> Many of the less expensive CT / AC ammeter setups are nonlinear at the
> bottom of the scale. You have to have enough current to get at least 10
> - 15% of a full scale reading to make them work accurately. This is
> because the meter movement inside the analog meter is actually a DC
> movement driven by a diode bridge, and you have to apply enough voltage
> to the terminals to overcome a couple of diode forward voltages to get
> the pointer to budge. Once the diode bridge in the input circuit is
> fully conducting, the readings will be valid.
and some AC meters use AC movements. Depending on
the type, these can give nonlinear indications, the scale will
be nonlinear...
> You said that with both the digital and analog meter connected to the
> CT, you got no readings. If you connected all three devices in series
> (which is the correct way), no current would flow until you had overcome
> the forward voltage of the bridge in the analog meter.
This would depend on where any shunt is located.
> If you are seeing a scale reading of 18 - 23 amps with the analog meter
> alone connected directly in your circuit without the CT in place, then
> it would indicate that you are actually running 1.8 - 2.3 amps in your
> NST primary, since 5 amps would drive the analog meter to full scale,
> or the 50 amp position.
>
> If you connect the current transformer to a good quality DMM set on the
> 10 amp scale, is should be linear down to very small currents.
best
dwp
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