Hi,
I would like to fit both the Arduino Uno and a 12 volt stepper motor into a single box with one powercord.
Output from the Transformer is 12v, 2000 mA. How do I split that between the Arduiono and motor, and will the Arduino run on 12 volts or do I have to decrease it to 5v?
Thanks,
Mike (beginner 
It will work fine on 12V.
You do not have to split it, just supply 12V to the power connector on the Arduino board, it will take what it needs.
The only problems you may have are if the motor takes such a big current that the voltage drops too low to supply the Arduino. This would cause the Arduino to black out and reset momentarily.
You can guard against this by Giving the Arduino its own reservoir of charge in the form of a large electrolytic capacitor (say 4700uF 16V) and a diode so the motor doesn't steal it.
|--------------- to motor
-------|>|--------- to Arduino
|
12V =
|
--------------------------
The Uno will work with +12vdc if powered throught the external power connector on the board. You will have to somehow
split the power between both the motor and the Uno by splicing the cable. Be sure you wire the correct polarity.
Lefty
It will work on 12v through the DC power jack.
Watch out! Some 12v power supplies put out 17v which could overheat the regulator.
Just get a regulated one. I found a lot of devices come with one. I took mine from a D-link AP. My external WD drive is 12v regulated.
Thank you all! This is a knowledge that could be expensive to learn the hard way!
If your stepper motor draws a lot of current, and many do, you might cause the arduino to crash when the voltage drops. And the voltage will drop, it is just a matter of how much. Some things that will help a lot is keeping the power leads short and twisting them together if you can. Use power leads that are as thick as practical. Leads that are too thing will increase the voltage drop. Adding capacitors across the input to the arduino will also help.
If your stepper motor is large and you have a heavy load on it it will make this problem worse.
Kevin
Except for the Arduino to crash/reset the stepper would have to pull the 12v supply down to 6v which is unlikely.
I am using an UNO board to provide PWM signal to four BLDC Hall Motor Controllers for changing the speed of four motors. Yesterday when I ran the test, the machine was running fine in first 30 minutes and then suddenly stopped. After checking, all four motor controllers were broken and the 12V power supply of the UNO board doesn't work too.
Do you think the issue can be caused by UNO board reset? I noted after the machine stopped, the battery voltage is at 23.64V, it should have a voltage of 25.8V after a full charge.
Oh come on, whatever do you mean by that? 
What does this have to do with a 12 V power supply?
Something that was missing from the previous discussion.
You can only use the "barrel jack" on a UNO or the "Vin" pin to supply 12 V (and no more) if you are not proposing to power anything of significance (which is to say, requiring more than 50 mA in total) from the output pins plus the "5V" pin.
You appear to be connecting relay modules to the 5 V pin. They require power as well as whatever other modules you have there; you need a proper 5 V regulator to power them, generally a 5 V switchmode "buck" regulator is what you need.
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