Battery Backup Circuit

[Suraj143]

I want to power my SIM800L GSM module & also it needs a battery backup in case of mains power failure. Is my diagram ok?

 

[rjenkinsgb]

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.

 

[Pommie]

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

Mike.

 

[rjenkinsgb]

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

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.

 

[Pommie]

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.

Of course, never thought of the charging requirement.

Mike.

 

[Suraj143]

Many Thanks for both of you.

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.

I redraw the same circuit (did not do any modifications). Do I really need a second diode?

 

[rjenkinsgb]

Do I really need a second diode?

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.

 

[Suraj143]

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.

Nicely explained. I'll definitely add a second diode. Thanks.