On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin 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.Long ago I had to fix a rack-mounted device that had failed. The failure had put enough current through a small leaded WW resistor that it glowed a fairly bright red. Though it had been running continuously for over a week, once powered down it measured within the resistance spec! Reliability testing for that kind of part mostly reflects the terminals. The cheaper swaged parts are considerably less reliable than the welded terminal parts. I don't know how the SMT parts are made, but you might want to investigate that.
exploding wirewound resistors
Started by ●September 15, 2017
Reply by ●September 18, 20172017-09-18
Reply by ●September 18, 20172017-09-18
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message news:g9jurc10cn36es7msu75e50iqgbh92f9fa@4ax.com...> Did you notice, on my sketch, the parts labeled CURRENT LIMITERS ? > >> >>And the manufacturer, let's not leave them out. Y'think they might know a >>thing or two about their product? > > Why are you being such a smarmy jerk? >Why are you conflating CURRENT LIMITERS with screwdrivers? "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message news:617trchvnvaprp8rhqf0lnucs3dnb5fveh@4ax.com...> I don't think so. You can fully charge an electrolytic cap and then > short it with a screwdriver (or a flashtube) and it doesn't seem to > mind.You wrote this, didn't you? Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●September 18, 20172017-09-18
Frank Miles <frankm@wanprc.org> writes:> On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin 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. > > Long ago I had to fix a rack-mounted device that had failed. The failure > had put enough current through a small leaded WW resistor that it glowed > a fairly bright red. Though it had been running continuously for over a > week, once powered down it measured within the resistance spec! > > Reliability testing for that kind of part mostly reflects the terminals. > The cheaper swaged parts are considerably less reliable than the welded > terminal parts. I don't know how the SMT parts are made, but you might > want to investigate that.Those green "vitreous enamelled" vishay wirewounds were pretty impressive. I remember putting them across car batteries until the coils glowed yellow through the molten glass. Never seemed to do them much harm :) -- John Devereux
Reply by ●September 18, 20172017-09-18
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.Most resistors that are joule rated, including wirewounds, are spec'd handle X joules for 1 second, and drop off hard for shorter durations. Some level off at some short time, and some keep tapering off forever as the times get shorter. We found a Vishay 3-watt part, WSN6927 at 0.33 ohms, that specs 75 j from 1 to 10 seconds, and 10j from 0 to 100 ms, with a slope in the gap. That makes our SSR circuit work. I'll get some and blow them up to see how much margin we'll have. Maybe pulse them for a few months to see if they fatigue. We'll put eight (!) ADCs on the board to measure channel ON voltage 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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 18, 20172017-09-18
On Mon, 18 Sep 2017 19:11:04 +0100, John Devereux <john@devereux.me.uk> wrote:>Frank Miles <frankm@wanprc.org> writes: > >> On Fri, 15 Sep 2017 13:16:39 -0700, John Larkin 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. >> >> Long ago I had to fix a rack-mounted device that had failed. The failure >> had put enough current through a small leaded WW resistor that it glowed >> a fairly bright red. Though it had been running continuously for over a >> week, once powered down it measured within the resistance spec! >> >> Reliability testing for that kind of part mostly reflects the terminals. >> The cheaper swaged parts are considerably less reliable than the welded >> terminal parts. I don't know how the SMT parts are made, but you might >> want to investigate that. > >Those green "vitreous enamelled" vishay wirewounds were pretty >impressive. I remember putting them across car batteries until the coils >glowed yellow through the molten glass. Never seemed to do them much >harm :)Eventually the glass melts and drips off. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 18, 20172017-09-18
>Most resistors that are joule rated, including wirewounds, are spec'd >handle X joules for 1 second, and drop off hard for shorter durations. >Some level off at some short time, and some keep tapering off forever >as the times get shorter.Ceramic has a high specific heat but very poor thermal diffusivity compared to metal. At short times all you have to work with is the metal itself. Cheers Phil Hobbs
Reply by ●September 18, 20172017-09-18
On Mon, 18 Sep 2017 15:35:01 -0700 (PDT), pcdhobbs@gmail.com wrote:>>Most resistors that are joule rated, including wirewounds, are spec'd >>handle X joules for 1 second, and drop off hard for shorter durations. >>Some level off at some short time, and some keep tapering off forever >>as the times get shorter. > >Ceramic has a high specific heat but very poor thermal diffusivity compared to metal. At short times all you have to work with is the metal itself. > >Cheers > >Phil HobbsThere are some impressive bulk-ceramic resistors that will absorb hundreds of joules in any time domain, but they are gigantic leaded parts. Wouldn't fit on a VME board and bad for vibration. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 18, 20172017-09-18
Den mandag den 18. september 2017 kl. 21.30.52 UTC+2 skrev John Larkin:> 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. > > Most resistors that are joule rated, including wirewounds, are spec'd > handle X joules for 1 second, and drop off hard for shorter durations. > Some level off at some short time, and some keep tapering off forever > as the times get shorter. > > We found a Vishay 3-watt part, WSN6927 at 0.33 ohms, that specs 75 j > from 1 to 10 seconds, and 10j from 0 to 100 ms, with a slope in the > gap. That makes our SSR circuit work. I'll get some and blow them up > to see how much margin we'll have. Maybe pulse them for a few months > to see if they fatigue. > > We'll put eight (!) ADCs on the board to measure channel ON voltage > 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 about a IR2127 to do the over current shutdown, seems like it would be cheaper than that fancy resistor http://www.datasheetdir.com/circuits/200/Using-The-Current-Sensing-Ir2125-Ir2127-Ir2128-Gate-Driver-Ics.jpg
Reply by ●September 18, 20172017-09-18
>There are some impressive bulk-ceramic resistors that will absorb >hundreds of joules in any time domain, but they are gigantic leaded >parts.A resistor of uniform cross-section will dissipate heat uniformly, if it's reasonably small. I have data showing this happening in ~10 ps time scales. It's only thermal conduction that's slow. Cheers Phil Hobbs
Reply by ●September 18, 20172017-09-18
On Monday, September 18, 2017 at 6:55:28 PM UTC-4, John Larkin wrote:> On Mon, 18 Sep 2017 15:35:01 -0700 (PDT), pcdhobbs@gmail.com wrote: > > >>Most resistors that are joule rated, including wirewounds, are spec'd > >>handle X joules for 1 second, and drop off hard for shorter durations. > >>Some level off at some short time, and some keep tapering off forever > >>as the times get shorter. > > > >Ceramic has a high specific heat but very poor thermal diffusivity compared to metal. At short times all you have to work with is the metal itself. > > > >Cheers > > > >Phil Hobbs > > There are some impressive bulk-ceramic resistors that will absorb > hundreds of joules in any time domain, but they are gigantic leaded > parts. Wouldn't fit on a VME board and bad for vibration. > >Carbon composite? or is that too noisy? George H.> -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
