12V wall wart battery backup

[AZdave]

I'm building a project which has a PIC16F877. It will store data averaging. I would like to have a battery back up so the data isn't lost duing a power outage.

The unit is powered by a 12v wall wart and stepped down by 78L05's for circuit power. Is there a battery backup system that will switch fast enough so the memory in the PIC won't be lost? I also was wondering if I should/could use a rechargable battery and trickle charge it to keep it at full charge?

 

[eblc1388]

Is there a battery backup system that will switch fast enough so the memory in the PIC won't be lost? I also was wondering if I should/could use a rechargable battery and trickle charge it to keep it at full charge?

I offer the following circuit. The battery will take over when AC supply is lost. The PIC won't even know the AC power has gone.

All the diodes D are 1N4001 and the resistor R is to set the trickle charging current of the Ni-Cd battery pack.

 

[Nigel Goodwin]

I'm building a project which has a PIC16F877. It will store data averaging. I would like to have a battery back up so the data isn't lost duing a power outage.

The unit is powered by a 12v wall wart and stepped down by 78L05's for circuit power. Is there a battery backup system that will switch fast enough so the memory in the PIC won't be lost? I also was wondering if I should/could use a rechargable battery and trickle charge it to keep it at full charge?

Yes you can use NiCd's for battery backup, you simply connect it's positive via a forward biased rectifier diode to the input of the 78L05 (negative of battery to 0V) - use either 8 or 10 NiCd's, to give enough voltage.

The incoming mains supply needs to be higher than the NiCd voltage - when the mains drops the rectifier diode conducts, feeding the 7805 from the battery.

To trickle charge the battery you simply fit a suitable value resistor in parallel with the diode from the battery.

To use non-rechargable batteries, leave the resistor out!.

 

[AZdave]

Thanks for the quick replies. eblc1388, The circuit drawing is a big help - thanks! Nigel, I have a couple 6-cell 7.2 volt (1500 ma!!) battery packs from my RC truck and figured I would use them. Connected in series they provide 14.3 volts.

Thanks again for the quick replies.

 

[Nigel Goodwin]

Thanks for the quick replies. eblc1388, The circuit drawing is a big help - thanks! Nigel, I have a couple 6-cell 7.2 volt (1500 ma!!) battery packs from my RC truck and figured I would use them. Connected in series they provide 14.3 volts.

In the diagram given above you probably don't need the extra diode to the left, if it's an unregulated supply the bridge rectifier inside the mains supply will already perform that function.

If you're using a 14.4V battery pack, you must make sure that the mains supply is higher than that - in order for the mains supply to feed the unit, and not the batteries. Plus it has to be higher to charge the batteries as well.

 

[AZdave]

Using L.Chung's circuit concept could I eliminate the 78L05's by doing it this way.

**broken link removed**

 

[Nigel Goodwin]

Using L.Chung's circuit concept could I eliminate the 78L05's by doing it this way.

Doesn't look a very good idea to me, and you're not eliminating the 78L05, you're simply moving it (or something very like it!) in to the 5V power supply. There's also no point in adding the diode wired like that either, all it's doing is dropping the PIC supply to 4.1V or so.

 

[AZdave]

Thanks Nigel, I'll go with L.Chung's circuit (minus the one diode), a 12V AC-DC regulated power supply and your recommendation of 8-10 batts.

 

[Nigel Goodwin]

Thanks Nigel, I'll go with L.Chung's circuit (minus the one diode), a 12V AC-DC regulated power supply and your recommendation of 8-10 batts.

If you're using a regulated 12V PSU, leave the extra diode in circuit - it prevents current from the battery flowing back into the PSU when the mains is off.

If you're using an unregulated PSU though, there's no need for the diode (although it does no real harm) as the bridge rectifier in the PSU does the same job.