Powering fans and arduino pro mini from single 12v supply
If you want to minimize power usage, the best thing would be to start with devices that use more power! :D

It's time for math! I'm going to use the generic L7805CV 5V, 1A linear regulator. Poking in the datasheet, we find that the quiescent (standby) current is max of 6 mA. I'm assuming this power drawn is 12V, not 5V.

Since a linear regulator is basically a variable resistor, we can figure out that the efficiency is:

Vout / Vin


...thus, our efficiency in this application is...

5V / 12V = 0.416666667% efficiency


... Read more here.

So, we can use the inverse of the efficiency function (Vout / Vin → Vin / Vout) to establish the amount of power it'll use. Final equation:

Power Used (W) = (12V / 5V) * Current Draw + (0.006A * 12V)


...which simplifies to:

Power Used (W) = 2.4 * Current Draw + 0.072 W


So, all we need to find is the power consumption of the Arduino. This forum post states the power consumption of different sleep modes:


SLEEP_MODE_IDLE: 15 mA
SLEEP_MODE_ADC: 6.5 mA
SLEEP_MODE_PWR_SAVE: 1.62 mA
SLEEP_MODE_EXT_STANDBY: 1.62 mA
SLEEP_MODE_STANDBY : 0.84 mA
SLEEP_MODE_PWR_DOWN : 0.36 mA


Let's give the Arduino a generous 15mA to sleep:

Power Used (W) = 2.4 * 0.025 + 0.072 W = 0.132 W


So, 132 milliwatts, idle. A bit further, we can calculate that, if your wall wart has an efficiency of 10%, it will actually draw 1.32 W from your wall socket. Using a general cost per killawatt hour ($0.125), we can calculate that it will cost you about $1.45 a year for that Arduino to be ran in sleep mode 24/7 (given that we used the maximums for everything).

It's pretty cheap, so I wouldn't worry about that power. A good switching regulator will cost more than to run it for a few years with a linear regulator. I'd use the onboard one.

If you still want to optimize things, the wall wart or the fan should be looked at first. Consider getting a switching wall wart instead; this will give you a higher return on your investment since both the fan and the logic use this.