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From: davet@tsdiag.ccur.com (Dave Tiller N2KAU)
Newsgroups: sci.electronics
Subject: Re: Current shunting
Message-ID: <1139@tsdiag.ccur.com>
Date: 16 Nov 90 16:13:17 GMT
References: <968@eplunix.UUCP>
Organization: Concurrent Computer Corp. Oceanport NJ
Lines: 31

In article <968@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
-Hello, folks: I have a problem here that maybe someone can point out
-obvious or old solutions to. We have a set of current stimulators
-that we use for work here. They are electrically isolated (running
-off separate batteries) and drive current signals from 1uA to 1 mA
-through nerve electrodes. We also have a 1K resistor and a 1uF AC
-coupling cap between the output and the electrode, so it goes like this:
-
-Iout---->Rtest---->Cap---->Electrode---->Ground
-     |         |       |             |
-    Lead1     Lead2   Lead3         Lead4
-
-We would like to put a current crowbar in: a circuit that would
-detect current above a preset level (positive or negative) and
-shunt Iout directly to ground, (to tie Lead1 to Lead2) and only 
-release that shunt when a panel button is pushed. Those current
-levels would be between 10 and 1000 uA.
-
-It would be possible to build up a very nice system to do this
-with a differential amplifier, a rectifier, and a relay,

Replace the relay with a small signal scr or triac with a spst momentary
contact NC switch in series.  Pushing the switch would open the current path
causing the scr/triac to 'let go', resetting the circuit.  The current levels
are pretty low, so I'm not sure you'll find an scr/triac that'll hold in
conduction at 10-1000uA. Hope this helps,
-- 
David E. Tiller         davet@tsdiag.ccur.com  | Concurrent Computer Corp.
FAX:  201-870-5952      Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117
UUCP: ucbvax!rutgers!petsd!tsdiag!davet        | Oceanport NJ, 07757
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