Battery Backup Circuit

Pommie said:

What is the point of the 317 when the 21V is fed around it?

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rjenkinsgb said:

It sets the float charge voltage for the 12V backup battery. The diode from the battery to 21V rail only conducts when the input power fails.

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rjenkinsgb said:

I'd advise adding another diode between the 317 output and the battery, then adjust the voltage to suit.

Connecting power to the output of many three terminal regulators, with a discharged smoothing cap at the input, can damage them.

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Suraj143 said:

Do I really need a second diode?

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rjenkinsgb said:

Yes, I'd say so, if you want absolute reliability. It will also prevent slow self-discharge via the regulator ref divider.

In a normal PSU, a reverse diode across the reg is OK as the only storage after the reg is from filter / smoothing caps; at worst a a very short pulse in case of a fault or input short.

You have a battery that can likely source 100A or more; connecting that with power off, or anything that shorts the input side, could blast the regulator.

Having thought about the setup a bit more, I'd also add an inline fuse to the battery negative cable, for fault and fire protection, eg. 5A or 10A.

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