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I have a resistor in a circuit that I've calculated will dissipate 225 mW, worst case, calculated for tolerance variation.

Most resistors are rated to dissipate 250 mW. This seems a bit close to the spec, but is the spec already given conservatively? How close to the spec do you allow before you jump up to 500 mW resistors?

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    \$\begingroup\$ I'm a hobbyist and don't need to count nickels. So I always use something rated for at least twice and hopefully three to four times the requirement. But the specifications for a resistor are for its typical usage, too. If it is an axial, then it's probably open air with a certain lead length and soldered to a board or tie-point. If you know something better about how it will be used, you can adjust. Safety is another point to consider. How bad will it be if "this resistor" fails? Depending on the answer, you might choose differently. \$\endgroup\$
    – jonk
    Jan 7, 2020 at 2:10
  • \$\begingroup\$ How often and for how long at a time will the worst case scenario occur? How critical is the application? In which exact way would the resistor failing open circuit affect the circuit? How about a short? How much airflow is there around the resistor? Does cost or space prohibit the use of a 500 mW resistor? Is lifting the resistor a few mm above the PCB viable? \$\endgroup\$
    – Dampmaskin
    Jan 7, 2020 at 2:14
  • \$\begingroup\$ Read the spec carefully. At what ambient temperature can the resistor dissipate 250 mW? Often the max dissipation is at something like 80C ambient. Also, for SMT, pad design and PCB temp is absolutely critical. Most dissipation will be through PCB. \$\endgroup\$
    – user57037
    Jan 7, 2020 at 2:41
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    \$\begingroup\$ @mkeith I've seen resistors specified for power at 50C or below. So read the specification really carefully. \$\endgroup\$
    – TimWescott
    Jan 7, 2020 at 5:10
  • \$\begingroup\$ Yeah, for sure. The SMT ones are reasonable. Some of the larger chassis mount ones need to be very carefully scrutinized. Some even require a heatsink to avoid savage de-rating to 25% or something like that. \$\endgroup\$
    – user57037
    Jan 7, 2020 at 8:23

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the power spec is not really a power alone specification per se, you gotta think of it as heat as well, if the resistor has proper cooling it should be fine. However, depending on the application you spec everything with a certain safety margin, I think for high grade circuits that would not be acceptable since they are in the range of 50% or so

Now, you gotta consider this can you afford the circuit going down and needing that resistor replaced? can your other components handle a short at that point given that resistor burns out? do you have enough space for 500mW instead? what is the role of that resistor to begin with... it takes a lot of considerations.

you are right at 10% safety margin, which for most common uses would be okay in my opinion, for something like military or circuits for automobiles it would not be acceptable design though.

you gotta look at each situation and assess the cost, risk, reliability and decide the margin with all of that in mind. Blindly using a margin is dangerous as well.

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My safety margin for resisters is 50% ,unless the client specifies otherwise.If the circuit boards are going into relatively small numbers of high value equipment being sold far away then it is better to be safe than sorry .Complex thermal analysis would be needed to justify pushing things harder .Product numbers would need to be very high to make this worthwhile.

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Considering 60'C can burn your finger and 85'C case temp is a prudent reliable max temp, consider your maximum ambient and max temp rise you can allow to meet 85'C .

Usually I derate resistor power 50% to avoid hot spots near other temperature sensitive parts. That allows it to rise 45'C from a max ambient of 40'C to reach 85'C rated at 75% so I have 25% margin on internal max temperatures of 155'C. ((I think ... going from memory, which isn't so hot) enter image description here

THT Specs

If you do not derate them it means they will rise 85'C above ambient. of 25'C so 50% will rise 43'C above 25'C or 68'C at room temp. Your choice is max ambient and max temp rise.

MTBF is degraded 50% with every 10'C temp rise. but resistors are very reliable to begin with, but the parts next to like e-caps might not appreciate it.

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Don't exceed 50% of the rating, if you're concerned. I have no issues with an upper threshold of 75%. If you only have a large stock of 250mW resistors, parallel them ( i.e. 4 1K resistors in parallel for 250ohm, for 1W).

Using resistors rated for higher power dissipation shouldn't really be necessary. In what usage scenario is this a problem?

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If the resistor leads are SHORT, and the 2 leads are soldered to large areas of copper so the heat is removed very near the resistor body, then why not go for 225mW?

Given each square of standard-thickness copper foil has 70 degree Centigrade thermal rise per watt, then a resistor dumping heat into 0.5" by 0.010 (50 squares) in each lead, has 25 squares effective thermal flow restriction, and 70 * 25 = 1,750 degree Centigrade temperature rise ----------- unless heat is dumped into underlying copper planes or there is air cooling...........at 1,780 C rise, simple radiation will be the exit path.

In other words, the rating of the resistor means little, if the thermal design is just punt and hope.

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