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ignitor continuity tester

J

Jamie Morken

Jan 1, 1970
0
Hi all,

I have an ignitor that has a resistance between 0 and 10 ohms and I would
like to output a voltage between ~0V and ~5V proportional to that resistance
(to check the ignitor resistance). The maximum current I can put through
the ignitor is ~0.5mA for safety. The circuit I have so far uses a
differential amplifier using an opamp:
http://www.rocketresearch.org/new/ignitortester.jpg

I need to squeeze four of these circuits into a small PCB area as this is
going on a small rocket electronics board. Any ideas on how to simplify the
circuit or other circuits that I could use to do this? Thanks for your
time,

cheers,
Jamie Morken
 
J

Jamie Morken

Jan 1, 1970
0
Ken Smith said:
If you make the +12V the return point for the meter, the circuit can be
made simpler. You need an op-amp with low bias current and that has a
common mode range up to the (+) rail.

I am starting to think that it may be easier just to have a digital output
which goes high if the ignitor is below 10ohms and otherwise goes low.
Assuming a max allowable current of 1mA through the ignitor, any ideas on
how to implement this? Thanks,

cheers,
Jamie
 
I

Ian Stirling

Jan 1, 1970
0
Jamie Morken said:
I am starting to think that it may be easier just to have a digital output
which goes high if the ignitor is below 10ohms and otherwise goes low.
Assuming a max allowable current of 1mA through the ignitor, any ideas on
how to implement this? Thanks,

I'd go for a current of 100uA.
This is a voltage of a millivolt.
If you are just looking for go/no-go, you need to detect 1mv, and
+-.25mv is probably OK.

Find a vendor who has a parametric seratch, and go and look for a 4 input
op-amp/comparator with a Vos lower than 250uv.
The first I found is the MXL1179, which easily beats this, at $3 per (1K).

Now, tie the ends of all 4 ignitors to 12V, and from each to 0V put
two 62K resistors in series (shorted resistor would be bad).
From each ignitors lower end, put 2 62K resistors to the inverting input
of each opamp.
Now, tie all the noninverting inputs together, and connect them to the wiper
of a 100 ohm potentiometer with one end connected to +12, the other to a
1M resistor to 0V.

This will give you four outputs that go high when the ignitor is good.
four diodes and a resistor will then give you one output that goes
low when any is bad.
 
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