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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I need to measure an AC signal that is between 0.5-2v. I am finding it difficult to come across an AC voltmeter in this range at a low price. However, I do have a DC meter. So, I set up a test circuit. I used a 120v/24v CT transformer and connected it to a variac and set it so I have 23v on the secondary. I connected a 1n4007 from each hot lead to the center-tap to create a full-wave rectifier. I connected one end of a 30k resistor to the two diodes and the other end to the center-tap. This, of course, cuts the voltage in half and the diodes have a 1.1v voltage drop. When I connect a digital voltmeter across the resistor and the center-tap I only measure 10v DC.
I guess that if I just use half-wave rectification between the hot leads I won't cut the voltage in half. If I place a capacitor across the load I can get a value that is close, but the capacitor captures the peak and not the RMS value. Notheless, I will still have the voltage drop of the diode which is significant for the voltages I intend to measure. Is there a better way to do this or a way to refine my approach?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Why not just use a digital multimeter? Even the cheapest ones measure AC voltage using a precision rectifier. The only gotcha is that the internal circuit full-wave rectifies and then averages the signal, and then scales it by a factor of around 1.1 (IIRC) so that the display reads the correct RMS voltage when the input is a sine wave. It won't read the RMS correctly for other waveforms.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Using a digitial multimeter defeats the purpose of "doing it myself." I think I will go along with using Schottky diodes. I have some with Vf = 0.2v when I measured them a while ago.
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measuring AC voltage with a DC meter
