Switching power sources, battery charging

[Demon]

Hi,

I'm designing a home alarm system and need to be able to switch during blackouts between a 15VDC power source and a 12V battery. I have found this design using mosfets:

http://saeedsolutions.blogspot.ca/#!...-supplies.html



I need to support 5A so I'm using IRFU6215 mosfets instead (150V 12A 110W). Comments and opinions appreciated.

Robert


EDIT: Changed to 15VDC source to remain above battery charging voltage.

[pedja089]

Why not to use just 2 diodes?

[Demon]

Actually, his article using diodes was the first one I looked at. But I eventually went with his transistor approach.

D'oh!

So, how do I go about selecting a diode that can take 15V 5A?

Robert


EDIT: Based on 1N4007 classification on Digikey as rectifier standard 1KV 1A, I searched on Digikey for DIODE - RECTIFIER - STANDARD - 5A and came up with these:

BY500-400 400V 5A thru-hole
S5GC-13-F 400V 5A surface mount
(I like to have both types)

I figure it's not like I have to go up to 1000V...?

[Demon]

Why not to use just 2 diodes?

Now I remember why I went with his transistor design, I need 12V for the siren.

If I use the diodes, the source must always be higher voltage than the battery. I end up with 15V from the main or "about" 12V from the battery. And then I can't add a 7812 voltage regulator at the end.

If I use transistors, I can use a 12V source. The OP AMP doesn't care if it's 12 or 15 volts, as long as it's there.

(and that's why I initially had a 12V source)

Robert

[pedja089]

You can't put 7812 when device is powered from accumulator. Because 7812 have voltage drop about 2V, so you need at least 14V on input to get 12Voutput, and 7812 is only 1A rated in to220 package, in to3 is 3A rated.
If you need all ways 12V on output, use buck/bust regulator, or use boost to get 15V and then use buck regulator to get 12V from 15V. This way your input voltage may be from 5-6V to 30-40V, depending on device. You also have simular PSU on ebay
http://www.ebay.com/itm/DC-Boost-Buc...item5af2b8bf04

[Charlie]

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.

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. And how does the battery get charged? 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. Or you might need to design either a switching supply, or a linear regulator that can work with a very small volt drop - again depending on that spec.

It's a fun problem.

[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