I am reporting a problem with an UNO R3 board. This is (supposedly) a genuine "Arduino.cc" board and not a knock-off. I bought it originally from either Adafruit or Sparkfun. I've been using the board for some projects over the last year on and off and have never had a problem. I currently have it powered from a 12VDC linear regulated supply connected to the Vin pin, with one of the power ground pins connected by to the supply ground.
Today I decided that I would try to connect some 3.3v circuitry to the board. So I ran a ground wire plus a power wire from the 3.3v power pin socket over to my breadboard. I could not get the project working and after some head scratching I started measuring things. This is when I discovered that the voltage coming out of the 3.3v power pin is actually 5v! Whoa there! Luckily all the circuity that I was connected to (except the last component) was 3.3v/5v rated so I likely did not kill anything.
I double checked that I had the wires connected to the correct places and remeasured. Still 3.3v. I then connected another UNO R3 board that I had not yet used (was still in its shrink wrapper) and measured that (powered by USB) and obtained 3.3v on the 3.3v pin, confirming that I can make the measurement correctly. So it seems the first board may have a blown regulator or something.
I thought I should post this in case others have experienced this problem and want to share their pain.
Any ideas on what could be causing this, if this signals any problems with the board, etc. ?
If you measure DC resistance (power off) from 3.3V to 5V, it's not a direct short is it?
The bad board has good 5V?
Both boards have the same P/N at the 3.3V regulator position?
Seems to me like 5v to 3v3 regulator is fried or not soldered. Check its input and outputs, check its model too (maybe its 5v regulator, fabrication issue). I wouod change your 12v reg to a 9v one, 12v is the upper limit, just one overvoltage and you can blow something on the board (and maybe happened to regulator).
@mart256: Not sure how there could be "overvoltage". I'm using a regulated power supply.
The board could have been made with a bad part. This is a cheap mass produced item. I doubt they test them before shipping. Could have been bad from the start.
No big deal. I will just not use this board for any 3.3v circuits. I switched to another UNO board that has a working 3.3v output.
charlielaub:
Both the 5v and 3.3v pins show 4.99 volts on my multimeter.
If they're showing the same voltage down to the 2nd decimal, there is a short somewhere. Could be the regulator is gone or it could be elsewhere on the board. Since there isn't anything on the board that uses 3.3v, its pretty likely the regulator.