I am building a portable light that will be powered from battery or a dc power supply if I am able to plug it in. I have a battery that is ran thru an over discharge disconnection switch, I then put a diode in series with the positive before landing on a 3 position selector switch with 2 NO contact blocks in one position for both the positive and negative. The other position is fed from a dc power supply(2 NO Contacts,+&-) and it also has a diode in series with its positive. My question is, if the contacts were to fail and stay closed from either supply and the switch were to be closed in the other direction are the diodes enough to protect the battery and or power supply from being backfed?

 

Yes, the diode in the battery circuit, by itself, will prevent any current from flowing into the battery. The diode in series with the power supply, will simply reduce the supply voltage supplied to the light. It provides no benefit at all.