# Connecting pin 5V to A0

Hello boys,

Can I connect a resistance directly between 5V and A0 of the board and reed the output ? Or will it burn the arduino ?

Thanks,
Preacher

Can I connect a resistance directly between 5V and A0 of the board and reed the output ?

You can put 5V into an analog pin even without a resistor: it's meant to tolerate thet.

When you put a potentiometer across 5V and 0V and the wiper to analog pin, which is a very common thing to do, and turn the pot to one extreme, you are actually connecting the pin to 5V anyway.

neiklot:
When you put a potentiometer across 5V and 0V and the wiper to analog pin, which is a very common thing to do, and turn the pot to one extreme, you are actually connecting the pin to 5V anyway.

Fair enough. Thanks for the answer, Best to save...

What are you actually trying to do?

Note that the allowable voltage range of the analog input depends on what voltage the microcontroller is running at. Most of the common AVR-based Arduino boards (e.g. Uno, Mega, Leonardo, Nano) run at 5 V and can take up to 5 V input to the analog pins. Other Arduino boards run at 3.3 V (e.g. Zero, MKR) and subjecting their analog pins to 5 V could cause damage.

Can I connect a resistance directly between 5V and A0 of the board and reed the output ? Or will it burn the arduino ?

It should read 1023 assuming the "5V" is exactly equal to (or slightly higher than) the power supply/reference.

The input can range between zero and Vcc (usually 5V). The datasheet for the ATmega chip says the "absolute maximum" is -0.5V or Vcc+0.5V.

Do you meant the "output" of the resistor? The Arduino has an input impedance of approximately 100 meghoms (essentially infinite impedance/resistance) so any "reasonable value" resistor won't give you a voltage drop and you'll just be reading 5V.

A resistor (along with the protection diodes built into the chip) will provide some over-voltage protection.

You're fine, unless the resistor is a low value and you accidentally set A0 to be an output, as opposed to input.

And of course you would ‘never’ connect 5v to A0 if A0 was every made an output.

Beginners often think that a single resistor will drop a voltage across it. It will not.

To drop a voltage you need current through it and if you simply connect a resistor of what ever value to between 5V and a measuring device you will see 5V.

Cue for pedantic idiots to say well if you use a 100M resistor you will see a change. Please try and resist.

Grumpy_Mike:
Beginners often think that a single resistor will drop a voltage across it. It will not.

To drop a voltage you need current through it and if you simply connect a resistor of what ever value to between 5V and a measuring device you will see 5V.
...

so... for example a resistor at an op-amp input will have the same voltage at either end of itself ?

BabyGeezer:
so... for example a resistor at an op-amp input will have the same voltage at either end of itself ?

Given no other connections to that input then yes.