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This is an "everyday" layman question, not a technical question.

I have an AC ceiling fan with three power settings (like some light-bulbs have three brightness settings). I also have a wall switch which is circular rather than on/off.

So I can either "ask" for less power (by changing the fan's setting) or "send" less power (by rotating the circular switch).

Is there an electrical difference between the two?

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Yes, there is a difference.

The circular thingy on the wall, called a dimmer, chops up the 60 Hz power sine wave in such a way that the overal apparent voltage available to the motor is less.

The knob on the fan causes different coils inside the fan motor to be switched in and out to trade off more torque versus lower power consumption in the motor.

Use the latter (the switch on the fan) to control its speed if you can. The dimmer does some nasty things to the power line waveform. This will not only make the fan run less efficiently, but also cause some radio interference. The people that designed the fan added different taps to the coils inside the motor for a reason. This allows still running the motor reasonably efficiently at the different speeds. The downside is that you only get a small number of fixed speed settings, whereas the dimmer can control the fan speed smoothly. However, it's a fan. Three speeds should be good enough.

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Without knowing the type of fan you are talking about this question can be difficult. I'll assume you mean an AC ceiling fan.

The wall switch is utilizing solid state device called a triac. This device will allow current to pass through it in only one direction at a time (the direction is controlled by a gate voltage).

Here is a triac turning on and off

in your wall switch the gate of the triac is hooked up to a basic series RC circuit. The knob controls a variable resistance which changes the RC time constant for the voltage on the capacitor. Once the voltage is past the gate's minimum turn on voltage the triac will conduct current.

In short a triac is simply opening and closing the circuit to provide a lesser voltage to the wall socket.

Now for the ceiling fan:

The ceiling fan is an AC motor (ill assume you know how those work) whose positive armature is connected to one of n leads (n being the number of fan speeds). Each one of these N fan speeds is connected across a resistor, with the leads decreasing from full 120V to 0V (or whatever the voltage of the motor is).

So in this instance the fan is supplied with a full 120V and is then the extra energy is dissipated in the form of heat through the resistors.

Long story short the dimmer switch limits the voltage by turning on at specific times in the AC cycle while the fan does it by dissipating the voltage through a resistor.

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  • \$\begingroup\$ Cool, I don't really understand this but it's inspiring how complicated a "simple" fan is… thanks … \$\endgroup\$ – isomorphismes Oct 21 '14 at 18:42
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The circular switch (aka a dimmer) controls the RMS voltage sent to the fan. As power is proportional to voltage-squared, the dimmer can adjust the power consumed by the fan.

The fan's 3 position switch probably adjusts the fan speed by switching in or out some components. A fan going faster will take more current than the same fan running slower. More current (at the same voltage applied) usually means more power consumed by the fan.

The "asking" or "sending" for less power idea sounds cool but in reality it's simpler than that once you realize that the circular dimmer has no idea what is connected to its outputs and the fan has no idea what is feeding voltage to it.

The likely electrical difference is that the dimmer sets the voltage supplied to the fan and the fan switch lowers the fans impedance thus necessitating a higher current being drawn from the voltage supplied.

Power = voltage x amperage

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  • \$\begingroup\$ Right, I wasn't trying to personify the electronics in any way, I just don't know the actual words (or "dimmer", either). Thanks! \$\endgroup\$ – isomorphismes Oct 21 '14 at 18:28

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