Battery charger for powersupply

For a simple burglar alarm I need 12 V and 500 mA max. and a batterybackup. I have found a 12 V lead flees battery 2,3 Ah and a batterycharger (600 mA at 13,8 V). Now I would like to ask if some one knows if it is possible to run the alarm direct over the charger and battery, and then save a 12 V power supply?

I am a hobbyist

Morten

The answer is maybe. If the alarm can tolerate +14 or more volts and possibly a negative voltage spike when the charger is unplugged then you do not need a PS. As you look at the alarm requirements you might get by with a series diode to eliminate the chance of a neg spike. If you need +3 or 5 volts for a MCU or logic a voltage regulator could be added.

In summary a battery charger and battery is a power supply, you may need a few power conditioning components after it depending on the requirements of the alarm.

Hi HMS

Thanks very much for the answer. The alarm runs 10 - 15 V/DC or 9- 12 V/AC. Think the series diode and and maybe a 15 V zener diode to ground can help. Just want to keep it as simple as possible.

Morten

morten said:

Hi HMS

Thanks very much for the answer. The alarm runs 10 - 15 V/DC or 9- 12 V/AC. Think the series diode and and maybe a 15 V zener diode to ground can help. Just want to keep it as simple as possible.

Morten

Click to expand...

If you go with the Zener you might want a fuse or a resistor in series with the input.

morten said:

The alarm runs 10 - 15 V/DC or 9- 12 V/AC. Think the series diode and and maybe a 15 V zener diode to ground can help. Just want to keep it as simple as possible.

Click to expand...

No need for the zener diode, and no real need for the series diode - the charger will prevent a negative spike, and the battery would further suppress [if it ever happened].

The diode and Zener are not needed if all of the remaining parts are functioning normally.
The diode protects the circuit if the battery is connected with reverse polarity. It also provides protection from the negative spike from a charger disconnect in the event the battery is not connected or has sat without being charged for a extended period and is completely discharged to the point where it is damaged. Without a healthy battery the charger would likely exceed +15v which is where the Zener and a fuse (or the series diode being sacrificed) are needed.

The battery is capable of high current in the event of a component failure or incorrect connections which could lead to fire if safeguards are not designed in. I understand this is a hobbyist design but if it was consumer product and the consumer unplugged it for 9 months and then plugged it in, and it was damaged (possibly with smoke coming out of it) because the battery was dead from not being charged he would likely buy another brand to replace it. A third scenario is all works well for a few years and the battery just dies from old age. If he plugged it back in without watching/checking it and it caused a fire you would pay if it was your product. I've put products through UL / CE / TUV and JET type acceptance and first time through it was a pain, but it has made me think beyond just getting a design to work.