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Resistance Measurement Circuit Help

Dan SNyder

Feb 2, 2015
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I'm trying to design a simple circuit to measure resistances between 100 and 100k ohms. I'm a tad rusty with electronics but what I've tried so far are a standard voltage divider and an attempt at a constant current source using an LM317 regulator. Neither circuit seems to reach a range vs accuracy sweet spot. Can anyone suggest a circuit that might serve me better? Ideally I would be able to adjust the accurate range using a voltage between 0 and 18 volts however if I have to adjust current output with different resistor values that's fine. The LM317 works this way however I can only really measure between 1-100 ohms with that circuit.
 

davenn

Moderator
Sep 5, 2009
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HI Dan
Welcome to Electronics Point :)

really trying to figure out what you are wanting to do ?
you do know there are multimeters with resistance measuring capabilities ?
show us a circuit of your project along with a better explanation :)

cheers
 

Arouse1973

Adam
Dec 18, 2013
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Hello

The reason you are probably having trouble is the head room voltage on the input. You will have to change the value of the current set resistor to allow you to measure at both ranges.

Example: if you had a current of 10 mA this would produce 1V across 100 R but would try and drop 1000 V across a 100 K. So you will have to keep fiddling with the resistor values to find a voltage you can measure.

You might be better off using a precision current source of say 100 uA and then use an opamp to amplify the voltage to a range you can measure. So with 100 uA you will get 10 V from a 100 K test resistor and 10 mV from a 100 R test resistor.

Adam
 

Dan SNyder

Feb 2, 2015
4
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So I'm taking a standalone embedded device which has a few digital I/Os and analog I/Os and I'm using it to measure the voltage and resistance of other circuits. I'm going to use it for sorting purposes (defective vs good parts). The reason why I can't use a multimeter is I'd like the process to be more or less automated via a script running on the embedded device. I also need to be able to take advantage of the http communications the device is capable of. So basically you can think of my circuit as:

upload_2015-2-2_15-29-50.png
 

Dan SNyder

Feb 2, 2015
4
Joined
Feb 2, 2015
Messages
4
The reason you are probably having trouble is the head room voltage on the input. You will have to change the value of the current set resistor to allow you to measure at both ranges.

Example: if you had a current of 10 mA this would produce 1V across 100 R but would try and drop 1000 V across a 100 K. So you will have to keep fiddling with the resistor values to find a voltage you can measure.


You might be better off using a precision current source of say 100 uA and then use an opamp to amplify the voltage to a range you can measure. So with 100 uA you will get 10 V from a 100 K test resistor and 10 mV from a 100 R test resistor.

Adam

This is more or less what I was seeing when fiddling with the LM317. Do you have a suggestion for a precision current source? The LM317 doesn't seem to provide such a current range.
 

Arouse1973

Adam
Dec 18, 2013
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Here is a little circuit for you. VS3 is the input voltage across the resistor. It gives you an output of approx. 1 V - 10 V for an input of 10 mV to 1 . You need to use a rail to rail opamp for this, Linear Tech are one such supplier. Output on this opamp in from pin 6.
Adam

OPAMP 10mV to 1V.PNG
 
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