Big crowbar module for <4VDC?
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From: Frithiof Jensen on 21 Dec 2008 12:20 "Joerg" <notthisjoergsch(a)removethispacbell.net> skrev i meddelelsen news:WFW2l.9412$D32.3143(a)flpi146.ffdc.sbc.com... > A transformer that overheats before the primary fuse goes is a serious > design flaw. Either the thermoswitch must open or the fuse. Ohh - But this is quite common: The typical wall-wart or 19,95 DVD player uses a transformer with so much leakage and resistance that it never blows anything; it just sit there and cook quietly until all the insulation is gone and it blows the fuse on the mains. > What else would protect upon overload caused be a faulty electrolytic? The fire brigade ;-) > Unless you pepper the whole unit with fuses. I would consider having either a fuse in the secondary or a thermal fuse on the transformer.
From: Joerg on 21 Dec 2008 12:34 Frithiof Jensen wrote: > > "Joerg" <notthisjoergsch(a)removethispacbell.net> skrev i meddelelsen > news:WFW2l.9412$D32.3143(a)flpi146.ffdc.sbc.com... > > >> A transformer that overheats before the primary fuse goes is a serious >> design flaw. Either the thermoswitch must open or the fuse. > > Ohh - But this is quite common: > > The typical wall-wart or 19,95 DVD player uses a transformer with so > much leakage and resistance that it never blows anything; it just sit > there and cook quietly until all the insulation is gone and it blows the > fuse on the mains. > Well, we are talking industrial grade power supplies here. Those that have to undergo real agency testing. >> What else would protect upon overload caused be a faulty electrolytic? > > The fire brigade ;-) > Then the client would have some questions ... >> Unless you pepper the whole unit with fuses. > > I would consider having either a fuse in the secondary or a thermal fuse > on the transformer. > A thermofuse is mandatory in many markets but secondary fuses are usually frowned upon. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Paul Hovnanian P.E. on 21 Dec 2008 13:46 Joerg wrote: > > Paul Hovnanian P.E. wrote: > > Joerg wrote: > >> Jim Thompson wrote: > >> > >> [...] > >> > >>> Simple crow-barring of a power supply output, without any well-defined > >>> relief/release mechanism seems down-right stupid to me. ... > >> The relief/release mechanism _has_ been shown in this thread. There is: > >> > >> a. Primary fuse > >> > >> b. Current sense resistor > >> > >> c. Transformer thermofuse > >> > >> Of course, all (not just one alone) need to be calculated, tested and > >> qualified to do a graceful job, meaning no loud bang or serious plume of > >> smoke or whatever else the client requires. The goal must be that only > >> the primary fuse goes as that is panel accessible by field service > >> folks. They often aren't allowed or equipped to open unit panels and > >> dive into the circuit board level. > > > > The longer the path current travels through the supply to the SCR > > crowbar, the less I want field techs putting the equipment back in > > service. > > > > The trick is to design the path so it can withstand that. There are > applications where a screw terminal coming loose should be correctable > without losing functionality of the whole installation. Else it would be > like an aircraft glass cockpit showing the message "An unknown error has > occurred and the program will shut down". But you ae stuck with an off the shelf supply. You are going to have to do quite a bit of reverse engineering to ensure that the part of the path you were not responsible for won't be damaged. Worst case is that it appears to work but fails and takes the load with it after the tech has packed his tools back into the van. > >>> ... Citing that > >>> everyone is doing it doesn't make it good... it only means it's > >>> "cheap" ;-) > >>> > >> Nope. If all the major manufacturers do it one should at least consider > >> it and find out why they do it that way. I know for a fact that they > >> also cater hi-rel markets. > >> > >> Just one more reason although I am sure your opinion is cast in concrete > >> here: If a sense line is shorted or one of the supply lines comes loose > >> a bit for some reason the supply will think it needs to shovel some more > >> coals ... crowbar comes. Both your crowbar and mine will tackle that. > >> But in my case the field service guy can just re-affix the wires and > >> swap a panel fuse. In your case the power supply box must be opened. > >> This is mostly not desired and field personnel is often not allowed to > >> do that -> PSU swap -> send back -> $$$ > > > > > > Ideally, to protect high value loads, it might be better to place the > > crowbar at the load and give them a supply of cheap power supplies. Or > > at least put a clamp on the load input to hold the voltage down until > > the crowbar in the PS can remove the supply. > > > > Crowbar at the load is ideal but in this case not quite feasible. The > supply must be super quite, hence a linear. Quiet as in you can't even tolerate the noise of an open SCR or zener at the load? > -- > Regards, Joerg > > http://www.analogconsultants.com/ > > "gmail" domain blocked because of excessive spam. > Use another domain or send PM. -- Paul Hovnanian paul(a)hovnanian.com ----------------------------------------------------------------------- Have gnu, will travel.
