Viewing an ohms change on an oscope??

Discussion in 'Electronic Basics' started by scanner80, Aug 6, 2005.

1. scanner80Guest

Hello,
I'm looking for a circuit I can build or equipment I can buy
so I can time how long an ohms change is. I would like to build it if
possible . I would like to view it on a scope.
One example would be a starting ohms reading from a device of 1355 ohms. It
will increase by approx. 20 ohms
and then return to 1355 ohms. The time it will take is approx. 19 seconds. I
will need to be able to see a change as small as 1 ohm if possible , but the
1355 ohms. I need to then measure the time with cursers on a scope.
I know a respiration monitor can see and display an ohms change , but I need
a way to exactly measure the change.
I will be greatful for any help.
Thank you,
Jeff

2. mikeGuest

Anything that takes 19 seconds and can be measured with cursors is gonna
take a digital storage oscilloscope. And you want > 0.1% precision.
This is gonna be one EXPENSIVE measurement.

I'd build a PIC processor controlled resistance bridge. Program that to
output the parameters of interest.
mike

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3. John PopelishGuest

Can this resistance be connected to some other components? Are you
familiar with a 4 resistor bridge?
If you connect your ~1355 ohm resistor in series with a fixed resistor
of nearly 1355 ohms, and connect that to a DC supply, say 10 volts.
Then connect another series pair of equal resistors (say, 10k each)
across the same supply. Now you can read or amplify and read the
voltage difference between the center nodes of these two resistor
pairs. It doesn't give you a direct measurement of resistance, but
with a bit of math, it is easy to measure the % change of any one of
the 4 resistors.

4. scanner80Guest

I just want to be able to measure the time it takes from when the resistance
leaves 1355 ohms and returns to 1355. I can do it with a meter and a
stopwatch, but I was looking for a more verifiable way to do it. I want to
remove human error. This is for calibration purposes.
Is there any type of simple timer or timer circuit that can will trigger as
the ohms changes and stop when it returns?
Jeff

5. scanner80Guest

The device will use an opamp circuit to create the resistances and the
change. Will the bridge setup work with that type of circuit with out
causing it to malfunction?

6. John PopelishGuest

My opinion on that would be worth more after I saw the schematic for

7. JamieGuest

hmm.
well.
unless your trying to get out of buying an expensive digital storage
scope, a line monitor unit (also expensive) would work! but you want to
hack out something and know a little computer programming, you could
make your self a VCO osc that would input to the sound card.
use some FFT coding to sample the frequencies in your time frame..
etc..

8. Jeff ThonGuest

Hi,

Have you considered a logging DMM? Your data will be in ohms. Sample times
can be set faster than the display update rate. The software can graph the
logged data and will have cursor capability so you can get your time as you
would have from the scope display. You can use the logging for any
measurement the meter is capable of for future testing. Additionally, you
have built in documenting. Way less expensive than a storage scope. Also,
you don't have to worry about the accuracy of any test gear you build.

Jeff

9. scanner80Guest

I was just thinking about that.
I work in a cal lab and have digital scopes, calibrators, and many other
cool toys. I don't have a logging DMM yet ,but but I can get access to one
to try it out.
If I can prove I need one , then they may get me one.
thank you so much for the idea. I confirms what I was thinking.
Jeff