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Is my multimeter broken?

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Phil.2007

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I've got a cheap multimeter (Velleman DVM890L) and recently I decided to measure the resistance of a microphone I've got to see if it equaled the quoted impedance (I'm trying to learn about all this) anyway, the ohmmeter kept creeping up and up, even with just the two test leads touching. I thought it was a dead battery so I've replaced it, but now it shows 0.63 ohms with the test leads touching - or 1.7 another time I tried it - does anyone know what's going on?

Thanks, and please bear in mind I only have a GCSE level of electronics knowledge...

Phil
 
Sorry, I should have clarified my answer - the fact that I was looking at impedance was inconsequential - the meter reads 1.4 ohms with the test leads touching each other, and even with a short length of new copper wire between the plugs the leads plug in to, it still shows a > 1 ohm resistance.

I also noticed after posting that it was doing that counting up and up, then down on its own...

On a side note, can you explain the difference between impedance and resistance... and why is impedance measured in ohms? Thanks
 
Speaker impedance is usually quoted at a 1000 Hz test frequency, for mid range speakers.
I' guess that the same applies to a microphone, It may even have some internal circuitery in it which throws you readings out.

Test on a couple of known resistors and see what your meter readings are.

How does your meter read on volts ? eg 1.5 or 9 Volt battery ?
 
Phil.2007 said:
Sorry, I should have clarified my answer - the fact that I was looking at impedance was inconsequential - the meter reads 1.4 ohms with the test leads touching each other, and even with a short length of new copper wire between the plugs the leads plug in to, it still shows a > 1 ohm resistance.

I also noticed after posting that it was doing that counting up and up, then down on its own...

On cheaper meters the switches and battery contacts can be of a low quality.
With a meter on ohms, resistance ranges, the meter leads have a voltage potential and when you touch the leads together or measure a resistor a small current flows thru these leads from the internal battery.
If any contact is poor in the 'test leads loop and meter' the indicated resistance will vary on the display.

Check the battery holder, internal fuse holder, if fitted and meter lead connectors.
 
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Inductance and capacitance change the effective resistance of a circuit depending on the frequency of the signal. You can not check the resistance of an electrical circuit with a multimeter when power is applied it could damage the meter. A non-zero reading with the leads shorted out isn't overly unusual, common meters aren't able to measure resistances that low, it could also be contact resistance from dirty probes. A way to check to make sure your meter is still working on the resistnace setting is checking it against 10 100 1k 10k 100k and 1meg resistors, or any resistors of a known value.
 
Phil.2007 said:
II thought it was a dead battery so I've replaced it, but now it shows 0.63 ohms with the test leads touching - or 1.7 another time I tried it - does anyone know what's going on?

Oxide on your test lead probe tip end. They are silver plated but the plating wear off very soon after some usage. They then oxidized and cause erratic zero ohm readings. Usually the problem is inconsistent value differs every time you touch the tips together.

Clean the tips and test again.
 
An ohm or two isn't unusual for cheap meters and cheap probes. Maybe it's actual resistance, or maybe it's calibration error. It can be a little unnerving, but for most readings that you take an ohm doesn't matter.
 
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