Be careful with wire-wound resistors - Page 1
Forum section: electronics / general

author: armandas
author_rank: Frequent Contributor
timestamp: May 02, 2012, 04:01:16 pm
content: Until today, I had no idea these resistors can explode violently when grossly overloaded.I was doing some testing today, where I put 2A at 24V across two 24R 10W resistors in parallel. I put them on a big heatsink and while they were getting hot, they were doing quite well. Then someone came and messed with the PSU thereby putting 5A at 60V through the two buggers. In a matter of seconds they exploded (with a flash) and emitted nasty-smelling smoke.PS: don't mind the mounting hole, that was done by me

author: Ajahn Lambda
author_rank: Regular Contributor
timestamp: May 02, 2012, 05:42:19 pm
content: Nice work.Given enough oomph, almost anything will go BANG!Did you manage to at least punch the person responsible for overloading the resistors?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 02, 2012, 05:45:40 pm
content: Quotechange "resistors" to "almost anything"

author: armandas
author_rank: Frequent Contributor
timestamp: May 02, 2012, 06:20:07 pm
content: Quote from: SigEpBlue on May 02, 2012, 05:42:19 pmQuote from: Mechatrommer on May 02, 2012, 05:45:40 pmSurely, normal 1/4W resistors would just melt? I'll try this tomorrowQuote from: SigEpBlue on May 02, 2012, 05:42:19 pmThat wouldn't be good for my career

author: IanB
author_rank: Super Contributor
timestamp: May 02, 2012, 06:24:39 pm
content: Quote from: armandas on May 02, 2012, 04:01:16 pmYou should obviously beat the person who fiddled with your power supply with a big stick (except if it was a child you should beat yourself with a big stick for leaving the apparatus unattended).But I don't get your numbers. You have some redundancy here. If you put 24 V across a 24 ohm resistor then 1 A will flow and the power dissipation will be 24 W. This is rather in excess of the 10 W rating. Of course if you put two resistors in parallel then the total current will be 2 A but each resistor individually will still exceed its power rating by a large margin.A good rule of thumb is to de-rate by 50%, so for a 10 W resistor don't rely on dissipating more than about 5 W continuously. That will give you a nice margin for lack of air circulation, higher ambient temperatures, radiation blocking and reduced stress on the component.

author: SeanB
author_rank: Super Contributor
timestamp: May 02, 2012, 06:34:49 pm
content: 1/4 W resistors just smoke and flame out, some glow red for a few seconds during this, but some just pop with a few brief flames. Carbon composition resistors, on the other hand, actually burn nicely as the wax used in the mix and cover burns. Wirewound white ceramic boats glow white before cracking.

author: armandas
author_rank: Frequent Contributor
timestamp: May 02, 2012, 08:20:42 pm
content: Quote from: IanB on May 02, 2012, 06:24:39 pmNot quite sure what numbers you don't get. I actually started with just a single resistor and was putting 24W through it. The second resistor came it when I wanted to double the current.Quote from: IanB on May 02, 2012, 06:24:39 pmI'm sure it's a good rule for production, but in this case, these resistors were just a part of a test device. I reckoned that with a heatsink, I could run them at 2-3 times the power rating for a while. I'm not saying that what I done was good in any way, the heatsink was pretty big and even that was getting warm after 10 or so minutes.

author: SeanB
author_rank: Super Contributor
timestamp: May 02, 2012, 08:41:09 pm
content: Best way for short term overloading is to drop them in a small pyrex dish filled around 3/4 with water. Works well and you can go to extreme overloads, provided the case does not boil the water too much. Of course low voltage and short term only.

author: IanB
author_rank: Super Contributor
timestamp: May 02, 2012, 08:44:08 pm
content: Quote from: armandas on May 02, 2012, 08:20:42 pmI eventually figured out what you were trying to say, but it was your use of words that was the problem. You described things in an imprecise way with words used out of context so I had to pause and scratch my head for a bit to figure out what you meant.For example, you said that you "put 2A at 24V across two 24R 10W resistors in parallel". But it doesn't make sense to specify the currentandthe voltageandthe resistance at the same time since one of them is fixed by the other two. This leads to a double-take and moment of confusion while trying to figure out what is going on.For instance you can put 24 Vacrossa 24 ohm resistor (in which case the current is known to be 1 A).Or you can feed 1 Athrougha 24 ohm resistor (in which case the voltage is known to be 24 V). When you feed current through something exactly the same current goes in as comes out.Or you can feed 24 Wintoa 24 ohm resistor (in which case we know both the voltage and the current). When you feed power into something it gets absorbed or transformed, it doesn't come out the other side.It may seem like I'm being pedantic, and maybe you will say that I am, but precise use of words and giving just the right amount of information helps tremendously with understanding. Giving too much information can often be as puzzling as giving too little information.

