resistors power rating - Page 1
Forum section: electronics / beginners

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 25, 2015, 09:13:48 pm
content: When replacing a through hole resistor on a board, in absence of schematics, its important toevaluate 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) productwith 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 documenthttp://www.sunroc.hk/index.php-view=article&catid=43-resistor-series&id=257-forming-resistor&option=com_content.htmIs it reliable.From it, I would deduce that my resistor is 1W . Is it OK ?

author: Joule Thief
author_rank: Regular Contributor
timestamp: October 25, 2015, 09:39:51 pm
content: 11 x 4 sounds like a 1 watt resistor.A Google search should return some general size / wattage relationships;

author: dacman
author_rank: Frequent Contributor
timestamp: October 26, 2015, 02:46:22 am
content: Do you have picture? If it were an MCRC series resistor from Newark or Farnell, it could be 1/2 watt.

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 26, 2015, 11:38:00 am
content: 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.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 26, 2015, 12:00:25 pm
content: 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 wThe two green resistors are 4 W (printed on the resistor)

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 26, 2015, 07:05:45 pm
content: One of these should do the job:http://www.gotronic.fr/cat-resistances-carbone-1-w-268.htmRussell.

author: tszaboo
author_rank: Super Contributor
timestamp: October 26, 2015, 09:35:23 pm
content: The typical voltage across it and the resistance value also can give you a very good clue.

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 27, 2015, 10:17:25 am
content: Quote from: NANDBlog on October 26, 2015, 09:35:23 pmDifficult 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.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 27, 2015, 03:16:36 pm
content: Quote from: rjeberhardt on October 27, 2015, 10:17:25 amIts 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.

author: tszaboo
author_rank: Super Contributor
timestamp: October 27, 2015, 04:19:24 pm
content: Quote from: JacquesBBB on October 27, 2015, 03:16:36 pmOr you can just follow the good old saying:"When in doubt, double thecapacitancepower rating!"

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: October 27, 2015, 04:47:56 pm
content: Quote from: NANDBlog on October 27, 2015, 04:19:24 pmIn 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.