So - i often work with DC current for robotics and the like, but now i have the problem that i'm having a 230V AC fan which shall be operated at two different speeds for an exhibition installation. I don't have a dimmer, only two 230V relais that are controlled by my microcontroller and that are connected to regular 230V AC powersupply. At one setting, the fan should rotate at maximum speed, at the other setting it should turn only very slowly.

Now, I'm not experienced with AC current electrics. I think I have to put a capacitor in series with the fan, as a capacitor in an AC circuit is kind of the equivalent to a resistor in a DC circuit, right?

But what capacitor to chose and what to pay attention to so that the whole thing won't catch fire?

//edit: the fan uses a "Shaded-pole" type of motor


  • 1
    \$\begingroup\$ Getting a fan to spin very slowly (such that blades are directly visible) is going to be a little tricky - you'd need to get below 1 rotation/s, and most non-stepping motors don't have any torque at such speeds. \$\endgroup\$
    – qdot
    Dec 20, 2011 at 11:27
  • \$\begingroup\$ the Wikipedia article of "shaded-pole motor" mentions "they are compatible with TRIAC-based variable-speed controls". Is it difficult to build that? \$\endgroup\$
    – Mat
    Dec 20, 2011 at 12:55
  • \$\begingroup\$ Not too difficult, I'll try to write up an answer for this, after you answer one more question - are you much better at understanding and debugging analog or digital circuits (there are two ways to approach it, one is the 'canonical' analog control, and the other one is the real-time digital control) \$\endgroup\$
    – qdot
    Dec 20, 2011 at 13:54
  • \$\begingroup\$ I'd prefere to do it analogous \$\endgroup\$
    – Mat
    Dec 20, 2011 at 20:48

1 Answer 1


Yes, you want a series capacitor. For 230v mains, it should be 400V rated (because of the peak being 1.4-ish times the RMS). If it's a typical 80- or 120-mm fan as used in computers etc, you'll probably want a cap of around 220nF to 2uF, depending on the fan and how much you want it to slow down. For a bigger fan you'd need more microfarads.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.