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I once tried a 10 watt resistor in series with the exhaust fan – I forget the value in Ohms that I used – which seemed to do the trick: The only problem being that a 10W resistor is fairly large, (I think the one I used was a wirewound ceramic out of the EHT circuit of an old TV.) not really cost-friendly if bought new, and can also generate more heat too depending upon its resistance.

I’ve also tried two fans in series but that was virtually a non-starter.

An idea I’ve just thought up this second, is to have a separate transformer with a 15 Volt or higher: 15 to 20 Volts should do the trick, secondary winding – It only need provide 500mA of current Min. – and use the AC half-wave potential to drive 2 fans at a lesser speed thus:

Connect a rectifier diode (I suggest a 1N4002) in parallel with small 10 nF capacitor across each fan opposing the polarity of the fan’s motor. e.g. connect the anode of the diode to the fan’s negative connection and vice-versa in each case.

Now connect both fans, including the two components that you’ve just connected in parallel with the fans as described above, in series so that the two negative leads from the fans are connected together/the two diodes’ anodes are effectively connected to one another via the fans’ series connection.

Connect both positive leads from the series-connected fans with the extra components added across the secondary winding of the transformer; it doesn’t matter which way round.

You should see that on power-up that both fans spin slowly and quietly. 15 Volts may not be quite enough voltage depending upon the type of fan used; so I would use an 18V secondary if possible.

How it works:

What you have flowing directly out of the transformer’s secondary winding is AC or Alternating Current. Alternating current changes polarity a given number of times per second depending upon its frequency – In this case 50 or 60 Hz.

When the polarity is one way round current flows through one of the diodes thus bypassing the fan that the diode is connected across. However since current can’t flow through the diode in series with it, due to it being against its own polarity, it flows through the other fan and causes it to start to turn. When the polarity is the other way round on the following AC half-cycle the opposite happens: The diode that was conducting no longer conducts, causing the electricity to flow through the fan; however the diode across the other fan that was previously not conducting and bypassing that fan is now able to do so, thus bypassing the fan that was originally active. The capacitors are just for interference-limiting, decoupling, and smoothing in a small way. Their reactance in this circuit is so miniscule as to be discounted and no effective alternative path is provided.

So basically the circuit that has just been constructed is sharing the entire available power between the two fans; thus causing each to run at half-power, effectively sharing the 18V between them at roughly 9V each.

As I say I’ve only just thought this up just before I wrote all this, so there are no guarantees: If you’d like to build the circuit and power up then please do: I’d be interested to hear your experiences.

If it works well and you like it then please feel free to describe my idea as FANtastic. (No fanny comments please.)

The italics above are to indicate a piece of text that has been altered: There was an editing error which left the text not making sense. This has now been corrected. Apologies.