I need to protect a 100 microamp moving-coil meter with a pair of back-to-back diodes. Can someone recommend suitable types ? I have various 1N400x types but I presume these would not be suitable.
Jim Hawkins
What is the voltage at 100 microamp? Or the resistance?
A 1N400x is a bit slow. But may be enough.
I would suggest using diodes that are fast enough like the good old
1N41418 or maybe some shottkys or zeners. Depends on the voltage. And on the maximum current that can flow through the diodes.P.
Most meters in this range drop a couple hundred millivolts at full scale. Rectifier diodes will probably be too leaky, try something in a small signal diode. I used to like the FDH300's but they were discontinued. You might get away with 1N4148's, check for nonlinearity approaching full scale. There used to be a device formulated just for this application, but the bottom pretty much fell out of the analog meter market with the advent of low power CMOS A/D converters.
Thanks for that Bill. I found that Fairchild-Direct still have a couple of thousand FDH300s in stock so I've ordered some.
Jim Hawkins
You could add an MOV in parallel to further protect the meter movement. Simply choose an MOV that clamps higher than the maximum voltage you plan on reading, yet low enough that the needle won't wrap around a peg before the protection kicks in..
An MOV as a backup to the diodes would be belt & suspenders...
John :-#)#
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