I looked at figure 15 of the LM340 datasheet,
the "shorted input protection diode" is in parallel with the regulator, not in series with it.
The diode addresses a weakness of the LM340: ?if the input of the LM340 suddenly drops to zero
then the output filter caps will discharge through the LM340 to the grounded input pin, with possibly enough current
to destroy the LM340. ?I have not seen that recommended on any modern linear regulators, and doubt it is needed.
Likewise most regulators don't need a protection diode across the output as shown in fig 26
(Though I suppose it could help if the battery leads are reversed, and the regulator does not block reverse voltages
like the LM2940* does, and the diode is big enough to not burn out.)
However, "a 1N400x in series?with the regulator input" as Arv describes it is a very good idea if you can tolerate the diode drop.
That will protect the regulator and everything behind it from reversed battery leads.
Or you could use an LM2940* (pin compatible with the LM7805), which has reverse protection built in,?
but does not pay for it with a significant voltage drop across the device.
Jerry, KE7ER
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On Sun, Jun 11, 2017 at 10:28 am, Arv Evans wrote:
45 years ago in the LM340 regulator days it was conventional to always put a 1N400x in series
with the regulator input.? The problem of shorted regulators passing full voltage through them was
well known and accounted for with the series diode.? Somehow we seem to have forgotten that
knowledge over the intervening 40+ years.? Doing this means that you have to account for the
additional 0.65 volts of diode voltage drop, but it is a small price to pay for the added security.
?
The datasheet also shows the shorted-input protection diode across the regulator device (Fig. 15)
and a protection diode across the output (Fig. 26).
Arv K7HKL
?
