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What's the purpose of diode D1 and D2 in the circuit

This is the circuit of a constant voltage battery charger, probably for floating a 12V sealed lead acid battery.

Many years ago the company I worked for constructed and sold many of these with almost identical circuit.

Diode D1 prevents reverse biasing of the LM317 if incoming power is switched off or fails, while the battery is

 still connected. Without D1, under this condition the LM317 is likely to release its smoke.

Diode D2 protects against reverse connection of the battery. If reverse connection does occur, (hopefully) a

 downstream fuse will blow and the charger stays intact. Personally I would use a more beefy diode in this

 location (say a 5A device) so that under short circuit conditions, it can blow a 2A fuse without damage to itself.

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