what is the purpose of a diode with the 7805 regulator?

[sunnystar]

what is the purpose of a diode at the input of the 7805 regulator? i have heard two reasons: 1) such a diode to be in place "in case of reverse current flow".

2) The idea of a backfeed diode like this is to discharge the output capacitor of the 7805 when the input is switched off. This prevents damage to the 7805 due to its output being at 5V when the input is at 0V.
which is the correct reason?

 

[esp1]

hi,
A reversed connected diode is often added, in parallel, to a Vreg as your example #2, to prevent the Vreg output being damaged due a reverse voltage, due to charged smoothing caps on the Vreg output, when the input to the Vreg is switched to 0V.

Also a forward biased series diode is added to the input to a Vreg to prevent reversed voltage input damage to the Vreg.

E

 

[Audioguru]

Why don't you read the datasheet for the 7805 voltage regulator IC? It clearly explains the use of diodes.

 

[sunnystar]

I downloaded the datasheet but there was no info of diodes in it

 

[D.A.(Tony)Stewart]

what datasheet ?

They all show diode requirements for reverse polarity output and reverse In-Out polarity.
As a series regulator, max current depends on source current.

https://www.sparkfun.com/datasheets/Components/LM7805.pdf Page 8