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Power Backup Circuit Help

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vladtess

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Hello fellows!

I need help creating a back up circuit.

Here is my current set-up:

I have 120V -> 12.6V (300 mA) transformer that will power 5V regulator for a microcontroller
It will also power a 12V regulator for a bit higher current device (controlled by microcontroller via a transistor).

Now the backup will consist of 8 REGULAR duracell non-rechargeable batteries that yield 12 volts of power.

Now, the thing is that I have a limit on the components. I only have various resistors, couple of general purpose NPN transistors and a bunch of low voltage 1A diodes.

What circuit design can you suggest? Thanks much in advance!!!
 
When on batteries, do you just need to keep the uC alive without running the power circuits, or are you trying to run everything?
 
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How long do you want it to work on batteries? Reason I am asking is do you want to make an efficient 12V to 5V switching converter to get the most out of the batteries?

Otherwise, post a circuit of the existing 12V and 5V regulators, and I'll suggest how to backfeed the 12V from the batteries. I'm assuming that the 12V power circuit is energizing a solenoid or motor, and is not particularly fussy about voltage?
 
Use a diode in the output of the battery pack before the reguators, thats the down and dirty way. You will likly need more over head than 12.6 volts to run a 12 volt regulater, can you use raw 12 volts unregulated? Whats this for? Andy
 
Im sorry that i reply much later (had a bad flu). Here's what the project is: its a PIC regulated lock system, that will unlock a .. well, lock upon entering the correct combination. I decided to use raw (unregulated) power supply for the lock. Here's a possible schematic that I might use. Attached!! sorry if a bit unclear!

How much I want it to work off the grid? well maybe a couple of days. But again. lock will only consume power when unlocking which will be a few seconds per door opening.

btw, here's the lock that I'll be using: https://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=160456050893&ssPageName=STRK:MEWNX:IT
 

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OK, your pick will always use some power limiting your battery life. What type of battery will you use? Your power supply should always be charging your battery and your battery should power the PIC and lock sort of like a lighted emergency exit sign. If this is for a door way then it should fail open not closed. Andy
 
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4pyros, what do you suggest i do? I need to be able to to open the door when there is no power. Thanks!!

I expect very rare outages that will last a short time
 
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If it is an egress door than it has to fail open, the lock in your link fails closed. If your circuit or battery fails and its all on the other side of the door than you will have to brake in.
what do you suggest i do?
Depends on what you are trying to do. If you can let us know what this lock is for we may be able to come up with a plan. Andy
 
Basically heres the thing. this is for an apartment door and it happens that we come in and come out way too often (i work a superintendent) and it gets annoying locking the door everytime we enter or exit. If power fails, i dont want the door to be open when i am away, even though if i go away far i would use regular mechanical lock. The ic will not consume much although i worry that the regulator may consume a lot. I plan to use the regular AA duracell batteries (12v) . Thanks for support!!!
 
Backup

Since you only use the batteries when power is lost I think you will be ok. I think AA are about 2200 MAH so probably at least 50 hours powering the PIC. I'm not sure I wouldn't replace the regular batteries with rechargeable just to avoid the time when they do go dead. I think you only need to do 2 things. Add a filter cap to your AC supply so the battery doesn't act as the filter. Then a diode so the battery is only used when there is no AC.
 

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You need a combo lock like in a car door, Your backup should be a key not a battery. Andy
 
Since you only use the batteries when power is lost I think you will be ok.
And if any of that fails during a fire and someone can not get out because the batterys are dead. I would not want that on my head. Andy
 
:eek:Guess you could put the lock on the outside so you don't lock yourself in or a little slider on the inside to push it back.
 
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:DI have the same problem with my smoke alarms but I found out that if I let the batteries go dead they will warn me all the time.
 
The fire/etc safety is important and I have considered it before even working on the PIC code. I will have a mechanical, electricity independent solution. I'll have a small lever that will have to be turned 90 degrees to release the door.

If I want to create a charging solution, a circuit where, as suggested by someone, the batteries will power the system and will be charged constantly, what would such a circuit be? Any web pages?

Also, what is a filter cap and how does it work?

Thanks!!
 
Filter

A filter cap is like a little battery. It helps to smooth out the AC from your bridge rectifier. In this case I chose 330 UFD so the voltage to your solenoid and battery would stay above 12 volts. Fig. 1. Without it the output voltage would look like Fig. 2. As you can see it drops to 0 volts in between cycles. In your case this would discharge the battery.
I'm not sure a rechargeable system is good because the voltage from rechargeable batteries is lower so you would maybe need more of them to power your solenoid. Having said that, if you use rechargeable I would use Ni-Cd just because you can trickle charge them all the time without damage. Fig. 3.
 

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You guys are very smart!! Thanks so much for your options. Just a few questions if I may. How did you calculate the correct capacitance, so i know in future. Also, I see R1, that is the load, or ports that connect to my system (PIC & lock)? And, the diode, may I use 1N4007 diodes for the bridge as well as for the battery backup? ThANKS!!!!
 
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