# What will be the reading of AC voltmeter if it is connected to a DC supply of 120V?

What will AC voltmeter show when connected to a DC source of 120V

• Kind of depends on the specific meter. My cheap one will read ~0V, the ones in the lab will (can if programmed) auto-detect and tell me the DC value. – Jon Custer Feb 18 at 16:20
• Would Electrical Engineering be a better home for this question? – Qmechanic Feb 18 at 17:40
• There is no single answer to this. It depends which kind of internals the AC voltmeter has. Either an AC voltmeter says there is 0V of AC because DC has no AC voltage components. Or it might give some other value if it uses some other way of measurement. – Justme Feb 18 at 20:46
• This appears to be a homework question, or at least it's a very common beginner level course question. Normally on EE.SE there is an effort requirement for a homework question that hasn't been demonstrated here. That said, you have 3 answers already. Typically in a course the correct answer to that question is that an AC voltmeter reads only the AC component of the voltage, so it will read 0V. – K H Feb 19 at 10:25

If you are using a scope with AC coupling, then it will read zero because it isolates DC, but if the AC coupling is off, then it will give you the true DC value.

If you are however talking about "AC Voltmeters" which are quite uncommon since one can easily use DC Voltmeters to read AC RMS; the ones I have dealt with use a rectifier, and holds to voltage across an RC channel and then reads the voltage as DC. The rectification can be done using a single diode or a full bridge rectifier, I found this circuit online:

Clearly this circuit will read DC values same as AC values, so long as the DC voltage is positive, if a full bridge rectifier was used instead of the single diode, then the absolute value of the voltage would be read.

Theoretically it should show the correct voltage of the source, because average, root mean square and maximum of the constant voltage is just that voltage itself. What else the AC voltmeter could be supposed to indicate?

However this depends. For instance, if the single diode is used to rectify the AC, the device still can be calibrated to indicate the value for AC properly but will show zero on one polarity. An old, analog (electromagnetic) indicator will likely display double voltage for another polarity.

Are you asking the question because you only have some kind of a meter that just measures AC? Why would you want to use it? You can buy a DMM with a 1000 VDC range for \$6.79, and it will also measure AC voltage and resistance in several ranges.

You must know this: you have an internet connection, so it must be a question to settle a bet or idle curiosity. The answers all answer the question to some extent: "It all depends." What meter?

• A DMM set to AC voltage qualifies as an AC Voltmeter for the purpose of the question. It will read the AC component of the voltage, so 0V. – K H Feb 19 at 10:27
• It won't necessarily read 0V. Both the other answers show how. It all depends. – John Connell Feb 19 at 13:08