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?

[Demon]

Amgen,

"Make voltage a little under the charged voltage of battery."

As I posted, battery voltage is not a fixed thing. It can vary from 11.something up to 14.5 volts under charge. That's the main reason why I have a hard time understanding how to use diodes.


Charlie,

I'm adding later the trickle charger shown in post #5 set at 12V 200mA.

The resistor might permit charging, but will it shut off when the battery will be full, or increase charge rate after battery has been drained after a power failure (I'm guessing no). Also, won't constantly charging the battery be bad for battery life over the long run? That would mean adding a sort of current monitoring circuit.

About decoupling caps on regulators, yup, leaving them out was intentional. It's the hexfets I'm interested in knowing if I have them wired properly. As far as I saw, I only need curent limit and pull-down resistors.

It's a Solex SB1270 12V 7Ah sealed lead acid battery.

Robert

[amgen]

Robert,
Generally for lead acid batteries, the charge ranges from 100% to 0% as the voltage falls from 12.7 down to 10.5......in that range. --see chart--
So from experience, if you charge and maintain the voltage at 13 to 13.5 volts, that's a fully charged battery, and not overcharged...and when you charge up to 14.5 volts, that's kind of stressing the battery over time and will reduce the life of the battery.
So, if you were to set the voltage from the regulator to say 13.8 to 14, the diode drops .3 or .6 volts depending on the diode type, and you will maintain the battery voltage to a fully charged and not overcharged state continuously.

don

[Demon]

Thanks.

Now I have to do some shottkey shopping. I think all I have are simple switching diodes.

Robert

[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.

[Charlie]

Charlie,
The resistor might permit charging, but will it shut off when the battery will be full, or increase charge rate after battery has been drained after a power failure (I'm guessing no). Also, won't constantly charging the battery be bad for battery life over the long run? That would mean adding a sort of current monitoring circuit.

About decoupling caps on regulators, yup, leaving them out was intentional. It's the hexfets I'm interested in knowing if I have them wired properly. As far as I saw, I only need curent limit and pull-down resistors.

It's a Solex SB1270 12V 7Ah sealed lead acid battery.

Robert

That battery can be left on trickle charge forever, which is how a UPS works (or the telephone network for that matter). There is really no need for a fast charge cycle after a power outage unless you get those outages really often. Say you do your calculations and you have 10 hours capacity in the battery. After a 4 hour outage, you are down to 6 hours capacity until it recharges. How likely is it that you would have an outage greater than 6 hours within the next few days?

So use a 14V regulator and any old diode you have around (4000 series will be perfect for this). You will then have 13.3 to 13.5 (depending on load) available for your siren when on house power, and between 12 and 12.2 available on battery (that will ramp down as the battery discharged. The siren will still work even when the battery is almost flat - no need for schottky diodes or other low forward drop devices.

The hexfets will work in that configuration, but you only need one of the 10K resistors to VSS, and I'd be temped to put a resistor in series with the gates in this application. I'd also change the logic so that you can turn both devices on to charge the battery somehow, maybe in some sort of duty cycle to pulse charge... actually, I'd just use the diodes, with the resistor around the second diode, value set to maybe 1K (need to think about it a bit more).

[Demon]

That battery can be left on trickle charge forever

That is what I needed to know.

There is really no need for a fast charge cycle after a power outage unless you get those outages really often.

No, not really. I have a generator for extended outages..

I can easily keep my little charger for new batteries, charge them, then replace the old one.

I still have a lot of work to do on the control panels. Then I'll be able to determine exactly how much maximum current the circuit will need. I'm aiming for 1A max (700-800mA with siren).

I still have a few unknowns, like the fire and CO detectors, window break detectors, etc.

Thanks guys. I'm really liking the diode technique now that I understand more what goes on.

Robert