When attempting to use that same technique to protect a high-powered
automotive accessory from reverse polarity, we (meaning myself and two
other engineers working on the project) discovered that big fuses are
horribly inaccurate, usually erring on the high side.  These were 100A
fuses, and in our test setup would usually survive 180 to 300A for many
seconds. They varied all over the place, even among fuses from the same
manufacturer.  Diodes capable of reliably blowing the fuses were too large
and expensive for this application.

So, we tested the device to see what would happen when someone
connected it backwards. The main concern was several large-ish electrolytic
caps.  Placing these in a metal bucket and connecting a car battery
backwards yielded a pretty strong bang with lots of debris in the bucket.
The 100A fuse was unharmed.  Definitely not the kind of thing we wanted to
happen to someone who's installing the thing in their car.  However, once
sealed in the product's extruded aluminum enclosure and epoxy
encapsulant, the results were not as bad.  It made a pop, cracked the
epoxy, and emitted a little smoke, but no projectiles or flames in any of the
tests using fully assembled devices.  Still not pretty, but much less likely to
cause injury.  The product was otherwise quite robust and could survive just
about any other kind of wiring error, short circuit, or other abuse.

The final decision was to leave the reverse polarity protection out and apply
really obvious warning labels. The connections were clearly marked with "+"
and "-". There's only so much idiot proofing you can reasonably do.

> I used to repair car audio equipment, and most of the cheaper head units
> used a 1N400X wired accross the supplies for reverse polarity protection,
> relying on the in-line fuse to blow.  Of course, lots of people don't bother
> with the fuse, or wrap it in tin foil, or put a 6 inch nail in.  Under these
> conditions I have seen many of these little rectifiers fail to a short
> cicuit mode, the PCB traces then (usualy) take the place of the fuse.
>
> Regards
>
> Mike Rigby-Jones
>


---
                                        Peace,
                                        William Kitchen
                                        bill@iglobal.net

The future is ours to create.