Big crowbar module for <4VDC?
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From: Paul Hovnanian P.E. on 20 Dec 2008 20:19 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. > > ... 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. -- Paul Hovnanian mailto:Paul(a)Hovnanian.com ------------------------------------------------------------------ 100 buckets of bits on the bus 100 buckets of bits You take one down, and short it to ground FF buckets of bits on the bus
From: Jasen Betts on 20 Dec 2008 23:50 On 2008-12-20, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: > > The 'evidence' case the republican Tommy refers to, does not hold if > for example 24 V is applied to the output of a 3.3V regulator with crowbar before > the regulator, as it may destroy the regulator anyways. > > Anyways, just short the output, there may even be fold back still working, > I have seen drifting supplies too, bit high voltage, crowbar active. > The old Philips K9 chassis had a crowbar in the series switcher 160V (IIRC) > output, to protect the horizontal output and every other thing in the set > getting supply from that. > It has a primary fuse too, but those switch mode would just go tick tick tick > if the transistors were not kaput. > Protect the load. if you put the crowbar after the regulator and someon econnects a 24V source (like a truck battery) I think your crowbar will have a little trouble.. in any case that sort of behavior is likely to void the warranty anyway.
From: Jan Panteltje on 21 Dec 2008 06:16 On a sunny day (21 Dec 2008 04:50:22 GMT) it happened Jasen Betts <jasen(a)xnet.co.nz> wrote in <gikhue$90b$2(a)reversiblemaps.ath.cx>: >if you put the crowbar after the regulator and someon econnects a 24V >source (like a truck battery) I think your crowbar will have a little >trouble.. in any case that sort of behavior is likely to void the >warranty anyway. Yes, it all depends... a good crowbar, like I have seen in some of the high current 5V supplies, would likely evaporate some battery leads... Thyristors are amazing. One case, that I have not seen mentioned, and that holds a very real argument for the crowbar at the output, is the circuit where diodes are used to the supply and ground, to limit an input. Now that is the wrong way to do it, one should use a transzorb or zener, but I have seen it proposed here. In case of some input accidently getting a surge (for example touching a mains wire) the low voltage supply will be lifted way above max spec, _if no crowbar at the output_. Blowing a fuse before the regulator does nothing in such a case. I sort of think, that when buying a supply with 'crowbar', you buy in fact 2 different things in one module. Like the real crowbar, where you throw a piece of copper over the terminals, you could sell a separate crowbar. In fact I was just thinking of doing some market research if I could sell some high power ones.
From: Joerg on 21 Dec 2008 11:25 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". >>> ... 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 21 Dec 2008 11:32
Jan Panteltje wrote: > On a sunny day (21 Dec 2008 04:50:22 GMT) it happened Jasen Betts > <jasen(a)xnet.co.nz> wrote in <gikhue$90b$2(a)reversiblemaps.ath.cx>: > > >> if you put the crowbar after the regulator and someon econnects a 24V >> source (like a truck battery) I think your crowbar will have a little >> trouble.. in any case that sort of behavior is likely to void the >> warranty anyway. > > Yes, it all depends... a good crowbar, like I have seen in some of the high > current 5V supplies, would likely evaporate some battery leads... > Thyristors are amazing. > > One case, that I have not seen mentioned, and that holds a very real argument for the crowbar at the output, > is the circuit where diodes are used to the supply and ground, to limit an input. > Now that is the wrong way to do it, one should use a transzorb or zener, but I have > seen it proposed here. > In case of some input accidently getting a surge (for example touching a mains wire) > the low voltage supply will be lifted way above max spec, _if no crowbar at the output_. > Blowing a fuse before the regulator does nothing in such a case. > > I sort of think, that when buying a supply with 'crowbar', you buy in fact 2 different > things in one module. > Like the real crowbar, where you throw a piece of copper over the terminals, > you could sell a separate crowbar. > In fact I was just thinking of doing some market research if I could sell some high power > ones. > If you had such a big crowbar module for sale we would have bought it. Seriously. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |