My arduino board had been working correctly until today at the 5v output it started pulling only 3.80v
Show us a good schematic of your circuit.
Show us a good image of your ‘actual’ wiring.
Give links to components.
Posting images:
https://forum.arduino.cc/index.php?topic=519037.0
Where are you measuring ?
Is that the 5V pin? How much voltage are you supplying?
Do you have anything "added" that might be drawing excess current?
If you provide your circuit, hardware details and code, we can have something to go on.
I am powered from the VIN pin with 5v and when feeding in this way it gives me 3.8v on the 5v pins but if I feed it from the board's own usb input I have 5v on all the 5v pins
The Vin pin goes to the onboard 5V regulator. The 5V regulator needs over 7V to its input to regulate properly. If you have regulated 5V, feed it to the 5V pin.
Can I power the board from a 5v pin with a power supply that I have that gives me 4.90v? because apparently something went wrong on the vin pin because it was working fine and now it gives me a lower voltage than the input voltage which is 4.90v, before it didn't matter to me and now it gives me around 3.80v
4.90V is close enough to 5V. Connect the 4.90V to the 5V pin not Vin. As long as the voltage is less than 5.50V connect to 5V. A 16MHz Uno or Nano will need a minimum of 4.5V.
groundFungus:
4.90V is close enough to 5V. Connect the 4.90V to the 5V pin not Vin. As long as the voltage is less than 5.50V connect to 5V. A 16MHz Uno or Nano will need a minimum of 4.5V.
So there is no problem in powering the board from a 5v pin? It is worth mentioning that from the board I need to power an obstacle sensor hc-sr04
gabrielnicolau:
Can I power the board from a 5v pin with a power supply that I have that gives me 4.90v? because apparently something went wrong on the vin pin because it was working fine and now it gives me a lower voltage than the input voltage which is 4.90v, before it didn't matter to me and now it gives me around 3.80v
This link here ([u]click here[/u]) shows options for powering an UNO.
The only thing is ------ for the DC power jack option ----- maybe 8 to 9 volt DC is best. There are reports of higher supply voltages, such 10 to 12 volt DC is still workable, but can lead to heating up - and might not be great for the arduino's regulator module.
Also - you mentioned you applied 5 volt to the Vin pin. So one helpful comment that I can make here is ----- in future, when you do something like that, make sure there are at least a few different tutorials sources (eg. online) that shows what level of voltage to apply. It just means - if you're going to apply 5 volt to a particular pin - even if it says Vin .... then get some information about the voltage levels and power requirements are ------ for that pin. Apply a voltage to the pin only when sure about the required voltage level(s).
gabrielnicolau:
So there is no problem in powering the board from a 5v pin? It is worth mentioning that from the board I need to power an obstacle sensor hc-sr04
I do get tired of explaining this, The simple answer is, please completely forget that the "Vin" pin or the "Barrel jack" on UNO exists. The point is this:
You have an ATmega328 microprocessor chip.
It is intended to run on 5 V. That is the design voltage, at least to use a 16 MHz clock which is what the Nano has. To use USB, it also has a USB interface chip which is - generally - designed to run on the USB voltage which is 5 V. That is 5 V OK? 
I repeat my "stock" explanation to this question every time. The "Vin" - or "Barrel jack" on a UNO - is nothing more than a legacy "novelty" from the times ten or twenty years ago when 5 V switchmode power supplies were not common as they now are. It allowed you to use unregulated 9 V "plug packs" (US: "Wall warts") which were the common supply for computer "phone" modems and ADSL boxes which contained heat-sinked 7805 regulators. But the regulator on a tiny Nano or even the UNO board has minimal heat-sinking and is only suitable for simple demonstrations of the basic board and a few LEDs.
For protection, there is a diode between the USB connector and the 5 V supply line (which is also the "5V" pin) in the Nano, so plugging 5 V power into the USB jack loses a significant part of a Volt before it gets to the 5 V line. According to the original circuit, this would be a SS1P3L with only 350 mV drop and capable of passing one Amp, but depending on what clone you have, this may be quite different.
So you can simply forget the on-board regulator and "Vin" and when you have a nice regulated supply of 5 V - generally from a switchmode "buck" regulator - you want to convey it to where it is actually required - the "5V" pin and your other modules. 
