Transistor use - Basic fan speed controller

Question

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.

To this: ^{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 + 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.

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

Answer 1

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.

Answer 2

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:

^{simulate this circuit – Schematic created using CircuitLab}