What is the diode for in 7805 5v regulator circuits?

[mth5044]

Is it necessary? On the data sheet, it just shows two caps:

http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMvHdo5hUx%252bJYj68h7lsS6Q8

but while searching for 7805 circuits a lot of them have diodes.. is that just for protecting the 7805 orr... I dont know. Thanks

[cloudscapes]

prolly reverse polarity protection

[mth5044]

Well that's lame and uninteresting 

[tempus]

I think it's there to protect the regulator in the event of the output voltage exceeding the input voltage.

[doitle]

I'm pretty sure it is to prevent damage if you accidentally plug in a DC jack that is not what you intended IE you built the circuit for center + and you plug in a center - jack.

[Paul Marossy]

I'm pretty sure it is to prevent damage if you accidentally plug in a DC jack that is not what you intended IE you built the circuit for center + and you plug in a center - jack.

+1

[R.G.]

In spite of the three terminal regulator being pretty good at low frequencies, people tend to load up its output with bypass caps for low impedances at high frequencies. I've seen as much as several thousand microfarads on the outputs of a three terminal.

When you turn off the raw DC supply, under some circumstances it can drop very quickly to near 0V. The output bypass caps hold up the output, and simply by turning off the power you have now reversed the voltage across the regulator - it's higher on the output than the input. This can do Bad Things to the chip that it can't protect itself against. A normally-reverse-polarity diode from output to input pin protects it.

[Rob Strand]

There's four different diode protection configurations for regulators:
- series with input         ;reverse input
- parallel with input       ;reverse input
- parallel with output     ;reverse connection output (usually only required for bench supplies)
- output to input.           ;prevent capacitive loads dumping current back through the device (common on LM317)
                                   ; also some protection against voltages driven into the output.

[mth5044]

Thanks for all the answers.. seems it has many uses