A-B 5069-IY4 Potentiometer Input

apearson · Jul 8, 2020

Hi folks,

I'm doing a valve positioning system (new plc replacing old discrete instrumentation), the old system has position feedback from valves in the form of a 3 wire potentiometer.

So, if I only use 2 of the wires so I now have a rheostat rather than a potential divider, could I input these into a 5069-IY4 set in 2 wire RTD mode?

I'm thinking set in Ohms rather than PT100 etc and scale 1-500ohm is 0-333 degrees, with the 135ohm pots I have, this should then equate to 0-90degrees valve position.

Any thoughts? I can't see a reason why it wouldn't work. I know there are some IR4 modules for the Micro's that can handle it so I'm assuming these can.

Thanks in Advance

Ash

geniusintraining · Jul 8, 2020

Hi Ash,

Does your rheostat have voltage? If yes why not use just as voltage

Page 10 https://literature.rockwellautomation.com/idc/groups/literature/documents/in/5069-in011_-en-p.pdf

apearson · Jul 8, 2020

geniusintraining said:
Hi Ash,

Does your rheostat have voltage? If yes why not use just as voltage

Page 10 https://literature.rockwellautomation.com/idc/groups/literature/documents/in/5069-in011_-en-p.pdf

@geniusintraining

It doesn't at the moment. It's just a passive variable resistor. I was thinking if I just connected the supply and the wiper, that would give me say 1-135Ohms and I could just handle the scaling.

I could do the maths on a voltage input but I think I'll need a strange supply to handle the 135Ohms as the input is only 0-10V

parky · Jul 8, 2020

You would probably be better to pad it with a parallel and series resistor as a PT100 input is 100 Ohm at 0 deg C and 137.8 at 100 deg. C. This way it would be easier to scale and zero ohms i.e. pot short could cause an error (depending on the card). so getting the pot to go between say 50 ohms and 150 ohms would stop the PT100 card from being out of range. to be honest anything between 50 & 150 ohms with a reasonable range to give you a count range you need would be best then scale it accordingly. I also suggest you run the third wire from the pot to the 3rd leg of the input card as this is effectively the noise & resistance cancellation so it acts like a PT100.
Edit: just realised that parallel resistor will make it non linear doh....

apearson · Jul 8, 2020

parky said:
You would probably be better to pad it with a parallel and series resistor as a PT100 input is 100 Ohm at 0 deg C and 137.8 at 100 deg. C. This way it would be easier to scale and zero ohms i.e. pot short could cause an error (depending on the card). so getting the pot to go between say 50 ohms and 150 ohms would stop the PT100 card from being out of range. to be honest anything between 50 & 150 ohms with a reasonable range to give you a count range you need would be best then scale it accordingly. I also suggest you run the third wire from the pot to the 3rd leg of the input card as this is effectively the noise & resistance cancellation so it acts like a PT100.
Edit: just realised that parallel resistor will make it non linear doh....

I was just thinking that about the parallel resistor, I think throwing a resistor in series to remove the pot short is a good shout as the card is 1-XOhms (I am assuming at the moment that the pot is 0-135 and not a none zero to 135). If I stick a 100R in I can scale between 100 and 235 as 0-100% open on the valve.

I'm not sure connecting the third wire of the pot will work, the three wires are Supply, Wiper and 135Ohm so that it can be used as a divider. My be worth running 2 legs to the common though as the third wire is general for lead resistance on a PTD. It measures up and down the two legs on the common side and then subtracts half that measured lead resistance from the PV to give an accurate temperature reading.

geniusintraining · Jul 8, 2020

Why not just put 10vdc to the pot and run your wiper back as a 0-10v input?

parky · Jul 8, 2020

I did not mean connecting the third wire to the pot 3 terminal, PT100 have a third wire connected to one of the two wires of the PT, this is checked at the PLC Card and reduces errors by compensating for lead resistance (don't think you will really need it though
PT100

apearson · Jul 8, 2020

@geniusintraining

Fair point, think that’s the route I’ll go.

A-B 5069-IY4 Potentiometer Input

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apearson

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