How do you calculate the power dissipated in a resistor?

Power is P = V × I = I² × R = V² / R. With 5 V across 100 Ω, the current is 50 mA and the resistor dissipates 0.25 W, exactly the rating of a common 1/4 W part, which will run hot. A common rule is to choose a rating at least twice the expected dissipation. The simulator shows power and estimated temperature as you change values.

How do you read a resistor color code?

On a four-band resistor, the first two bands are digits, the third is the multiplier and the fourth is the tolerance. Brown, black, brown, gold reads 1, 0, ×10, ±5 %, which is 100 Ω ±5 %. Five-band resistors add a third digit for precision values. The simulator draws the matching bands as you type a resistance.

What does resistor tolerance mean?

Tolerance is how far the real resistance may differ from the marked value. A 100 Ω resistor with ±5 % tolerance can measure anywhere from 95 to 105 Ω. Standard value series follow from it: E24 values are spaced for 5 % parts and E96 values for 1 % precision parts.

Resistor Simulator, Ohm’s Law and I–V Curve

What is a resistor?

Working principle

A resistor is a passive two-terminal component that opposes the flow of electric current and converts the associated electrical energy into heat. Its value, resistance, is measured in ohms and depends on the resistive material, its length and its cross-sectional area. Unlike capacitors and inductors, a resistor stores no energy and introduces no phase shift between voltage and current, which makes it the simplest and most predictable component in any circuit.

Ohm's law

The relationship between the three quantities is Ohm's law: voltage equals current times resistance. Hold the resistance constant and current rises in direct proportion to applied voltage, which plots as a straight line through the origin — the signature of a linear, or ohmic, device. This is exactly the behaviour the simulator's characteristic curve shows, and it is what lets a resistor be used to set a precisely known current from a known voltage.

Power rating and heat

Power dissipated in a resistor is voltage times current, equivalently current squared times resistance. Because the current term is squared, doubling the current quadruples the heat. Every resistor carries a power rating — a quarter watt is common for through-hole parts — and exceeding it causes drift, discolouration and eventual failure. The rating assumes free air at room temperature, so a resistor in a hot enclosure has to be derated accordingly.

Types and applications

Carbon film resistors are cheap and general purpose, metal film types offer better tolerance and lower noise, and wirewound resistors handle high power. Variable versions include potentiometers and rheostats, while thermistors and light-dependent resistors change value with temperature or light. In circuits, resistors set bias points, limit LED and base currents, form voltage dividers and RC timing networks, and terminate transmission lines.

Frequently asked questions

How do you calculate the power dissipated in a resistor?: Power is P = V × I = I² × R = V² / R. With 5 V across 100 Ω, the current is 50 mA and the resistor dissipates 0.25 W, exactly the rating of a common 1/4 W part, which will run hot. A common rule is to choose a rating at least twice the expected dissipation. The simulator shows power and estimated temperature as you change values.
How do you read a resistor color code?: On a four-band resistor, the first two bands are digits, the third is the multiplier and the fourth is the tolerance. Brown, black, brown, gold reads 1, 0, ×10, ±5 %, which is 100 Ω ±5 %. Five-band resistors add a third digit for precision values. The simulator draws the matching bands as you type a resistance.
What does resistor tolerance mean?: Tolerance is how far the real resistance may differ from the marked value. A 100 Ω resistor with ±5 % tolerance can measure anywhere from 95 to 105 Ω. Standard value series follow from it: E24 values are spaced for 5 % parts and E96 values for 1 % precision parts.

Fixed resistor

Voltage–current characteristic