped over and that will give 
you the current ratio.  If you measure 1 volt [hence one amp] through 
the resistor with 10 amps in the wire you'll know the ratio is 10:1 [10 
amps in primary wire for 1 amp in secondary circuit], etc.  As long as 
the secondary load resistor is small enough to keep the maximum voltage 
to a couple of volts you should be OK.  The current ratio [1/turns 
ratio] will hold for any secondary load which keeps the voltage below 
core saturation.

Ed

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla