Resistor power dissipation calculator
A resistor run at its rated power gets very hot. Specifying twice what you need is normal practice.
ohm
mA
V
%
100% means specify twice the calculated power.
Power from the current
0.423W
- Power from the voltage
- 0.423
- Rating to specify
- 0.846
- Voltage across the resistor
- 14.1000 V
- Current implied by the voltage
- 30.000 mA
Show the working
- Resistance = 470
- Current = 30
- Or voltage across it = 14.1
- Design margin = 100
- i_a = 30 / 1000 = 0.03
- Power from the current = 0.03^2 * 470 = 0.423
- Power from the voltage = 14.1^2 / 470 = 0.423
- Rating to specify = 0.423 * (1 + 100/100) = 0.846
- Voltage across the resistor = 0.03 * 470 = 14.1
- Current implied by the voltage = 14.1 / 470 * 1000 = 30
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- P = I^2 R = V^2 / R
Standard ratings are 1/8, 1/4, 1/2, 1, 2 and 5 watts. Round up to the next one above the rating shown.