At 09:02 AM 3/6/02 +1100, you wrote:

>I've never seen this approach with 78xx regs - interesting. I've only
>seen it with 317 types.

It's valid- look at the internal schematic for the 7805, here's
a pointer to one, and analyze what happens for Vout > Vin.

http://www.onsemi.com/pub/Collateral/MC7800-D.PDF

It's only a concern under the following conditions:

Substantial capacitance on the output (but if you need it, you
need it)

AND either

A)      A very heavy load on the unregulated supply such that the
         current from the resulting dv/dt at switch-off could
         cause excessive current through the regulator. This is
         rather an unlikely scenario.
Or

B)      There is some possibility that the input could be short-
         circuited and there is no series diode. This is more
         likely where you'd use it. If there is a bridge or
         polarity protection diode, it's not a concern unless
         there is a possibility that it could be shorted on the
         *other* side of the diode.

Not all data sheets cover this issue, so it's a good point. Like
an input capacitor, it's wasteful to put it in there if it is
unnecessary.

(BTW, on the previous post, 470uF is about 10 times too small
for 50/60Hz mains filter for typical 1A supply, also the ripple
current rating will not likely be high enough to be reliable).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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