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I used my simple digital multimeter to measure the real output of several small PSU's I have got at home. They all should give just direct current and show some values in VDC mode, but the multimeter also shows nonzero values when positioned in VAC mode. But not on all power supplies. Why is this?

For example a 12 V PSU shows 12 VDC, but it also shows 25 VAC, but other 12 V PSU shows just 0.00 on VAC with just slowly alternating +/- sign. Is this some sign of imperfect PSU, a bad output capacitor, or is it that a SMPS always shows something at VAC due to its noise, while linear PSU not showing anything at VAC....?

Update:

I did more testes with various PSU's. I get around 215% of DC voltage in VAC mode on my multimeter. This applies to all PSU's I've tested except one which shows 0.00 VAC - maybe that was caused by switched polarity. It shows 0.00 when I switch probes.

I also tested wall outlet - it shows nice 225 VAC, so the multimeter seems to be working correctly when there is a real AC.

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3 Answers

A DC power supply that shows a greater AC voltage than the rated DC is probably not very well-designed, or due for replacement.

That apart, many SMPS designs will show some AC ripple (that your multimeter reads as AC signal), and in some cases this ripple is higher when the supply is not loaded, or lightly loaded.

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Reading 25 VAC on the output of a 12 VDC power supply is definitely wrong. Unfortunately, from what you tell us it's hard to determine what exactly is wrong. Perhaps that supply is just broken.

The best thing to do would be to look at its output voltage on a scope. Then you can see for sure what is going on. There are other ways to get some idea about a AC signal. For example, put a speaker in series with a 1 kΩ resistor on the supply output. If it really has such large AC and it is in the audible range, then you'll definitely hear it. If it is really putting out 25 VAC RMS (which I have a hard time believing), then a 1 kΩ resistor will dissipate over 600 mW, which will make a ordinary "1/4 W" resistor get very hot quickly. If the voltage is really that large, you'll hear something with a 10 kΩ resistor in series with the speaker too.

You can also try putting some capacitance on the supply output and see what that does to the meter reading. To be safe, get capacitor rated for 50 V at least. You probably need 10s of µF before anything much happens. If this supply is truly broken, it could blow up the cap though. Again, a scope would tell us what's really going on.

Added:

I just had another thought as to what is going on. 25 VAC from a 12 VDC supplies seems a bit unbelievable, even for a busted one. I'm guessing your meter isn't really connectect accross the supply output properly. Does this supply possibly have 3 terminals? I have seen some where it's a bit confusing which two are actually the supply output and that the third is the wall plug ground. There is usually a strap you can clamp between the wall ground and one of the two supply ends. When this is not strapped and you put the meter between either output and the wall ground, you can get exactly what you are seeing. There will be some capacitance in the supply to the hot side of the AC line, and this will add a common mode signal onto the supply output. It is high impedance, so not really a problem. If you put a 10 kΩ resistor accross the meter when reading the AC voltage and it drops down a lot, then that's what's happening.

Added 2:

From your latest experiments, it sounds like the DC blocking cap isn't in series when you are doing the AC tests. Look closely at your meter. Does it only have two places to plug the leads into, or are there two or more jacks for the red lead depending on what you are trying to measure? When taking AC measurements, not only make sure the dial is on AC volts, but also that the leads are plugged into the correct places for AC voltage measurement.

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If I understand it correctly, I need a bipolar capacitor with tens micro farad capacity and high voltage rating. That's definitly out of my scope. I did more tests - see update in my question - it seems to me that it's all caused by the multimeter itself, not a problem with my PSU's. –  Al Kepp Dec 29 '12 at 20:09
    
I wonder if the meter might be measuring AC by half-wave rectifying the input signal and measuring the linear (non-RMS) average voltage, and scaling the result? Feeding 1 volt RMS into such a system would yield about .46 volts after filtering, so if the meter assumes AC voltage is 2.1x the measured value that could explain the result. –  supercat Oct 31 '13 at 20:40
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Cheap multimeters usually can't accurately measure an AC signal with a large DC component. A two times multiple of the DC voltage is a common erroneous result. It has nothing to do with your power supply, but with the circuit the multimeter uses to measure AC voltages. See this question and this post on adafruit which also had this problem.

I don't know exactly what causes the 2x factor, but maybe someone with more knowledge of the methods used to measure AC voltages in entry-level meters could help.

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