resistors power rating

[JacquesBBB]

When replacing a through hole resistor on a board,  in absence of schematics, its important to
evaluate the  power rating of the resistor in place,
but I cannot find good information on this point. Can you help ?

I  understand that size may not be sufficient as it will evolve with time,
but  let us assume that we are in presence of a modern ( < 20 years) product
with some other resistors on the board that can provide additional clues.

Is there a good reference somewhere  for the relation between size and power rating ?


In particular, the resistor I am looking for at present is  4mm x 11mm .  What could be its power rating ?


Edit :  I have since found this  document
http://www.sunroc.hk/index.php-view=article&catid=43-resistor-series&id=257-forming-resistor&option=com_content.htm
Is it reliable.

From it, I would deduce that my resistor is  1W . Is it OK ?

[Joule Thief]

11 x 4 sounds like a 1 watt resistor.

A Google search should return some general size / wattage relationships;

[dacman]

Do you have picture?  If it were an MCRC series resistor from Newark or Farnell, it could be 1/2 watt.

[rjeberhardt]

The power rating for a given size will depend on the technology.  A carbon film resistor will have a lower maximum temperature, and thus power rating, than a metal film resistor of the same size.  Without analysing the circuit to determine the dissipation in the resistor I would just replace it with a metal film resistor of about the same dimensions.

Russell.

[JacquesBBB]

Thanks for the advices,

Here is the  picture  of the board. The bad resistor is R32 on the upper left corner.

The two  small resistors close by are probably  1/4 w
The two green resistors are 4 W (printed on the resistor)
 

[rjeberhardt]

One of these should do the job: http://www.gotronic.fr/cat-resistances-carbone-1-w-268.htm

Russell.

[tszaboo]

The typical voltage across it and the resistance value also can give you a very good clue.

[rjeberhardt]

The typical voltage across it and the resistance value also can give you a very good clue.

Difficult to do until you've replaced it with a good one!  The only foolproof way would be to draw out a schematic and analyse it to calculate the power dissipated - also a bit difficult while some components are unidentified.

Having worked for Philips in the 1970s that picture brought back some memories.  Most of those components were very familiar and are definitely Philips brand.

Russell.

[JacquesBBB]

The only foolproof way would be to draw out a schematic and analyse it to calculate the power dissipated - also a bit difficult while some components are unidentified.

Having worked for Philips in the 1970s that picture brought back some memories.  Most of those components were very familiar and are definitely Philips brand.

Its a power board of an INduction cooking table. IX3 from Brandt.
Her is the schematics : R32 and R31 are the same large 68k resistor.

I have the schematics, but  no BOM.
 

[tszaboo]

The only foolproof way would be to draw out a schematic and analyse it to calculate the power dissipated - also a bit difficult while some components are unidentified.

Having worked for Philips in the 1970s that picture brought back some memories.  Most of those components were very familiar and are definitely Philips brand.

Its a power board of an INduction cooking table. IX3 from Brandt.
Her is the schematics : R32 and R31 are the same large 68k resistor.

I have the schematics, but  no BOM.

Or you can just follow the good old saying:
"When in doubt, double the capacitance power rating!"

[JacquesBBB]

Or you can just follow the good old saying:
"When in doubt, double the capacitance power rating!"

In the present case,  this is what I will do. Even if most probably  the resistor is a  1W carbon resistor, I will change it with a  2W metal film resistor of the same size (unless I have the 1W in my junk box).

But The interest of the discussion was more general than solving this specific case, thanks to all the contributions.