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I know that with DC you can measure current by putting your meter in series with the positive supply.

I have a project running off a low voltage AC supply. Can I do the same?

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2 Answers 2

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Many (most inexpensive?) multimeters can only measure DC current - in that case, no, you can't.

However, if your meter can measure AC current, you would connect it in series with the circuit, as you would with DC.

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AC current ranges are often not available on cheaper meters.

A "work around" that works with almost any DMM and is sometimes superior is to use a small series resistor and to measure the voltage drop across it with an AC voltage range on the DMM.

Most cheaper DMMs are "three and a half digit" meaning they display values from 0000 to 1999. Some meters have a 200 mV AC lowest AC voltage, range and others 2 VAC. A few very cheap ones may only have 20 VAC minimum.

What value series R to use depends on current and tolerable voltage drop. For eg a load drawing up to 100 mA and with a say 12VAC supply then 0.1 VAC drop in a sense resistor would usually be tolerable and higher may be.
0.1 VAC is 0.1/0.2 = 50% of a 200 mV range and gives 1000 counts (pout of 2000 possible) so in this case woukld allow a 100 mA/1000 = 0.1 mA resolution with the right resistor.
To do this select R so V_r = 0.1V at max crrent. So R = V/I = 0.1/100 mA = 1 Ohm.

If only a 2VAC minimum range was available then a 1 Ohm resistor will give 1 mA resolution in the above example.

A 20 VAC minimum range gives 10 mA resolution - use a larger resistor for more voltage drop or (better) get a slightly better meter.

Usually sense resistor value of 0.1 Ohm and 1 Ohm are useful. These allow easy conversion from current to voltage. If doint this regularly use a good precision resistor and perhaps select one from a batch that is most accurate. Note that power dissipation is usually not an issue but if you do overheat the sense resistor its value will often be affected.

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