Vin and barrel jack are nothing but trouble. Always use 5V pin or USB. Even something as jank as 9V battery connected to external 7805 into 5v pin is arguably better than using Vin. Again, Vin and barrel jack are nothing but trouble.
Also, something I learnt just few weeks ago: ICSP header (the 3x2 pin header) of UNO and NANO has 5v and GND pin. Seems obvious, but not for me at that time.
I think that in electronics ------- one main teaching I want to spread around for beginners is ------ specifications. Know the specifications - voltage requirement, current requirement --- and power handling. Operating voltage and ratings. Basically - not only know operating voltages and currents, but also how much power a device is meant to either use, or how much it can deliver.
Although - yes ---- I do agree that it was pretty poor form in that the arduino barrel jack specified input voltage range is specified as 7 to 12 volt. But it was then mentioned somewhere that voltages getting above say 10 volt at Vin can make the regulator run fairly hot. And nobody bothered to just get in there and change the specified input voltage for barrel jack to be say 7 to 9 volt DC ----- for good measure. And not sure if they even specified the recommended power rating of the DC power supplies to be used on the barrel jacks.
I do get tired of explaining this, The simple answer is, please completely forget that the "Vin" pin or the "Barrel jack" on UNO exists. The point is this:
I do get tired of hearing it.
@OP,
Is there anything you aren't telling us ? (is there , by any chance, ANYTHING connected to the arduino when this
happens ?)
Did you try a DIFFERENT USB source ?
Did you measure the USB input voltage ?
You can test the 5V regulator by unplugging the USB and connecting 6+Vdc to the Vin pin.
With nothing connected to Vin or Barrel Jack, the 5V you say is 3.8V is NOT from the arduino regulator.
It is the USB INPUT voltage , due to the design of the arduino. It uses the USB 5V unless there is an external input
greater than 6.6Vdc .
Arduino schematic
raschemmel:
I do get tired of hearing it.@OP,
Is there anything you aren't telling us ? (is there , by any chance, ANYTHING connected to the arduino when this
happens ?)
Did you try a DIFFERENT USB source ?
Did you measure the USB input voltage ?
You can test the 5V regulator by unplugging the USB and connecting 6+Vdc to the Vin pin.
With nothing connected to Vin or Barrel Jack, the 5V you say is 3.8V is NOT from the arduino regulator.
It is the USB INPUT voltage , due to the design of the arduino. It uses the USB 5V unless there is an external input
greater than 6.6Vdc .
Arduino schematic
What's up dude. There is nothing connected to the board other than the voltage which is almost 5v it is exactly 4.91v and it gives me a voltage loss on the 5v pins of almost 1v, it gives me 3.88v. I tried a regulated 7v supply and still have a loss of almost 1v on all 5v pins. The problem is in the Vin pin since magically out of nowhere it started to have a 1v loss because before when feeding from that pin all the 5v pins were throwing me the same input voltage and now not. I have tried to power from usb B on the board with the pc (ie constant 5v) and I have the 5v on all 5v pins.
I hope you hadn't applied 7 volt to the '5V' pin before.
If you have a regulated 5 volt DC power supply, you can try connecting that supply across the '5V' pin and arduino GND.
Also - if possible, get 1 or 2 spare arduinos. Can be nice to have back-ups, in case this one has hardware failure.
"What's up dude ?"
If you have 5V on the 5V pin when using USB then
you're 5V regulator maybe damaged.
Check U1 input and output voltage with an external
DC input greater than 6.8Vdc.
If the input voltage is the same connected as it is not connected but the output is 3.8V then it's bad.
If you had a short on your board it would not be
5V with USB power so that's good.
If it's bad you can replace it with this pin for pin
compatible switching 5V regulator if you can solder
You'll have to connect wires to the pins and use
double sided tape to mount it on your arduino.
Otherwise just replace the regulator
"https://www.digikey.com/en/products/detail/murata-power-solutions-inc/OKI-78SR-5%2F1.5-W36-C/2259781?utm_adgroup=Power%20Supplies%20-%20Board%20Mount&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Supplier_Murata%20Power%20Solutions_0811_Co-op&utm_term=&utm_content=Power%20Supplies%20-%20Board%20Mount&gclid=EAIaIQobChMIq4_Zgt7Q7QIVZAnnCh0hxQ-LEAUYASABEgJCXPD_BwE"