I want to have my arduino hook up with a wall adapter and a battery just in case. But i want to make sure the battery still have a charge.
I just want to know if schematic is OK or if i break any laws.
If you have any comment you can leave them too.
Should be okay. I would use schottky diodes with low forward voltage drop, like 0.31V.
Too pricey?
How about something a little more reasonable:
A little more than a run of the mill 1N4004, but you gain 0.65V of voltage loss back:
The Zener is backwards.
The Zener will dissipate about a watt of power, big waste. Even a linear regulator would be preferable.
2xAA cells is not enough voltage, Arduino requires 5V.
Silicon diodes drop ~0.7V so 2xAA will only deliver about 2.3V or so to the Arduino.
I made so change i put a schottky diodes has ask by CrossRoads and i put my zener on the good side.
The Zener will dissipate about a watt of power, big waste. Even a linear regulator would be preferable.
Can a LM317 do the job ? or a LM350 ?
Arduino chip ( atmega328pu-p) can operate with a voltage of 1.8v to 5.5v so i should be alright.
Letnic:
Can a LM317 do the job ? or a LM350 ?
Probably. How much current is required?
Arduino chip ( atmega328pu-p) can operate with a voltage of 1.8v to 5.5v so i should be alright.
The 328P needs at least 3.8V to operate at 16MHz, 2.4V to operate at 8MHz, and can only operate at 4MHz with 1.8V. See the datasheet, section 29.3.
Can a LM317 do the job ? or a LM350 ?
Probably. How much current is required?
i will probably need around 30 -50ma
Arduino chip ( atmega328pu-p) can operate with a voltage of 1.8v to 5.5v so i should be alright.
The 328P needs at least 3.8V to operate at 16MHz, 2.4V to operate at 8MHz, and can only operate at 4MHz with 1.8V. See the datasheet, section 29.3.
I need to be at 8MHz so i will need 2.4v or more. So i will put 3 AA battery.
I run an arduino directly from a small lead (12v) acid battery, via a switching regulator, power goes out arduino stays on!
How do you recharge the battery when it occasionally needs it?
How do you recharge the battery when it occasionally needs it?
i change those battery, they are not rechargeable.
edit:
I run an arduino directly from a small lead (12v) acid battery, via a switching regulator, power goes out arduino stays on!
you always recharge your battery with a battery charger ? but for my project a 12V battery is too big.
CrossRoads:
How do you recharge the battery when it occasionally needs it?
Lead-Acid batteries take a float charge for an indefinite period right? This is basically the concept behind a computer uninterruptable power supply. The lead-acid battery is being constantly charged at a low rate (a "maintenance" charge), and the load side of the UPS is powered by the battery using a power inverter. That way the battery is 100% charged when you lose the power, and there is no delay caused by any kind of "switch" to battery power.
The recharging question was to cjdelphi, sorry about the confusion there.
CrossRoads:
The recharging question was to cjdelphi, sorry about the confusion there.
I have a 19v 300ma charger and a little switching regulator to float the battery at 14.5v.
Letnic:
I need to be at 8MHz so i will need 2.4v or more. So i will put 3 AA battery.
I have a project that runs on 3.3V @ 8MHz that has a battery backup. Here is the power supply schematic. A 9V wall wart provides main power; I used a 1N400x as I wasn't worried about a little voltage drop there. A Schottky diode minimizes voltage drop from the battery and I also used a low-dropout regulator. I used 4xAA to ensure a longer run time, but 3xAA will probably work. I'd test it to make sure I get the run time I need. If current requirements are 50mA or less, it should run quite a while, this circuit can supply considerably more than that.
If I had it to do again, I'd use a 5V wall wart, a switching regulator, and 2xAA but that's another discussion 
Running at 3.3V and 8MHz involves replacing the onboard regulator and some open socket flashing doesn't it? I'm interested because of the power saving, I saw the 5V 16MHz standard uses a relatively large supply current (80mA) and I don't really need a 16MHz chip for some applications like IR remotes, that are always on. (Sorry, not trying to derail the thread, just thought there may have been some progress in low power Arduino).
emueyes:
Running at 3.3V and 8MHz involves replacing the onboard regulator and some open socket flashing doesn't it? I'm interested because of the power saving, I saw the 5V 16MHz standard uses a relatively large supply current (80mA) and I don't really need a 16MHz chip for some applications like IR remotes, that are always on. (Sorry, not trying to derail the thread, just thought there may have been some progress in low power Arduino).
