THE LIMITS OF THE CURRENT
Let’s start by remembering that you cannot pretend to power a load requiring a certain current, without the source being capable of delivering it, and that any load draws the power it needs and not the available one. Let’s see some examples, assuming that Arduino will draw the maximum possible (200 mA), the rest is then absorbed by the external components:
A – Global current consumption of 400 mA; in this case it is possible to power everything via USB, on the condition that the computer’s port (or the power supply with USB output, used as its substitute) is capable of delivering all the 500 mA considered as maximum limit;
B – Global current consumption of 600 mA; in this case it is NOT possible to power the circuit via USB since we saw that the limit set by the PTC protection fuse is of about 500 mA; we have then to resort to an external source applied to the JACK port, one that is capable of guaranteeing a current that is greater than the one required by at least 20-30%, thus for a total of at least 600-700 mA, and as stated before the exceeding current available is not used. The voltage for the peripherals can be drawn from the 5 V pin. As already seen, however, the USB connection may be kept for the programming of the micro or for the usage of the serial monitor.
C – Let’s see the case in which the total drawing is 1A; even in this case we have to resort to an external voltage to be applied to the JACK, but as we will see in the next paragraph we are operating at the extreme limit of the regulator, thus it will be needed to separate Arduino’s powering from the one of the external loads.
In the case in which there are no peripherals requiring a voltage greater than 5 V, for example 12 V, it is needed to resort to an external power supply, applied to the JACK socket, but to understand how to draw the two voltages one must always assess the currents drawn by Arduino and the 5 V and 12 V loads. Let’s see some examples:
D – Let’s see the case of Arduino (200 mA), with 5 V (50 mA) peripherals and 12 V (300 mA) peripherals; by applying a 12 V power supply with at least 1 A maximum current, the limits of the regulator (that we will see in the next paragraph) enable us to power Arduino and its 5 V and the 5 V peripherals; as regards the 12 V peripherals, on the other hand we could draw that voltage from the Vin pin;
e – Let’s see the case of Arduino (200 mA), with 5 V (300 mA) peripherals and 12 V(1A) peripherals; by applying a 12 V power supply with at least 2 A maximum current, the limits of the regulator (that we will see in the next paragraph) will impose to power Arduino alone with its 5 V, while it will not be possible to use the Vin pin (see following NOTE), at this point it a different solution should be found.
NOTE: let’s consider another factor, this time concerning the Vin pin; apparently there should not be limits of deliverable current, in addition to the one imposed by the applied power supply; however all the current will have to flow within the protection diode, that is anyway quite sturdy, but de facto one has to consider that the copper track, that connects the regulator’s input to the Vin pin placed on Arduino’s POWER header, is not very thick and consequently may not endure high currents, as it would risk burning.