Switching power sources, battery charging

[Demon]

Robert, if your siren needs an absolute minimum of 12 V, it's likely not going to work anyway with a 12V battery, as the battery could get under 12V fairly quickly when being discharged depending on the load and it's capacity. I think you need a proper spec for what the circuit produces i.e. "Must operate from a 12V sealed lead acid battery and/or an external power supply between 12V and 16V and must produce 12 V regulated to +/- 0.5 V"... that sort of thing. Further, you need to specify with tolerances, the current needed and for how long, and under what conditions. If you plan to mount it outside in a garage or something, the temperature range will be important too....

These are piezo residential alarm sirens, not humungous horns. I have an old DSC1000 system in the house and it works fine using such a horn and a 12V 4Ah battery. I am upgraded to a 12V 7Ah with this project.

My plan is to make 3 identical systems; house, garage, shed, all interconnected. If one system is triggerred and disabled, the other systems will still be triggerred (making it very difficult for someone to overcome them all quickly). It's also possible the cables between the buildings could get damaged.

Here is a pic of the current system to give you an idea how the installed system operates:

¨... The circuit shown is way too complicated for what it does - effectively switching the load to one supply or another. You would not need 2 FETs for each leg - power VFETS have a saturated resistance of a few milliohms depending on the device chosen. At 5A you wouldn't even need a heat sink. ...

I liked this circuit because it seemed so much simpler than so many other designs I've googled; some even used mechanical relays. So how would I go about connecting VFETS, and which model? Remember this is my first foray in MOSFET territory so schematics help me understand (I'm visual by nature).

¨... And how does the battery get charged? ...

My initial idea was to keep things simple and bought this universal floating charger:
http://www.rpelectronics.com/fc-612c...ger-6-12v.html


I wouldn't mind handling the charging myself if I find a design I can manage.

¨... You might be able to connect the battery charger to the battery and power from the battery at the same time (like a car) depending on your spec. ...

Wouldn't that shorten the life of the battery?

¨... Or you might need to design either a switching supply...

Schematics I've seen so far seem complicated. I would have liked to keep things reliable and simple if at all possible.

¨... or a linear regulator that can work with a very small volt drop ...

I don't think that would be good when the system would run off the battery. I would think I need all the juice I can muster under blackouts.


I'm embarking on this adventure because the DSC1000 is erratic after 20 years of service. The neighbours and family don't enjoy listening to a siren at 3am. That and it only has 4 zones.

Robert