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How should I measure half-wave rectified current?

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If I'd like to monitor the actual rms current going into the battery from a battery charger using a series resistor and a single diode, how should I approach it?

I've tried my generic true RMS DMM in both DC and AC modes and both settings registers different readings and neither is consistent with what I expect. I place a known, accurate shunt and a resistive load in series with the diode. By measuring the peak voltage at shunt, I know that Vrms should be 1/sqrt2 x peak, but my DMM doesn't reflect that.

The DMM actually doesn't register anything in true RMS AC mode with a true DC, yet DC mode wouldn't register correctly uni-directional, rippled voltage.

Reply to
itsme.ultimate
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The RMS value of a sine wave is 0.707 x peak, but you're looking at a half-wave rectified waveform: one hump instead of two per period ... lower RMS value.

The charging current waveform will be even less sinusoidal: zero until the source voltage exceeds battery terminal voltage by a diode drop, then a pulse of current that tails off to zero when source voltage drops below battery voltage again.

"True DC" has zero AC component, so its true RMS AC value is zero.

A "unidirectional rippled voltage" has a non-zero DC component as well as AC harmonics. I would expect the meter to show (probably different) non-zero RMS values for "DC" and "AC" modes.

Reply to
Roby

If you know the maximum voltage, then the RMS current for that waveform is one half of the sine wave.

The half wave rectifier output current looks like a sine wave, but only half the time, the other half of the sine wave is set to zero - therefore the RMS current is one half of what you would expect from the full sine wave.

Reply to
AverageGuy

Try again, this time square the two currents in the period before averaging and taking the square root.

Reply to
stevenal

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