Protection Diodes on 3 Terminal Regulators.

[Lee Leduc]

I have a question about protection diodes used on fixed and adjustable 3 terminal regulators.
The attached image shows the datasheet recommended diodes for these 2 regulators. Should D?? be added to the adjustable regulator to protect it if the output falls below ground (negative) as D2 protects the fixed regulator?

Thank you.

[Gyro]

D?? isn't normally used unless there is also a negative rail in the system that might pull the output negative (maybe at power down). It really depends what the regulator is powering, you would probably want to include it if you were making a bench psu for instance.