# Help needed with low value potentiometer

Discussion in 'General Electronics Chat' started by hspalm, May 10, 2011.

1. ### hspalm Thread Starter Active Member

Feb 17, 2010
201
8
Hello,
On an automation assignment we have to simulate a PT100 temperature sensor with a potentiometer. First problem is that these potmeters are overly expensive compared to standard values like 100 Ohm, 1K ohm etc. We need about 20 Ohm. If we wire a standard value, say 100 Ohm in paralell with a lower value fixed resistor we obtain the right values, but the increase per turn is now logarithmic.

Is there a clever way of solving this without buying the expensive potentiometers? We need linear increase in resistance because our front panel will not look good for presentation with a logarithmic scale.

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,442
1,628
What does this pot drive? Perhaps if we saw what the pot sees we could come up with something clever to use a high value resistor to simulate the same output when connected with the rest of the circuit.

Example: The resistor is driven by a 50 mA constant current source, so as the resistance changes from 0 to 20 ohms the input voltage changes from 0V to 1V. Then you could use a higher value pot as a voltage divider, and an op amp unity gain amp to give the same drive voltage. This keeps your front panel nice, the control linear, but does mean you need a power source into the "sensor simulator."

Jul 7, 2009
1,585
141
Why don't you define your problem exactly? The reason I say this is I don't understand what the problem is. An IEC60751 RTD changes its resistance from 100 ohms to 197 ohms in the range 100 to 250 deg C. So why not just use a 100 ohm fixed resistor in series with a 100 ohm pot? Or, tell us the resistances you need to simulate.

4. ### hspalm Thread Starter Active Member

Feb 17, 2010
201
8
The device reading the potentiometer is a Omron TS652, http://cospa.ru/userfiles/database/itemimages/CJ1W-TS562_InstructionSheet.pdf.

It's run off 24v, but I do not know, it's not stated, what current is passed through the potentiometer. Surely this is something I can measure. My bet is a constant current source because it is often necessary to wire long cables to the pt100 elements in industrial applications...

The TS652 is rated -200 to +650 degrees Celsius, but since our application is inside a hotel room, we are only interested in temperatures just below freezing point up to about 30 degrees. That's why we have now set the limit 98.04 to 111.67 Ohms, the equivalent of -5 to +30 degrees Celsius. Hence the variable resistance of ~10 Ohms.

5. ### Jaguarjoe Active Member

Apr 7, 2010
770
90
That Omron part can also work with PT1000 RTD's. That would bump your resistance up 10X.

Mouser has 20 and 50 ohm linear pots at \$4.00 each if you're stuck with PT100.

6. ### hspalm Thread Starter Active Member

Feb 17, 2010
201
8
Thank you for noticing that! I read it, but it never occurred to me that was the solution right in front of my nose. Thank you everyone!

7. ### Jaguarjoe Active Member

Apr 7, 2010
770
90
My wife needs to see this. She is convinced that all I ever notice is other women's breasts