How hot is too hot for a resistor ? - Page 1
Forum section: electronics / beginners

author: ez24
author_rank: Super Contributor
timestamp: May 15, 2015, 04:40:34 am
content: HiI used 2 - 8 ohm 100 watt resistors in parallel at 4.3 amps at about 19 v (laptop power supply) = about 74 wattsI did not use them long because the temp of the resistors got to 200F very fastSo I was wondering how hot should I allow a metal 100 watt chinese aluminum (with small fins) power resistor get to ? I was using them at 50 watts (2 - 100 watters in parallel) , but I do not know how hot I should allow them to get toSimilar tohttp://www.ebay.com/itm/1PCS-High-Power-1-0-Ohm-1R-100W-Watt-Aluminum-Shell-Case-Wirewound-Resistor-5-/381091958208?pt=LH_DefaultDomain_0&hash=item58bad9cdc0Anyone know hot is acceptable for a 100 watt Chinese metal power resistor? This would be without a fan.thanks

author: coppice
author_rank: Super Contributor
timestamp: May 15, 2015, 05:04:08 am
content: How big a heat sink is it attached to? Without a fan you need a pretty big sink to get rid of 100W, if you want things to stay cool. As for maximum temperature, that should be part of the device's spec. Some of the all ceramic designs can be run like an electric fire, with the resistive element glowing red. It reduces the lifetime, which may or may not matter to you. The resistance will also change quite a bit, which also may or may not matter to you.

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: May 15, 2015, 07:56:07 am
content: i would try high quality power resistors rather than crappy eBay Chinese resistors

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 15, 2015, 08:25:43 am
content: Quote from: ez24 on May 15, 2015, 04:40:34 amMay i ask about the application were you need this?There might be a easy solution to your problem, slim change but might.

author: tron9000
author_rank: Frequent Contributor
timestamp: May 15, 2015, 08:27:36 am
content: http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0188087/?searchTerm=HS100+1R+J&relevancy-data=636F3D3126696E3D4931384E4272616E644D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5B5C707B4C7D5C707B4E647D5C707B5A737D2D2C2F255C2E5D2B2426706F3D3526736E3D592673743D4B4559574F52445F4D554C54495F414C5048415F4E554D455249432677633D4E4F4E45267573743D4853313030203152204A26&sra=pThe resistor above gives you some quick specs, though for you ebay special I'm not sure if they'd be identical, but how hard is it to copy a 100W alu clad resistor?this one has 1.0degC/W, and a max operating temp of -55 to 200 deg C36.98W per resistor, call it 37Wtemp rise above ambient at that power per resistor = 37deg C = 1.0degC/W * 37Wcall ambient 24degC therefore surface of resistor = 61degCHowever your getting 93.33DegC? which means your chinese knock off probably has a worse thermal resistance than that above. Seems my question has been answered - guess it is hard to copy a 100W resistorI would not exceed 150DegC (302F) for this one. I wouldn't trust it seen as there are no specs given on thermal resistance. If in doubt, bolt it to a big chunk of metal! But for the extra couple of bucks, I'd buy the branded ones from a reputable supplier

author: coppice
author_rank: Super Contributor
timestamp: May 15, 2015, 08:33:42 am
content: Quote from: tron9000 on May 15, 2015, 08:27:36 amIts doesn't have 1 deg C/W. They say its only rated for 100W when mounted on a 1 deg C/W heat sink. Few of these power resistors are designed to work at their rating without being bolted to a substantial heatsink.

author: wraper
author_rank: Unknown
timestamp: May 15, 2015, 08:34:33 am
content: Quote from: tron9000 on May 15, 2015, 08:27:36 amWhere did you get this 1.0degC/W from? All what written there is:QuoteRead: it needs 1.0°C/W heatsink to operate at full power with 25°C ambient temperature.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2015, 09:18:03 am
content: Metal case resistors are potted with epoxy or something like that. 150C max internal temperature. They have pitiful ratings when used without a huge heatsink.Ceramic core (silicone and vitreous enamel coated) resistors are capable of operating at incendiary temperatures, at least for short periods (say, a few minutes). Ratings of 225C and up are typical. The high surface temperature gives them a much smaller profile for the same power dissipation, and they're usually not much more expensive.Tim

author: tron9000
author_rank: Frequent Contributor
timestamp: May 15, 2015, 09:39:52 am
content: Quote from: wraper on May 15, 2015, 08:34:33 ammy bad. Still it says not to exceed 200degC (392F) in the datasheet. plus give him something to think about if and when he does decide to bolt to a heatsink.

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 15, 2015, 10:47:26 am
content: Ok... i will just say it.If you only need the resistor as a dummy load for test purposes, or just for a short time, a bucket/bottle of water and you old toaster will make a great 500W power resistor with no danger of overheating.Rip the heating wire out of your toaster, stuff if in the bucket till you reach the right resistance and pour water on that.Your lead wires have to connect to the resistance wire under water, if you feel fancy make yourself a jig out of some old wood to wrap the wire around so it will not shift and change resistance.

author: Psi
author_rank: Super Contributor
timestamp: May 15, 2015, 10:55:28 am
content: QuoteIf you hear any sizzling when touching it with a wet finger, its too hot.If you smell burning, its too hotIf you see flames, its too hotIf you hear fire trucks, its too late.

author: tron9000
author_rank: Frequent Contributor
timestamp: May 15, 2015, 11:32:02 am
content: or just solder some wire to it and when the solder melts again - then its too hot!

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 15, 2015, 11:58:02 am
content: Quote from: tron9000 on May 15, 2015, 11:32:02 amYes, just like this:

author: tron9000
author_rank: Frequent Contributor
timestamp: May 15, 2015, 12:15:29 pm
content: Quote from: PA0PBZ on May 15, 2015, 11:58:02 amIs that a sort of thermal cutout? like a bi-metallic strip?

author: bktemp
author_rank: Super Contributor
timestamp: May 15, 2015, 12:23:26 pm
content: Quote from: tron9000 on May 15, 2015, 12:15:29 pmThe solder at the top end works as a thermal fuse: When it melts, the springy contact moves away and disconnects as depicted in the drawing.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 15, 2015, 12:23:47 pm
content: Quote from: tron9000 on May 15, 2015, 12:15:29 pmNo, it is just the solder melting and the spring action. Found a lot in old color TV's, and fixable with a soldering iron.However, I have no idea if special solder/low melting point was used. Fixed a lot of them in the 80's.

author: tron9000
author_rank: Frequent Contributor
timestamp: May 15, 2015, 12:26:49 pm
content: never seen them like that before, I suppose the type of solder would dictate the trip temp. I suppose not many around now: limited applications, possible RoHS?

author: SeanB
author_rank: Super Contributor
timestamp: May 15, 2015, 04:44:02 pm
content: Was common until fusible resistor technology advanced to the point where you could get a unit that would reliably ( as in more than 99.9% would) handle the rated power, yet would fail open circuit with no or minimal smoke emission with about 3 times the power, but with a lifetime that would give years of service at rated power.As to the OP, you MUSt run a metal clad resistor on a heatsink ( and a pretty big one for high power ones) to get rated power. Otherwise you are derated to about 10% of the rating, for the 100W unit it is about 15W in free air without a heatsink.