Hi, i have almost fried my r3 when i putted 12v 3A current into it. Small smoke come from it and protection diode flied out. I soldered it back and runned arduino on 5v usb. Its working but Is extremely overheating. Can somebody advice me where Is problem. I think probably voltage regulator Is destroyed but i don't know.
Sorry to say, but, the problem began with whatever you were doing before it started.
Care to go into that a bit?
jurij734:
... when i putted 12v 3A current into it....
You can't put current into it. It could be a 12V, 300A supply and the Arduino will only draw the current it needs, and no more.
A schematic, not a Fritzing LEGO picture, would answer an awful lot of questions.
You damaged some other component on the board which is now near-shorted internally and is drawing tons of current and turning it into heat. The opamp for the power switching circuit is hosed for sure (the L led is stuck on too isn't it?) Assuming the Leo buffers it like the uno - I don't know if it does, hardly anyone here seems to use them, probably because for all the use cases that a leo is an appropriate board for are beter served by either an UIno or a Micro.....
Discard the board and purchase another. Leo clones are under $7/ea shipped, it's not worth your time even if you value your time at minimum wage and had the skills you would need to repair it. Which, based on the fact that you're asking questions like this and talking about putting current into the board, you do not. Trying to fix boards that you trashed through electrical abuse just isn't worth tthe time or the effort. I don't even bother with it, and usually when a board fails in my possession, there's no mystery in my head about what I trashed and how I'd go about fixing it...
Frankly, I;m amazed the 32u4 is still alive...
DrAzzy:
Frankly, I;m amazed the 32u4 is still alive...
We don't know 
OP did not state what was overheating so it might very well be the 32U4.
What is overheating?
So I'll add a little. The 32u4 main microprocessor overheats. behind the mosfet which spins vusb I measured the voltage around 8v at the usb connection.
Hi,
Welcome to the forum.
Please read the post at the start of any forum , entitled "How to use this Forum".
OR
http://forum.arduino.cc/index.php/topic,148850.0.html.
What was your project?
What were the component in circuit at the time?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Thanks.. Tom... 
and protection diode flied out.
That is an extremely violent event and suggest something serious like a shorted capacitor or reverse polarity connection.
Was there anything else connected to it at the time that could have caused excess current to flow.
But as other have said throw it away. If you are getting 8V at the USB connector then you could damage your computer if you plugged it in for programming.
Hi,
Small smoke come from it and protection diode flied out.
Sounds like more than 3A reverse polarity connection.
Did you test the protection diode before putting it back in?
Did you put it in the correct way?
Can you please post a picture of your damaged board?
Thanks.. Tom...
This is where spare boards always come in handy. So, grab a spare Leonardo of the same kind and don't connect it to anything just yet. Then use a multimeter to measure the output voltage of the 12V DC power supply (the one with the 3A power rating). If the voltage measurement is ok, then see what happens when you use that DC power supply to power the second (spare) Leonardo. Just a power-up test only. Have no other components (resistors, capacitors etc) connected to the Leonardo.
Also - ensure that the polarity is correct for the DC power supply, and that the Leonardo is powered with the correct polarity.