Main Site Documentation

Max Current from the +5V pin


I don’t seem to be able to find this in the Documentation


the answer is actually another question - how much can your power supply provide?

For Example, if you use USB only to power things, then the 5v supply can only give ~500ma; if you use a 2a power supply then theoretically you could draw 2a, depending how you connect all that up. Often the limitation is the vregs on the board.


Interesting that is very different from most Microcontrolers. So the +5V pin is more of a bus connected to the power supply?


Not really - The 5V is either regulated from the power supply using a 5V regulator, that has not a big current capability / and power dissipation. Or if you power it via USB, it is connected to your USB port.

Let’s say your fez board is using 150mA :

  • if external power used :by the difference between this voltage and 5v times the total output current (watt power dissipation capability of the onboard 5v regulator). If you power it with a 6v5 power source, you will be able to use more current available than if powering it with 9v : much more power to dissipate into heat :wink: Example : power 9v, fez 150mA. The reg will have to dissipate : (9-5)*0.15=0,6 Watt. It’s about the limit you can dissipate out of it. So now let’s use a 6,5DC power: (6,5-5)*0.15=0.22 w. You should be able to pull 0.3 watt more, so about : 0.3/(6,5-5) = 0,2A so 200mA.
  • if usb power: usualy about 500ma (USB typical max current) - current used by your few boad. So about 300 mA.

Anyway, don’t expect more than 200 or 300 mA to be secure !


Great thanks!