Power and component ratings
A resistor has a heat budget
Power is the rate of energy transfer. P denotes power; a watt (W) is one joule per second. A resistor converts electrical energy into heat. Its power rating is an operating limit under specified conditions. A joule is a unit of energy; a milliwatt (mW) is one thousandth of a watt.
EXPERIMENT BENCH
Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.
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Why it works
Ideal steady voltage and resistance. Temperature and cooling are not simulated; the rating is a comparison, not a prediction of failure.
P = V²/R and I = V/R. Doubling voltage at fixed resistance makes four times the power. Rating use = 100 × P/rating.
Difficulty
Build · Build it
Power on at 5 V, 1000 Ω and a 0.25 W rating. Read 25 mW.
Need a hint?
5 V; 1000 Ω; 0.25 W → 25 mW
Explore · Change one thing
At 9 V and a 0.25 W rating, keep power at or below 125 mW while current is at least 10 mA.
Need a hint?
9 V; 800 Ω; 0.25 W → 101.25 mW; 11.25 mA
Solve it · Solve the challenge
At 12 V and a 0.25 W rating, obtain 170–190 mW with current no greater than 20 mA.
Need a hint?
12 V; 800 Ω; 0.25 W → 180 mW; 15 mA
Extra reading and circuit recipes
A resistor has a heat budget
A useful pattern
At fixed voltage, twice the resistance means half the current and half the power.
Beyond this model
A real rating depends on ambient temperature, mounting and the manufacturer’s derating curve.
A useful trap to avoid
A 0.25 W rating does not mean the resistor always consumes 0.25 W.
Voltage doubles while resistance stays fixed. Power…
- doubles
- quadruples
- halves
Read more
Voltage is squared in P = V²/R; a factor of two becomes a factor of four.