Big crowbar module for <4VDC?
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From: Joerg on 19 Dec 2008 20:34 Chris Jones wrote: > Joerg wrote: > >> Jim Thompson wrote: >>> On Sat, 20 Dec 2008 00:03:42 +0000, Chris Jones <lugnut808(a)yahoo.com> >>> wrote: >>> >>>> Jim Thompson wrote: >>>> >>>>> On Thu, 18 Dec 2008 16:10:36 -0800 (PST), Tim Williams >>>>> <tmoranwms(a)gmail.com> wrote: >>>>> >>>>>> On Dec 17, 7:12 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- >>>>>> Web-Site.com> wrote: >>>>>>> Second question. You _are_ aware that the crowbar goes on the >>>>>>> _input_ side of the linear regulator, aren't you ?:-) >>>>>> Hey Jim, you respect AoE2 last I heard, right? Why, then, do they >>>>>> also recommend putting the crowbar at the output? Which is, >>>>>> incidentially, the logical place to put it when you want to protect >>>>>> $10k equipment powered by said supply anyway. Equipment >> supply, >>>>>> protect equipment. QED. >>>>>> >>>>>> Tim >>>>> Where did you hear that I respected AoE? >>>>> >>>>> It's nothing more than a compendium of circuits... somewhat on a par >>>>> with Ideas for Design, are they not? >>>>> >>>>> Crowbar on input side of a linear regulator is ideal for speed and >>>>> efficacy of fuse blowing. >>>>> >>>>> Crowbar on the output side is a prayer... hope it can sink more >>>>> current than the faulty regulator can deliver, and that the resultant >>>>> heating won't violate the various regulatory rules that Joerg worries >>>>> more about than good engineering practice... when that melts, then >>>>> there's the spike that takes out $BIG load anyway ;-) >>>>> >>>>> But I suppose there _are_ people who fret over spending $1 to protect >>>>> $BIG load <smirk> >>>>> >>>>> ...Jim Thompson >>>> A lot of people have learnt a lot of very useful things from AoE, and >>>> the world is significantly better off for its existence. >>> Indeed. >>> >>>> I do agree that the fuse and crowbar SCR should preferably go before the >>>> regulator, especially if the transformer is not protected by a fuse in >>>> the secondary that will always blow before the transformer overheats >>>> (which is difficult unless the transformer is grossly oversized), and/or >>>> a thermal cutout in the windings. >>> Thank you! Finally someone understands! >>> >> A transformer that overheats before the primary fuse goes is a serious >> design flaw. Either the thermoswitch must open or the fuse. > Yes, but it is not at all trivial to get it to work without the > thermoswitch. If I were to hang my crowbar on someone else's PSU, then I > guess I might feel duty bound to know the likely and less likely outcomes > of the crowbar being activated, in the presence of whatever fault inside > the PSU was the original cause of the overvoltage (so maybe the current > limit doesn't work right anymore, etc.). Perhaps that is doing the PSU > designer's job for them. > Most of my designs require a thermoswitch to be embedded in the transformer. Usually self-reset types are not allowed. IOW once such a transformer has overheated it's toast. But the building didn't burn down. >> What else would protect upon overload caused be a faulty electrolytic? >> Unless you pepper the whole unit with fuses. > > I agree that the transformer should ideally be protected from fire in the > case of a long term short circuit appearing at its output, as well as any > current between normal and the short circuit current. The worst case is > probably not a short circuit (if this blows the fuse) but rather whatever > current nearly but not quite blows the fuse (and there is some > manufacturing variation in fuses too, even the non-user-replaceable ones). > I am wary of the possibility that the original design did not necessarily > achieve this degree of protection. I am not sure what is legally required > but I have seen some commercial designs that worry me. Admittedly it is > mostly smaller transformers than the one you are looking at, where I worry > that the primary current with a nearly-shorted secondary is not quite > enough to blow a primary fuse. In things like wall warts I am much more > reassured when there is a thermal cutout embedded in the primary winding as > well. Maybe I'm being paranoid and the flame retardants in the modern > insulation are possibly better than I have assumed. I have had an old > waxed-paper transformer catch fire in some audio equipment (without blowing > the fuse), fortunately whilst I was there in the room to unplug it, but > that experience did make me look at transformers with more suspicion. > Once the power went and I heard a loud boom. Then screams. Looked out the window and the slope across the valley was ablaze. Hot flames licking up the hill, and