Herremans Kurt wrote:

> I think the contrary...
> The diode is there to blow the fuse. Why use a diode across the
> terminals and not in series ?

Because a series diode does absolutely NOTHING to limit transients ascompared
to some form of transorb.

> The 0.7V voltage drop across the series
> diode might be too much (especially in 12V devices), when connecting a
> diode in shunt this problem is omitted.

And when dealing with automotive applications (12V devices), the 0.7Vdrop
means very little.  After all, the logic circuitry is further regulated
down to a lower voltage such as a CLEAN 5V. Lets not forget that when the
car is running, the voltage is actually more like 13.6V at the battery.

> The consequence is that you blow
> the fuse when you connect the power wrong.

As I said, "Although it may be true that this will work, I don't think
that is the reason WHY they put it there."  After all, there is a safety
factor if they don't have a fuse or have bypassed the fuse.  It could
overheat the wires causing insulation to melt and short.  Do you want
your device being responsible for causing the fire that burned down
someone's car.  The SAFE circuit is to have a series diode for reverse
polarity protection with a transorb (or zener or reverse biased diode)
right after that to ground for transient protection.  Add a big cap in
parallel with the transorb to help filter things out and you now have a SAFE
and SOMEWHAT effective 12V automotive power input circuit.

> Mark Devin Newland wrote:
> >
> > Although it may be true that this will work, I don't think
> > that is the reason WHY they put it there.  Typically,
> > a manufacturer will put a reverse biased diode across
> > the power supply terminals as a form of transient
> > protection.  If a voltage spike appears on the positive
> > terminal greater than the reverse breakdown voltage
> > of the diode, the diode will bleed the excess to ground.
> > It may not be as good as a 79 cent transorb but for 7
> > cents the diode is ALOT better than nothing.
> >
> > Mike DeMetz wrote:
> >
> > > > Provide the simplest protection against accidental
> > > >     reverse connection (a series diode), mark the
> > > >     terminals clearly '+' and '-', color the terminals
> > > >     red and black respectively, and the intended User
> > > >     will understand what's required.
> > > >
> > > Another method I've seen on CBs is to connect a diode across the power
> > > terminals reversed biased. Connecting backwards blows the fuse.