A resistor restricts the flow of current. It is a basic component of all electrical devices, even the filament in a light globe is a resistor. Resistors are defined by their material type, resistance, power rating, and tolerance.

Resistor Types

There are four main types of resistor: carbon, film, wirewound, and semiconductor. Carbon resistors are tiny insulated cylinders of carbon and ceramic dust with a terminal at each end. They have colored bands to show their resistance and tolerance values.

Film resistors are similar to carbon resistors, but use a wound metal film instead of dust for better tolerance. Wirewound resistors are also similar, but use a wound metal wire for low resistance and higher power rating.

Variable resistors, also called potentiometers, allow the resistance to be changed while in circuit. They are used in devices that control current, such as light dimmers, volume dials, and gamepads.

Thermistors change their resistance according to temperature. They are used in devices which detect heat, such as fire alarms and thermostats.

Resistor Markings

Carbon and film resistors have their resistance and tolerance values marked on the case by colored bands. The values of these bands are found on a resistor color code chart.

Other resistors have their values marked with three or four digits. These are usually the first two numbers of the resistance value followed by a power-of-ten multiplier, except when the decimal point symbol "R" is present.

Tolerance is the allowed variation from the marked resistance value expressed as a percentage. Carbon resistors typically have a value of 5% or 10%, while film resistors are 1%, and there are special resistors that go even lower.

Resistor Circuit Theory

The circuit symbol for a fixed resistor is a short zig-zag, while a variable resistor has an arrow on the zig-zag, but the thermistor has a line through a rectangle.

When resistors are connected in series, the total resistance is the sum of the resistance values. Connected in series means placing the resistors in a line and joining their terminals end-to-end.

When resistors are connected in parallel, the total resistance is the multiplication of the resistance values, divided by their sum. Connected in parallel means placing the resistors side by side and joining the terminals on each end together to form one terminal.