# Electrical question: fan + dimmer switch? **URL:** \ **Category:** Factual Questions\ **Created:** [February 19, 2010, 10:41pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705 "2010-02-19T22:41:00Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 19, 2010, 10:41pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/1 "2010-02-19T22:41:00Z")
I have a sandblaster which is illuminated by a light box. Said light box contains two “300-watt-equivalent” CFL’s in it. I have installed a fan to keep them cool. [This is the spec sheet for the fan.](https://www.alliedelec.com/Images/Products/Datasheets/BM/ORION_FANS/592-2001.PDF) The fan is noisier than I’d like, and I’m pretty sure I’ve got lots of margin to reduce the airflow in the box without cooking the CFL’s. Question #1: can I put a standard wall-mount dimmer switch (i.e. something from Home Depot) in-line with the fan and achieve a reduced fan speed that way without any problems (motor somehow overheating, etc.)? Question#2: If that will work, then what’s the name for the type of wall-mounted dimmer switch you can buy at Home Depot? I’d like to get the same device from Radio Shack or Allied Electronics, but in a panel-mount option; I don’t feel like cutting another rectangle out of my light box to install another electrical junction box. Thanks… ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [February 19, 2010, 11:03pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/2 "2010-02-19T23:03:29Z")
You can’t use a dimmer switch with a fan. The speed of the motor is determined by the line frequency, which is not changed by a dimmer. You’ll reduce the torque of the fan which may reduce its speed somewhat depending on conditions, but it’s not a good way to reduce the fan’s speed. They do make fan speed controllers for AC motors. I’m not sure if they sell them at home depot but they are commonly used in ceiling style house fans. ---
**Author:** ![Deeg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/deeg/32/2955_2.png) [@Deeg](https://boards.straightdope.com/u/Deeg)\ **Post date:** [February 19, 2010, 11:15pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/3 "2010-02-19T23:15:53Z")
> [@engineer\_comp\_geek](#): > > You can’t use a dimmer switch with a fan. The speed of the motor is determined by the line frequency, which is not changed by a dimmer. You’ll reduce the torque of the fan which may reduce its speed somewhat depending on conditions, but it’s not a good way to reduce the fan’s speed. Is that true for all fans? If I put a dimmer between the plug of a window fan and the outlet will I be able to slow it down? ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 19, 2010, 11:20pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/4 "2010-02-19T23:20:45Z")
I think I just found part of my answer [here.](http://en.wikipedia.org/wiki/Shaded-pole_motor) > [@Wikipedia](#): > > Moreover, [shaded pole motors] are compatible with TRIAC-based variable-speed controls, which often are used with fans. So are the typical dimmer switches at Home Depot “TRIAC-based?” ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [February 20, 2010, 12:26am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/5 "2010-02-20T00:26:16Z")
> [@Joe Frickin Friday](#): > > [This is the spec sheet for the fan.](https://www.alliedelec.com/Images/Products/Datasheets/BM/ORION_FANS/592-2001.PDF) There are lots of DC fans available in that size/cfm range. Little variable voltage DC power supplies, while not quite a dime a dozen, are pretty cheap. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [February 20, 2010, 3:07am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/6 "2010-02-20T03:07:44Z")
> [@Joe Frickin Friday](#): > > I think I just found part of my answer [here.](http://en.wikipedia.org/wiki/Shaded-pole_motor) > > So are the typical dimmer switches at Home Depot “TRIAC-based?” NO they are resistance dimmers. you can not use a dimmer on a fan with out problems you need a speed a speed control. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [February 20, 2010, 3:20am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/7 "2010-02-20T03:20:21Z")
> [@Snnipe\_70E](#): > > NO they are resistance dimmers. you can not use a dimmer on a fan with out problems you need a speed a speed control. IANAElectrician, but I put a dimmer switch on a bathroom fan and never had a problems with it. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 20, 2010, 4:10am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/8 "2010-02-20T04:10:04Z")
> [@Snnipe\_70E](#): > > NO they are resistance dimmers. Are you talking about a [potentiometer?](http://en.wikipedia.org/wiki/Potentiometer) > [@wikipedia](#): > > Potentiometers are rarely used to directly control significant power (more than a watt). Instead they are used to adjust the level of analog signals (e.g. volume controls on audio equipment), and as control inputs for electronic circuits. **For example, a light dimmer uses a potentiometer to control the switching of a TRIAC and so indirectly control the brightness of lamps.** If you’re talking about a pot doing the dimming directly, it seems like the pot would get smoking hot when trying to dim a 100-watt bulb. If Wikipedia is right, then I guess a pot is involved, but only to control the TRIAC, which is what does the actual power management (and is what the dimmed appliance - bulb, fan, personal vibrator - would see). It’s starting to sound like run of the mill dimmers are indeed TRIAC-based, but I’m still looking for more confirmation before I buy parts and/or risk my fan… ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [February 20, 2010, 5:05am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/9 "2010-02-20T05:05:14Z")
