Russell McMahon wrote:

> Someone else noted the need for reverse voltage protection diodes. These are
> covered in the data sheet. The need arises when the input voltage falls much
> more rapidly than the output and the output cap discharges "backwards" into
> the regulator. This can occur if the input is shorted suddenly OR if the
> output load is small relative to the output cap provided while the input can
> discharge rapidly when turned off - either because there is little or n
> input cap or because some other load rapidly discharges the input cap.


In the case I mentioned with a large series resistor
(like 56 ohms etc), you can sometimes dispense with
the diodes as the resistor will protect the 7805
if the input cap is shorted. It is only a good solution
where you need <100mA from the 7805.

I first saw this technique in some "student proofed"
logic trainers which used a 100 ohm resistor in front
of each 7805 reg, and the regs sagged at currents
over 60mA. More recently i've seen this all the time
in TV sets etc where they use a series resistor before
the regs as the resistor takes the bulk of the
dissipation and saves the cost of a heatsink etc.
I always use a series resistor now any time the 7805
has 12v or higher input voltage.
-Roman

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