Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!zaphod.mps.ohio-state.edu!think.com!eplunix!raoul
From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: sci.electronics
Subject: Current Shunt Problem
Keywords: Current, Crowbar
Message-ID: <971@eplunix.UUCP>
Date: 23 Nov 90 17:23:29 GMT
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 38

In article <968@eplunix.UUCP> you write:
>>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).

A re-examination of the circuit shows that I made a mistake in the drawing:
It should read (to tie Lead2 to Lead4) in the following configuration.

Iout---->Cap---->Rtest---->Electrode---->Ground
     |       |         |             |
    Lead1   Lead2     Lead3         Lead4


Also please note: Since the cap is between the stimulator and ground,
this thing only passes AC signals. This means that the SCR 
solutions people proposed will not remain crowbarred, but will reconnect
when the Iout goes negative.

There have been some interesting proposals, which I'm investigating.
Some of them fail on power requirements, or require re-configuring the
the electrode in the circuit which I really can't do. There are a couple
of them I'm investigating that might work. Thanks for the help, folks.

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