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From: Winfield Hill
Subject: Re: Strange going-on with a floating MOSFET gate
Date: 5 Jan 2003 16:28:17 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
Norman Yarvin wrote...
> Winfield Hill wrote:
>> Tony wrote...
>>> Winfield 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 (voltage).
> Whether this is the correct explanation depends on the relative
> sizes of the different capacitances involved ... 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.
Yes I have thought of such a scenario; it has a certain appeal.
But a primary problem is that everything is presumed to be in
equilibrium in the instant before the breaking contact, thus no
current is flowing (ignoring ac pickup), and therefore (at least
in the dc case) the argument for charge transfer at the moment
of severed contact fails.
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