On Tue, 13 Dec 2005 20:23:52 GMT, Ignoramus29580
wrote:

Let's say that I have a cable and I want to measure the AC current
going through it. Up to, say, 100 amps.

I could use a current transformer, right?

If I have a say 200:1 current transformer, then on a 100 amp AC
current it would want to produce a 0.5 amp current. Then if I stick,
say, a 1 ohm resistor across it, it would produce 0.5*1 = 0.5 volts
AC across the resistor.

Is that right?



While this is correct it's not the best way to use a
current transformer as a measurement device. This because most
meters capable of reading a few volts AC FSD are messed up by the
forward drop of diodes used to rectifiy the AC.

The trick is to use it as a true current transformer without
an intermediate voltage transformation. Feed the output of the
current transformer directly into a full wave rectifier - silicon
diodes are OK because forward voltage drop is not important.

Short circuit the rectifier output directly through a DC
AMMETER. The ammeter will then read directly the MEAN value (0.90
x RMS) of the secondary current. The voltage drop of the
rectifier diodes will slightly increase the voltage drop at the
current transformer primary but will not affect the current
transformation ratio. For sine wave input waveform the equivalent
RMS current should be read as 1.11 x the indicated DC value.

Jim