On Thu, 21 Sep 2017 01:12:07 -0700 (PDT), tabbypurr@gmail.com wrote:>On Thursday, 21 September 2017 04:16:01 UTC+1, John Larkin wrote: >> On Wed, 20 Sep 2017 08:57:59 -0700 (PDT), Lasse Langwadt Christensen >> <langwadt@fonz.dk> wrote: >> >Den onsdag den 20. september 2017 kl. 17.23.27 UTC+2 skrev John Larkin: >> >> On Wed, 20 Sep 2017 06:37:09 -0400, Phil Hobbs >> >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 09/19/2017 05:55 PM, John Larkin wrote: > >> >> >> We got some TE 3-watt parts, 0.33R, SMW3 series. I kept increasing >> >> >> joules per shot. At 10.8J, the resistor went POP but kept working. The >> >> >> case has a small crack on top and little puffs of smoke came out every >> >> >> pulse. >> >> >> >> >> >> It seems to have run out of smoke now. Still works. >> >> > >> >> >There's also non-magic smoke. >> >> > >> >> >Cheers >> >> > >> >> >Phil Hobbs >> >> >> >> I'm conjecturing that the nichrome element is now nestled in a little >> >> coccoon of carbon, not much left to boil out. >> >> >> >> I need a MELF wirewound, exposed wire on the outside and ceramic >> >> inside. >> >> >> > >> >http://www.firstohm.com.tw/phocadownloadpap/01_resistors/SWM20160721.pdf >> >> I emailed them about joule rating, or samples to test. >> >> The coating looks organic. Maybe they would sell me uncoated ones. > >Then you'd get damp ingress, corrosion etc. You need to go back to the 1920s for uncoated resistors. > > >NT0.33 ohms of ceramic and nichrome shouldn't mind a little humidity. -- John Larkin Highland Technology, Inc lunatic fringe electronics
exploding wirewound resistors
Started by ●September 15, 2017
Reply by ●September 21, 20172017-09-21
Reply by ●September 21, 20172017-09-21
On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin <jjlarkin@highland_snip_technology.com> wrote:> > >Lots of people make parts like this: > >https://www.digikey.com/product-detail/en/bourns-inc/PWR5322WR250JE/PWR5322WR250JETR-ND/2022962 > >The overload specs are almost always 5x rated power for 5 seconds, so >a 3 watt part can absorb 15 joules in that time. I wonder what happens >for shorter time overloads? Constant joules? > >Has anyone experimented with stressing this sort of small wirewound >resistor? > >I'll probably order some and blow them up. We'll be using low ohms, >0.2 maybe, so we'll need a lot of current. There's a car repair place >and a motorcycle shop on our block, so maybe I could buy or borrow a >battery. For pure fast joules, I could just discharge some big >electrolytics.OK, I give up on surface mount. I blew the top off the 5-watt power-pad Vishay at 20 joules. The gianytish TE 5-watt part failed overnight at 0.05Hz and 21 joules per shot. It blew a tiny hole in the side and shot some goo out. My production folks will grudgingly allow me to use an axial leaded part; I'll buy them some cookies or something. I tried a 0.1R 3W axial wirewound; it worked for a while at 40J but made a smellable event every shot; it failed open at 50J. I've ordered some vitreous enameled parts to try (and bigger electrolytics!) Maybe the glass coating will hold the molten nichrome in place, or at least conduct heat a little better than the smelly organic coating. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 21, 20172017-09-21
On Thursday, 21 September 2017 18:58:38 UTC+1, John Larkin wrote:> On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin > <jjlarkin@highland_snip_technology.com> wrote: > > > > > > >Lots of people make parts like this: > > > >https://www.digikey.com/product-detail/en/bourns-inc/PWR5322WR250JE/PWR5322WR250JETR-ND/2022962 > > > >The overload specs are almost always 5x rated power for 5 seconds, so > >a 3 watt part can absorb 15 joules in that time. I wonder what happens > >for shorter time overloads? Constant joules? > > > >Has anyone experimented with stressing this sort of small wirewound > >resistor? > > > >I'll probably order some and blow them up. We'll be using low ohms, > >0.2 maybe, so we'll need a lot of current. There's a car repair place > >and a motorcycle shop on our block, so maybe I could buy or borrow a > >battery. For pure fast joules, I could just discharge some big > >electrolytics. > > OK, I give up on surface mount. I blew the top off the 5-watt > power-pad Vishay at 20 joules. The gianytish TE 5-watt part failed > overnight at 0.05Hz and 21 joules per shot. It blew a tiny hole in the > side and shot some goo out. > > My production folks will grudgingly allow me to use an axial leaded > part; I'll buy them some cookies or something. I tried a 0.1R 3W axial > wirewound; it worked for a while at 40J but made a smellable event > every shot; it failed open at 50J. I've ordered some vitreous enameled > parts to try (and bigger electrolytics!) Maybe the glass coating will > hold the molten nichrome in place, or at least conduct heat a little > better than the smelly organic coating.Why not use SMs in parallel? NT
