/diy/ - Do It Yourself

>>2599399
Capacitors are not the best solution for powering a device while the main power source is off, you use batteries (rechargeable if possible) for that. If your arduino uses maybe 1 mA at 5V you could get away with using a capacitor, but if it draws something like 1A, there is no way you can power that with just a regular capacitor, you'll need a pretty big capacitor that would be significantly larger than a simple battery with a charging circuit would be, and most likely more expensive as well, and you still have to deal with the inefficient conversion. There is a better way to accomplish what you want, which could be a nice project: ditch the voltage regulator and use a buck converter with a rechargeable battery backup system with a charging circuit. It's a bit more complicated but would be the ideal solution. Should you still want to use a cap and diode, here is what you do:


1. Measure the current that the arduino draws (preferably peak current to have error margin), this is important. You could try to measure this with an ammeter or you might get lucky reading the datasheet
2. Measure the voltage, this is also important (voltmeter or datasheet)
3. Determine the resistance = Voltage/current
4. Determine the capacitance using the RC time constant by solving equation shown in pic related where:
* Vs is the initial voltage of the capacitor at the interuption of the battery power (Vs = Vbattery(12v) - Vdiode(~0.7V for silicon).
* Vc is the voltage at which your capacitor voltage is no longer enough to output 5V across your regulator (check the datasheet, different for each regulator, usually 0.3-0.7V above the output voltage, i.e. 5.3 to 5.7v)
* R is the resistance found in 3)
t is the time for the capacitor to output 5V, in this case you stated 30 seconds.
* C is the capacitor value you need in Farads ( convert to uF).

Not the best way, but very cheap with minimal components. Critical: Diode required to prevent cap discharge during cranking.