Path: utzoo!utgpu!cunews!spock!grayt
From: grayt@spock (Tom Gray)
Newsgroups: sci.electronics
Subject: Re: Current Shunt Problem
Keywords: Current, Crowbar
Message-ID: <5468@smithd>
Date: 26 Nov 90 14:29:40 GMT
References: <971@eplunix.UUCP>
Reply-To: grayt@smithd (Tom Gray)
Organization: Mitel. Kanata (Ontario). Canada.
Lines: 51

In article <971@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
<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


Well am I the only one. The simplest trick would be to detect the signal
and use it to trigger a latching relay. i don't know what people have
againast relays. They are cheap and do perform the job.
you can buy very cheap relays that will switch 1000V safely.

If you don't want to use an SCR because of the changing polarity
of current, you can use a triac. Just as easy to use and
will handle aC.