author: T4P
author_rank: Super Contributor
timestamp: May 02, 2012, 09:17:04 pm
content: I don't actually get anything , unless the voltage was 10V 1Amp and 10R resistance then will you hit max rated dissipation ...You put 24V 1amps into a 24R 10W resistor ... that's not 10watts that's 24watts.By the way is the 10W rating open case? If it was on a heatsink i understand what you are getting at, which i just read upwards.But yeah 60V @ 5Amps means 2.5amps per resistor and that is 150W , any 10W resistors will blow it's guts out at that sort of power dissipation .That's was the max of your power supply right ?But anyway, i am pretty stoked to see a metal case wirewound spit out it's guts ...

author: wkb
author_rank: Super Contributor
timestamp: May 02, 2012, 09:43:43 pm
content: Quote from: SeanB on May 02, 2012, 06:34:49 pmTry a 12V halogen car headlight bulb when discharging a reasonably size flash capacitor (Metz 45CT hammerhead flash for the curious).. The very robust crystal(?) glass of the bulb simply exploded in tiny fragments.. with an impressive BANG.. wow..Next time I used some mains bulbs :-) instead of a random grab from what was on the table available and what I assumed to be a 220V bulb...

author: IanB
author_rank: Super Contributor
timestamp: May 02, 2012, 10:54:11 pm
content: Quote from: wkb on May 02, 2012, 09:43:43 pmLet's see now. A 12 V 60 W bulb has a filament resistance of 2.4 ohms at working temperature. By the normal rule of thumb that would be about 0.25 ohms when cold. If we apply 300 V across 0.25 ohms we get an instantaneous power dissipation of 300^2/0.25 = 360 000 W. So you just turned a 60 W bulb into a 360 kW bulb. No wonder it exploded...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 02, 2012, 11:08:58 pm
content: Been there, done that!It's tempting to assume that this type of resistor can tolerate a huge dissipation for a short time due to the aluminium, but not the case.It can get even more fun - in my case I was using one to ballast a big argon laser, the end shot off, trailing the resistance wire begin it like a taser. Just to add to the fun it had about 340V of rectified mains on it.....

author: Psi
author_rank: Super Contributor
timestamp: May 03, 2012, 01:12:45 am
content: Quote from: mikeselectricstuff on May 02, 2012, 11:08:58 pmhehe, that sounds both awesome and scarey

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 03, 2012, 07:30:29 am
content: Quote from: armandas on May 02, 2012, 06:20:07 pmtry 10ohm 1/4W on mains live to neutral. wear eyeprotection beforehand.

author: SeanB
author_rank: Super Contributor
timestamp: May 03, 2012, 08:10:59 pm
content: Try putting a fluorescent starter across the mains. Never did find the case, glass or capacitor.......The dog was looking funny at me for days........ I did have the bakelite base though, with some rather melted pins still attached.

author: aargee
author_rank: Frequent Contributor
timestamp: May 03, 2012, 09:39:12 pm
content: I've never seen inside these 'heatsink' power resistors before, so it relies on air conduction to the case to dissipate the heat? Or is there some intermediary (oil like) between the resistance element and the case?

author: amyk
author_rank: Super Contributor
timestamp: May 04, 2012, 07:07:08 am
content: It blew open probably because of the pressure generated by the heating. If it was vented it would just get hotter until it fuses or glows brightly (or both, usually the latter followed by the former).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 04, 2012, 08:30:34 am
content: Quote from: aargee on May 03, 2012, 09:39:12 pmIt's some kind of solid potting compound.

author: robrenz
author_rank: Super Contributor
timestamp: May 04, 2012, 11:35:42 am
content: In a reasonably high quality resistor it should be a high thermal conductivity ceramic potting compound like these.The 810 in thishttp://www.cotronics.com/vo/cotr/pdf/801.pdforhttp://www.sauereisen.com/adhesives/sensors_resistors.aspxorhttp://www.aremco.com/product/a4/

author: T4P
author_rank: Super Contributor
timestamp: May 04, 2012, 01:18:51 pm
content: Quote from: SeanB on May 03, 2012, 08:10:59 pmClassic inductive storage or a electronic starter ?

