A resistor limits current and creates a defined voltage drop. Fixed resistors are the most common, but potentiometers, thermistors and light-dependent resistors are also resistors whose value can be adjusted or changes with a physical quantity.
The four-band colour code
In the common four-band scheme, the first two bands give significant digits, the third is a decimal multiplier and the fourth is tolerance. The sequence is a long-established industry convention; it is still worth confirming the value with a meter when the circuit matters.
- Black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.
- Gold as a multiplier means ×0.1; silver means ×0.01.
- Gold as a tolerance band means ±5%; silver means ±10%; brown ±1%; red ±2%.
- A missing fourth band usually means a wider tolerance, so verify before relying on it.
Decoding a resistor
Problem. A resistor shows brown, black, red and gold. What is its nominal value and tolerance?
Solution. Brown = 1, black = 0, red = ×100, gold = ±5%. Value = 10 × 100 = 1000 Ω = 1 kΩ, tolerance ±5%, so the real value lies between 950 Ω and 1050 Ω.
Power rating
Resistor dissipation
Problem. A 100 Ω resistor carries 20 mA. What power does it dissipate?
Solution. P = I² × R = (0.020)² × 100 = 0.0004 × 100 = 0.04 W. A resistor rated well above 0.04 W gives margin for tolerance and temperature.
- The first two colour bands are digits; the third is a multiplier.
- The fourth band gives tolerance, not a digit.
- Dissipation is P = I²R = V²/R = V × I.
- Measure resistance with the component isolated from the circuit.
Worked example
A 470 Ω resistor carrying 15 mA dissipates P = (0.015)² × 470 = 0.106 W, so a 0.25 W part provides reasonable margin in normal ambient conditions.