>
> >Certainly you want to prevent catastrophic problems, like fire, but not
> >much sense in adding components, which have their own failure rate, to
> >protect against the results of failure on the other parts, when the end
> >result is that it's still broken.
>
>The idea is to blow the fuse before the zener blows. If it works right,
>only the fuse is broken.

Ok, the first case is just the reg, which might fail..  If it fails, the
product is broken.

In the second case, we have the reg, plus the fuse, plus the zener.
All of these devices fail in the direction that kills the product.
So, without knowing anything about the rest of the system, the failure
rates multiply, and your system is significantly less reliable than any of
the three components by it's self.

I understand what you're doing technically, what I'm asking, is wether it
really is doing anything for you.

Designing the fuse in could be a bit tricky, you'll have to have enough
current capability in the power supply, to blow the fuse, when it's in
series with the zener.
If you're lucky, the zener will fail shorted somewhere in the middle due to
dissipation limits, and the fuse will then blow.
Otherwise, you've now got a nasty load on the power supply, making more of
a mess.

LittleFuse has a PDF on their web page called Fuseology, a very worthwhile
read.

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