How to slow down a fan.

I bought a small cooling fan at Radio Shack to cool down some of my stereo components, but when I hooked it up it was louder than I had expected. I was about to return it and look for a quieter unit, but before doing that I thought I’d check with my friends here at the Dope.

Is there a simple way to slow it down? It’s a 115VAC, 130 mA fan, and I think it would probably provide adequate cooling at half its rated speed (3,100 rpm), and obviously be less noisy.

Can I wire in a resistor or something? If so, what specs, and how do I wire it? Or should I just spring for a quieter unit like this one? The one I have has a rated noise level of 36dB.)


Carefully wire a rectifier diode in series with one side of the supply voltage. It blocks every other half cycle instead of just dumping power into a resistor as heat. The diode should be rated at least 200 PIV and ½ amp. The 1N4003 diode is very common, you should be able to get them at Radio Snack.

I wonder if slowing it down would actually have an affect on the level of noise, or would it just change the pitch?

Let us know how this works.

Did you think of the obvious? Wire a ceiling fan control switch, or possibly a simple dimmer switch to the cord?

Ok I forgot what forum I was in

By obvious I meant, if you are about to try soldering anything to the motor I figured you had at least a working knowledge of electricty.

Make any and all connections in an approved box.

Firmly install the switch in the afforementioned box. Make sure the cord is secured on both ends of the box to prevent shorts.

What if I’m wearing shorts?

Heh he, then don’t stand close enough to cause a short. There may be a stray wire .

Gotta say this,

Hey DWC, it’s been a while. I dont get around as much as I used to and if i do you usually have the building questions covered enough that I can’t see saying anything but “What DWC said”.

Anyway nice seing you on in my time zone.

The typical dimmer switches are really cheap, and produce a lot of stray electrical noise. Putting one of them in close proximity to your stereo equipment is likely to produce lots of humming, buzzing and other sounds you don’t want from your stereo.

You mean the theory of this, right?

If you have a source of 12 volts DC handy, go to the computer shop and get a computer case fan. Some of these little beasties are nearly silent as-is, and they’re easily slowed down to become completely silent.

Actually, a popular trick to slow down and quite computer fans is to run them at 7 volts (the difference between the 5 volts and 12 volts that a computer power supply puts out) so you really can run these things at anything from 7 to 12 volts, depending on what’s available to you. If you’ve got a PC as part of the entertainment system, some computer power supplies even have extra power jacks on the back for fans to cool the cabinet the PC’s living in.

Most AC cooling fans are induction motors. Thier speed is controled mostly by the line frequency, not the voltage. Since dimmers and such do not change the frequency, they have little effect on speed.

Note that I didn’t say “no effect”. Reducing the voltage does lower the available torque which increases the slip of an induction motor, so you can slow them down a little bit, and IME even a small reduction in speed reduces noise noticeably.

I’ve done this using a series capacitor. Note that this must be non-polarized, rated to handle the voltage, and be low disipation. Mylar or polypropelene types would work. You might find something at radioshack, but you shouldn’t expect anyone there to advise you on the suitability.

ETA: feeding a AC motor through a rectifier = bad idea.

This is the fan in question: As you can see, it is a computer case fan, but I got a 120V unit because 12V is not as readily available in the stereo rack as 120V. It comes with bare wires, and my plan was to hook up a plug and stick in the back of the rack. Switches, dimmers, and boxes weren’t part of the plan, but wiring a diode into the line looked like a quick and easy solution.

So are the rest of you saying that Patty O’Furniture’s idea won’t work or isn’t a good idea? I was just about to go out and get one of those diodes.

I tried adding a diode to a cruddy old squirrel cage fan. that was blowing too fast. It didn’t slow it significanly. I ended up using 4 10 watt X 5 ohm resistors to get the speed where I wanted it. They look like this , and are available at radio shack. Be sure to get ones that’ll handle the wattage. They get pretty warm.

Well, if the fan keeps coming at you, you could always hire security and get a restraining order.

Apparently you bought it without reading all of the reviews that say “it’s loud!” :smack:

If you want to keep it, there are speed controls available for ceiling fans. These are in-wall mounted devices that would replace a standard switch, but they can probably be used for your small fan. Only drawback is they cost about $30, and are not directly amenable to your purpose as they’re meant to be installed in an electrical wiring box.

Alternately, a 12 volt power supply and a quite PC fan (such as one of the Zalman models) will also cost about $30 at Radio Shack, but it’ll be simpler to set up.

Yet another option is to get another of those 120-volt fans and wire the two in series so they each get 60 volts. No promise that two at half power will be significantly quieter than one at full power, though, but I’d expect them to be quieter this way.

No, I just happened to be at the store and picked it up. I didn’t even notice that the box had a dB spec until I had goten it home and found out first hand how loud it was.

Back to the store it goes. Thanks, all, for the advice.

Probably won’t work. Most such fans are controlled by the 60Hz line frequency, rather than voltage (as mentioned by KevBo just a few posts previously). Running them at half-voltage will just make them labor, heat up, and possibly seize up and burst into flames. True, they will be quieter at that point, but not in a good way.

I would just get a bigger amplifier. I used to put fans on most of my enclosures for cooling as well. Of course at the time I had a 1250 Watt main amp with a 500 Watt amp for the rear channels. I never heard any fan noise! :smiley: