I need precision variable resistors for a programmable analog computer that I'm trying to construct.

An analog computer requires variable resistors that can be set with precision.

I don't want to use digital components because the have discrete steps and are not pure resistors (have capacitance)

My first thought was to connect a dc motor via reduction gears to a precision rheostat -- wire that to a digital ohm meter and build a control circuit that dials the rheostat to the desired value.

Are there off the shelf components that I can buy or will I have to construct my own.


  1. On using a MOSFET as a variable resistor -- based on this post, it looks like it will be difficult to deal with the nonlinear behavior -- plus the ohomic range seems narrow.

  2. I looked at digitally controlled rheostats but they have several limitations including the fact that they are active solid-state components and that they have discreet steps (typically 256 and 1024 on the high end.

  • \$\begingroup\$ Is solid state really not an option? There are analog solid state solutions. \$\endgroup\$ Sep 21, 2016 at 2:07
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    \$\begingroup\$ Make up your mind -- in one paragraph, you say you don't want anything with discrete steps, but in another, you say you'll use a digital ohmmeter. How exactly do you intend to specify the desired value of the resistance? \$\endgroup\$
    – Dave Tweed
    Sep 21, 2016 at 2:07
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    \$\begingroup\$ A MOSFET is basically a voltage controlled resistor. All you need to do know is combat the odd curve. \$\endgroup\$
    – Bradman175
    Sep 21, 2016 at 2:10
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    \$\begingroup\$ Sounds to me that this is just a dream project from someone who hasn't tried to do the math and found out what kind of precision and accuracy you need. \$\endgroup\$
    – pipe
    Sep 21, 2016 at 2:23
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    \$\begingroup\$ A JFET and an op amp. \$\endgroup\$ Sep 21, 2016 at 2:50

1 Answer 1


There is indeed - this motorized potentiometer. Note that it has two separate resistive tracks. Or had you not realized that you have to disconnect a resistor from its' circuit to measure it?


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