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Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
I've ordered several panel amp meters for a project I am working on, and was wondering, how well will they handle being over-stressed? I have one 50uADC meter that will between two 500Meg resistors to measure a maximum of +-18kV, and a 25A AC meter to measure the input to the souce from the wall. If, for some reason, I manage to put too much amperage into the meters, what are the chances of destroying them?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
A common trick for protecting sensitive moving-coil meters is to put two silicon diodes in parallel with the movement, one pointing each direction. They don't conduct in normal use (or shouldn't: check the voltage required for FSD) but they stop you burning out the coil or pegging the needle hard enough to damage it.
AC moving-iron meters are pretty tough, I don't think you would break one from overcurrent, short of overheating the coil enough that something melted.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
If I recall correctly, the resistance of my 50uA meter is 2k, so it would need a mere 100mV across it to go to FSD. So you are saying to use something like a 1n4002 (I have a handful in my stash), which doesn't really start conducting appreciable current until a few hundred millivolts? According to Fairchild's datasheet, it conducts 10mA at 600mV at 25C, but that is as low as the graph goes.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
Firefox wrote ...
So you are saying to use something like a 1n4002 (I have a handful in my stash), which doesn't really start conducting appreciable current until a few hundred millivolts?
yes.
BTW if you hook the meter between the two resistors, it will be floating at half the potential you are measuring. not sure if thats a good idea
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
I plan on using the meter between equal positive and negative potentials, so it will hopefully be close to or at ground. In any case, I don't intend to get near the meter with anything it is measuring powered, so it may be a non-issue anyway.
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Robustness of Panel Meters
