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Old 18th Apr 2017, 12:55 pm   #7
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Another FET voltmeter

Quote:
Originally Posted by MrBungle View Post
I checked leakage through the zener on the breadboard (with a marginally superior HP3478A ) and the FSD voltage at the FET is 1v... I blew the front end FET up twice on the breadboard (at 100v in 1v range) and this stopped it from blowing up so I'm not 100% convinced it isn't necessary.
You're right, 1V on the Zener shouldn't upset anything!

I'm surprised at the blowing up - after seeing your circuit, on the 1V range you still have 2M of series resistance in series with the gate! I'm tempted to find a few FETs and try and replicate your results. Thanks for sharing this saga, much appreciated! Learning a new failure mode is always worthwhile...
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