Determine power rating of giant resistor - Page 1
Forum section: electronics / repair

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 13, 2016, 11:26:37 pm
content: I have a giant resistor that is wire wound with braid copper wire that looks like 14 or 16 gauge. It is about 6inches long by about an inch in diameter, and has ceramic ends, likely a ceramic core. I believe it is coated in a plastic sleeve.It seems to be about 14ohms. Trying to determine the power this bad boy can dissipate. I was pumping 24v at about 1.7 amps (40watts) for about thirty seconds. It was getting warm, not crazy hot, but quite warm.Any guesses? It makes a nice load resistor, but I want to know much load it will take.Thanks.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 13, 2016, 11:49:25 pm
content: We'll need to see pictures. But braided copper wire? Doesn't seem likely that it would be copper. There wouldn't be enough resistance. The plastic sleeve might be silicone. Is it a bit soft and squishy?Ed

author: TheMG
author_rank: Frequent Contributor
timestamp: May 14, 2016, 12:19:16 am
content: A picture is worth a thousand words.The plastic sleeve might just be heatshirnk tubing or something that someone added, or it might be a silicone coated resistor meant for relatively low power dissipation levels.Wires don't normally come pre-attached to power resistors.Again, hard to say without a picture. There should be some markings on the resistor with resistance value, part number, tolerance, and usually the power rating. Could be underneath the plastic if it's something someone added after the fact.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 01:59:53 am
content: I will take a pic. The thing is pretty old... 60s at the latest, possibly 40s or 50s. It is from an old building alarm system.The wire is braided, not just twisted, but braided. Pretty sure it is copper, due to the cyan corrosion on one area.There are no markings. The sleeve covers the wire directly. It was probably a custom part for this old alarm system...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2016, 02:11:46 am
content: About 100W.There should be no  copper involved, only alloys, and at that, only for the end terminals. The element is likely nichrome.Plastic doesn't sound right for the era. Are you sure it isn't vitreous enamel (ceramic glaze)?Tim

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 14, 2016, 03:20:15 am
content: Upon a closer inspection it is indeed ceramic glaze. It is also 12ohm.Here are some pics.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 14, 2016, 05:19:05 am
content: Yeah now that I see it, around 100W would be a safe bet.Note that such high power resistors are designed to withstand high temperatures and will get VERY hot during prolonged operation at or near the maximum power rating. More than hot enough to boil water. This is perfectly normal, so do make sure to keep anything that can be damaged by heat and any combustible materials away from the resistor when in use.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 14, 2016, 06:06:23 am
content: 100W is probably a conservative rating. Digikey shows 1.0" x 6.7" at 160W or 200W.One minor nuisance with yours is that it looks like the mounting points are also the electrical terminals. If so, make sure your mounting arrangement is electrically isolated and capable of taking lots of heat. Is that a bolt through the center? You might be able to work out a mounting arrangement by replacing the bolt with a threaded rod and a few nuts.Ed