Veiwing an ohms change on an oscope????

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- scanner80
  
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posted
18 years ago

Sat, Aug 6, 2005 1:56 AM

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 most important thing is to see the reading change from 1355 and return to

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

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- scanner80
  
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posted
18 years ago

Sat, Aug 6, 2005 4:29 AM

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- CWatters
  
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posted
18 years ago

Mon, Aug 8, 2005 2:58 PM

resistance

Try putting the resistor in series with a 1mA current source to produce an output that varies from 1355mV to 1375mV. Then feed that into the scope. Adjust the gain and offset of the scope to display 1355mV at the bottom and

1375mV at the top. Most scopes have 10 grid squares vertically so you should be able to see 1mV (=1 Ohm) changes easily.

If you can't adjust the offset on the scope enough then feed the output into an opamp configured as a subtractor...

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Set it up to subtract a constant 1355mV from the input...

1355-1355=0 1375-1355=20 so the resulting output would be 0-20mV where 0mV corresponds to 1355 Ohms and 20mV = 1375 Ohms.

You might be able to use the scopes trig level and trig output (if it has one) to control a timer/counter.

Check that the source resisrtor isn't temperature sensitive. If it is then you might be able to compensate for this by using a similar part (with a constant resistance) to produce the 1355mV needed for the subtractor.

Colin