Why is my multimeter giving false readings?

March 16, 2011 at 07:53:31
Specs: Windows XP
I'm using a RadioShack multimeter which claims to measure current up to 10A. When I hook up the meter to a 9V battery in series with a 1 ohm resister, I'd expect 9 amp, but I get a reading of less than 1 amp. Can anyone help me understand this? I suppose the meter itself may have resistance to lower the amps, but if this is the case, how could it ever give accurate results of what's happening in the circuit without the meter in place?

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March 16, 2011 at 07:58:27
You honestly expect a small 9 volt battery to be able to supply 9 amps? The most you're going to get out of a 9 volt battery is going to be at best a couple hundred milliamps. A NiCad 9 volt may give you an amp or two for a few seconds.

"I suppose the meter itself may have resistance to lower the amps"
When measuring current, the meter should basicaly present itself with zero resistance, thus producing current flow. When measuring voltage it should have an extreamly high resistance so it doesn't load the circuit.

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March 16, 2011 at 08:47:35
Thanks. Understanding the limitations of a 9V battery is helpful.

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March 16, 2011 at 10:02:41
You can be thankful your experiment didn't work as expected. If the battery were actually able to provide 9 amps into a 1 ohm load the resistor would have to dissipate 81 watts. Unless it was a very high wattage resistor it would burn out very quickly. Some resistors can literally burn in such cases. Some batteries can explode with a sufficiently high load.

The internal resistance of a meter depends on it's type and the scale in use. Low current ranges will have higher resistance. Usually the resistance is low enough not to effect circuit operation but this is not always the case. The electronic meters are better but they are of course more expensive.

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