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I am fairly new to Arduino. When I am experimenting with circuits, do i need to worry about shorting out a pin if I accidentally connect the +5V pin on Arduino to an input pin on Arduino without a resistor? I just want to make sure I avoid destroying the board when learning how to create circuits.

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No, no short. Arduino (Atmega) pins default to inputs. Pins configured this way are said to be in a high-impedance state. Input pins make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 megohm in front of the pin.

Answer provided by Look Alterno and converted from a comment.

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  • However be careful that your code does not set any such pins to outputs.
    – Nick Gammon
    Commented Nov 7, 2017 at 20:15
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Apply 5V directly, without any resistor:

Arduino power

5 V socket

it is directly connected to the regulator’s output, thus the 5 V to power external loads to Arduino can be drawn from it. In the case voltages are not applied to the USB Port or to the JACK socket, the 5 V socket can be even used to power Arduino directly, if having an external stabilized 5 V source. One has to consider that, in general, regulators do not like voltages being applied to their output, but in this particular case this situation turns out to happen even when powering Arduino from the USB port, therefore we may assume that the designers judged this problem as harmless. Even in this case there is no form of protection, since both the diode and the PTC fuse are found above this socket and thus they do not have any active function. As in the case of the Vin socket, the voltage negative pole can be found on the board’s GND sockets.

NOTE: regardless of the input used, Arduino has a 3.3 V output socket to power loads operating at this voltage; in fact a second regulator, right for the purpose of generating 3.3 V, is directly connected to the 5 V. This socket cannot be used as input.

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  • So if I were to connect the +5V pin to an input pin (Say pin 7 just as an example) and the board is powered through a USB connection to a computer, would this short out the board?
    – theNewGuy
    Commented Nov 7, 2017 at 17:13
  • No, no short. Arduino (Atmega) pins default to inputs. Pins configured this way are said to be in a high-impedance state. Input pins make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 megohm in front of the pin
    – user31481
    Commented Nov 7, 2017 at 17:17
  • My suggestion use resistors (220 ohm or higher) in series with the inputs this will help protect you from mistooks. A 220 Ohm resistor will limit the current to about 31mA with a fault to +12. A higher value will be ok. Once you get comfortable with this you will no longer use need resistors. The current path is in the pin, through the "protection diode" to Plus supply (VCC) pin. Depending on your design this can cause the VCC on the board to rise.
    – Gil
    Commented Sep 20, 2020 at 3:21

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