Reply by ●September 21, 20172017-09-21
On Thursday, 21 September 2017 15:23:26 UTC+1, John Larkin wrote:> On Thu, 21 Sep 2017 01:12:07 -0700 (PDT), tabbypurr wrote: > >On Thursday, 21 September 2017 04:16:01 UTC+1, John Larkin wrote:> >> The coating looks organic. Maybe they would sell me uncoated ones. > > > >Then you'd get damp ingress, corrosion etc. You need to go back to the 1920s for uncoated resistors. > > 0.33 ohms of ceramic and nichrome shouldn't mind a little humidity.No. The higher the R the more vulnerable it is. NT
Reply by ●September 21, 20172017-09-21
On Thu, 21 Sep 2017 11:38:32 -0700 (PDT), tabbypurr@gmail.com wrote:>On Thursday, 21 September 2017 18:58:38 UTC+1, John Larkin wrote: >> On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin >> <jjlarkin@highland_snip_technology.com> wrote: >> >> > >> > >> >Lots of people make parts like this: >> > >> >https://www.digikey.com/product-detail/en/bourns-inc/PWR5322WR250JE/PWR5322WR250JETR-ND/2022962 >> > >> >The overload specs are almost always 5x rated power for 5 seconds, so >> >a 3 watt part can absorb 15 joules in that time. I wonder what happens >> >for shorter time overloads? Constant joules? >> > >> >Has anyone experimented with stressing this sort of small wirewound >> >resistor? >> > >> >I'll probably order some and blow them up. We'll be using low ohms, >> >0.2 maybe, so we'll need a lot of current. There's a car repair place >> >and a motorcycle shop on our block, so maybe I could buy or borrow a >> >battery. For pure fast joules, I could just discharge some big >> >electrolytics. >> >> OK, I give up on surface mount. I blew the top off the 5-watt >> power-pad Vishay at 20 joules. The gianytish TE 5-watt part failed >> overnight at 0.05Hz and 21 joules per shot. It blew a tiny hole in the >> side and shot some goo out. >> >> My production folks will grudgingly allow me to use an axial leaded >> part; I'll buy them some cookies or something. I tried a 0.1R 3W axial >> wirewound; it worked for a while at 40J but made a smellable event >> every shot; it failed open at 50J. I've ordered some vitreous enameled >> parts to try (and bigger electrolytics!) Maybe the glass coating will >> hold the molten nichrome in place, or at least conduct heat a little >> better than the smelly organic coating. > >Why not use SMs in parallel? > > >NTEach SSR channel will get 0.85 square inches on the board, and that allows two DPAKS, the gate drive transformer, and the wirewound resistor on the top side. We'll put the small stuff on the bottom. It's a VME module, so one has to be careful about putting parts on the bottom side, specifically nothing tall or easy to shear off. I don't think I could fit many joules of thickfilm resistors in the available board area. I'm spiking an axial wirewound hot each shot, and it has a lot of mass. I don't know what a, say, 1206 could absorb, but I'd guess millijoules. Does anybody know? -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 21, 20172017-09-21
On Thursday, September 21, 2017 at 7:23:26 AM UTC-7, John Larkin wrote:> On Thu, 21 Sep 2017 01:12:07 -0700 (PDT), tabbypurr@gmail.com wrote:> >> The coating looks organic. Maybe they would sell me uncoated ones. > > > >Then you'd get damp ingress, corrosion etc. You need to go back to the 1920s for uncoated resistors.> 0.33 ohms of ceramic and nichrome shouldn't mind a little humidity.There's a nichrome-to-other-metal weld, and an other-metal-to-copper wire bond, so there's dissimilar metals involved; a little humidity can be a problem, eventually. Better battery packs are assembled with nickel straps, 'cuz the nickel-to-nickel spot welds aren't subject to corrosion due to dissimilar metals.
