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If building a breadboard Arduino and powering it with a computer power supply that outputs 5v, do I still need a voltage regulator?  Is there such thing as a 5v to 5v regulator?
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Well yes, but only for isolation purposes.  If the computer PSU is outputting a clean 5V then it should be OK (some only run properly under a moderate load, note, so do check the open-circuit output voltage).

(You can get isolated switch-mode 5V to 5V DC->DC converters - one use is to allow an opto-isolated differential line drivers/receivers to float - long cable runs can cause voltage differences that would otherwise inject unwanted currents into the line)
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Quote
long cable runs can cause voltage differences that would otherwise inject unwanted currents into the line

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Is there such thing as a 5v to 5v regulator?
A capacitor ?
With long cables it makes a difference where they are located.
Having two ( a big electrolytic + an affordable ceramic ) "regulates" distortions of different frequencies. 
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Using an external regulated DC power supply to provide power for your breadboard circuitry is fine without any need for isolation. Do however be sure you have a wire from the negative terminal of the external power supply wired to an arduino ground pin if you are using the arduino to input or output signals to your breadboard circuitry.

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If building a breadboard Arduino and powering it with a computer power supply that outputs 5v, do I still need a voltage regulator?  Is there such thing as a 5v to 5v regulator?

You can connect it directly. I would, however, include a 1 amp fast blow fuse in series with the 5V line. Computer power supplies can put out 25 or more amperes at 5 volts and something less than a DEAD short (like a thin wire cliplead short) will result in a spectacular cloud of smoke followed by flames.

You may be safer to use a wall wart or build your own supply.

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