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Dimmer Switch

  1. May 15, 2005 #1


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    I heard that a dimmer switch/knob/potentiometer used to control the lights in a house, for example, uses the same amount of power no matter if it is just barely on, or turned up all the way, so it really doesnt matter (energy usage wise), what the switch is turned to, it will use the same amount of energy?
    I would think this is false, since power is volts * amps, and the voltage is pretty much constant, say "out of the wall" voltage of 110 volts, then amps is the only thing that would effect the power consumption.
    Amps are calculated by volts over resistance, and we already assumed volts were constant, so ressitance is therefore the only thing effecting power consumption.
    As you turn the dial on the potentiometer, the resistance increses or decreases, thus making the power consumption go up or down depending on a lower of higher resistance.

    So is my conclusion correct, or the person who I heard this from, who claims to have gotten it from an elecrtician?
  2. jcsd
  3. May 15, 2005 #2


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    Total BS. A dimmer switch in a house is more than just a resistor inserted in series with the load. At one time there were dimmers like this however. I recall seeing a very old lamp that had a rotatable base on the socket which was a variable resistor. The thing ran VERY hot. What happens in a situation like that is all of the load current is also passing through the resistor and any time you pass current through a resistor energy is dissipated in the form of heat. Heat means waste unless you are trying to heat your house with it. The total current used with one of these in dim mode is still less than full current.

    The modern dimmer switches are actually just that, switches. They allow the current to fully flow for a set amount of time in each AC cycle. Set to really dim and they will allow the current to flow for only the tail end of half of the cycle before the voltage crosses zero. When the voltage passes through zero the device that does the switching opens up and is not turned on until the same point in the next half of the cycle. These dimmers are essentialy pulse width modulators. They are either on or off but the perception is a dimmer.
  4. May 15, 2005 #3


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    so it is like turning the light on an off very very quickly, not allowing it to get up to its full brightness at each cycle.
  5. May 16, 2005 #4
    Modern dimmer switches implement semiconductor-based hysteresis devices called thyristors (specifically TRIACs) which take advantage of break-over voltages much like neon-tubes and lightning bolts do.

    Read about them:
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