A resistor opposes current, turning electrical energy into heat. It sets bias currents, drops voltages, divides signals, limits current through an LED and, in larger form, soaks up power in a dummy load. Its value is in ohms (Ω), following Ohm's law.
Reading the colour code
Small resistors are too tiny for printed numbers, so they wear bands. The first bands are digits, the next is a multiplier (how many zeros), and the last is tolerance. Precision parts use a third digit band. Common tolerance bands are brown ±1%, red ±2%, gold ±5% and silver ±10%.
Start from the end with the bands bunched together. The widely spaced band at the far end is the tolerance.
1st digit
2nd digit
Multiplier
Tolerance
Resistance4.7 kΩ
Colour
Digit
Colour
Digit
black
0
blue
6
brown
1
violet
7
red
2
grey
8
orange
3
white
9
yellow
4
gold
×0.1 (multiplier), ±5%
green
5
silver
×0.01 (multiplier), ±10%
Surface-mount resistors print numbers instead: 472 means 47 followed by two zeros, 4700 Ω or 4.7 kΩ.
Choosing one
Value. Resistors come in standard steps, so you pick the nearest. The common E12 series (typically ±10% parts) runs 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, then repeats ×10. The E24 series (typically ±5%) adds the values in between.
Power. The resistor turns I² × R into heat. A 100 Ω resistor across 12 V dissipates 12² ÷ 100 = 1.44 W, so a 1/4 W part would burn. Choose a rating at least twice the calculated dissipation, and expect a hot resistor to be too hot to touch.
Construction. Carbon film and metal film are the everyday types, metal film being tighter and quieter. Wire-wound resistors handle high power but are coils, so they have inductance and are poor at RF. Specially wound non-inductive power resistors are used where RF matters.
Real parts have hidden extras. At RF they start to matter.
Variable resistors
A potentiometer has three terminals: the two ends of a resistive track and a moving wiper. Used as a volume control, the wiper picks off a fraction of the voltage. Wired as a two-terminal rheostat, it simply varies the resistance.
Slide the wiper: the part of the track in the circuit (amber) is the resistance.