Alt-BEAM Archive
Message #05750
To: beam@sgiblab.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Mon, 16 Aug 1999 22:24:41 -0700
Subject: [alt-beam] Re: MOSFET sensor thingy
Hi Richard,
2N7000 is a FETLINGTON, something like a low power IGBT and is more
complex than a MOSFET and a lot more complex than a JFET. Looks like
the reverse connections are actually correct. The normal connection for
a N channel FET common source amplifier is S = gnd (neg) , D = (pos)
signal out and normally has a resistor to +V and G = input. This is
similar to a NPN amplifier but with E=S,C=D, B=G. The 2N7000 gate is
insulated and can be extremely high input impedance capable of trapping
a charge for a long time which turns the mosfet partially on. But some
manufacturers add some protection devices connected between G and S
which leak off any charge and shut the fet off. Some mosfets are
specifically designed for charge amplifiers and these are ideal for
your application This type of mosfet always has lot's of warnings about
damage from static discharge because there is no protection from
overvoltage. JFET's are not as high input impedance so not as
sensitive for your app but they are quite rugged and they are normally
"on" when the gate charge is zero. The interesting thing about JFETs is
they are generally completely reversible ie swap source and drain
without affecting performance.
all for now
wilf
Richard Caudle wrote:
> -----------------------------------------------------------------------
> Evening all,Yet another in a long line of strange things... Today at
> work, I came across a circuit for detecting electrostatic fields.
> Since I have need for such a thing, I tried to build one. The only
> problem is that the required components weren't available (what else
> is new?). It called for an n-channel FET and all I could come up with
> was an n-channel MOSFET (2N7000). I got the pinout from Digi-Key and
> proceeded to build the circuit. Needless to say, it didn't work. So I
> decided that the pinout must be wrong. I turned it 180 degrees and it
> started working, but not like the description said it would. The
> closer I moved my hand to the 'antenna', the higher the voltage went.
> I put in an LED so I could watch it. It was sitting at about 530mV
> until my hand got about 10" away and it began to rise. At about 4
> inches, it went to about 1.3V and when I would touch it, it jumped to
> 4.8V (input V=5VDC). The strange thing about it: The 2N7000 is
> backwards! At least I think it is. The S (source?) pin is on the
> negative side of the supply and the D (drain?) pin is on the positive
> side. The gate is acting as the antenna. Maybe I've got the terms
> wrong. It's still a cool thing. I was thinking it could be used as a
> 'real' antenna for a walker or popper. It could trigger a Nu that
> operates the reverser. Something that deserves further
> investigation(IMHO). Richard Caudle
> richard@cqc.com
> Home ICQ - Frankendaddy
> -----------------------------------------------------------------------
> Guardians of the sacred words: Nee, Ptang, and NeeWhon!
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