This does not mean you won’t be able to use the USB data lines – those will work fine – it’s just the USB won’t be providing any power to the board. IF the voltage at above that switch is greater than 6.6V, then it opens the switch, which shuts off the power supply line form the USB cable – what this mean? It means, if you are powering using Vin or the DC jack, then the power from the USB cable will be disabled. That switch is actually a P-channel MOSFET, that you can think of as a switch – either being open (no current able to flow), or closed (current able to flow). Notice the dotted connection between the DC Jack and Vin circuitry to what we have drawn as a switch. Also, with no diode, there is also no voltage drop. What you don’t have on the Vin pin is any reverse polarity protection – because there is no diode protection there – so you’ll want to make sure to connect positive voltage to this pin. The voltage requirement is the same as with the DC jack, because you’ll notice Vin is supplying power to the 5V regulator as well – in fact powering with Vin is almost exactly like powering with the DC jack. Vin is a pin on the Arduino power rail where you can directly hook a power supply line into the board. If you use the DC jack, you may want to bump up the input voltage a bit. If you provide the DC jack with 7 volts, the 5V regulator will see an input of 7V – 0.7V = 6.3 V, which is getting uncomfortably close to the minimum 5V regulator input of 6.2V. There’s one more important thing to mention: the diode in line with the DC jack has a voltage drop of roughly 0.7 volts. Notice that the 5V regulated power supply also power the 3.3V regulator on the board, which then provides power to the 3.3V pin. So if you go attaching a bunch of stuff to your 5V power rail that draws a bunch of current, like say servos that are going to be under a high load, then you’ll be drawing more current than what the regulator is rated for, and you’ll burn up the part – something you’d want to avoid. In fact, the components on the Arduino UNO board itself will be using around 25mA of current. What’s the big deal about the current? Well here is the deal – all the electronics stuff you attach to your Arduino board is going to consume current. OK – one more thing about that 5V regulator, it can provide up to 1A of current. Programming Electronics Academy members, check out the Powering Projects Course to learn more about powering your projects.
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