Big crowbar module for <4VDC?

All you care about is the I^2*t rating of the fuse versus that of the SCR.

Make sure the traces can support the expected crowbar current!

In the Master's designs there is only a mild click when the fuse "flashes" ;-)

...Jim Thompson

Second question. You _are_ aware that the crowbar goes on the _input_ side of the linear regulator, aren't you ?:-)

Short series loop of input electrolytic and fuse and SCR... phzzzzt ;-)

...Jim Thompson

"And let there be light...' If only for a second. :)

Ideally yes. Reality, no. Most crowbars are across the output terminals. They do their job because typically the supply is toast anyway if the crowbar has to kick in. IOW one of the pass transistors has fused shut and then it doesn't matter where the crowbar sits.

And that's the problem ... the SCR needs to be huge. There is a really fat one in the supply (>>TO220) but to get there would require major surgery.

Nonsense! Regulator loop fails, pass device can't pass enough current to blow fuse.

Again nonsense! Sounds like you've never actually designed a crowbar-protected supply ??

I did my first 33 years ago, before simulation capability, worked perfectly ;-)

Doesn't take a big "fat one".

The clue to your learning process is I^2*t

...Jim Thompson

Not nonsense at all. Here's why: The resistance of the pass device, for whatever reason it is there, is desired. It makes sure the SCR doesn't turn into a plume of smoke. The pass devices will likely blow but who cares? Saving the $0.50/each pass devices is not the issue, saving a $10k load is the issue.

Also, with OEM supplies you do not have access to the input side. Else the warranty is toast. Unless qties are huge and you can negotiate that "minor" inconvenience away. Not all issues are electronic ;-)

Trying to short out a bank of serious electrolytics is IMHO not a smart thing to do. First, they guarantee max ESR but usually never a min ESR. Secondly, the trace resistance max can be calculated but not the min because PCB fabs usually do not guarantee a max copper weight. You've spec'd 2oz but what if it's a lot more? As an aside you'd also run the risk of exploding an electrolytic and if something bad results from that, oh boy, plaintiff's counsel would be all over you because I am sure they can source the same state-of-the-art I have (schematics of major supply mfgs) and use that as exhibits.

Hint: There are emitter load share resistors and current sense resistors which further limit the damage. Puff of smoke versus major kablouie ;-)

Au contraire. And they all tested out fine. I would never, ever, consider placing a crowbar directly across a bank of serious input side electrolytics. That is IMHO a recipe for disaster.

It does. There is a good reason why they selected a much fatter one on this 18A supply. If I could get at it I'd be home. And if there ain't any COTS modules I'll have to.

I know about that. But business reality in systems architecture is often like this: There is no fuse before the regulator, only one before the transformer. Meaning the caps are free to dump their charge and good ones do so with gusto. You can place a fuse between the crowbar and the output of the supply but you must assume that it'll be an automotive rated slow fuse and that the pass devices have fully fused shut. IOW worst case, only load share and current sense resistors left in the game until the fuse decides to finally blow.

I have here various schematics of OEM linears from reputable US manufacturers (unfortunately not at liberty to post on the web). All have the SCR connected to the output rails, just like I always did. Now all those manufacturers can't be wrong, can they?

That statement proves you don't understand I^2*t

For crowbar use, there should be... makes for trivial protection of the, as you say, $BIG load ;-)

They're probably all Democrats ;-)

...Jim Thompson

Again, show me a commercial grade linear supply from Condor, PowerOne or whatever that has an extra fuse between capacitor bank and pass device. With a good bank of caps well in excess of a total of 10,000uF a TO220 device doesn't stand much of a chance. You wouldn't even here phssst, just *KABLAM*. I will not do that.

I assume we'll never reach agreement on this one. Anyhow, as far as I am concerned I'll continue to side with the design engineers at the major power supply manufacturers.

Hanging the crowbar onto the output makes it even more trivial. Which is why that's how it's done :-)

I hope none of them read that ;-)

"SLOW" ??? Dum, dee, dum, dum ;-)

I guess there's no convincing you that the crowbar goes (properly) before the linear, all the energy to blow the fuse (between capacitor bank and linear) comes from the capacitor bank, with properly sized fuse (fuseI^2*t

Mine, too. If you have fuses after the caps your crowbar solution is (probably) going to be ok. If not you better have good PL coverage and warning labels in English and Spanish ;-)

I was doing system level stuff before you were born ;-)

That's where they belong.

...Jim Thompson

But that doesn't mean us younger guys couldn't find a more cost efficient topology :-))

Hint: A decent fuse costs quite a bit and fuses always attract the scrutiny of the agency compliance inspector. More paperwork, more Dollars. And one likes to minimize scrutiny.

I guess this is an area where the two of us will never see eye to eye ...

Aha! You don't care what burns up... as long as it's not the $BIG load ?:-)

"...fuses always attract the scrutiny of the agency compliance inspector..." sounds like a business I don't want to play in.

Fuses are for safety, n'est ce pas ?:-)

And, done properly (aka "my way"), crowbars are _very_ fast at accomplishing disconnect.

...Jim Thompson

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

...Jim Thompson

Pardon my quoting Maxim ;-)...

or Hobby Projects

or Fairchild

http://74.125.95.132/search?q=cache:yk_Mxz8_OisJ:

(back into the PDF)

For switchers I tend toward redundancy which can be guaranteed to take out the appropriate failure paths... but this designing, unlike that for linear supplies, must be on a case-by-case basis.

...Jim Thompson

And, and... for those that would impugn my systems experience...

From 1962-1970 I designed primarily ASIC's for automobile application, wherein I defined the overall system.

From 1970-1973 I designed hybrid SYSTEMS for (primarily) military application.

From 1973-1987 I was about 50% discrete systems / 50% ASIC's.

From 1987 to present I'm mostly doing ASIC's with an occasional system design, but even with the ASIC's I dominate making the whole system play as desired... _never_ trust a system architect... every time I do they "f-bomb" something up ;-)

...Jim Thompson

The suitability will depend on the application circuit - and access in the application. Location and method will vary. Add-on modules assume no internal access, save the supply output terminals.

For linears with built-in crowbar circuitry, blowing the primary fuse from the regulator input is traditionally the safer and more predictable method. At lower voltages, higher currents and under more complex load conditions, other methods might make more sense, considering advances in power mosfet technology and integrated power supervisory bells and whistles.

RL

No, clients do not care. Whether a regulator loop, a transistor or a transformer failed doesn't matter. The power supply is toast and will be swapped. Of course this is very different on my own designs where power supplies are residing on the same board as the other stuff. Those usually don't get crowbars but FET disconnects.

Then better stay out of med, aero and so on.

Yes, but only where needed. The fact that they are safety devices is exactly the reason for said scrutiny. I remember a situation where I told a someone umpteen times to replace a resettable thermo-fuse with a simple and cheap non-resettable one. "But then the transformer will be dead" ... "Yes, but else you won't pass med cert in Canada". They kept the resettable version, went to the test lab and promptly ended up with egg in the face. I heard that their Canadian sales reps were very pissed.

So are mine. Except that the pass devices are toast afterwards.

Hey, maybe you should have done an apprenticeship under Earl William Muntz before going to MIT ;-)