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#51800 05/09/05 07:58 PM
Joined: Nov 2004
Posts: 53

How do incandescant light dimmers function? I have a customer with a custom range hood that the fabricator installed a dimmer to control the speed of the exaust fan. This is incorrect, right. It works now, but won't it cook the motor sooner than later. What about wall devices that control the speed of a ceiling fan, how do they operate by controlling the voltage?


#51801 05/09/05 10:19 PM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
A simple version of an Incandescent Lamp Dimmer would be a very large Rheostat (Variable Resistor).
Since this is a very wasteful method to dim lamps, it is not used very much anymore!

The basic "Solid State" Incandescent Lamp dimmer performs a function, known as "Chopped Wave".
This "Limits" only a portion of an AC wave to flow, which reduces the overall efficiency of the Lamp, and at the same time, reduces the amount of power the Lamp can draw - so it (the Lamp) gets dimmed.

Chopped Wave dimmers cause Lamp fillaments to "Sing" acrossed certain levels.

Another method, which does not incorporate any SCRs or TRIACs (Solid State controls), is an AutoTransformer with a "Wiper" contact.

I am sure there is (or has been) a saturable reactor type dimmer scheme.

I am thinking the "Actual Motor Speed Controllers" which look much like Lamp Dimmers, and are for use with Ceiling Fans (PSC Motors), are either "Pseudo Reactors" , Autotransformers, or Voltage Reducing Solid State types of control, as this is the most likely method used to control PSC and Shaded Pole Motors.

Is there a Variable Frequency type Speed Controller, in the form of the typical wall switch dimmer?


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#51802 05/10/05 05:46 AM
Joined: Aug 2001
Posts: 7,520
To expand upon this, the basic circuit used in the modern triac dimmer is fairly simple in its basic form.

The triac is a semiconductor device which will conduct only after a suitable trigger voltage has been applied to its gate terminal. It then continues to conduct until the through current drops to zero, which of course will occur at the end of each half cycle.

The resistance of the dimmer control is placed in series with a capacitor so that adjusting the control alters the rate at which the cap will charge on each half cycle. The voltage across the capacitor is then used to trigger the triac into conduction (via another semiconductor device known as a diac).

Thus as the resistance is increased, the capacitor takes longer to charge each time and the triac is triggered into conduction at a later part of the half-cycle.

You can find many links to dimmer notes and circuits here:

#51803 05/10/05 02:54 PM
Joined: Aug 2001
Posts: 335
Now that they all told you how the dimmer works, lets look at the fan issue. From experience and from talking to several hood manufacturers in the past, as well as two pond pump manufacturers ... no the dimmer won't hurt the small (emphasis on small) motors. I have been told by all of them that these shaded pole motors are the world's most inefficient motors anyway and the lower voltage will not cause them to get beyond the capacity of their windings. We all know this is in theoretical conflict with George Simon (Ohm) but in real life it works.
BTW most of these dimmers are generic, low dollar dimmers that reduce the voltage ... you can verify this with your DMM.

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