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[on:]

Could somebody tell me if I'm asking too much from my Uno... 

I'm using a 12v (500mA) power supply, which is in fact giving me 17.6v, so I'm using a voltage regulator http://www.datasheetcatalog.org/datasheet2/8/0ishsf7y9sp31h690e60g8gclc3y.pdf which give me a stable 11.94v.

Connected to the Uno I have:
- SD card for data logging http://www.ebay.ca/itm/330658250543?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
- 3 digital thermometers http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf
- LCD screen (2*16) running thru a shift register
- clock http://www.ebay.ca/itm/170794823401?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
- 12v relay to activate 2 pc fans.  Those fan run directly from the voltage regulator
- 120v relay to activate a bigger fan http://www.datasheetdir.com/VB-6TCU-E+download

When I'm running the Uno thru the USB port power, everything works fine (the LCD screen is a little dimmer) and no overheating (perfectly cold to the touch.  But as soon as I modify the circuit and put the 11.94v in the Vin and GND, then the board start overheating (very hot to the touch).

Thanks for your help.

[Reply #1 on:]

I don't think your meant to connect 12V to the Vin header. Put a little 2.1mm power plug on the 12V and plug it into the UNO power socket where it will get regulated down to the 5V arduino expects.

[Reply #2 on:]

I don't think your meant to connect 12V to the Vin header.

Yes you can.

Basically you are drawing too much current. Also you supply is sagging hence the backlight going dimmer. Get a better supply with more current capability. And lower voltage if possible.

[Reply #3 on:]

The Vin header connects to the input of the regulator and should be used with great caution as there is NO REVERSE protection at that point. Connecting there will allow the UNO to work fairly well on 6V in BUT be aware that the ALS1117 data sheet makes no mention of input reverse voltage protection so any misconnection (Reversing the wires) is likely to be A MAJOR ISSUE...

Bob

[Reply #4 on:]

there is NO REVERSE protection

Yes but there is no reverse polarity protection on the 5V or on many other supplys.

While I am by no means advocating it the ATMEGA processor is rather good at withstanding reverse polarity if you don't apply it for too long. Especially if the power supply has some current limiting. I know, I was that stupid.