I need to find something like this but that is automatable, over I2C or GPIB or something.

Is there a name for this? I am having problems finding something. Worst case I can buy this and hook up a motor to it, but I imagine a cheap packaged solution already exists, I just don't know the name for it.

Looking for these specs: 7Vrms (so ~20W at the low end of 3ohm, <1W above 50ohm) 3ohm-100ohm in ~20 log spaced steps Automatable

A decade box could also work if it were automatable. They seem to be very expensive ($6k) for both automatable and specced for power.

The specific application is for loading a sigma delta (Class D) op amp (an audio amplifier). The output is differential and switching, a 1kHz AC sine wave is the resulting waveform after low pass filtering. A constant DC current sink will not fit the bill unfortunately. Open to other solutions though!

Rheostat 25W

  • \$\begingroup\$ Do you need the analog control knob aspect? If not, you're looking for a simple digital potentiometer. \$\endgroup\$ – alzee Jul 8 '16 at 18:40
  • \$\begingroup\$ This question will be tough to answer. What are your real requirements? 3-100Ohm @ what wattage? When you request a value, what tolerance do you have for accuracy? \$\endgroup\$ – pgvoorhees Jul 8 '16 at 18:46
  • \$\begingroup\$ @pgvoorhees 3-100ohm at 7Vrms, so at the low end, that will be about 20W for the 3ohm case, 50+ ohms will only require <1W \$\endgroup\$ – SwimBikeRun Jul 8 '16 at 18:54
  • \$\begingroup\$ @SwimBikeRun Honestly, unless you want to build some kind of wiper driver, I think you might be in for something like a class-AB amplifier. \$\endgroup\$ – pgvoorhees Jul 8 '16 at 19:00
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    \$\begingroup\$ You can build a resistor network and use a CMOS multiplexer to select the resistor you want. \$\endgroup\$ – Captainj2001 Jul 8 '16 at 19:06

The part you are describing is simply called a "motorized rheostat". It's not a common component, but does exist as a special-purpose item for certain industrial applications.

Control of motorized rheostats tends be pretty crude. Don't expect digital inputs; what you get is likely to be more along the lines of a pair of limit switches.


Rheostat means current controller. If you want to control a HIGH current, for a DC motor, the usual approach is a PWM (pulse-width modulation) drive style of motor controller. That relies on the motor to be inductive, and/or have a flywheel characteristic. Rheostats for lighting AKA dimmer circuits (often AC phase control) are very different. For power-handling efficiency, big power resistor controls are rare nowadays, but they traditionally did both those tasks.

A DC motor speed control using I2C, like this http://www.ti.com/product/DRV2604/datasheet comes close (it only likes 5.5V input, though).


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