Hello all! First time poster and Arduino / electronic noob with a question on my first project.
Basically in my project I am trying to create a circuit that will switch to a battery (9v) when power is cut from the main power source (12v wall-wart).
I have attached an image of my schematic so far.
This circuit works and does what I want it to do however the voltage change is drastic (instant drop from 12v to 9v) from what I have read the Arduino's power regulator doesn't like this.
(To avoid confusion the resistor on the far right is my load in order to simulate the Arduino)
I have the thought of using a capacitor over the power and ground lines going to the Arduino to help smooth this transition, however I have little (read no) experience with capacitors.
Will using a capacitor solve my voltage drop issue?
Is there a better way to do the power switch?
Regulators are designed specifically to cope with widely fluctuating input voltages (its their whole function!), this shouldn't be an issue at all. No need for any more capacitance than already in the circuit. Diode supply switching like this is fine (you lose 0.7V or so to the diode forward drop, this can be reduced by using schottky diodes with their lower voltage drop (0.3 to 0.4V).
The caveats about PP3 sized 9V batteries don't apply so much to rechargeables BTW - these should be able to source somewhat more current. Neither sort have much capacity which is another issue if used as mains-replacement. Larger 9V batteries are available, in the UK these are called PP6 and PP9, but they are rather specialised. I tend to use a bunch of (rechargeable) AAs in a battery holder (more capacity, you get to select the voltage by the number of cells).
I will hopefully be receiving my Arduino today and can run some tests with my current circuit.
From what both of you have said I may go with AA's simply for the cost effectiveness.
The actual length of time the project needs to run on battery power is very short, maybe a few seconds.
I am also going to try and include a way for the Arduino to turn off the power supplied from the battery until main power is resupplied. This way the battery doesn't drain every time the Arduino looses power.
If you don't expect to lose power very often, a lithium PP3-size 9v battery may be a good option. They are quite expensive but have a long shelf life, a capacity of 600mAh or more, and can supply quite a lot of current (unlike the alkaline version). See http://data.energizer.com/PDFs/la522.pdf for an example.
The problem with using NiMH cells as backup batteries is that the constant overcharging wears them out. That's why PCs use lithium coin cells to back up the clock, not rechargeable cells.
