That's what I was afraid of. But wouldn't an open circuit show 0 volts?
I've never heard that about DMMs. I'll have to get a test light.
Yes, a true open circuit will read zero volts, but..............
Google the voltage divider rule, which come from Kirchoffs voltage and current laws. The DMM
looks like a 11,000,000 ohm resistor. If a power circuit has 1 ohm resistance (WAY high for a circuit that must supply a number of amps), one eleven millionth of the voltage will drop across the 1 ohm resistance, and the rest across the DMM
, so the DMM
will effectively read the correct voltage. If the power circuit has 11,000,000 ohm resistance (such a circuit can only supply microamps at 12 V), the voltage will be divided equally between the circuit and the DMM
, and the DMM
will read 6 V with a 12V supply to the circuit. This is why a test light should always be used to test power circuits. A test light, which can draw around an amp, will load the circuit sufficiently so that even a few ohms resistance in the power circuit will cause the light to glow dim or not at all.