Litmux Studio

LessonPower and energy · 13 min

Heat in resistors and power ratings

Every watt a resistor takes becomes heat. The resistor warms up until it loses heat to the air and the board as fast as it makes it. Too much power and it gets hot enough to discolour, drift in value, unsolder itself, crack or burn - and sometimes the circuit board around it. Each resistor therefore has a power rating: the most it can safely handle.

The bigger the body, the more heat it can get rid of.
ResistorTypical rating
Small through-hole (the common one)1/4 W (0.25 W)
Larger through-hole1/2 W, 1 W, 2 W
Ceramic "cement" or wirewound power resistor5 W, 10 W and more
SMD 0402 / 0603 / 0805 / 12061/16 W / 1/10 W / 1/8 W / 1/4 W
SMD 25121 W - 2 W
Worked example 1: the LED resistor

The 470 Ω resistor for a red LED on 9 V carries 15 mA. Which power rating?

  1. Its voltage is 9 − 2 = 7 V (not 9 V - the LED takes 2 V).
  2. P = V × I = 7 × 0.015 = 0.105 W.
  3. A 1/4 W resistor gives a margin of 0.25 / 0.105 = 2.4×.

Answer: 1/4 W is fine.

Worked example 2: dropping voltage with a resistor

A 9 V, 300 mA radio is to run from a 12 V battery through a series resistor. Which resistor, and which rating?

  1. The resistor must drop 12 − 9 = 3 V at 0.3 A: R = 3 / 0.3 = 10 Ω.
  2. P = V × I = 3 × 0.3 = 0.9 W.
  3. With a 2× margin: a 2 W resistor.

Answer: 10 Ω, 2 W. But the radio's current rises and falls with the volume, and the resistor's drop with it - a voltage regulator does this job much better.

12 V across 100 Ω: P = 144 / 100 = 1.44 W. A 1/4 W resistor would burn within seconds; this one is a 3 W part (a 2× margin).
12 V across 100 Ω: P = 144 / 100 = 1.44 W. A 1/4 W resistor would burn within seconds; this one is a 3 W part (a 2× margin).

Where you meet it

Power supplies

Big ceramic 5 W - 10 W resistors limit the switch-on surge or slowly empty capacitors after power-off (bleeder resistors).

Current sensing

Low-value shunts (0.01 - 0.1 Ω) carry amps; their rating is chosen with P = I² R.

Load testing

Technicians test a battery or power supply into a known power resistor - a "dummy load" - and watch the voltage.

Practice 10.1 A flows through a 100 Ω resistor. What power rating would you choose (with a 2× margin)?

  1. 1/4 W
  2. 1/2 W
  3. 1 W
  4. 2 W
Hint

P = I² R, then double it.

Show the answer

Answer: D. 2 W

P = 0.1² × 100 = 1 W. With a 2× margin: 2 W.

Practice 2What is the largest voltage a 1/4 W, 1 kΩ resistor can take (at its full rating)?

Hint

P = V² / R, so V = √(P × R).

Show the answer

Answer: 15.81 V

V = √(0.25 × 1000) = √250 = 15.8 V.

Challenge 3A 47 Ω resistor is rated 0.5 W. Keeping a 2× margin (use at most half the rating), what is the largest current it should carry, in mA?

Hint

Allowed power = 0.25 W. I = √(P / R).

Show the answer

Answer: 72.9 mA

I = √(0.25 / 47) = √0.005319 = 0.0729 A = 72.9 mA.