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?
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.