# Varistor/Rheostat

#### NatashaK

Joined Nov 19, 2015
2
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

Thanks

Natasha

#### WBahn

Joined Mar 31, 2012
29,147
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.

https://en.wikipedia.org/wiki/Varistor

#### R!f@@

Joined Apr 2, 2009
9,877
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 ?

#### NatashaK

Joined Nov 19, 2015
2
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?

#### Alec_t

Joined Sep 17, 2013
13,773
Do you really mean 'varistor', or are you referring to a 'potentiometer' (as shown in your circuit)?

#### R!f@@

Joined Apr 2, 2009
9,877