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Someone mentioned to me that they wanted to replace a fan switch with a potentiometer to adjust the fan speed. This seemed a reasonable request, and being eager to learn, I drew up a schematic using a simulator (colour has been used to avoid any confusion).

It went from this: switch Switch.

To this: potentiometer Potentiometer.

The circuit appeared to work, but got laughed at because, "Running 600mA through a potentiometer is not a good idea." and, "A waste of a good pot."

So I read about it, and (without really finding an explanation as to why) I discovered that a transistor is typically required. This seems weird to me because I was under the impression that transistors were much more sensitive than potentiometers, and easier to ruin.

Anyway, I tried implementing a transistor. No doubt there are better (more sophisticated, more components, etc.) circuits for achieving the same goal, but for now I just wanted to make sure I was using the transistor correctly:

potentiometer+transistor Potentiometer + NPN Transistor.

My understanding is that the potentiometer now controls the transistor, rather than the fans directly. And the transistor is basically like a variable gate between the fans and their power source.


The Result: Current across the potentiometer has dropped from 600mA to 6mA, but the maximum fan speed/RPM has dropped by ~50%.

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    \$\begingroup\$ Tip: add the Watt rating of the fans. And possibly what make and model you have of the NPN and potentiometer. \$\endgroup\$ – Dejvid_no1 Oct 22 '17 at 10:53
  • \$\begingroup\$ @Dejvid_no1 It doesn't really seem relevant to the actual question, so I left it out for the sake of simplicity. This is just one example. But for arguments sake, I was working with a hypothetical value for the fans: 12V 100mA. As for the transistor, I don't have/know the specific model; its just a generic, default transistor used by the software (EveryCircuit). \$\endgroup\$ – tjt263 Oct 22 '17 at 11:05
  • \$\begingroup\$ Also, at this stage, the potentiometer has no model either. It's just a hypothetical, generic 260Ω variable resistor. The question is really about the correct use of a transistor in this, or a similar situation. Specific values surely change from circuit to circuit, but I imagine the general concept remains the same; like for a lighting dimmer switch, for example. So, correct me if I'm wrong, but this is really just one of many examples I could have chosen, to give some context for the question. \$\endgroup\$ – tjt263 Oct 22 '17 at 11:22
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You need a PNP transistor for the high side switch. Your circuit only "works" because current flows across the BE diode - it is forward based after all.

Remember that for an NPN transistor, the base must have a potential of ~0.7V above the emitter, which is only possible in your circuit when CE resistance is high enough.

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  • \$\begingroup\$ Okay, that doesn't seem right. And I got confusing results with a PNP transistor. But you probably know better than I. If that's the case, the question remains, however: The correct configuration is... If you can show the correct configuration, and it works, I'll mark your answer correct. \$\endgroup\$ – tjt263 Oct 22 '17 at 11:34
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    \$\begingroup\$ The PNP will need its base resistor connected to 0V, and the emitter connected to 12V. The load is at the collector. \$\endgroup\$ – Turbo J Oct 22 '17 at 11:36
  • \$\begingroup\$ I didn't know that re: ~0.7V, by the way. Also, what is BE & CE? \$\endgroup\$ – tjt263 Oct 22 '17 at 11:37
  • \$\begingroup\$ A potentiometer is also a resistor. You should not use just a potentiometer because the potentiometer would generate more heat than it could dissipate, and the potentiometer would "fry". \$\endgroup\$ – mguima Oct 22 '17 at 12:52
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In both cases (a potentiometer and a transistor), a compensation voltage regulation method is used. Which leads to significant dissipation of heat if not on a potentiometer, then on a transistor.

It is better to use pulse width (PWM) regulation.

PS: To simulate the case with a transistor can be as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The circuit drawed above by AltAir is simple and efective. But use a power transistor because the transistor may get hot, that's why a PWM should be used instead. I would add just an advice to the OP: if you use this circuit, or even if you use a PWM, have in mind that most of motors can't be controlled in a linear way, especially if they weren't designed for this. Expect to be able to control the motor only in the 40%-100% range of the maximum speed, 20%-100 if you have luck. \$\endgroup\$ – mguima Oct 22 '17 at 13:04
  • \$\begingroup\$ Another nitpick: This circuit will not work if the fans have a speed sensor output connected. You will need a high side transistor in this case. \$\endgroup\$ – Turbo J Oct 22 '17 at 14:02

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