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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?

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  • \$\begingroup\$ Turns out I could just watch the voltage drop across my power supply go from 5.09V unloaded to 5.19V when it was charging the battery. Still looking for an answer though =) \$\endgroup\$ – nvuono Mar 6 '11 at 6:39
  • \$\begingroup\$ measure the resistance between the two ports for current while your multimeter is in resistance measurement. The resistance measurement normally uses the voltage prong and that will have the current side "off". The resistance on that side should still measure as a short. This is not a perfect test, but a quick one that can give 95% confidence. \$\endgroup\$ – Kortuk Mar 6 '11 at 10:00
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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.

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  • \$\begingroup\$ really, you need to use a third meter to check to make sure that the second is not causing the problem. \$\endgroup\$ – Kortuk Mar 6 '11 at 9:57
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Just hook up a light bulb or LED through the current connections on the meter and observe that it remains lit when the meter is powered off.

Or pop the case open and observe the wiring of the current connections. Current is inferred by measuring the voltage drop across big hefty resistor (but with quite low resistance). You should see a big thick wire (a low-value resistor called a "shunt") joining the common and current sockets.

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There are multimeters with separate inputs for voltage and for current measurement and there are multimeters that have a single input for both.

With those that have a single input, current will flow through the meter only if it's in current range.

With those that have separate inputs, current will always flow when the red lead is plugged into the current measurement plug.

However, there will flow a very small leakage current even in voltage mode, but that is usually negligible.

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