Hello everyone. I'm supplying 12V through jack connector to my Arduino Uno. I've checked 12V get to the board correctly but when I check output voltage is only 2,1V.
However if I use the USB connector to the PC, the output is right. 5V.
Does anyone know what is the problem? I know jack connection has an Voltage regulator, but if broken I suppose it would give 0V, not 2.
Thanks in advance.
What is connected to the UNO's output, 5V and 3.3V pins?
What "output voltage" and under what conditions?
An I/O pin written high with nothing connected (except for a multimeter) should read nearly 5V.
When you put a load on the output pin the voltage can drop. The "absolute maximum" output current is 40mA (1) which means a minimum resistance of 125 Ohms. And with a 125 Ohm (or lower resistance) load I'd guess the voltage drops somewhat.
...If you connect an LED without a current limiting resistor, the voltage will drop, you'll get excess current and possibly damage the LED or the Arduino.
What does the 5V pin read?
(1) I believe the Uno R4 is rated for lower current.
Nothing. Only the voltimeter. I've taken away every wire to components. It only has the supply wire and my voltimeter in the 5V or 3,3V and ground. Btw, when checking 3.3V pin it gives 2,1V and when checking 5V pin also gives 2,1V
Where is the meter's black probe connected?
The black is connected to the GND beside 3,3 and 5V pin
What do you read at the VIN pin?
11,75V in the Vin
Hmmm, sounding like a bad voltage regulator, sorry that's all I can do from here. 
Anything getting hot?
Mmmm I think you are gonna be right.
This is the voltage regulator, right?
I've checked voltage between the pin at the left and the pin in the middle and it's 2,1V and between at the left and the right 11,75V.
So maybe I will have to buy a new regulator and solder it.
Don't know in which moment it has got damaged.
Is that heatsink on the device below the USB socket "factory placed" ?
Yes, it was already placed with the board when it was bought (Amazon buying)
Is the power LED on? Does the board work?
The LED is off when supplying from jack. But it is on when supplying from USB
Nothing is connected to the board, right?
What has been connected to the board.
12volt on the DC socket is borderline too high. At that voltage you can't draw more than 100mA from the Uno for relays/sensors without overheating and maybe killing the voltage regulator.
Leo..
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I think I understand what happened.
For a millisecond while setting the wires I touched with the wire from 5V the wire from ground, a light touch of milliseconds and I think it was when led stopped lighting. So pretty sure voltage regulator died. I'll solder another one on the board.
And about what I'm supplying, I'm supplying 2 stepper motor drivers (A4988 or TMC2209). Only the drivers, the motors are supplied directly with the same 12V battery that I was using for supplying Arduino through the barrel jack.
If only these two drivers are being supplied with the 5V pin, there is no problem to use the jack with 12V?
I don't think that is what burned out the regulator.
If only these two drivers are being supplied with the 5V pin, there is no problem to use the jack with 12V?
Well, something burned out the regulator. Maybe you have the drivers wired wrong.
Right. As much as we try to avoid it, it will usually not destroy a regulator to short circuit it for a few milliseconds, because it has built in over current protection and thermal detection (well, most of them do).
I hate to admit it, but I have from time to time shorted the supply. The key thing is, how long before you realize what you've done. 