My project consists of a NEMA17 stepper motor controlled by a A4988 driver and Arduino Uno.
12V, 2 amps psu for the motor
5V, 2.4amp powering the arduino through the usb connector.
I need to free up plug space and was wondering if I could get rid of the 5V psu and run the arduino off the 12v psu.
I have very limited experience in electronics so please excuse any silly questions.
If that is everything in the circuit, then yes.
To power the Arduino Uno, you can use a voltage regulator, like this one, to efficiently step down 12V to 5V.
Apply the 5V to the Arduino 5V pin, and connect all the grounds.
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Yes it should work nicely. With 12 supplying Vin you have very little extra energy on the 5V pin of the arduino so avoid using it to power external devices if possible.
12 volts is the maximum barrel or Vin voltage for the UNO.
I would go with @jremington's suggestion and use a buck regulator to step down the voltage more efficiently that wasting it as heat on the UNO.
It would also provide a bit of isolation from trash and transients that might be in the 12 volts.
You could use more and larger filtering capacitors.
pololu.com is reputable and has a wide variety of inexpensive buck regulators, choose one that woukd be loafing providing the required current, like 2/3 or even just 1/2 capacity.
OIC @jremington has gone the step further at pololu to point out an exemplar.
If you randomly get a cheap regulator from here or there, you must verify performance with the expected input voltage and load. You may find that the specs are aspirational, you may find you got what you paid for.
Don't ask me how I know. 
a7
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Thanks, I tried that and it worked,
Well first it caught fire, but that was because I put one of my +ve and -ve connections in the wrong way round.
When I wired it correctly everything worked.
It still worked, after it caught fire? I'm impressed!
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