Resistance check for SLC 500

I need to check the resistance of a part and feed that information into the PLC. My resistance varies from 20k to 40k. A 1746-NR4 RTD resistance card will only read to 3k. I think I can buy a meter to read it that has a 4-20ma output and use an analog card. Does any one have any other ideas or know of a meter that might work well.

Thanks
John

Would something like this work? Then you could use a 4-20mA analog input card.

http://www.api-usa.com/pdf/api/api4003gi.pdf

How accurate does it need to be? In simple terms do you need to know it is 20,102 ohms or can you get away with saying 20k, 22k, 24k, 26k, or 20, 24, 28, etc.

I have a simple idea but hesitate to state it yet, much depends on the resolution involved.

Thanks I'll take a look

I think within 1k would be good

I needed a resistance transducer to measure from 0 to 1 meg and I couldn't find anything that covered that range. I finally used a five volt power supply and a voltage divider, with the load being measured forming the second resistor in the divider. For improved accuracy I used two analog inputs, one to measure the actual output of the power supply and one to measure the voltage drop across the tested load. The voltage drop across the first resistor can be determined as V_ps - V_test. With that you can determine what the resistance is. Select the value of the first resistor to best suit the range you are measuring.

Look under potential divider on this web page for some information that might help you with this method - it shows how to solve a voltage divider, its not hard, but just in case you need it its there.

sounds like what i did, and it was for SLC500:

http://forums.mrplc.com/index.php?download=666

Panic,
Thats very similar to what I did. I even wrote the code in almost the same way using the same equation, Rx = R3*V2/(V1-V2)

The main differences in my applicaiton is that I used a seperate 5V power supply and also I used only one analog input. During the time between parts the circuit is opened by a relay as there are times in the process when a potential should not be present at the test studs and one stud is not isolated from the frame because of mechanical loads on it. So between parts I read the power supply output at the 1K ohm current limiting resistor with a single analog input. When a part is inserted I read the voltage again at the same point.



It worked well enough for this application and it was very simple to do.

yup, that's what i had initially but eventually i tried to get rid of
the extra power supply since I had space restrictions, 24V was already there
as well as spare analog input (ok, also two more resistors...).