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I'm trying to design a circuit based on an ATMega328 (from an Arduino stand-alone type kit), and was planning on adding a USB connector and powering the board from a generic USB phone charger type device.

Do I need to add a voltage regulator to my circuit, or can I just assume the charger will give a good enough 5V supply?

The instructions I found at http://arduino.cc/en/Main/Standalone show how to add a regulator, but I'm not sure if I need it.

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Be warned that, while the spec might require regulation, many/most cheap chargers won't actually regulate the output well, if at all. For example: righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html –  Bob Feb 22 at 17:07
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Yes, USB charger is a regulated power supply. USB devices expect around 5 V on the Vcc line. Do note that the regulation on some of the cheaper USB power supplies is pretty bad, especially if you're near the power supply's maximum current.

The best way to be sure if you need a regulator is to check the regulator and what your power supply is providing. If you have a multimeter, use it in the voltage mode on the two outer pins if the USB connector of the power supply. You should get around 5 V.

In the guide you linked to, the regulator used is of 7805 pattern. Here 78 is the series number and 05 is the output voltage of the particular regulator.

Such regulators need higher input voltage of around 7 to 8 volts (2 to 3 volts above the output voltage) to be able to regulate properly. Basically, this type of regulator, called linear regulator, turns the excess voltage into heat and provides 5 V output (for 7805). If the input voltage is too low, it won't be able to give correct output voltage.

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USB will provide a good power supply (5V and 500mA for USB 2.0) and based on my experience it is better than batteries.

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This does not answer the question that was asked. –  David Feb 22 at 19:36
    
I mean the voltage is already regulated thats why you dont need to add voltage regulator –  Mohammed Abualgassim Mar 1 at 23:19
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