I have a 3 dial motor control assembly that is using 3 potentiometers to control 3 independent 110v motors speed.

The label on the pots read TO50K 1339 (first O has a "S" like shape inside). I have not been able to find any spec sheet on this specific pot but in readings with a multimeter I can the following data.

  1. ~50V depending on pot position
  2. 0.05A
  3. ~30K Ohms (at max, of course ranges based on pot position)
  4. 2.5 Watt (based on V*I)

So with all this in mind. Would it be possible to replace the analog pots with digital ICs? I have been trying to match the data with an IC but cannot find anything with the correct voltage or wattage that seems to be required. End of the day, I would want to control the 3 dials with an Arduino board so I can monitor and change motor speed over the internet.

So if anyone has a moment and can help map out a circuit for a single or for 3 pots I would really appreciate the help.

Thank you!


1 Answer 1


Probably not, practically.

It sounds like you have thyristor phase controllers and the pots would not only have to be inconveniently high voltage and power, but also you'd need to add isolation from the mains for safety.

You'd be better off (and much safer) finding a motor speed control that has an isolated low-level control input.

Edit: Speed controls of this type energize the field coil (if any) with a constant voltage and control timing of pulses to the thyristors supplying the armature based on the input speed command signal and motor current (to maintain a more constant speed). By controlling the timing relative to mains zero crossing, the average armature current can be controlled, and by measuring the armature current (a proxy for torque) the speed can be regulated fairly well without a tachometer.

For a controller with an external low-level input it would be designed from the ground up to use low level signals and something like an optocoupler or a small transformer would be used to transfer the control signal (perhaps 0-10V) to the mains circuitry using a linear optocoupler or a PWM scheme.

Trying to add a digital pot to one of these things (in my experience) would be like trying to add a digital pot to a wall light dimmer- really totally unsuitable and you're better off starting from scratch, with a much more complex design.

  • \$\begingroup\$ That was what I was beginning to think. Without you looking it up for me, can you point me in the right direction for a motor speed controller and/or a working circuit of how it would be in place of a pot? Thanks \$\endgroup\$
    – cgaubuchon
    Commented Nov 12, 2014 at 17:54
  • \$\begingroup\$ Please see edit above. I did look up a commercial isolator for a major brand of controller and it has about 40-45 parts in it-- not so simple. \$\endgroup\$ Commented Nov 12, 2014 at 18:02

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