> [@Joe Frickin Friday](#): > > Are you talking about a [potentiometer?](http://en.wikipedia.org/wiki/Potentiometer) > > If you’re talking about a pot doing the dimming directly, it seems like the pot would get smoking hot when trying to dim a 100-watt bulb. If Wikipedia is right, then I guess a pot is involved, but only to control the TRIAC, which is what does the actual power management (and is what the dimmed appliance - bulb, fan, personal vibrator - would see). > > It’s starting to sound like run of the mill dimmers are indeed TRIAC-based, but I’m still looking for more confirmation before I buy parts and/or risk my fan… Most small dimmers that I have seen are variable resistance. They are usually limited to 300 watts. But it has been afew years since I have torn one apart and now that I think about it today they are probably triac’s. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [February 20, 2010, 5:34am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/10 "2010-02-20T05:34:53Z")
I just used a dimmer to run an electric drill motor on a El-Cheapo drill press attachment. I didn’t run it very long and was using it more for the off/on feature instead of locking the drill trigger each time. > [@](#): > > engineer\_comp\_geek > You can’t use a dimmer switch with a fan. The speed of the motor is determined by the line frequency, which is not changed by a dimmer. You’ll reduce the torque of the fan which may reduce its speed somewhat depending on conditions, but it’s not a good way to reduce the fan’s speed. I know understand why the power was lower than it normally is:dubious: ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [February 20, 2010, 5:52am UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/11 "2010-02-20T05:52:17Z")
> [@Gbro](#): > > I just used a dimmer to run an electric drill motor on a El-Cheapo drill press attachment. I didn’t run it very long and was using it more for the off/on feature instead of locking the drill trigger each time. > > I know understand why the power was lower than it normally is:dubious: Most drill motots are series motors so a dimmer could be used. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [February 20, 2010, 4:38pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/12 "2010-02-20T16:38:40Z")
A dimmer switch is a poor way to control a fan but a much worse way to control a universal motor. The speed and torque of the series motor are related such that slowing it down a bit means reducing it’s torque a lot. Fan controllers are small VFDs, variable frequency drives. Those are much better for slowing down induction motors. The motor will still run hotter than usual at lower frequencies because the low volt/hertz ratio means the current will stay higher than usual in the motor. Probably not a big deal on a little fan motor. If it were me though, I would go with a DC fan and variable power supply as mentioned by **Squink**. That would be an almost permanent solution. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 21, 2010, 5:11pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/13 "2010-02-21T17:11:33Z")
> [@HongKongFooey](#): > > Fan controllers are small VFDs, variable frequency drives. Those are much better for slowing down induction motors. The motor will still run hotter than usual at lower frequencies because the low volt/hertz ratio means the current will stay higher than usual in the motor. Probably not a big deal on a little fan motor. So do they actually put VFD’s in those 3-speed box fans that you can buy at Target/Kmart/Walmart? ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [February 21, 2010, 6:07pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/14 "2010-02-21T18:07:50Z")
> [@Joe Frickin Friday](#): > > So do they actually put VFD’s in those 3-speed box fans that you can buy at Target/Kmart/Walmart? No those are normally wound in a manner that the number of poles can be changed. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [February 21, 2010, 7:45pm UTC](https://boards.straightdope.com/t/electrical-question-fan-dimmer-switch/529705/15 "2010-02-21T19:45:26Z")
> [@Joe Frickin Friday](#): > > So do they actually put VFD’s in those 3-speed box fans that you can buy at Target/Kmart/Walmart? No but I thought I had one from Honeywell for ceiling fans. I must be mistaken since all I can find that looks similar is a digital thermostat I took apart. It works by pulse width modulation but costs more than a new fan would. I might play with it though and see if I can control a fan with it. 😃