Theoretical questions about potentiometers

Discussion in 'Homework Help' started by fektom, Dec 5, 2012.

  1. fektom

    Thread Starter New Member

    Aug 20, 2012
    16
    0
    Hello everybody!

    I need help with potentiometers, because this page discusses this topic on several pages, but I've got further and further questions.

    First of all lot people says, that you can not use a potentiometer in an AC environment, because normally the potentiometers are designed with low power rating. Even if I have a 100Kohm linear potentiometer, if I use this with 220V AC, still means 0.484W which is too much if I have a potentiometer with max 0.2W power rating.
    I say ok, but theoretically, if I use only 9v AC, is there any obstacle to use my potentiometer in AC environment?

    Secondly, I have seen a normal room lamp without a normal 2 position switch, but with a switch like a linear potentiometer which is very similar to a drawings can be found on: http://www.allaboutcircuits.com/vol_1/chpt_6/1.html (second coloured figure). Is there any chance that the AC lamp I've seen used this potentiometer or rheostat technic written and taught by this page? Or do these lamps using something different technology to control the strength of the light? If yes, what is the name of the technology?

    And last but not least I have a theoretical question. If I have a voltage divider circuit build with a 100Kohm potentiometer, I can divide the voltage with the rotary switch. Suppose, I set the rotary switch to have about 6.75V from 9V battery. It means 75% voltage drop for the first two terminals. But this means also, that I use 75Kohm resistance from the 100Kohm.
    Now, I want to put a light bulb into this 75% branch. What if the light bulb's resistance is much smaller than the 75Kohm potentiometer resistance? Theoretically, it means that I "shorted" the branch, and the potentiometer created 6.75V branch has no more 6.75V. Is that correct? If yes gives the light bulb any light?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    1) A potentiometer can be use in DC and AC. The limitation has nothing to do with AC or DC.
    Any limitation has to do with the maximum power the potentiometer can dissipate, in other words, the maximum current that the pot can handle. 220VAC or DC is a lot of voltage for any pot, resistor or electronic component. Calculate the current through the device and hence the power.

    Power P = I x V.

    2) The potentiometer you see on a light-dimmer switch does not conduct the total current current flowing through the lamp. It controls the gate current through a triac which is a lot lower than the lamp current. The technology is called THYRISTORS.

    3) If you are using a potentiometer to control an output voltage, a rule-of-thumb is to make the resistance of the pot less than one-tenth of the load resistance.

    So if your lamp is about 100Ω your pot would have to be about 10Ω which is going to draw a lot of current, get very hot and waste a lot of energy.

    If you want to put a variable resistor in series with a 100Ω lamp, then the resistor should be about 1000Ω max.
     
    fektom likes this.
  3. fektom

    Thread Starter New Member

    Aug 20, 2012
    16
    0
    Very useful answer, Thank you!
     
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