# use of variable resistor

• December 2nd, 2009, 09:56 AM
Heinsbergrelatz
use of variable resistor
for differing the brightness of the lamp, can use we a variable resistor in a circuit?
also besides from a variable resistor(if it can be used), what other electronic component can be used? how about a potential divider?
• December 2nd, 2009, 10:21 AM
DrRocket
Re: use of variable resistor
Quote:

Originally Posted by Heinsbergrelatz
for differing the brightness of the lamp, can use we a variable resistor in a circuit?
also besides from a variable resistor(if it can be used), what other electronic component can be used? how about a potential divider?

You can use a variable resistor, aka rheostat.

You can also use a variable transformer to control the voltage.

A voltage divider is just a series resistor. The variable resistor is a variable voltage divider.

You can also use a lower wattage light bulb.
• December 2nd, 2009, 10:38 AM
Heinsbergrelatz
oh i see, thank you
• December 2nd, 2009, 12:15 PM
Bender
A variable resistance is quite inefficient though, and is rarely used. You waste the same order of magnitude of energy in the resistor as you use in the lamp.

More usual is a chopper, which very quickly turns off the voltage and on again, lowering the mean effective voltage.
• December 2nd, 2009, 12:25 PM
Heinsbergrelatz
wait can you explain the ineffecient part again, for the variable resistor please? :D
• December 2nd, 2009, 02:16 PM
Harold14370
Quote:

Originally Posted by Heinsbergrelatz
wait can you explain the ineffecient part again, for the variable resistor please? :D

The resistor dissipates power without providing any light. It just heats up.
• December 2nd, 2009, 02:24 PM
Janus
Quote:

Originally Posted by Heinsbergrelatz
wait can you explain the ineffecient part again, for the variable resistor please? :D

The resistor itself will use up energy.

For example: If your variable resistor is set so that a 100 watt bulb shines at 1/4 its brightness and uses only 25 watts, the resistor itself will use 25 watts. Thus you are reducing the light by 1/4 but only reducing your energy usage by 1/2.
• December 3rd, 2009, 06:55 AM
Heinsbergrelatz
so is it still preferable to use the variable resistor despite its disadvantageous factors?
• December 3rd, 2009, 07:27 AM
Bender
No

The only advantage a variable resistor has compared to a chopper is that a chopper is bad for the 'quality' of the AC voltage, but that's rarely an issue.
You might also find the slight buzz a chopper makes annoying. You can hear this very clearly if you use a dimmer at home.
• December 5th, 2009, 11:14 AM
Heinsbergrelatz
oh ok, thank you for the answer