Replacing wire ceramic resistors with equivalents - Page 1
Forum section: electronics / projects

author: nuno
author_rank: Frequent Contributor
timestamp: March 08, 2018, 03:27:45 am
content: Do you see a problem replacing a typical wire ceramic 5W resistor (those white blocks) by an equivalent series of 2 carbon (3W) ones? (assuming no problem with voltage & current withstanding)

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2018, 03:52:52 am
content: Carbon composition resistors dissipating significant power can 'cook' the binder and decrease in resistance till they overheat and burn up, and when they do so, can do significant damage to the board and surrounding parts. It will do for testing, but I wouldn't let it out of the shop like that. Non-flammable metal film resistors would be more acceptable but have a lower pulse overload capability than wirewounds.

author: Nusa
author_rank: Super Contributor
timestamp: March 08, 2018, 06:58:12 am
content: Generally speaking, replacing a 5W part with a lower-rated 3W part is not a good idea. If the previous part failed from heat, its a REALLY BAD idea.Now if the replacement resisters were 3W appropriate values inparallel(making it a 6W replacement), you might be ok.

author: nuno
author_rank: Frequent Contributor
timestamp: March 08, 2018, 11:23:42 am
content: Quote from: Ian.M on March 08, 2018, 03:52:52 amSo that's the same as saying a 3W *carbon* resistor isn't really up to its spec, or it's the failure mode that worries you? I would be increasing the total wattage from 5W to 6W, since the original 5W resistor burnt (actually, 2 of them burnt, and they were in series with nothing else in between, which I find rather odd but I don't see any other problem on the board and it works).The main reason for the swap is actually that I can't get the original resistors in time and budget, but it wouldn't hurt to increase wattage a little too.Quote from: Nusa on March 08, 2018, 06:58:12 amI don't see a difference using a series or parallel configuration, the idea was to use 2 x 3W resistors in series, so that's 6W, each resistor with half the value of the original one, of course.

author: mikerj
author_rank: Super Contributor
timestamp: March 08, 2018, 12:00:09 pm
content: Quote from: Nusa on March 08, 2018, 06:58:12 amSeries or parallel makes no difference, the power dissipation is shared between the resistors in either case.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2018, 12:03:23 pm
content: Carbon what, carbon film? Carbon comp?Don't recall seeing either in 3W sizes, new (not that I go out of my way to look for comp, mind), parts in that range are usually metal-oxide or the like.But anyway, as long as ratings are met, no problem.Tim

author: nuno
author_rank: Frequent Contributor
timestamp: March 08, 2018, 12:11:13 pm
content: I don't have a datasheet, the description just says "carbon". 250V max.

author: Benta
author_rank: Super Contributor
timestamp: March 08, 2018, 12:47:33 pm
content: Depends on the application. Wirewound resistors are very resilient to current pulse overloads. Carbon types don't like that kind of abuse.

author: Nusa
author_rank: Super Contributor
timestamp: March 08, 2018, 12:51:23 pm
content: Quote from: mikerj on March 08, 2018, 12:00:09 pmOops, I apparently got my rules of thumb confused. Now that I've checked myself with the math, you guys are right, 6W either way. Apologies.