Dual power source

[Demon]

I've been reading a lot of threads and googling about this. I'd like to use HEXFETs as switches between sources. this is for a residential alarm system; 12V for siren, 5V for logic.



The built-in diode drops very little voltage and is intended to prevent reverse current. I kept things simple to illustrate, I'll add fuses, caps, etc later.

Comments? Suggestions?

Robert


Edit: U? R? Q? BT? Are unnumbered labels from QCAD that I forgot to erase from image.

[Demon]

I suppose I could use a 7814 instead of 7812 and replace the N channel hexfet with a diode; cheaper and just as good. The siren will work just as well with a bit more than 12V.

Forgot to say, only expect to power less than 1A with this switcher.

[amgen]

is not just 2 diodes giving same function ? shotkey diodes have only .3 volt drop.
when the 18 volts (from ac supply) drops out, the battery voltage starts giving current.
don

[amgen]

ok, boost up 7812 output by voltage divider or add diodes to gnd leg of 7812 (each diode adds about .6 volts). Make voltage a little under the charged voltage of battery. Use shotkey blocking diodes (only .3 volt drop). Add resister across battery diode to allow slow charging of battery.
disregard previous post.
don

[Demon]

I only need 2 of these circuits; one for the house and one for the garage. Both alarms are independant with sirens but both will communicate status to each other making it difficult to disable both at the same time.

1. I was planning on using a wall adapter trickle charger. I figured it wasn't worth reinventing the wheel when I can buy one for $29.95 locally; gut the circuits and place it in a metal shield within the alarm box.

http://www.rpelectronics.com/fc-612c...ger-6-12v.html




2. My 12V 7Ah battery is at 13V right now and it has been sitting on the shelf for at least a month since last charge. I took readings on first charge and initial charging was at 13.5V, intermediate was at 14.25V and trickle was at 14.5V (using 200mA setting). That could potentially place the battery voltage higher than mains side when trickling.

3. I looked at my old DSC1000 alarm circuit and it doesn't seem to use 2 diodes to switch sources; the circuitry seems more complex (18V from transformer in mains breaker box enters at bottom left). Same with the more advanced circuits on google. If 2 diodes would be good enough, I would think it would be common usage; less parts = more revenue. They have at least one IRFZ22 N-CH HEXFET (bottom left) and a LM317 regulator (on heat sink) and a 7805 at top left.



4. I can afford the HEXFETs, I already have some on hand and really like the "no doubt about it factor"; either it was ON or OFF regardless of voltage when controlled properly and have built-in shottkeys, switch fast enough and are rated for much more current and voltage than I will use.

Mains voltage ON opens mains N-CH HEXFET and shuts battery P-CH HEXFET.
Mains voltage OFF shuts mains N-CH HEXFET and opens battery P-CH HEXFET.

The concept seemed simple to me when I first thought of it, and then I became unsure the more I googled. I'm just not sure on the resistors and exact setup, should I also have caps in there? The googles I found were vague on some aspects, and not all of them were the same.

Robert

[Charlie]

Robert, like many problems there are several possible solutions. Your solution offers no advantage I can see, and being more complex has more places to fail. And you will need still more complexity to charge the battery, or do it manually and risk needing it precisely when you've got it out of the circuit. But with a bit of fiddling, it can certainly be made to work.
Don's solution is simple, more robust, and has the added advantage of trickle charging the battery.
Also, both circuits are missing decoupling caps for the regulators (presumably for simplicity in the drawing).
What sort of battery? Sealed lead acid / gel cell I presume?

[rsocor01]

I've been reading a lot of threads and googling about this. I'd like to use HEXFETs as switches between sources. this is for a residential alarm system; 12V for siren, 5V for logic.

Have you considered using a relay module? I found this nice module on ebay for $6.50.

http://www.ebay.com/itm/For-Raspberr...ht_4576wt_1133

According to their description

This is a 5V 8-Channel Relay interface board, Be able to control various appliances, and other equipments with large current. It can be controlled directly by Microcontroller(Arduino , 8051, AVR, PIC, DSP, ARM, ARM, MSP430, TTL logic)

They also have a 12V version of the module.