Question about variable resistors

[maverick99]

I'm building a circuit in one of my classes and it asks for a variable resistor. On the schematic it's value is 47kohms. On the circuit diagram I'm building this from, it says to adjust VR1 (variable resistor) until a diode comes on. First off, what exactly is a variable resistor? Second, how do i adjust it??

 

[Delta]

It's like a typical resistor but you can adjust its resistance yourself. You need to find youself a variable resistor marked with 47kohms which means its resistance can be adjusted between 0and 47kohms.

See below for further details:

http://www.kpsec.freeuk.com/components/vres.htm

 

[maverick99]

well crap i didn't know they were the same things as pots. Thanks

 

[nykill]

u find various kinds of pot with different ressitance. pick any one, supply a constant current of some amperes and connect a volt meter parallel to the pot, with the current supplied and adjust the pot with the reading on the volt meter so u get the result of the resistance required with the formula V/ i = R

 

[Rogue Physicist]

well crap i didn't know they were the same things as pots. Thanks

Very often a 'variable resistor' in a circuit is a 'pot' (linear potentiometer), simply a carbon strip with a sweeping contact. In low voltage, low current applications this is usually the case, such as in audio mixers, or stereo tone controls.

However, (not always obvious from the circuit diagram), sometimes a wire-wound resistor is required with a screw-down or soldered set of taps. Most linear pots cannot handle any serious kind of voltage or amperage without frying or arcing. Even in seemingly 'low' voltages in the range of say 120 VAC house currents (dimmer switches) or low current situations like guitar amp balance circuits for high voltage output stages, ordinary pots will simply present a fire-hazard and danger to other components.

You need to be sure of voltages and currents to know if a 'pot' is a safe application.

Secondly, because most high power resistors have to be wire-wound copper coils with sweeps, they have frequency-response side-effects that have to be taken into consideration. That is, they will have an impedance as well as a resistance, which affects both frequency response and phase of voltages and currents in the circuit.