DIYbanter - Dumb "current transformer" questions

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
any 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. Because a moving coil meter responds to the average
value of the current it will 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 will accurately read as 1.11 x the indicated DC
value.

Jim

P.S This is a repeat post as the original seems to have got
lost.
As others have commented a current transformer must never drive
an open circuit.

If you're switching the meter between current transformers with
this scheme use a relatively low FSD current meter -perhaps 1 to
10 ma, Make up a set of separate 5A shunts. Terminate each
current transformer with a full wave rectifier loaded with one of
these 5A shunts. This keeps the current transformer loaded while
the meter is switched across one of the other shunts.