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Topic: power source: wall charger AND battery (Read 1 time) previous topic - next topic

karlok


they will be at least 1 volt over 5 VDC those pins are made for

Oh, I forgot this.
I will think about a solution for this when I have finished the actual idea.
using Arduino Uno Rev 3

karlok

I have still a problem:
I am afraid that the current "I" is the sum of the two power supplies. I already know that the voltage "U" will not be the sum because the power sources are switched parallel.
Is this really the case, and if it is the case: Do I have to be afraid that the Arduino cannot work with that?
using Arduino Uno Rev 3

Far-seeker

#12
Aug 28, 2012, 04:24 pm Last Edit: Aug 28, 2012, 04:26 pm by Far-seeker Reason: 1
You don't need to worry about this in this case.  While it is true that the current at a node is the the sum of all currents flowing into it, in this case these currents shouldn't be excessive.  

This is partially because both the wall wart and the battery are (more or less) constant voltage, variable current power sources.  While it is possible to draw upto some limit of current from either source, neither source is trying to force a specific level of current into the circuit (as would be the case for a constant current source).  So the actual current drawn is going to be determined by the load the rest of the circuit presents to the power source; the most the power source can supply if positive is shorted to negative, a little bit less if the load is slight, or not a lot if the load is large.

The other reason it's not going to be an issue is the built-in current limiting features of standard Arduino boards.  Because the power is going through the voltage regulator circuit, it will basically only draw the current necessary to power the board and anything powered from the board.  Now a short either on the Arduino board, or in another part of the circuit could possibibly cause damaging amouts of current to be drawn, but unless there is a problem like that the current won't be excessive.

Of course, this isn't the case if you want to attach 5 VDC to the +5V pin, or 3.3 VDC to the +3.3V pin .  Then you would need some way to limit the current to below the microprocessor's rated maximum current (this information will be in the datasheet for the specific microprocessor chip in question).

karlok

Thank you very much! Now, I feel sure that this will won't cause trouble.
I think this is a very easy way if you want to have more than one power source
However, can I be really sure that the battery's voltage is always less then the one from the wallwart? It is very important for me that the battery is only used if the main power supply isn't working anymore. (I want that the batteries aren't even used if everything is normal.)
using Arduino Uno Rev 3

Far-seeker

With a design this simple, being completely sure the batteries won't draw any current while the main power is still on isn't possible.  1.5 VDC is only the nominal voltage for alkaline primary cells. A completely new battery may actually measure higher than 1.5 VDC on multimeter, and one that's partially discharged will measure somewhat less than 1.5 VDC.  So it is possible for a very fresh set of six alkaline AAs (e.g. if they each are about 1.6 VDC) to initially have higher voltage than a 9 VDC wall wart outputs (which may also be slightly above exactly 9 VDC).  However, most of the time the batteries will have at least slightly lower voltage, and that should be enough to keep any current drawn from the batteries very small unless the main power goes out.

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