I've just got a cheapo < $20 digital multimeter here and was curious if anyone could tell me whether current might still flow through it after i turn the dial so that the power is off.
Some background: I have a wireless bluetooth mouse that always gave me problems charging because the contacts don't seem to line up properly. The mouse itself doesn't seem to give any indication whether it is charging or not but I am able to watch current start to flow on my multimeter when I move the mouse into just the right position within the cradle to make a proper connection. Now that it is seated correctly I'd like to just leave it charge for a while without disturbing it by disconnecting the leads and reconnecting the +5V wire.
I'll probably figure something out for this specific problem (just disconnect the leads from DMM and jumper them together for now) but I'm still kind of curious as to the original question of whether or not current flow is completely shut off when the DMM power is off.
It might vary by model but maybe somebody out there with 2 multimeters could test this for me?
Best Answer
In short, yes.
The way a digital multimeter measures current (not a loop current meter) is that it measures the voltage drop across a small precision resistor. For reliability, simplicity, and repeatability, no switches or contacts are placed in series with this shunt resistor. Some meters will have a fast-acting fuse to protect the shunt resistor. Basically, the switches will control the electronics hooked up to the shunt resistor, but they won't disconnect it.
You can perform an experiment with your meter as some degree of proof that this is true. If you configure your meter to read resistance, put the positive probe into the positive current probe socket. You should measure a very small resistance. This shows that the meter doesn't disconnect the shunt resistor while in a separate mode. It would be a small leap of logic to say that if it didn't switch off the shunt resistor then, it probably couldn't when the meter is off.
Alternatively, you could measure the current using a second meter.