Main Site Documentation

Max Current from the +5V pin


#1

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


#2

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.


#3

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


#4

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 !


#5

Great thanks!