As a reference, I'd like to know how much current I can draw from the 5V and 3.3V pins on the Mega 2560 before I need to add my own power system to my project.
For the 5V it depends on the power source. If USB, I think around 500mA. If powered through the power plug or Vin the 5V comes from the little 5V regulator on the board. The max recommended dissipation for the regulator is 1 Watt. So the max current is 1W / (Vsupply - 5V). For 12V into Vin that works out to 1/(12-5) = 1/7 or 143mA*. If powered by an external 5V supply then the current is what that supply is capable of.
I don't expect the 3.3V regulator to supply more than 50mA.
*The Arduino uses about 50mA of that so, really, there is less than 90mA or so available for other things.
groundFungus:
I don't expect the 3.3V regulator to supply more than 50mA.
The Uno/Mega datasheet states 50mA, but that is wrong.
The 3.3volt regulator one an Uno/Mega is rated for 150mA.
Note that it takes it's power from the 5volt rail.
An Uno/Mega uses about 50/70mA for itself.
Leo..
Thank you both for your replys.
The Mega 2560 schematic is rather confusing.
It shows the USB power coming in through a fusible link and it's named USBVCC.
But that is where it stops.
The actual input to the 5V regulator NCP1117ST50T3G is shown as coming from
X1 POWERSUPPLY DC21MMX(what's that? Is that derived from VIN? It obviously
must be running from the USB if there is no VIN.).
The output of the 5V regulator is powering both the ATMEGA2560-16AU and the
ATMEGA16U2-MU chips so there is already a load on the 5V regulator before
it goes to the 5V pins.
3V3 is also derived from the regulator 5V output through a LP2985 regulator.
So it would appear that any load on the 3V3 will add to the load on the 5V regulator.
Also, I'm sure the MEGA chipset is a variable load depending on its
activity so there probably isn't a hard and fast number of milliamps that could be
drawn by a project. At best, it would be the USB 500ma minus the MEGA load.
Might be interesting to just take some measurements.
Thanks again.
MANoyes:
But that is where it stops...
Hope not.
USB power goes to a P-channel fet switch (T2), before it connects to the 5volt rail.
It's gate is controlled by the LM358 (IC5B) op-amp, that switches the fet off with more than 6.6volt on V-in.
The 5volt pin is the power rail. It powers (almost) everything on the Mega.
The MCU is not throttled, so draws about the same current all the time. Active or not.
Leo..