There are 3.3V 8MHz Arduino models; the project I mentioned above was a custom board. I measured an Uno at around 40mA, but nearly three-quarters of that is stuff on the board other than the microcontroller. For low-power work, I always go with a custom board.
I need to be at 8MHz so i will need 2.4v or more. So i will put 3 AA battery.
I have a project that runs on 3.3V @ 8MHz that has a battery backup. Here is the power supply schematic. A 9V wall wart provides main power; I used a 1N400x as I wasn't worried about a little voltage drop there. A Schottky diode minimizes voltage drop from the battery and I also used a low-dropout regulator. I used 4xAA to ensure a longer run time, but 3xAA will probably work. I'd test it to make sure I get the run time I need. If current requirements are 50mA or less, it should run quite a while, this circuit can supply considerably more than that.
If I had it to do again, I'd use a 5V wall wart, a switching regulator, and 2xAA but that's another discussion smiley-grin
why there so much capacitor on your schematic ?
what is V+ for ?
Why don't you use a 3.3V zener diode ?
In my project i need a way to check the voltage on the battery and if the main power is down. There are a easy ways of doing this ?
edit : adding schematic version 3
emueyes:
Running at 3.3V and 8MHz involves replacing the onboard regulator and some open socket flashing doesn't it? I'm interested because of the power saving, I saw the 5V 16MHz standard uses a relatively large supply current (80mA) and I don't really need a 16MHz chip for some applications like IR remotes, that are always on. (Sorry, not trying to derail the thread, just thought there may have been some progress in low power Arduino).
See here: Gammon Forum : Electronics : Microprocessors : Power saving techniques for microprocessors
I did a temperature/humidity logger: Gammon Forum : Electronics : Microprocessors : Temperature and humidity sensor - battery powered
That uses an estimated (average) 36 µA of power. Whilst asleep (most of the time) it uses 6 µA. That runs off 3 x AA batteries. If you sleep the device the need to drop down to 3.3V (in itself not a bad idea) is somewhat reduced because the power usage while asleep is pretty much the same anyway (very low).
Letnic:
why there so much capacitor on your schematic ?
C6 is probably a bit larger than needed, but it's like chicken soup, it can't hurt. Most linear regulators should have a ceramic capacitor on the input and output for high-frequency bypass (C2, C4). C5 was actually not part of the power supply, but a bypass for an IC elsewhere in the circuit.
what is V+ for ?
Specific to my project, can be ignored.
Why don't you use a 3.3V zener diode ?
Zeners are inefficient regulators, especially at higher currents (as we saw earlier in the thread). To regulate supply voltages, it's much better to use a device that is designed for that purpose. It will be more efficient and probably more accurate than a Zener.
In my project i need a way to check the voltage on the battery and if the main power is down. There are a easy ways of doing this ?
Actually my project had a check for the mains power, see circuit below. Note I do use a 3.3V Zener there to limit the input voltage to the MCU (this is a very low-current situation). My circuit didn't check battery voltage though. An easy way would be as shown below, just connect an analog input to the battery. I might prefer to devise some sort of load test though. That would probably require a MOSFET, load resistor, and another MCU pin.
All schematic posted above lacks proper power input protection. Ferrite bead, fuse, TVS, EMC... Please read:
http://nandblog.com/power-input-protection/
Also, if you don't want tot waste battery power on the Schottky diode, use a mosfet instead.
http://nandblog.com/reverse-power-supply-connection-circuits/
Moderator edit: Changed mailto tag to url tag (Nick Gammon)
NANDBlog edit: 
wrong button...
NANDBlog:
All schematic posted above lacks proper power input protection. Ferrite bead, fuse, TVS, EMC... Please read:
http://nandblog.com/power-input-protection/
Also, if you don't want tot waste battery power on the Schottky diode, use a mosfet instead.
http://nandblog.com/reverse-power-supply-connection-circuits/Moderator edit: Changed mailto tag to url tag (Nick Gammon)
Interesting blog, I'll have to try the MOSFET trick. Regarding input power protection, I wasn't going for UL certification or anything 
Anyway the input was from a wall-wart. It should be reasonable to assume that since it's certified then all is well, but I know better than to bet on either the competence or the scruples of whoever built it. But sometimes good enough is good enough.