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Re: how to use a current transformer



Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 05:03 PM 9/2/2003 -0600, you wrote:
>Original poster: coils-at-dslextreme-dot-com
>Thanks for the responses,
>
>So I need a resistor across the CT leads, soldered on preferably.  Does
>the value of this resistor establish the Volts and power at the CT leads?
>2ohms -at- 100:5A = 10v, 50w
>.2= 1v, 5w
>.02= .1v, .5w
>no resistor = dangerous 20x volts
>
>I'm not clear on what value I should use to be safe and to properly load
>the CT.

Use the smallest value you can where your volt meter can still read 
accurately.  The "burden" is proportional to the resistance (the current is 
fixed, by the turns ratio), so smaller is better.  Diminishing returns when 
the meter can't accurately read the tiny voltage.



>Once I have a suitable resistor in place, can I read the output
>with my handheld meter?

You bet.


>  Finally, at 100:5 A full scale, and I am
>anticipating 30 amps through the CT (one pass), should I hope for 1.5 amps
>at the CT leads?

Precisely... so, with a 0.1 ohm resistor, you'll see 0.15 Volts (and it 
will dissipate about a quarter of a watt).


>Thanks again,
>
>Thad Howard