Resistance (R) is the property of a material to oppose the flow of electric current. You can calculate it using three equations, depending on which quantities you know:
Power rating is the maximum power a resistor can safely dissipate without being damaged. The actual power dissipated is calculated using:
Always choose a resistor with a power rating at least twice the calculated dissipation for safety.
Exceeding a resistor's wattage can cause failure.
A circuit has a voltage of 12 V and a current of 0.02 A. What is the resistance?
Solution: R = 12 / 0.02 = 600 Ω
Power dissipated: P = 12 × 0.02 = 0.24 W
A resistor has 10 V across it and dissipates 2 W. What is the resistance?
Solution: R = 10² / 2 = 100 / 2 = 50 Ω
Power dissipated: P = 2 W (given)
A resistor dissipates 5 W with a current of 2 A. What is the resistance?
Solution: R = 5 / (2²) = 5 / 4 = 1.25 Ω
Power dissipated: P = 5 W (given)
Once the value is known, several other properties decide whether the part actually works in circuit.
Dissipation is P = I2R = V2/R. A resistor’s rating assumes a specified ambient temperature, usually 70 °C, and must be derated above it — many parts reach zero rated power around 155 °C. Common practice is to run at no more than half the rated power, which keeps the surface temperature sensible and markedly extends life.
Tolerance describes the value at room temperature when new. Temperature coefficient, in ppm/°C, describes how it drifts afterwards. A ±1% resistor with a 100 ppm/°C coefficient shifts a further 0.5% over a 50 °C rise, so for anything precision the coefficient often matters more than the initial tolerance.
| Type | Characteristics |
|---|---|
| Carbon film | Cheap, noisy, poor tolerance; general purpose only |
| Metal film | Low noise, good tolerance and tempco; the default choice |
| Wirewound | High power, but inductive — unsuitable at high frequency |
| Thick film (SMD) | Compact and cheap; moderate precision and pulse handling |
Calculate dissipation from P = I²R or V²/R, then select a part rated at roughly twice that figure. Ratings assume a stated ambient, usually 70 °C, and must be derated above it.
It is the drift in resistance per degree, in ppm/°C. A 100 ppm/°C part shifts 0.5% over a 50 °C rise, which can exceed its initial tolerance and matters more in precision circuits.
Metal film for most purposes, given its low noise and good stability. Wirewound where high power is needed and inductance is acceptable, and thick-film SMD for compact general-purpose work.
Because they are physically coils and therefore inductive. In snubbers, current-sense positions or any fast-switching circuit they ring; non-inductive or film types are required.
A maximum voltage independent of power rating, set by the physical construction. High-value parts reach it easily — a 10 MΩ resistor may be limited to 200 V even though that represents only 4 mW.
Yes. A short high-energy pulse can destroy a part whose average dissipation is well within rating. Inrush limiting and snubber positions need explicitly pulse-rated resistors.