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Stir plate fan makes a constant whine

So, I recently made my own stir plate, using parts I scrounged up around the house and some stuff I bought on Amazon:

  1. 120mm case fan, 12V, 200mA (free)
  2. Hard drive magnet (free)
  3. A 7.5V, 0.5A wall-wart type power plug (free)
  4. A PWM dimmer ($6)
  5. Magnetic stir bar ($2)

When I connected the power source directly to the fan, the fan would spin smoothly and silently, but was just too fast. I tried put a couple flashlight batteries in the circuit, it either didn’t do enough or did too much, because the fan either wouldn’t spin or spun too fast still.

So I broke down and spent money on this project, and picked up a PWM driver with a dimming knob (actually intended for LED lights, and such).

Now it functions great (and I have control over the speed), but the fan makes this awesome high-pitched whine that neither my spouse nor my dog are particularly pleased about.

At present, my “stir plate” is literally just a fan with a magnet underneath an actual kitchen plate, upon which I set the flask, so I’m planning to built some sort of little box for it to contain the sound. My suspicion is that the “pulse” part of the Pulse Width Modulation is happening at some sort of audible frequency, so I’ve essentially just built the world’s most annoying plasma speaker. I’ve left it going for about half an hour now, and apart from getting more obnoxious as time goes on, it doesn’t seem to be doing much else–no fire or sparks or melting parts, or anything.

But since my electricity knowledge consists of what I learned in second semester physics, I figured I’d better check and see if anyone has any thoughts, explanations, or advice.

Probably is the frequency of the PWM with the fan.
A simple rheostat would have worked.

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