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2012, 03:27:03 pm
content: Quote from: Dave.S on May 04, 2012, 01:18:51 pmI would guess the traditional gas discharge tube. As soon as it switches on the contacts will close and make a short circuit. A short circuit across the mains is nearly always exciting.

author: metalphreak
author_rank: Frequent Contributor
timestamp: May 04, 2012, 04:44:20 pm
content: I remember seeing a video a long while ago where someone made a video of "blowing up leds". He was putting a not-too-outrageous voltage through it (but clearly well past what its rated to) and the top part of the led blew off straight towards the camera, giving him quite a scare

author: Ajahn Lambda
author_rank: Regular Contributor
timestamp: May 04, 2012, 05:06:10 pm
content: Hehe, yes, it's always the shrapnel that scares me, not the fire and smoke.

author: T4P
author_rank: Super Contributor
timestamp: May 04, 2012, 06:34:03 pm
content: Quote from: IanB on May 04, 2012, 03:27:03 pmRight i just remembered , it's that starter one uses with preheat fixtures?

author: SeanB
author_rank: Super Contributor
timestamp: May 04, 2012, 07:10:26 pm
content: Yes preheat bipin starter.Best for a bang is a glass frit sealed tantalum solid slug capacitor.

author: nanofrog
author_rank: Super Contributor
timestamp: May 04, 2012, 07:19:45 pm
content: Quote from: mikeselectricstuff on May 02, 2012, 11:08:58 pmShame you weren't able to make a vid and post it.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: May 04, 2012, 07:31:40 pm
content: One of my fingerprints is permanently modified due to touching one of these resistors under load.

author: xavier60
author_rank: Super Contributor
timestamp: May 15, 2018, 12:57:01 pm
content: A while back, I was testing the fusing characteristics of some 39 ohm 3W metal oxide resistors I got on ebay. When I put them directly across 240VAC mains, they would explode but with not much flash, it was the ceramic rod that was violently shattering.I have just found this photo of a failed resistor in the mains input circuitry of a clothes dryer control PCB. I notice that the ceramic rod has also shattered.I'm wondering if this an intentional characteristic that causes the fault current to be quickly interrupted rather than sustaining an arc?https://www.mikrocontroller.net/attachment/277685/DSC02293.JPG

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2018, 01:32:13 pm
content: Quote from: armandas on May 02, 2012, 04:01:16 pmIt's happened to me before.The problem is, when these types of resistors overheat, the organic compounds undergo pyrolysis, releasing various flammable, toxic, nasty gases, which can't escape. The pressure builds, until the casing can't take it and they're released with a bang. In the most extreme cases the gasses ignite, resulting in a bright flame, which only lasts for under a second.

author: CopperCone
author_rank: Super Contributor
timestamp: May 16, 2018, 02:08:12 am
content: how come they dont use refractory powders as an insulator and thermal transfer medium like a cartridge heater?

author: vk6zgo
author_rank: Super Contributor
timestamp: May 16, 2018, 02:46:55 am
content: My best effort was many years ago, in my first job.We had some valve ( tube ) type "radiogram" chassis that the Boss decided to sell off cheap.Yours truly got the job of testing them.I looked at them, & found a length of "figure 8" cable coming out of one side, which I thought was the loudspeaker connection.I duly connected this up to a 12" speaker. & turned on the radio.There was a sound like a heavy calibre rifle shot, then silence.People appeared from everywhere!It turned out that length of "figure 8" was to supply mains to the record player when the radio was switched on.The speaker showed no external signs of its trauma, but the voice coil was open circuit.Amazingly, I didn't get the sack!

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2018, 12:15:05 pm
content: Quote from: CopperCone on May 16, 2018, 02:08:12 amI don't know, cost? The thermal expansion coefficient of the materials would have to be matched or course.Quote from: vk6zgo on May 16, 2018, 02:46:55 amI don't see why they would sack you for that. The speaker couldn't have cost that much to replace and I'm sure people have made more expensive mistakes than that.

author: woodchips
author_rank: Frequent Contributor
timestamp: May 16, 2018, 12:43:20 pm
content: As has been mentioned, these aluminium cased power resistors are hopeless at overloads.This is why I always use the wire wound and vitreous enamel covered resistors, they die gracefully, still hot but gives you a second or two to turn it off!