Resistors: 0.4W size compared to 0.25W size

[tron9000]

(I did think of this until now)

So just doing a bit of stock taking and found a box of 0.4W resistors

now compared to a 0.25W one they are noticeably smaller. They are both metal film type, same resistance, same tolerance, but the one that takes more power is SMALLER?

Does anyone know why this is? You usually associate larger size with more power don't you?

[cowana]

It all comes down to the maximum temperature.

For a given power dissipation, the smaller one will get hotter - but as long as it can cope with that, there isn't an issue.

So although they're both metal film, I'd imagine there are some subtle differences in the materials used which allow the 0.4W one to have a much higher maximum temperature.

Andy

[RogerRowland]

... either that, or you mislabelled the boxes 

[tron9000]

... either that, or you mislabelled the boxes 

nope I checked the part number on the suppliers site!

[freddy114234]

 mainly due to the  thermal conductivity of  ceramic core(as the same size, the  percentage of  aluminum oxide   is proportional to  the power rate ).

[Dr. Frank]

The specification of max. power dissipation of a resistor is always coupled with its max. body temperature and in turn with its stability.

These parameters also depend on the technology (Thin Film, Thick Film, or others), and the case size.

For SMD components, the cooling pads on the PCB play an overwhelming role in the cooling aspect.
Small resistors can not transfer as much heat over their soldering junctions, than bigger ones, and heat up more, reducing their max. power, although the naked components may have the same specification.
 
Here's an example for leaded resistors, with randomly chosen values:
If you have a thin film (precision) resistor of a certain case size, you may apply 0.25W for 125°C max. hot spot temperature, which gives 0.1%/yr. stability, and 0.4W for 155°C hot spot temperature and  0.5%/yr. stability ("Power Mode").

Thick Film resistors of the same case size may even allow higher power dissipation, first due to the higher robustness of the "Thick" film, and second, because thick film resistors are mostly less stable, anyhow.

The specification of the manufacturers are mostly valid for the naked component only, but do not account for the cooling problem by air flow or by PCB cooling areas.


These arguments may well explain the characteristics of your resistors.

Frank