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I'm very new to electrical engineering in general and picked up an Arduino recently. I have a couple questions:

  1. What will happen if I short 5V/3.3V/Digital pin output directly to ground? Will it damage the board or does the Arduino board have protection against it?

  2. If the above causes damage to the board or is bad practice, can I put a resistor between the power and ground to fix it? If so, how many ohms should the resistor be?

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  • \$\begingroup\$ External source may or may not have over-current protection, but the 5V/3V Voltage regulator on arduino board has fuses for protection from over-current. They limit the max source/sink current for each digital pin and Vcc/GND. \$\endgroup\$ – Mitu Raj Feb 22 at 10:50
  • \$\begingroup\$ the answer to the two questions at (1.) depend on whether the pin is configured as an input or an output, and if configured as an output, whether the output is set high or low ...... the resistor mentioned in the two questions at (2.) would not do anything to prevent any damage caused by grounding the pin \$\endgroup\$ – jsotola Feb 23 at 2:20
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Shorting

What will happen if I short 5V/3.3V/Digital pin output directly to ground? Will it damage the board or does the Arduino board have protection against it?

As much current as your 5V source, or your 3.3V regulator, or that digital can source will flow.

5V: that's what you can directly plug in via USB or barrel connector. So, the effect of this will depend on the external power supply/battery pack. But you might damage your board, or your power supply. Don't do this. A lot of current will flow, and things will be dangerously hot. Some Arduino boards have a fuse against this, some don't. Even if yours has a fuse, it will be no fun replacing that.

3.3V: you will force a poor 3.3V power supply into its knees. I don't know of cases where that has actually lead to permanent damage, but that's just luck, probably. Avoid this.

Digital Pin: The different Arduinos are actually mostly built from relatively robust microcontrollerss, but: Microcontrollers have datasheets where the maximum current you may source from a pin is specified, and your exceeding that rating, definitely, if you're shorting to ground. Don't do this.

Resistor

If the above causes damage to the board or is bad practice, can I put a resistor between the power and ground to fix it? If so, how many ohms should the resistor be?

Yes, because remember: V=I·R, so I = V/R, so the more resistance, the smaller the current that flows.

Your question, however, makes no sense: There's no point in connecting power -> resistor -> ground, unless you want to build an electrical heater. So, the optimal answer would be "the resistance should be infinite, i.e. don't do that connection at all".

What you, often, however need is some pull down resistor or voltage divider or such to ground, because you want to do something with the output of a digital pin. Without knowing what that doing is we can't possibly tell you what the right resistor would be!

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  • \$\begingroup\$ So I know it was a bad idea but I was curious so I shorted 5V directly to ground while my Uno was plugged in to my computer via USB and I just got the USB disconnect sound and everything is fine. Is that because the USB detected too much current and shut off? Also I want to connect power -> resistor -> ground because if I pass current through a coin it registers as a touch on a capacitative touch screen. Is there a better way to do this? \$\endgroup\$ – Caders117 Feb 22 at 7:21
  • \$\begingroup\$ yes, power supplies in USB hosts have fuses that can reset themselves and/or shut themselves off when they detect overcurrent. \$\endgroup\$ – Marcus Müller Feb 22 at 7:26
  • \$\begingroup\$ Notice that it's called "capacitive" touch screen because it's capacitive, i.e. the actual voltage on the coin doesn't matter, only that it's connected to a larger mass. So, I don't really understand what you're trying to tell me about this coin, or what senses the touch (you haven't mentioned anything about that in your question), but I'm sure you should ask a new question where you explain, in detail, what you're trying to build. Avoid the XY problem! \$\endgroup\$ – Marcus Müller Feb 22 at 7:28
  • \$\begingroup\$ My bad thanks for the information though! \$\endgroup\$ – Caders117 Feb 22 at 7:30

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