fast. Luckily the fire department was there in minutes. A major utility transformer had decided to go lalaland. >>>> It can be even more difficult to choose the rating of the fuse in the >>>> primary of the transformer such that it will always blow when a steady >>>> overload appears at the regulator output, yet does not blow in normal >>>> circumstances due to inrush current. >>>> >>>> Unless there is a thermal cutout in the transformer itself, it may be >>>> possible in some cases to set fire to the transformer without first >>>> blowing >>>> the primary fuse. That could damage the expensive load by purely >>>> thermal means (flames, etc.), even if no electrical damage is caused. >>>> >>>> It is probably worthwhile working out exactly how the current is going >>>> to be >>>> cut off after the crowbar fires. Putting the fuse and crowbar switching >>>> device before the regulator (obviously with the voltage sensing >>>> connection after the regulator) sounds like one good way whereby the >>>> likelihood of blowing the fuse cleanly could be predicted, with a >>>> suitably large >>>> capacitor and SCR. Other arrangements might require more analysis >>>> and/or testing. >>>> >>>> Chris >>> My approach had the storage cap, the fuse, and the SCR in a _very_ >>> tight loop, with wide traces. I have even been known to solder bus >>> bar on top of this path to ensure that high currents are reached >>> quickly ;-) >>> >> Just received another schematic, for da big mother of all linear >> supplies. The one that needs modding. The SCR is, tada, you guessed it, >> on the output. Rock-solid American manufacturer, in business since 1973. >> Good products, top notch customer service (I had the schematic within >> the hour). >> >> The SCR is a serious edition, size of a silver Dollar. > How do they cut off the current after the SCR fires? Is it the primary > fuse, or maybe the pass transistor emitter bondwire? I guess they tested > it at some stage. > The primary fuse, usually. Sometimes the current sense resistor which just opens. However, it must be designed and rated for such an event. Those are the minor things often overlooked by younger designers. Back in Europe they used to have versions with a spring-loaded solder joint but those could be outlawed by now. The buck grazing on the hilltop on the other side of town might get lead poisoning from the miniscule plume ... >> If you do the super high current spike thing (I wouldn't, but you know >> that by now) make sure they never put in a plain old glass fuse. >> Exploding glass is no fun. >> > The fuses that they use in mains plugs in the UK are surprisingly robust. > I'm not sure what the official rating is but they are made of ceramic not > glass and they are filled with sand. They are used in such large numbers > that the price is very very low. > 20mm or 1-1/4" fuses can also be had with sand-fill. But you know how it goes. A nuisance trip, no spare, quick dash to Home Depot. Only glass fuses to be had and, voila, it works again and everybody forgets about it. Until the day Jim's crowbar decides it's time for da big one. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 19 Dec 2008 20:59 On Fri, 19 Dec 2008 17:34:17 -0800, Joerg <notthisjoergsch(a)removethispacbell.net> wrote: >Chris Jones wrote: >> Joerg wrote: >> >>> Jim Thompson wrote: >>>> On Sat, 20 Dec 2008 00:03:42 +0000, Chris Jones <lugnut808(a)yahoo.com> >>>> wrote: >>>> >>>>> Jim Thompson wrote: >>>>> [sniped Joerg's smell of burning transformer :-] >>> >>> The SCR is a serious edition, size of a silver Dollar. >> How do they cut off the current after the SCR fires? Is it the primary >> fuse, or maybe the pass transistor emitter bondwire? I guess they tested >> it at some stage. >> > >The primary fuse, usually. Sometimes the current sense resistor which >just opens. However, it must be designed and rated for such an event. >Those are the minor things often overlooked by younger designers. Back >in Europe they used to have versions with a spring-loaded solder joint >but those could be outlawed by now. The buck grazing on the hilltop on >the other side of town might get lead poisoning from the miniscule plume ... Wait for Obama's "green team" :-( [snip] > >20mm or 1-1/4" fuses can also be had with sand-fill. But you know how it >goes. A nuisance trip, no spare, quick dash to Home Depot. Only glass >fuses to be had and, voila, it works again and everybody forgets about >it. Until the day Jim's crowbar decides it's time for da big one. Funny. In mucho testing I've never fractured the glass. That would only happen if enough time elapsed to produce heating... like the way you do it ;-) (My method hardly disturbs the voltage on the cap ;-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine Sometimes I even put it in the food
