I just got a rheostat from radio shack to connect to my homemade stir plate. The rheostat looks like:

enter image description here

(from here)

My question is, how do I set it up? I want to use it to vary the fan speed.

I am using the wall plug from and old DSL modem as a power source (12V output) and an old CPU fan (12V input).

Which is the positive terminal, negative terminal, and ground?

  • \$\begingroup\$ What is the power of your stir plate? Are you going to connect it in series with the rheostat? \$\endgroup\$
    – Eugene Sh.
    Commented Feb 18, 2015 at 21:37
  • \$\begingroup\$ Anyway, in this type the two external pins are the + and - terminals (symmetric), and the middle one is the variable. \$\endgroup\$
    – Eugene Sh.
    Commented Feb 18, 2015 at 21:45

2 Answers 2


You put the resistor in series with the Fan. Keep in mind to not exceed the maximum current (around 300mA) of the rheostat.

There are no positive or negative terminals on a resistor. Internally, it's just a resistive plate with a kind of "wiper" connected to the middle terminal. So you connect one of the end terminals and the middle one. For example +12V to left terminal and the fan to the middle terminal. Than you can change the fan speed. If you think you have to turn in the wrong direction to increase/decrease the speed, just connect the +12V to the other terminal.


Connect the rheostat stat in series with one motor lead. Use one of the end terminals and the middle terminal; leave the other end terminal disconnected.

Note that this rheostat is good for a maximum current of 346 mA.

  • \$\begingroup\$ So I'm confused about which terminal is which. \$\endgroup\$ Commented Feb 18, 2015 at 23:41
  • \$\begingroup\$ What I mean is I feel like I got two answers. Can I just clip the positive wire coming from the power adapter? \$\endgroup\$ Commented Feb 18, 2015 at 23:51
  • \$\begingroup\$ Do you know what you are doing? I have a feeling that you are going just fry the rheostat in the best case. What currents you are going to have? \$\endgroup\$
    – Eugene Sh.
    Commented Feb 19, 2015 at 0:32
  • \$\begingroup\$ You're right, I don't have any idea what I'm doing. Clipped the connection on an old power adapter and connected it to my motor. I actualmy have the rheostat in place and working. Plugged in to the wall. \$\endgroup\$ Commented Feb 19, 2015 at 1:04

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