Variable resistors consist of a resistance track with connections at both ends and a
wiper which moves along the track as you turn the spindle. The track may be made from
carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances).
The track is usually rotary but straight track versions, usually called sliders, are also available.
Variable resistors may be used as a rheostat with two connections
(the wiper and just one end of the track) or as a potentiometer with all three
connections in use. Miniature versions called presets are made for setting up circuits which will not
require regular adjustment.
Variable resistors are often called potentiometers.
They are specified by their maximum resistance, linear or logarithmic track, and their physical size.
The standard spindle diameter is 6mm.
The resistance and type of track are marked on the body:
4K7 LIN means 4.7k linear track.
1M LOG means 1M logarithmic track.
Some variable resistors are designed to be mounted directly on the circuit board.
Others are for mounting through a hole in the case containing the circuit,
use stranded wire to connect these variable resistors.
Linear (LIN) and Logarithmic (LOG) tracks
Linear (LIN) track means that the resistance changes at a constant rate as you move
the wiper. This is the standard arrangement and you should assume this type is required if
a project does not specify the type of track. Presets always have linear tracks.
Logarithmic (LOG) track means that the resistance changes slowly at one end of the track and
rapidly at the other end, so halfway along the track is not half the total resistance!
This arrangement is used for volume (loudness) controls because the human ear has a logarithmic
response to loudness so fine control (slow change) is required at low volumes and coarser
control (rapid change) at high volumes. It is important to connect the ends of the track the
correct way round, if you find that turning the spindle increases the volume rapidly followed
by little further change you should swap the connections to the ends of the track.
This is the simplest way of using a variable resistor.
Two terminals are used: one connected to an end of the track, the other to the moveable wiper.
Turning the spindle changes the resistance between the two terminals from zero up to the maximum resistance.
Rheostats are often used to vary current, for example to control the brightness
of a lamp or the rate of charging a capacitor.
If a rheostat is mounted on a printed circuit board all three terminals are usually soldered for better mechanical
strength. The third terminal serves no electrical function but is usually linked to the wiper terminal.
Variable resistors used as potentiometers have all three terminals connected.
This arrangement is normally used to vary voltage, for example to set the switching
point of a circuit with a sensor, or control the volume (loudness) in an amplifier circuit.
If the terminals at the ends of the track are connected across the power supply then the
wiper terminal will provide a voltage which can be varied from zero up to the maximum of the supply.
Presets are miniature versions of standard variable resistors.
They are designed to be mounted directly onto circuit boards and adjusted only when the circuit is built.
For example they may be used to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit.
A small screwdriver or similar tool is usually required to adjust presets.
Presets are available in vertical and horizontal styles, they are electrically identical but make sure you buy the correct
type for your circuit board layout. Horizontal presets provide better mechanical strength on a circuit board.
Presets may be open (no case) or enclosed within a plastic case to protect them from dust and dirt.
Presets are cheaper than standard variable resistors so they are often used in educational and hobby projects.
Multiturn presets are used where very precise adjustments must be made. The screw
must be turned many times (10+) to move the slider from one end of the track to the other,
giving very fine control.
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