What is a Variable Resistor?
A variable resistor is a type of resistor whose electrical resistance value can be adjusted on demand. Variable resistors are used in an electronic circuit to adjust circuit resistance as a means to control the voltage or current within a circuit (as per Ohm’s Law).
The electrical resistance is varied by sliding a wiper contact along a resistance track. Sometimes the resistance is adjusted at preset value as required at the time of circuit building by adjusting screw attached to it and sometimes resistance can be adjusted as when required by controlling knob connected to it.
The active resistance value of the variable resistor depends upon the position of the slider contact on the resistance track.
It mainly consists of a resistance track and a wiper contact. The wiper contact moves along the resistance track when the adjustable component is adjusted.
There are mainly three different types of resistance track used in this resistor they are carbon track, cermet (ceramic and metal mixture) track and wire wound track.
Carbon track and cermet track are used for high resistance application whereas wire wound track is used for low resistance variable resistor.
The resistance tracks generally are of a circular shape – but a straight track is also used in many cases.
Variable Resistor Connection
It is used as a rheostat when one end of the resistance track and wiper terminal are connected to the circuit and other terminal of resistance track remains open.
In this case the electrical resistance between connected terminal and wiper terminal depends upon the position of the wiper (slider) on the resistance track.
A variable resistor can also be used as a potentiometer when both ends of the resistance track are connected to the input circuit and one of the said ends of resistance track and wiper terminal is connected to the output circuit.
In this case all three terminals are in use. Sometimes in electronics circuit there may be requirement of adjustable resistance but this adjustment is required only once or very often. This is done by connecting preset resistors in the circuit.
Preset resistor is one kind of variable resistor whose electrical resistance value can be adjusted by adjusting an adjustable screw attached to it.
Types of Variable Resistor
Resistance track wise there are mainly two types of resistance track available one is linear track and other is logarithmic track.
In linear track the resistance value varies linearly with changing slider position on the track. That means the resistance and the position of slider, form a straight line characteristics curve.
When the resistance of variable resistor, varies logarithmically with position of the slider contact on the resistance track, the track is referred as logarithmic track.
The resistance value and type of track are marked on the resistor itself. For example when a variable resistor is marked as 5K9 LIN means it has maximum 5.9 kilo Ω resistance and has a linear resistance track.
Again when a resistor is marked as 2M LOG, it will have maximum 2 mega Ω resistance and it has logarithmic track. Preset resistors are linear track type.
ut the variable resistors used for volume control in sound system are mainly LOG type as our ears have logarithmic response to the loudness.
In a GOL resistor, the resistance changes slowly at beginning and rapidly at towards end of the track.
Uses of Variable Resistors
A variable resistor can be used mainly in two different ways. When one end of resistance track and wiper terminal is connected with circuit then current through the resistor limits according to the position of the wiper contact on the resistance track.
As the wiper contact slides away from the connected end of the resistance track, the resistive value of the resistor increases and current goes down through the circuit. That means the variable resistor behaves like a rheostat.
Another use is as a potentiometer. In this case the two ends of resistance track are connected with a voltage source. Hence voltage drop across the resistance track is equal to the value of voltage source.
Now the output or load circuit is connected across one end of the resistance track and wiped terminal. Hence voltage across the load terminals is the fraction of source voltage and it depends upon the position of the wiper terminals on the resistance track.
This is another widely used application of variable resistors. This is needless to say, potentiometers are used to control voltages whereas rheostats are used to control electric currents.
Preset Variable Resistor
This is micro version of variable resistor. Preset resistors are directly mounted on circuit board and adjusted only when the circuit is built.
There is an adjustable screw attached to the resistor and a small screwdriver is required to adjust this screw for desired resistance value.
These resistors are quite cheaper than standard variable resistor available in the market.