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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
exploding wirewound resistors
Started by ●September 15, 2017
Reply by ●September 15, 20172017-09-15
John Larkin wrote on 9/15/2017 4:16 PM:> > > 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?I think that is 75 joules, no?> 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.Really? You need to borrow a $50 battery? -- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
Reply by ●September 15, 20172017-09-15
On Fri, 15 Sep 2017 16:24:39 -0400, rickman <gnuarm@gmail.com> wrote:>John Larkin wrote on 9/15/2017 4:16 PM: >> >> >> 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? > >I think that is 75 joules, no? >Right, 75. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 15, 20172017-09-15
No, not constant energy. A survey of datasheets shows about a P ~ t^(1/x) trend, for x = 2 or 3. At time scales short enough where only the wire is heating up, it's probably also short enough that skin effect is relevant, and so the trend continues (give or take a hiccup as it crosses over from thermal to skin-effect diffusion) down to very short timescales indeed (ns). There's also a sweet spot of energy capacity, with respect to resistance value. A high value wirewound has less metal than a small one, because the wire is thinner and the winding fill factor is the same. On the other hand, really small ones don't have a high fill factor (just a few turns), or a single wire link is used. If you need a shitton of energy, fast, there are bulk resistors. Ohmite makes some that are frequently stocked at the usual suspects, or the really big ones (often in puck form) are special order, from who knows where. On the other hand, if you have the pulse time and budget to do it, you can use a switching circuit to chop up the pulse and pump it into a capacitor (switching is necessary to implement an equivalent resistive input), then bleed that off over time. Uh, needless to say... this is more feasible in a controlled surge situation (say, inrush current limiter) than not (say, lightning). Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com "John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message news:8qcorcltubv26s40iuturktvar28ihrt7j@4ax.com...> > > 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. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com >
Reply by ●September 15, 20172017-09-15
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.The hard plastic case TH Ohmite's go pop when over stressed. Wear safety glasses, seriously, that plastic can fly. Cheers
Reply by ●September 16, 20172017-09-16
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. > >Use of an electrolytic / high voltage capacitor for energy source is the classical "WOW" method (and seem a preferred way also).
Reply by ●September 16, 20172017-09-16
On Fri, 15 Sep 2017 23:32:18 -0700, Robert Baer <robertbaer@localnet.com> wrote:>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. >> >> > Use of an electrolytic / high voltage capacitor for energy source is >the classical "WOW" method (and seem a preferred way also).Yeah, it looks like the best way to get hundreds of amps in the millisecond down time frame is to parallel a bunch of electrolytic caps. Maybe a sheet of copperclad FR4 with a lot of caps soldered or bolted to the connections on both sides. Where's my Dremel? I have a lot of 22,000uF 50V parts in stock, surplus from a discontinued product. Each one can store 27 joules. Even 100 J makes an impressive bang. A cap bank, and a pulsed series mosfet or so, maybe some current-limiting resistors, could explore the power-time destruction envelope of my wirewould resistors, or of other parts. Might be handy to have around. https://www.dropbox.com/s/z12fml3yy8n33lq/High_Current_Pulser.JPG?raw=1 More voltage might be good. I'd have to buy some caps. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●September 17, 20172017-09-17
On Sat, 16 Sep 2017 11:01:31 -0700, John Larkin wrote: Isn't such a sudden discharge likely to damage the caps? -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
Reply by ●September 17, 20172017-09-17
On Monday, September 18, 2017 at 12:21:52 AM UTC+10, Cursitor Doom wrote:> On Sat, 16 Sep 2017 11:01:31 -0700, John Larkin wrote: > > > Isn't such a sudden discharge likely to damage the caps?How? If you don't heat up the contents of the capacitor all that much, how is the sudden discharge going to mess anything up? -- Bill Sloman, Sydney
Reply by ●September 17, 20172017-09-17
On Sun, 17 Sep 2017 14:21:45 -0000 (UTC), Cursitor Doom <curd@notformail.com> wrote:>On Sat, 16 Sep 2017 11:01:31 -0700, John Larkin wrote: > > >Isn't such a sudden discharge likely to damage the caps?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. I'd have to use a lot of caps in parallel, to get enough coulombs and low ESR, so no one cap would see all the load current. A cap bank like this can magnetize anything ferrous. Just a few turns of fat wire, and a couple thousand amps, makes a lot of field. -- John Larkin Highland Technology, Inc lunatic fringe electronics