measure resistance using 2 wire method

[Justin986]

Hello, am I using the 2-wire method of measuring resistance across a photocell correctly? When i run i I get a sinusoidal reading from my max range of 15k ohms to my min range of 1k ohms. Why isnt this working?

[mikeporter]

What kind of box do you have? How is it wired?

Mike...

[Bob_Schor]

What is the circuit?  I see what looks like a USB-6009 (but, of course, you don't show enough to really identify it) with two green wires that seem to go into Analog In 0 high and low.  One wire seems to go to a pair of resistors that go to something (can't tell what that is), then another pair of resistors and the other green wire.  Four more green wires go to other analog inputs, but we don't see where they are connected.

How are you expecting to measure a voltage (which appears to be across four resistors + a "something", with no obvious voltage source)?  One obvious voltage source that you can certainly measure by putting a resistance across a differential amplifier is "noise", including 60Hz pickup from flourescent lights, A/C power, etc.

Just because you tell the DAQ Assistant (I assume that's what you are using) that you want to measure Resistance, you still need to set up the circuit properly.  Without a diagram or a better series of pictures (showing all the wires, with all of the components identified), I can't tell what you are doing, so I can't say that your results aren't expected.  

Bob Schor

[billko]

A good rule of thumb is to be able to take the measurements manually before attempting to do it with software.  That way you can eliminate setup issues.

[dmsilev]

Based on your front-panel settings, you should have an external 1 mA current source in your circuit as well as the DAQ measuring the voltage. That's how a simple 2-wire resistance measurement works: send a known current through a device and measure the voltage across it.

The circuit should be:

I+ --->(1)->unknown resistance->(2)---->I-

where I+ and I- are the positive and negative terminals of your current source and (1) and (2) are where you connect V+ and V- of your DAQ.

If you don't have an external current source and you know roughly the range of resistance of your unknown, you can use the 5V output of your DAQ and what's called a current-limiiting resistor to make an ..OK current source:

+5V -> R1 ->(1) -> unknown -> (2) -> GND

R1 should be large compared to the unknown, at least 10x bigger. The current in the circuit can then be approximated as 5V/R1, and the resistance of the unknown is then (again approximately) R1* (V12 / 5 V).