I'm looking to find a way to operate a small servo using a 4-20ma linear analog signal generated by a PLC in an industrial setting.

The purpose of this is to allow for automation for a task that currently is done by manually turning and adjusting a potentiometer with removable dial. Basically, I'm trying to ghetto together an oldschool Motor Operated Potentiometer (MOP) so it can be removed quickly and easily without affecting the operation of the original process.

I've spent hours looking for servo controllers/encoders that are capable of this, but I haven't been able to find any. Any way I could get pointed in the right direction would be fantastic. Surely such a thing must exist! Thanks so much!


1 Answer 1


The most simple way, is to let the current run over a shunt and just use the voltage as input signal. Voltage controlled motorcontrollers exist like "sand at the sea"
Basically you loose the safety feature of the current signal to detect cable break. So far industrial application you need a more sophisticated approach. If you do not this feature anyway then you should be fine with the shunt. I use for a similar application I use e48-127 Ohm resistors. I get output signal between 0.5 and 2.5 volts.

  • $\begingroup$ This is a great idea, I appreciate the response! I don't suppose you'd be able to recommend a cheap(ish) controller that uses voltage as an input? My experience with servos is pretty much non-existant. Thanks so much! $\endgroup$ May 7, 2015 at 11:43
  • $\begingroup$ Sure I know some...but this depends a lot on your setup of the motor. Do you want to turn the motor in only one direction. which voltage does this motor use. How much power? Lot of different parameters I would need to know. $\endgroup$
    – TobiasK
    May 7, 2015 at 12:00
  • $\begingroup$ The motor I'm using is a 4.8-6v motor that I'd like to be driven in two directions. I've done some digging around and I found something that can output 0-10VDC rather than a 4-20ma signal, so I could skip the shunt middle man and just use that, perhaps as an input. Thanks so much again! $\endgroup$ May 9, 2015 at 20:07

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