Resistor Measuring

[CTITech]

Hi I got a New task today, its seems rather simple in itself by Im lost. Im using the NI cdaq-9172 with a NI 9215 module. I need to measure some resistors for a job. The 9215 only measures up to 10k and the range im looking for is 1k- 300k. I was thinking about putting some voltage across each resistor and measure the current and just have the program do the math. The only problem is i dont know where to start, alittle lost :headbang:

[Mark Smith]

I'm no expert here, but it seems like you could put a precision resistor in parallel w/ the resistor under test and use

Req = Rt * Rp/ Rt + Rp where Req is the resistance measured, Rt is the DUT, and Rp is the precision resistor. Then solve for Rt. Size the precision resistor so that the Req is always under 10K even when testing the 300K resistors - this should be a little more than 10K for the precision resistor.

Mark

[CTITech]

Hey thanks im goin to try that today and ill let you know how it comes

[TobyD]

QUOTE(mesmith @ Feb 21 2008, 06:51 AM)

I'm no expert here, but it seems like you could put a precision resistor in parallel w/ the resistor under test and use

Req = Rt * Rp/ Rt + Rp where Req is the resistance measured, Rt is the DUT, and Rp is the precision resistor. Then solve for Rt. Size the precision resistor so that the Req is always under 10K even when testing the 300K resistors - this should be a little more than 10K for the precision resistor.

Mark

That's the same idea I had. You'll lose some resolution in your measurements, but it should still be close enough to get the job done pretty well.

[CTITech]

The application is medical and mil, so I need presion. So im open for options. :headbang:

[eaolson]

QUOTE(CTITech @ Feb 21 2008, 01:08 PM)

The application is medical and mil, so I need presion. So im open for options. :headbang:

The 9215 is a voltage-measurement board, correct? Could you put a precision, known resistor in series with your unknown, apply some voltage across both, measure the voltage drop across the known resistor (giving you current) and then measure the voltage drop across the unknown (R=V/I)?

[Mark Smith]

QUOTE(CTITech @ Feb 21 2008, 12:08 PM)

The application is medical and mil, so I need presion. So im open for options. :headbang:

If you need traceability and precision for a mil spec/medical measurement, then you really ought to consider using a calibrated DMM. The time you'll save in configuring the measurement and the added confidence you'll have (and paperwork trail to justify that confidence) will be worth the money.

Mark