[sdiy] circuit protection question

[David G. Dixon]

> It seems to me that the reverse diodes are doing the protective work
> for the module. What are the polyfuses doing? I'm figuring that they
> protect the PSU? I wondered if they protected the circuit in different
> ways. So I tried an experiment and put an op amp in backwards. It
> smoked.

I believe the diodes are the main protective elements.  The polyfuses mostly
protect the diodes, which would have to pass a lot of current without them,
and may still have to, briefly, while they trip.

The polyfuses will only protect the circuit if the power is plugged in
backwards.  If the power is plugged in correctly, then there is nothing for
the polyfuses to do.  If a chip is plugged in backwards, there is really no
way to protect it.
 
> How about if someone plugs the power cable in and they are off to the
> side one pin?
> Say...+15V header is now connected to PSU ground and PSU ground and
> -15V are connected to the header ground and the -15V header pin is
> unconnected (the power connector is shifted one over where it should
> be).

Now the +15 and GND are shorted together, which may trip some protection
mechanism at the PSU.  The diodes will do nothing in this situation, because
unless the connector is rotated 180 degrees from the way it should be, they
are still reverse biased, regardless of which direction the connector is
offset.