Voltage or Current Controlled Variable Resistance

[Adam Lethbridge]

I need a circuit that takes either a 4-20mA or 0-10V signal and converts it to about 100-200ohm resistance. I need to simulate an RTD with an analog output from a PLC. I was reading on here about something similar using a Voltage Controlled Resistor Fet but need some more information. I know basic circuit building only and have only taken a 1 year digital electronics course. Any information would be greatly appreciated. Sorry if I broke any rules since this is my first time being on this site. Thanks!

 

[MikeMl]

Adam, in order to suggest something, I would have to see how the varying resistance ties to other circuitry. For example, is one end of the varying resistance grounded? Does current only flow one way? Is it driven by an AC signal? etc.

 

[Adam Lethbridge]

The variable resistance circuit will tie right back into a RTD card in a PLC. Here are some specs I pulled from the RTD card.
https://www.electro-tech-online.com/custompdfs/2010/01/MAN0073-11.pdf

 

[dougy83]

If I were to make a big assumption: the Common point is GND and the Excitation pin provides a constant current, then you can likely feed the 0-10 V into the sense pin through a resistor divider (use a pot to adjust the sense voltage). You can also connect the Excitation and Common pins to GND (pin 1). It's worth a try at least.

PT-100 current of 330uA at 200ohms --> 66mV at the sense pin. A 1k resistor in series with the 0-10V line to the sense pin and a 100ohm pot (short its wiper to one of its other terminals) between the sense and common pins should give a max voltage of 90mV.

 

[Adam Lethbridge]

Ok, that was a little over my head. Your saying all I need to do is put a POT in parallel with the input and the card will read the 0-10V as an RTD? Thats seems a little too easy.

 

[dougy83]

Ok, that was a little over my head. Your saying all I need to do is put a POT in parallel with the input and the card will read the 0-10V as an RTD? Thats seems a little too easy.

I think you got my drift OK. The pot (100 ohm) goes in parallel with the input of the card, and there is another resistor (1k ohm) in-line (series connection) with the 0-10V input. If it works and is too easy, that's a good thing, right? If it doesn't work, then you haven't wasted much time or money.

 

[Adam Lethbridge]

Ok I can test this theory tomorrow. Before I do can this hurt the RTD card in any way? Last thing I want to do is blow a channel on a $900 PLC card.

 

[dougy83]

I wouldn't think that it would hurt the channel; if it was my card I'd do it. Just wire it correctly.

Whatever you do is always at your own risk (as with most things in life).