RAMPS 1.4 question

From a straight functional perspective - you can.

But - the 5V rails on the RAMPS are actually powered via the underlying arduino megas 5V regulator - The ramps passes power to the arduino via VIN and then draws the 5V back via the 5V pins...

This means it really comes down to the regulator on the arduino mega or clone you are using - usually - the MC33269-D ( http://www.onsemi.com/pub_link/Collateral/MC33269-D.PDF ) So a normal arduino is really only rated to 800ma max - some clones bump this to 1A+ like the Taurino Power (not the Classic!)...

Given all the other 5V stuff drawing power (the drivers, the mega itself, etc) - 340ma is probably pushing it. Better off just using the RRD fanextender board and running them 12v but with PWM would be my suggestion.. but short term - yes it will work but just will potentially load you mega's regulator a lot and cause power supply ripple depending on the quality of the capacitors filtering the 5V on your specific mega...

And aux ports use - expansion really... there is/was an extruder breakout and things similar to the extrudrboard (for the printrboard)... can't recall who did it though.

(EDIT: do also remember - arduino's traditionally don't like 12V - they'll do it; but that 7V voltage drop has to get burnt up as heat in the regulator.. so using 12V to power them in the first place really heats up that regulator... (another reason the taurino/clones swap out the regulator and talk of 'being able to be powered from 12v reliably'...) .. so be wary of taking it all the way to max on the regulator... its already going to be toasty )

2 fans and they are gcode driven - but ultimately just additional pins... so you can also just set the pins up to be whatever under the firmware, so they can just be always on - or you can set them up to be 'controller fan' 'extruder fan' and only turn on when neccesary based on movement/temp thresholds.

As for using it to drive additional extruders - you can turn heaters on/off because yes they are ultimately just 2 pair of extra pins - but you can't actually drive a driver for it etc... thats what the extrudrboard etc addresses by mimicking not only the mosfet but also all the stepper-driver pins

hangon - i think we've crossed wires here. Yes you can support 2 extruders on the ramps, via the D9/D10 . This has nothing to do with the fanextender. When you said the fanextender can support extra extruders - I was assuming you where  referring to using the fan extender to control *extra* extruders - as in more than the 2 that are there already.

The fan extender can support 2 fans, yes. The first fan is configured by the same as the RH fan control button, and the extra fan is gcode controlled or can be used elsewhere in the firmware using its pin directly. (Marlin has, for a while now, allowed far more intelligent fan control setup - so that the electronics fan can be configured to only come on when there is motor movements and then stay on for 90secs or whatever you set; and likewise, it can setup any extruder cooling fans (not the Print fans...) to only turn on when the extruder is past 50°C or whatever you set... This is what I was referring to as well in regards to options you can exploit in the firmware with the fanextender...)

At the end of the day, the fan extender is a dual-output additional low-current MOSFET setup borrowing pins from the servo header... you can then choose to use it in a multitude of ways and not just extruders or print fans.

yes.

But like I said - they are just outputs ; not something specifically only for extruders - all the extender is is a low-power switched pair of mosfets that can be gcode controlled - you can use them for anything or any types of fans...