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# "Explain why resistors sometimes burn out when they run close to their power rating" Watch

1. The Q is in the title.
Would appreciate the help as I cannot find the answer
2. (Original post by Danny.L)
The Q is in the title.
Would appreciate the help as I cannot find the answer
It's because any stated resistor value is most likely not to have that exact value of resistance, but will fall within a range of actual values governed by the manufacturers stated production line tolerances.

That means any given resistor may have an actual resistance as much as 20% higher or lower than the nominal value would suggest.

Then, if the nominal chosen resistor is operated close to it's maximum power limit, it runs the risk of overheating if the actual value of that resistor is less than that calculated as optimal and safe for the circuit in question. i.e. it ends up dissipating more power than calculated for the nominal value.

More detailed description

Resistors must be manufactured to a cost, hence in a process where the actual value of any single resistor produced may fall within quite a wide range of values.
The manufactured components will consequently have a spread of values. Values falling within a given range are lumped together and sold as having a mean value within a specified 'tolerance range'.

e.g. an E12 range of resistors has 12 mean values per decade of resistance stated and sold as: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 & 82 ohms, with all values repeating as the decade multiplier increases: 1's, 10's, 100's, 1000's, 10,000's etc. A tolerance band is also printed on the resistor to show the range of values the resistor falls within. i.e. 1%, 2%, 5%, 10% etc. (The tighter the tolerance band, the higher the cost.) For the E12 range, the tolerance is +/- 10%. E24 range is +/- 5% etc.

As an example, a 33K ohms +/-10% resistor can have an actual value anywhere between 29.7K to 36.3K ohms, but is still classed as 33K ohms. The resistor below that in the same E12 range would be 27K ohms which can have an actual resistance anywhere between 24.3K ohms to 29.7K ohms. You should now see why the nominal values are chosen to not overlap.

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Updated: January 22, 2015
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