power rating of smd resistors

[joniengr081]

I am wondering about the power rating of the smd package resistors for example 0603, 0805 and 1206. I have seen somewhere that the power taring is a function of tolerance. For example resistance tolerance 1% resistance points to higher power rating as compared to 0.5 resistance tolerance which is less power rated.

How can I find the rule or a table in which power rating of some common packages are given with tolerance.

[Psi]

I have seen somewhere that the power taring is a function of tolerance. For example resistance tolerance 1% resistance points to higher power rating as compared to 0.5 resistance tolerance which is less power rated.

Nope, the tolerance of a resistor has nothing to do with power rating.

If you have a look at cheapest resistors on digikey you will see what the typical power ratings are for each size.
You can get higher power smt resistors within the same size category. They are made of materials that can run hotter without failing and they will cost more. (ceramic etc)

[ArdWar]

"power rating is a function of tolerance" is probably true in a sense that heating a resistor will change its resistance value. Hence dissipating too much power will cause too much change of resistance.

This is indeed true, but if you found yourself at this point you're probably already nitpicking about PPMs and stuffs well past the "will this resistor burns up at this power" stage.

[Vovk_Z]

How can I find the rule or a table in which power rating of some common packages are given with tolerance.

Look at a datasheet of any general-use type resistors.

[Siwastaja]

We don't need any more "rule of thumb" tables or advice, which is almost always incorrect, and even if correct, outdated. Just look at the datasheets. Distributors also collect the datasheet values into their parametric search systems. You can instantly see what combinations of power ratings, tolerances and temperature coefficients are available and at what cost.

You can pretty much assume that when running at full rated power, resistor will be at full rated temperature, and knowing TC, you can then calculate extra error from this heating, which comes on the top of the tolerance. You can then choose if you try to find lower TC resistors, or ones with higher power rating and larger package so you can derate more to reduce heating.

[joniengr081]

If the resistance of smd resistor is defined with tolerance 200ppm/°C.

At 50 degC it will be 10000 ppm/50°C.
Percentage = 10000/10^6 * 100
Percentage = 1 %

At 100 degC it will be 20000 ppm/50°C.
Percentage = 20000/10^6 * 100
Percentage = 2 %

and so on. This shows that tolerance is temperature dependent.

But this is also true that smd resistors are available in more then one power rating. For example 1206 is available in 250 mWatt and also in 500 mWatt. What the difference between them other then power rating ?

[Siwastaja]

For example 1206 is available in 250 mWatt and also in 500 mWatt. What the difference between them other then power rating ?

The 500mW part is internally constructed to survive higher temperature. Therefore, unless it has better temperature coefficient to compensate, it will also see larger shift in resistance, when used at maximum rated power. If the two parts are used at same power, there probably is not much difference in internal temperature. There might be; the higher rated part maybe also dissipates the heat better through the pads, but I would not count on it; most of the power rating advantage comes from survival of higher temperatures.

Terminology fix: 200ppm/°C is not "tolerance", it's temperature coefficient (TC). Tolerance is just the initial variation within the batch of manufactured units.

[mikeselectricstuff]

For example 1206 is available in 250 mWatt and also in 500 mWatt. What the difference between them other then power rating ?

The 500mW part is internally constructed to survive higher temperature. Therefore, unless it has better temperature coefficient to compensate, it will also see larger shift in resistance, when used at maximum rated power. If the two parts are used at same power, there probably is not much difference in internal temperature. There might be; the higher rated part maybe also dissipates the heat better through the pads, but I would not count on it; most of the power rating advantage comes from survival of higher temperatures.

Terminology fix: 200ppm/°C is not "tolerance", it's temperature coefficient (TC). Tolerance is just the initial variation within the batch of manufactured units.

the amount of copper it's attached to will have a far greater effect than tolerance. Stated resistor power ratings are rarely achievable in practice

[MathWizard]

Now I have some sample books of SMD parts, but they are all low power parts. Over the years, I've hardly killed any passive SMD parts. But yeah, I guess on some circuits I design for SMD, I'll still probably have to use some through hole stuff. Even with through hole, I have lots of 1/8th and 1/4W stuff, but only a few 1/2, 1W resistors on hand in my collection.

I guess just sensing the temperature of SMD parts with a finger, is different than much larger area THH parts. I would expect higher temperatures for the same power dissipation over a smaller area.

I'm not looking forward to burning my fingers on SMD resistors, them, so I better use lots of them.

[Doctorandus_P]

How many fingers do you have? 

[MarkT]

Thermal cameras are very handy for this.  Basically size your resistors to run cool, and provide enough copper / thermal vias for the hot devices...