12v supply with battery backup

[pixman]

Hi

HAS ANYONE GOT A SIMPLE CIRCUIT FOR A MAINS TO 12V DC SUPPLY WITH BATTERY BACKUP AND CHARGING FOR THE BATTERY. THE CIRCUIT I AM USING DOES NOT DRAW MORE THAN 400mA. THE BATTERY I WOULD LIKE TO USE IS SLA 12 0.8AMP.

THANKS

 

[ericgibbs]

Hi

HAS ANYONE GOT A SIMPLE CIRCUIT FOR A MAINS TO 12V DC SUPPLY WITH BATTERY BACKUP AND CHARGING FOR THE BATTERY. THE CIRCUIT I AM USING DOES NOT DRAW MORE THAN 400mA. THE BATTERY I WOULD LIKE TO USE IS SLA 12 0.8AMP.

THANKS

hi,
This is a simple SLA charger circuit.

 

[pixman]

Thanks eric
am i correct in saying the secondary side of the transformer must be 30v or can i use a lower voltage around 18v.

 

[ericgibbs]

Thanks eric
am i correct in saying the secondary side of the transformer must be 30v or can i use a lower voltage around 18v.

hi,
Its a centre tapped 15V 0V 15V transformer.

If you have a 18V transformer it can be used but the by pass transistor will run hotter. OK

On the circuit I have posted I used a dual 15V secondary transformer connected in parallel.

 

[Mr RB]

A simple system is just buy a 13.8v regulated DC supply and connect it to your 12v battery. Then run the load from the battery.

Most of these supplies won't drain the battery when their mains power is disconnected, but you might want to check that. It's also possible to put a diode between the supply and battery and tweak the supply voltage up +0.6v with the internal trimpot.

(edit) This is pretty much the same thing that Eric's circuit does.

 

[pixman]

Mr RB

So if i use a lm317 type power supply and set the output voltage to 13.5 volts it should work.

 

[ericgibbs]

Mr RB

So if i use a lm317 type power supply and set the output voltage to 13.5 volts it should work.

hi,
Check that the LM317 will withstand a reverse voltage of ~12V when the mains power fails and the battery is still connected.

Also if you use your 18V transformer, check the heat dissipation of the LM317 when charging a 'low' battery.

Lets know how it goes,

 

[Mr RB]

If using a LM317 I would use a series diode on the output, that should protect from any battery drain issues.

A fuse (2A ?) might be a good idea too in case you accidentally connect a big battery in reverse.

 

[CRISespecialist]

hi mr.pixman,mr erick has good idea from what he give to you,schematic doagram you can fallow that,instead of 15v center tapped,if you cannot find in the market,you can use 12v center tapped, as my idea for replacement,but observed the output if gitting hotter or not,with a load.
cris...

 

[pixman]

i tried the lm317 power supply and it works very well. i put an amp meter in line and as the battery charges
the current drops away. till almost zero. its an analoge meter so not sure what it stays at. will use a digital
meter and check what it is.
i put a diode on the output so current drains back into the power supply and a 1 amp fuse.
the battery i will be using is only 1,2 ah. i still want to try ERIC's one.

David

 

[jst3712]

hi,
This is a simple SLA charger circuit.

Hi Eric. I was just reading through this thread.....
Could you please explain what the "Bypass Transistor" does and also the 27 ohm resistor? Current limiting?

Also, how would one control the voltage drop that's caused by the load? That would have a negative effect on the charging process, right? (if voltage falls slightly below 13.5v).
Thanks.