Reply by ●September 21, 20172017-09-21
"whit3rd" <whit3rd@gmail.com> wrote in message news:1df0ebcf-71e7-40c2-8587-ed6876599a86@googlegroups.com...> There's a nichrome-to-other-metal weld, and an other-metal-to-copper wire > bond, > so there's dissimilar metals involved; a little humidity can be a problem, > eventually. > > Better battery packs are assembled with nickel straps, 'cuz the > nickel-to-nickel > spot welds aren't subject to corrosion due to dissimilar metals.I have some open-frame wirewound (almost rod-wound) resistors with corrosion. Green spots. Like you sometimes see on ruddy old chrome bathroom fixtures. (It's the color of chrome oxide, of course.) Although, to be fair, those resistors did survive the numerous chemical experiments I did in high school. HCl fumes are bad for a lot of metals, in fact; who'd have thought? :-) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●September 22, 20172017-09-22
On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin <jjlarkin@highland_snip_technology.com> wrote:> > >Lots of people make parts like this: > >https://www.digikey.com/product-detail/en/bourns-inc/PWR5322WR250JE/PWR5322WR250JETR-ND/2022962 > >The overload specs are almost always 5x rated power for 5 seconds, so >a 3 watt part can absorb 15 joules in that time. I wonder what happens >for shorter time overloads? Constant joules? > >Has anyone experimented with stressing this sort of small wirewound >resistor? > >I'll probably order some and blow them up. We'll be using low ohms, >0.2 maybe, so we'll need a lot of current. There's a car repair place >and a motorcycle shop on our block, so maybe I could buy or borrow a >battery. For pure fast joules, I could just discharge some big >electrolytics.Well, my theory about the virtues of vitreus-enamel wirewound resistors is literally exploded. One thing we have learned is that if you want to survive pulses, buy 5% or 10% parts. The 1% wirewounds are trimmed somehow, probably grinding, so have weak spots in the wire. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 22, 20172017-09-22
On a sunny day (Fri, 22 Sep 2017 12:38:51 -0700) it happened John Larkin <jjlarkin@highland_snip_technology.com> wrote in <l7pascpqsc0ms74ihtadn05l86oboeso68@4ax.com>: Well, my theory about the virtues of vitreus-enamel wirewound>resistors is literally exploded. > >One thing we have learned is that if you want to survive pulses, buy >5% or 10% parts. The 1% wirewounds are trimmed somehow, probably >grinding, so have weak spots in the wire.In the old audio power amp days wound those emitter resistors myself on a high value carbon one.
Reply by ●September 24, 20172017-09-24
On 18/09/2017 20:30, John Larkin wrote:> On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin > <jjlarkin@highland_snip_technology.com> wrote: > drops fast, to shut things down quick if a customer puts a hard > voltage across one of our SSRs. The series resistor buys us enough > time to do that. >How fast will you switch off the fets? If too fast then any external series inductance could overvolt the fets, or are you placing transzorbs/movs across the SSR? piglet
