Hi,
There is a chance that leakage current from the diode caused a problem, but this usually only happens when the circuit to be protected has relatively high impedance for minus inputs. Just in case that happened, the solution is two diodes: one to protect against reverse polarity, and the other to protect against the first diode negative leakage current with reverse polarity.
Code:
+ o-----D1*-----+------o to regulator + input
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|
*
D2
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- o-------------+------o to regulator - input
* Asterisks mark cathodes
The other type of solution, the diode across the input with fuse trick, has to be applied very carefully because the PC board traces leading to the diode can actually vaporize while the fuse is in the process of blowing open. I dont think that should be needed here anyway as that should only be necessary when there are two power sources that can connect to the circuit such as a regular input and a battery (as in a battery charger). The series diode solution doesnt work on the output of the battery charger, so a diode across the output has to be used, with the proper precautions of course.
Another cute trick that is used:
Provide the end user with a switch that connects the input power to the input of the regulator circuit. Before the input power is turned on, the switch is moved to the 'open' position. Also connect an LED with resistor across the input before the switch, preferably a big red LED, that only lights up when the polarity is reversed. Provide another green LED that only lights up when the polarity is correct. The user, seeing the green LED is lit, then turns the switch to the 'on' position. If the user sees the red LED lit, he reverses the input polarity and checks again for the green LED.
This solution is used when very high currents are involved so that no diodes with their wasteful voltage drop need to be used. Only the switch has to be rated for the whole current, and only then in the 'closed' position (it doesnt have to actually switch high currents, just be able to conduct them).
This solution can save lots of power and avoid having to use very high current diodes but it does rely on the correct operation by the end user. If this is a problem, a relay can be used instead with the coil powered from the input supply voltage and a diode to allow only one polarity to energize the relay coil.