Battery Supply question.

[Dusty]

Hi all, I'm quite new to this ...I have a stand alone Shed alarm ,its a quite simple but effective unit, entry time fixed ~15 sec. exit about the same 110dB alarm . It's powered by a 9v battery this lasts about 3 months.
I have a large (24 Ah) rechargeable 12v battery, I would like to use this to power the alarm circuit. is there an easy way to do this ?
The circuit may work with 12v but I expect the timing would change...
A Dc to Dc converter is probably the way to go, but I'm not that good , would this be an extra drain on the battery (how efficient are these?).
Thanks,

 

[Resqueline]

It depends on the type of regulator how much current it draws by itself. Expect 3mA extra for a 78L09 regulator. Other types may draw much less.
It also depends on the temperature how large the self-discharge current in the 12V battery is. (I guess it's a SLA.)
If the temperature is high then you'll need to recharge it every 3 months anyway.
A 9V alkaline is around 500mAh, so the alarm draws 500mAh/(3monthsx30daysx24hours)=0.23mA.
The alarm (w/a 7809 regulator) would in theory last 24000mAh/3.23mA=7430hours=10months on the 12V battery, but this requires a low ambient temperature. Also, a SLA battery doesn't get a long life unless it's discharged only half-way before being recharged again, resulting in only 5 months of use.
My suggestion is that you're better off putting 6 alkaline AA's in a battery holder to give you 9V & 2000mAh. That should make it last one full year w/o problems.

 

[Laplace]

The simplest solution would be to try operating it from 12 volts and see if the timing changes. For instance, RC time constants don't necessarily change with supply voltage. The next simplest solution might be to insert a voltage dropping diode coming from the battery, e.g., a Vishay 1N5225B 3.0 volt zener diode should be good for up to 100 ma current.

 

[roltex_rohit123]

Hi all, I'm quite new to this ...I have a stand alone Shed alarm ,its a quite simple but effective unit, entry time fixed ~15 sec. exit about the same 110dB alarm . It's powered by a 9v battery this lasts about 3 months.
I have a large (24 Ah) rechargeable 12v battery, I would like to use this to power the alarm circuit. is there an easy way to do this ?
The circuit may work with 12v but I expect the timing would change...
A Dc to Dc converter is probably the way to go, but I'm not that good , would this be an extra drain on the battery (how efficient are these?).
Thanks,

post your schematics then we'l if your ic is capable of working on 12 volts. you could directly change the 9 volt to 12 volt. if you it is a comercial one then they make it such that it cannot be modified.

 

[55pilot]

If you decide to apply 12V to a 9V input, the timing may be the least of your concerns. You may kill the device because you may over-voltage one of the input caps or you may generate too much heat in the input voltage voltage regulator.

If the input goes to a diode then a 10V cap and then a 7805, you could kill both the cap and double the power drop in the 7805.

On the other hand, if the design is robust, you may get away with it with no adverse consequences.

---55p