Suraj said:

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

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.