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The recommended voltage for an arduino Vin is 7-12V. The Vin can't be outside 6-20V for proper operation. If Arduino uses 5V on its rails and for its logic, why is 7V needed?

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    \$\begingroup\$ The on-board 7805 regulator, which provides regulated 5V to the rest of the board, needs a minimum of 7V to work. See it's data sheet. \$\endgroup\$ Jun 20, 2013 at 14:38

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That 7V is fed to an 7805-style voltage regulator, which provides the 5 the Arduino is running on. You could apply 5V to the appropriate pin directly, but

  • it must be a real stable 5V (not some wacky unregulated wall-wart)
  • you'd loose the current limiting of the 7805
  • the 7805 in the Arduino must be protected by a reverse diode, check the circuit
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    \$\begingroup\$ Great answer. thank you very much. I'll avoid wacky unregulated wall-warts. \$\endgroup\$ Jun 20, 2013 at 17:44
  • \$\begingroup\$ You should not connect 5volts to the 5V directly AND connect USB at the same time. There are no protections for that setup. \$\endgroup\$ Jun 20, 2013 at 19:16
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    \$\begingroup\$ Hence the only 'appropriate pin to connect the 5V to' would be the USB 5V pin :) \$\endgroup\$ Jun 20, 2013 at 19:27
  • \$\begingroup\$ @john Perhaps you misunderstood Mr. Ooijen's answer. If you apply a 5V adapter to the device then it should be regulated 5V. If you apply a voltage that is in the proper range, then it can be an unregulated wall-wart (transformer + rectifier in a box). These are not really wacky; they are what they are, and fit devices that are designed to use them. \$\endgroup\$
    – Kaz
    Jun 21, 2013 at 0:41
  • \$\begingroup\$ With 'wacky' I referred to the wall-warts that claim to output a certain voltage, but contain only a trafo, rectifier and buffer capacitor. Those should advertise itself as 'unregulated'. As Kaz points out they can be perfect as 7-12V input for an Arduino, but (even though they sometimes claim to produce 5V) they are not suitable to supply the 5V. I have even seen this in a small power supply withh USB output :( \$\endgroup\$ Jun 21, 2013 at 5:36

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