From: Norman Yarvin
Subject: Re: Strange going-on with a floating MOSFET gate
NNTP-Posting-Date: Sun, 05 Jan 2003 17:36:47 EST
Date: Sun, 05 Jan 2003 22:36:47 GMT
In article ,
Winfield Hill wrote:
>> Winfield Hill wrote:
>>> But what I didn't expect was what happened immediately when
>>> removing the clip lead to the gate. Without fail the current
>>> would instantly drop by about 2x, indicating a roughly -75mV
>>> change in gate voltage. ...
>> If you offered your hand (and croc clip) back to the
>> gate (without actually connecting to it) did you get
>> the corresponding +75mV change on the gate voltage?
> Generally the new readings were stable, movements of my hand
> nearby and other activity had little effect, that's why I went
> ahead and let the meter log 2 hours of data. The FET's tiny
> antenna wire and all that. But I haven't tried your idea.
Remember that it's not potential that's conserved, it's charge. Removing
the clip lead removed nearby charges, thus changing the potential
Whether this is the correct explanation depends on the relative sizes of
the different capacitances involved -- the gate capacitance versus the
capacitance of the air gap, as measured at the first instant that current
could no longer flow between the alligator clip and the transistor, as
the clip was pulled away.
I'd expect Tony's experiment to fail, by not producing as much voltage
change as was lost, since the capacitance you could produce by getting
the alligator clip close by hand would be less than the capacitance of
the miniscule gap at the first instant the clip lost contact.
Norman Yarvin firstname.lastname@example.org