From: Joerg on 19 Dec 2008 21:21 Jim Thompson wrote: > On Fri, 19 Dec 2008 17:34:17 -0800, Joerg > <notthisjoergsch(a)removethispacbell.net> wrote: [...] > [snip] >> 20mm or 1-1/4" fuses can also be had with sand-fill. But you know how it >> goes. A nuisance trip, no spare, quick dash to Home Depot. Only glass >> fuses to be had and, voila, it works again and everybody forgets about >> it. Until the day Jim's crowbar decides it's time for da big one. > > Funny. In mucho testing I've never fractured the glass. That would > only happen if enough time elapsed to produce heating... like the way > you do it ;-) > I don't think a TUEV medical device inspector would let that fly. > (My method hardly disturbs the voltage on the cap ;-) > Blowing fuses fast and with extreme current is dangerous. You need a special high energy fuse and preferably sand-filled: http://assets.fluke.com/Appnotes/2041429_w.pdf The other reason why I do not like fuses on DC circuits is arcing. Given a sufficient voltage the fuse could just let St.Elmo dance for a while. Phssssssssssssssss ... dazzling blue light, a whiff of ozone, then a stench ... -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 19 Dec 2008 21:33 On Fri, 19 Dec 2008 18:21:55 -0800, Joerg <notthisjoergsch(a)removethispacbell.net> wrote: >Jim Thompson wrote: >> On Fri, 19 Dec 2008 17:34:17 -0800, Joerg >> <notthisjoergsch(a)removethispacbell.net> wrote: > >[...] > >> [snip] >>> 20mm or 1-1/4" fuses can also be had with sand-fill. But you know how it >>> goes. A nuisance trip, no spare, quick dash to Home Depot. Only glass >>> fuses to be had and, voila, it works again and everybody forgets about >>> it. Until the day Jim's crowbar decides it's time for da big one. >> >> Funny. In mucho testing I've never fractured the glass. That would >> only happen if enough time elapsed to produce heating... like the way >> you do it ;-) >> > >I don't think a TUEV medical device inspector would let that fly. > > >> (My method hardly disturbs the voltage on the cap ;-) >> > >Blowing fuses fast and with extreme current is dangerous. You need a >special high energy fuse and preferably sand-filled: > >http://assets.fluke.com/Appnotes/2041429_w.pdf > >The other reason why I do not like fuses on DC circuits is arcing. Given >a sufficient voltage the fuse could just let St.Elmo dance for a while. >Phssssssssssssssss ... dazzling blue light, a whiff of ozone, then a >stench ... Didn't we start this discussion protecting a 3.3V linear regulator? You keep changing the game to protect your argument ;-) Besides, I should have pointed out, I use 220V fuses for this scheme. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine Sometimes I even put it in the food
From: Joerg on 19 Dec 2008 21:42
Jim Thompson wrote: > On Fri, 19 Dec 2008 18:21:55 -0800, Joerg > <notthisjoergsch(a)removethispacbell.net> wrote: > >> Jim Thompson wrote: >>> On Fri, 19 Dec 2008 17:34:17 -0800, Joerg >>> <notthisjoergsch(a)removethispacbell.net> wrote: >> [...] >> >>> [snip] >>>> 20mm or 1-1/4" fuses can also be had with sand-fill. But you know how it >>>> goes. A nuisance trip, no spare, quick dash to Home Depot. Only glass >>>> fuses to be had and, voila, it works again and everybody forgets about >>>> it. Until the day Jim's crowbar decides it's time for da big one. >>> Funny. In mucho testing I've never fractured the glass. That would >>> only happen if enough time elapsed to produce heating... like the way >>> you do it ;-) >>> >> I don't think a TUEV medical device inspector would let that fly. >> >> >>> (My method hardly disturbs the voltage on the cap ;-) >>> >> Blowing fuses fast and with extreme current is dangerous. You need a >> special high energy fuse and preferably sand-filled: >> >> http://assets.fluke.com/Appnotes/2041429_w.pdf >> >> The other reason why I do not like fuses on DC circuits is arcing. Given >> a sufficient voltage the fuse could just let St.Elmo dance for a while. >> Phssssssssssssssss ... dazzling blue light, a whiff of ozone, then a >> stench ... > > Didn't we start this discussion protecting a 3.3V linear regulator? > > You keep changing the game to protect your argument ;-) > No, just wanted to point out another risk for folks who adopt this design strategy and then use it on a 48V telco supply. > Besides, I should have pointed out, I use 220V fuses for this scheme. > I still wouldn't do it, unless it's sand-filled fuses. Anyhow, solution is found. Got schematic, will use existing SCR and install a nice TL431-based trigger when at that client next time. Can be done without disassembly. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |