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Resistance measurement

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  • MVP 2023

Continuity is easy - just connect +24V to one side of the wires one at a time and see if you get it on the other side.  If you have a bundle to check you can switch wire feeds in with relays and make sure you only get one signal on a DI module connected to all wires.

The RTD module only returns temperature from a resistance of about 25 ohms to 267 ohms.  You could reverse-calculate the resistance, but I don't think this is the range you're looking for.

Resistance is not so easy.  Many moons ago I rolled my own ohmmeter with a DC power supply, current limiting resistors, voltage inputs on an analog module, and current sensors into another module. 

I was trying to measure to .01 ohm resolution on wire to make fuses and it kind of turned into a trainwreck.  I learned about thermal drift of a multi-component system the hard way - I could not keep the thing calibrated.

I eventually bought a really nice four-wire ohmmeter with a serial port and multiplexed it between the wires under test - the manufacturer liked it so much they asked if I would write an app note about it-


So this is an area I actually know a little more than the average bear about.

This approach may be overkill for what you're trying to do.  Can you be very specific about what you want?

Joe T.

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