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I have a wired heater controller attached on the wall. I lifted the controller and saw two power lines connected to it on the back. I was not sure how to measure the voltage. This Quora answer says:

IN ac mode, if it reads as zero then the output is either DC or Zero. If it is not zero then anyways the output has ac

But when I measured it with a cheap multimeter XL830L. When the knob at V~ 600 or 200, it read "20.0". Then I turned the knob to V- 200 or 20, it read 10.0 (but periodically dropped a little bit).

After measuring that, I completely detached the controller, and saw the back panel. There were + and - etched near where the cables were connected on the back. Then why did it read "20" when I the knob was at V~ 200.

Damn Vegetables
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  • Just to add some links to similar previous questions - (a) here's a recent question, where DC measurements on the AC 200V range, were about twice the expected value: "[Multimeter reading double voltage](https://electronics.stackexchange.com/q/521475/101852)" (the OP kindly self-answered after realising they were using an AC voltage range to measure DC voltage); (b) the accepted answer here gives another explanation of why the meter displays an AC voltage value for a DC voltage: "[Measuring AC voltage from DC battery](https://electronics.stackexchange.com/q/101418/101852)". – SamGibson Sep 21 '20 at 20:06

1 Answers1

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Good that you included a link to the manual. The lowest ACV range on that multimeter is 200VAC so it seems they've just used a series diode to read the AC voltage rather than the more usual precision rectifier, thus saving a few pennies.

The average voltage of a half-wave rectified sinusoidal waveform of x volts RMS is about 1/2.2 (\$\sqrt{2}/\pi\$ to be exact) times x, so they need to jig the reference lower by that ratio to make it read higher (to approximate the correct RMS reading on the display. That accounts for the higher reading when DC of the correct polarity was applied- also the diode drops a bit of voltage so it will be closer to (Vin(DC)-0.7)*2.22. That increasing inaccuracy at low voltages is why they don't provide a lower AC range than 200VAC. With 10V applied I get a predicted reading of 20.6, close enough.

If you had reversed the leads it probably would have read zero.

Don't believe everything you read on the internet (including here). Many multimeters are as described, but not all.

Spehro Pefhany
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