wrote:
Ummmm, don't these things almost always have a Zener on the output ?

That would have had to gone open. Usually I see them dead shorted if they get overloaded. That's because the pass transistor is shorted which made the Zener short, which means it is shorted all the way around.

Of course on any given Thursday run there could be some units that have an open Zener. And you'll never know it unless the rest of it fails.

Just love this business.

I've never seen integral over voltage protection on the 78/7900 devices,
but they all behave differently since they're so generic any that you can
get known good ones as well as sketchy ones.

Fairchild even claims this on their datasheet (from
http://www.fairchildsemi.com/ds/LM/LM7805.pdf)


The LM78XX series of three terminal positive regulators
are available in the TO-220 package and with several
fixed output voltages, making them useful in a wide
range of applications. Each type employs internal current
limiting, thermal shut down and safe operating area pro-
tection, making it essentially indestructible.

I doubt "essentially indestructible" applies to the no-name ones with
poorly molded cases and crappy printing. If you can barely package a
semiconductor, there's no way you care about the dies going inside them.

In really plain open style frame linear power supplies, I've seen the 723
regulators blow up far more often than the aux wimpy outputs with the
single to220 regulators. If only switching power supplies all had marked
parts and were fixable.