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I know resistors have a maximum power rating in watts. But if my pack says 1/8 watt does that mean 0.125 watts max, or does it mean 1 watt up to a maximum of 8 watts?

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    \$\begingroup\$ What does "1 watt up to a maximum of 8 watts" mean? \$\endgroup\$
    – user253751
    Oct 2, 2016 at 21:30
  • \$\begingroup\$ I meant was a it a range of suitable working power values from 1 watt up to 8. \$\endgroup\$ Oct 2, 2016 at 21:32
  • \$\begingroup\$ It's .125W. It means that if the resistor dissipates more power than that (V^2/R or I^2*R) it will become a heater, then a smoke generator. \$\endgroup\$
    – Ian Bland
    Oct 2, 2016 at 22:27
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    \$\begingroup\$ @RenegadeAndy would that mean that its not able to work from 0 Watts to 0.999999 Watts? \$\endgroup\$
    – Passerby
    Oct 3, 2016 at 1:48

2 Answers 2

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Easy answer. 1/8 W means 0.125 W.

But what does that mean?

The datasheet will, or should, have details about that. An exhaustive search of two different resistor types in my inventory shows that it means that it can dissipate up to the rated power as long as the ambient temperature is kept below 70 °C.

Note that the resistor will likely elevate the "ambient" temperature.

If you want to operate it at a higher ambient temperature, you have to derate it - that is, use less power. Good resistor datasheet will have curves for this as well, they are pretty simple:

enter image description here

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Simply put, it means the resistor can safely convert electrical energy to heat at the rate of one eighth of a watt, forcing above that, the resistor will heats up excessively.

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