Discussion in 'Homework Help' started by NatashaK, Nov 19, 2015.

  1. NatashaK

    Thread Starter New Member

    Nov 19, 2015
    Hi All

    First post here, so please go easy on me! :)

    Im having a bit of trouble with a practice paper question I have been given.

    The question is:

    Explain its operation and suggest one advantage of this type of circuit over a rheostat operated circuit.


    Now I understand that a varistor changes its output as voltage increases/decreases and is dependant on voltage.

    Also that rheostats control the resistance without interrupting the current flow..

    But I fail to see why that would be better in this light dimmer module?

    Any help would be greatly appreciated


  2. WBahn


    Mar 31, 2012
    Look over the description of how a varistor behaves and what kinds of things it is used for. Then ask whether that is a good match for a circuit intended to provide for an adjustable light intensity.
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Varistors are used for Over voltage protection and surge suppression.
    A rheostat is just one big variable power resistor.

    The circuit Natasha showed can use a rheostat instead of a variable low power resistor.
    The Rheostat is big and consumes power and should be rated according to load.
    The circuit uses little variable resistors, The transistor does all the extra power dissipation.
    Ask ur self this. What is expensive and bigger ?
  4. NatashaK

    Thread Starter New Member

    Nov 19, 2015
    Thanks very much R!f@@, Thats a hard name to type! haha

    Something big, bulky and a power consumption is not good for a lighting circuit, I now understand :) - Thanks very much

    Just to clarify here, Power comes in from the top, the switch opens and closes the circuit, then the base needs to have a voltage higher than the diode (typically 0.7v to open the diode), which is controlled by the variable resistor on the dimmer, then current will flow to illuminate the lamp?
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Do you really mean 'varistor', or are you referring to a 'potentiometer' (as shown in your circuit)?
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Your Welcome
    Yup. Something like that. The Pot controls the base voltage, there by controlling the CE current flow.
    But typically the load should be in the collector not in the emitter.