Power supply 0-30 volt reverse voltage protection
This is how I retrofitted an old bench supply that I need to tolerate reversed batteries connected to the output (without forcing current through them or having fuses/circuit breakers pop). 

A fet switch optocoupler controlled the drive to the output stage.







simulate this circuit &ndash; Schematic created using CircuitLab

Normally D1 is energised, and the output stage gets drive.
When the output goes below 0, D2,D3 take the current from opto D1, disconnecting the drive to the output stage. D2 lights during reverse protection.
D3 protects D2 from 30V- LED's don't have high reverse voltage rating.
D4 can be 0-2 diodes to set the voltage around 0, where it cuts off.

You need to watch for other components that might be affected. C1 is the usual electrolytic capacitor across the output. Adding C2 makes it bipolar, so -ve does not destroy it. However electros are not like diodes. They do not conduct reversed. On my supply C1 is a 63V electro. A 12V battery can be put reversed across the power supply. The 63V electro does not mind -12V :I did not need C2.