From: Joerg on 21 Dec 2008 16:42
Paul Hovnanian P.E. wrote: > Joerg wrote: >> Paul Hovnanian P.E. wrote: >>> Joerg wrote: >>>> Jim Thompson wrote: >>>> >>>> [...] >>>> >>>>> Simple crow-barring of a power supply output, without any well-defined >>>>> relief/release mechanism seems down-right stupid to me. ... >>>> The relief/release mechanism _has_ been shown in this thread. There is: >>>> >>>> a. Primary fuse >>>> >>>> b. Current sense resistor >>>> >>>> c. Transformer thermofuse >>>> >>>> Of course, all (not just one alone) need to be calculated, tested and >>>> qualified to do a graceful job, meaning no loud bang or serious plume of >>>> smoke or whatever else the client requires. The goal must be that only >>>> the primary fuse goes as that is panel accessible by field service >>>> folks. They often aren't allowed or equipped to open unit panels and >>>> dive into the circuit board level. >>> The longer the path current travels through the supply to the SCR >>> crowbar, the less I want field techs putting the equipment back in >>> service. >>> >> The trick is to design the path so it can withstand that. There are >> applications where a screw terminal coming loose should be correctable >> without losing functionality of the whole installation. Else it would be >> like an aircraft glass cockpit showing the message "An unknown error has >> occurred and the program will shut down". > > But you ae stuck with an off the shelf supply. You are going to have to > do quite a bit of reverse engineering to ensure that the part of the > path you were not responsible for won't be damaged. Worst case is that > it appears to work but fails and takes the load with it after the tech > has packed his tools back into the van. > It already has an SCR across the output, like all good linears do. All I have to do is mod that as well (since it seems nobody has external modules big enough). >>>>> ... Citing that >>>>> everyone is doing it doesn't make it good... it only means it's >>>>> "cheap" ;-) >>>>> >>>> Nope. If all the major manufacturers do it one should at least consider >>>> it and find out why they do it that way. I know for a fact that they >>>> also cater hi-rel markets. >>>> >>>> Just one more reason although I am sure your opinion is cast in concrete >>>> here: If a sense line is shorted or one of the supply lines comes loose >>>> a bit for some reason the supply will think it needs to shovel some more >>>> coals ... crowbar comes. Both your crowbar and mine will tackle that. >>>> But in my case the field service guy can just re-affix the wires and >>>> swap a panel fuse. In your case the power supply box must be opened. >>>> This is mostly not desired and field personnel is often not allowed to >>>> do that -> PSU swap -> send back -> $$$ >>> >>> Ideally, to protect high value loads, it might be better to place the >>> crowbar at the load and give them a supply of cheap power supplies. Or >>> at least put a clamp on the load input to hold the voltage down until >>> the crowbar in the PS can remove the supply. >>> >> Crowbar at the load is ideal but in this case not quite feasible. The >> supply must be super quite, hence a linear. > > Quiet as in you can't even tolerate the noise of an open SCR or zener at > the load? > No, I was referring to "cheap supplies". Got to be ye olde LM723 as a controller, very nice noise performance. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |