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From: Norman Yarvin
Subject: Re: Strange going-on with a floating MOSFET gate
NNTP-Posting-Date: Mon, 06 Jan 2003 10:19:15 EST
Date: Mon, 06 Jan 2003 15:19:15 GMT
In article ,
Winfield Hill wrote:
> 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.
But that's the point: you don't need charge transfer at the moment of
severed contact, to produce a potential change. Just removing a nearby
charge will produce a change in potential; this is basic electrostatics.
The nearby charge in this case is the charge on the newly disconnected
lead wire. If you moved that wire back to where it was -- just a hair
away from touching, which you could never do by hand -- you'd get the
voltage back. (At least if this is the only thing going on.)
In general, most of the time you don't have to worry about where the
charges are in a circuit, just what the voltage is; but the charges on an
object of uniform voltage are by no means uniformly distributed. They
concentrate, for instance, at sharp points. If you were to surgically
excise those points, and remove them, the voltage on the remainder would
Norman Yarvin email@example.com
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