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I have a 20" Box Fan, that has 3 speeds. It's a commerical model from Lasko, heavily built, puts out a lot of air... too much actually for my normal needs.

So I'd like to add speeds in addition way slower than what it has. As far as I can tell it's a straight power cord to the motor with a 3 way switch which I assume is some sort of resistor bank that limit the power to the motor.

In a perfect world I could have a similar switch with 6 or more settings, or alternatively another switch that works with the existing one, to multiply the 3 speed setting, which I think can be done by resistors in parallel or series.

I know a little about electronics, but not enough as to the best way to go from here. Part of me was thinking of a lamp dimmer but doubt those can handle the inductive load of a fan, or an AC fan speed control like this http://www.amazon.com/Speed-Control-Cord-AC-Cooling/dp/B00809BQZG

Here's a pic of the project if curious, it's DIY allergy filter using the box fan and a decent hepa furnace filter. https://instagram.com/p/z07BOOJ0pX/?modal=true

  • Are you interested in making your own corded-fan-control-switch? I dunno 'bout that one you linked, it gives you a Euro-style receptacle (for CPUs; what it's for) and warns that it might not work all fans. – Mazura Mar 5 '15 at 7:06
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    The common three speed box fans do not use a "resistor bank" to provide the various speeds. Instead the three speed selector switch engages various combinations windings inside the fan motor to get the fan speeds. – Michael Karas Mar 5 '15 at 11:35
  • Also note that the speed controller that you linked is not even recommended by the seller. They say that it will work with some fans up to 120mm (~4.7") in size. – Michael Karas Mar 5 '15 at 11:51
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    Depending on the electronics inside the fan, a "router speed control" might work... (no personal experience with this, so ymmv) – Aloysius Defenestrate Mar 5 '15 at 15:07
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I used the variac and got the noise level down from 74-84db to 45. Pleasant white noise, vs Is that Industrial Music you're playing?

I researched various approaches, that fall into the basic camps of a variable resistor (like the sun court-variable speed, speed bully) and variable transformer.

The resistor ($20-$30) absorbs part of the power, thus generates heat, and the reviews of which almost always were super low, with burnout of the device and or fan, heat noise etc.

After trying to find the power requirements of the fan (which was not well documented anywhere), i figured it was about 1.5 AMP Max. So I got a 3AMP Variac Variable Transformer ($65) which there are several that all look like rebrands of the same thing from China. It is big, beautiful in an industrial manner, smooth dial, and works perfectly silently (no buzz hum), so I can choose whatever noise/airflow I need. That said it's not calibrated correctly, so 100% on it's dial does not correspond to 100% when no variac is used, but it does go above 100% to make up for it.

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  • Far more lyrical than my crass cut and paste. A variac is indeed as close as you can come to perfection in this application. – Some Guy Mar 19 '15 at 23:49
  • @SomeGuy I do thank you for your informative answer, it was the super low feedback that got me to post, as the resistors appraoch are seductively low priced but usually suck in reviews, which dissuaded me from my initial enthusiasm for going that route. – Troy Gardner Mar 20 '15 at 5:24
  • I was raised in the 'entertainment industry' lighting which covers everything from Mrs. Smiths 3rd grade talent night to live television. I love all of it. The autotransformer was state of the art for that entire world in the old days, (well hollywood has their own strange ways from standards created by Edison.) In many ways magnetics is still the best answer to a problem. Your autotransformer will work in 50 years, try to find a switching supply that will do that. The pic is college stuff from the 80's love it, likely 300 amps worth. flickr.com/photos/some_guy/16685244310 – Some Guy Mar 20 '15 at 13:51
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The Easy answer, $20.00 just plug in, small http://www.amazon.com/Suncourt-Variable-Speed-Fan-Controller/dp/B006L87LK

A Good answer:$20.00, and you supply your own box and plugs http://www.amazon.com/KB-Electronics-8811012-Variable-K177-1005/dp/B000F9DAL2

The Perfect answer, $65.00 you supply nothing pretty package plugs, fuse, big knob. http://www.amazon.com/Parts-Express-Variac-Variable-Transformer/dp/B00BXJYO6I

What you want is a speed control for a shaded pole motor. Anything listed as a 'silent' ceiling fan control should be fine.

You can get these thing anywhere, it was just easy for me to put amazon links.


The typical fan speed control is different windings inside the motor - Michael is right on there.

Aloysius, is clever, I have seen a few of his answers,and the logic is good (and gives the ymmv, easier to trust someone that knows what they do not know, and will admit it.) The router speed control is a great idea, it would likely even work for a time. The problem is that the router motor has brushes, which can be controlled, using less expensive parts. It can also be done in a way that it would work for this problem. Who knows what is inside the magic box. Indeed, my 'easy' answer may not have a real long life, mystery is part of life.

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