Arduino Uno - 45 Projects for Beginners and Experts
Figure 53. Power over USB: follow the USBUCC wire. The only components between the USB connection and the +5 volts pin is a fuse and a mosfet. The fuse is a polyfuse of 500 mA, and this corresponds to the maximum current that a normal USB connection on a PC can supply.33 The FDN340P mosfet accepts a continuous current of 2 A according to the datasheet, so that is high enough, but we should also check the heat dissipation. The FDN340P is not equipped with a heatsink, and according to the datasheet this limits the heat dissipation to PD = 0.5 W. Since the voltage drop over a mosfet is about 0.7 V we can calculate the maximum current with this formula: P=V*I With
P = the power (W) V = the voltage (V) I = the current (A)
Entering our data yields: Imax = PD / V = 0.5 / 0.7 = 714 mA That is more than the polyfuse can handle, so the maximum current is limited to the polyfuse maximum current, which is 500 mA. The Arduino itself needs some of that power as well by the way. As a rule of thumb the Arduino Uno uses about 50 mA. The rest can be drawn from the +5 volts pin. Do note that anything connected to other pins must also be powered from that same 500 mA! 6.0.2. An external power supply
With an external power supply we mean one that is connected to the power supply plug on 33 A polyfuse is an automatic fuse. When the current exceeds a set value the fuse opens, breaking the circuit. Remove the over-consumer and after some time the polyfuse closes again. Unless the current was so large that the polyfuse itself was damaged.
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