Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!think.com!eplunix!raoul
From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: sci.electronics
Subject: Current shunting
Message-ID: <968@eplunix.UUCP>
Date: 14 Nov 90 21:00:57 GMT
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 34

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, but
there are problems with that. I have to stuff this inside an already
built unit, so my board space is very limited. It cannot draw
more than a couple of mA quiescent current, since that would drain
the batteries too quickly. It cannot have a common mode input 
impedance of less than 100 kOhm, since that would divert some of 
the output current away from the electrodes.

If there is no clean way to do this in small and low-power
circuits, I'll have to build up another set of modules for this. 
Frankly, I'd rather not. Any ideas out there?

-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu