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In Reply to: RE: My meter is scaled to 200 ma. posted by DAK on June 19, 2017 at 02:51:02
Is this a panel meter you're discussing? If so, I would expect a 200mA meter not to need an external shunt. The only way to be certain is to rig up a variable power supply and fixed resistor for a test. For example, connect a 100 ohm resistor in series with the meter, then slowly begin applying voltage. If the needle goes to full scale quickly, it needs a shunt. Otherwise, it should read half scale (100mA) when you reach 10V.
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Buy Chinese. Bury freedom.
Follow Ups:
I have a ton of old panel meters. Most labeled "mA" that have a low scale (500mA or less) are 200mV DC meters without an internal shunt.
Amp meters that are above that always have an external shunt. Far cheaper to test than to assume. :-)
"Most labeled "mA" that have a low scale (500mA or less) are 200mV DC meters without an internal shunt."
If I understand, you're saying the meters are mostly 0.2V FS, and it's the difference in coil resistance that provides different FS current readings. That would mean a 200mA DC panel meter is likely to have a coil resistance of one ohm. Is that correct?
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Buy Chinese. Bury freedom.
Most old panel meters need an external resistor network to function correctly. My advice is to test it carefully before putting it in a circuit and destroying it. But yes, if the coil resistance is 1 ohm then it could be used directly as a low current ammeter.
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