raoul@eplunix.UUCP (Nico Garcia) (11/15/90)
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.eduwhit@milton.u.washington.edu (John Whitmore) (11/16/90)
In article <968@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes: > We have a set of current stimulators... 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. Would an SCR crobar suffice? You can get SCR-output optoisolators that would allow you to put a completely isolated crowbar on each circuit. If the holding current is as low as your current source, this would take a single pulse to turn "on", and would remain shunted until Iout was removed. If that won't work, you could use a darlington optoisolator and just keep the LED input turned on (circa 1 mA). Either way, you'd have to have an op amp to detect the fault and a latch or a one-shot to trigger the shunt. John Whitmore whit@milton.u.washington.edu
davet@tsdiag.ccur.com (Dave Tiller N2KAU) (11/17/90)
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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