Measuring AC current on DC meter

Discussion in 'Electronic Basics' started by Suraj, Jan 12, 2004.

1. SurajGuest

I was wondering if I could measure AC current drawn by a device on my
digital multimeter which has DC current measurement (10A Max). We have AC 50
Hz @220 V here. As far as I understand, the current wil change direction 50
times in a second, therefore the DC meter should positive and negative
readings alteringly (If I can see it and if the sampling rate of the digital
meter is high enough?)

Should I try it? How are AC ampere meters different? do they calculate the
integral of the Sine Wave?

Thanks,
Suraj

2. John PopelishGuest

Passing AC through a DC ammeter will not hurt the meter, as long as
you do not exceed its rating, but it will not give a useful result.
The DC meter displays the result of an average over some time interval
(perhaps as long as a quarter second), so the average of alternating
half cycles will be close to zero. AC meters rectify the signal into
a unidirectional magnitude and average that. Then the result is
scaled to produce the effective current (the DC current that would
produce the same heat in a resistor if the current were DC, with the
assumption that the current is sinusoidal). Really good AC current
meters square the instantaneous current, average the current squared
over some time interval, and take the square root of that average to
get an effective current that is accurate even if the waveform is not
a sine wave.

One fairly accurate way to read AC current with a DC meter is to pass
the current through the window in a current transformer, put a bridge
rectifier on the transformer output, with a scaling resistor on the
output of the rectifier. With the right choice of scaling resistor,
your DC meter will read a fractional multiple of the AC current, based
on the number of turns on the current transformer. THis approach has
the added feature that it isolates the metering from any voltage on
the current carrying conductor.