Driving VCC directly.
July 19, 2011 07:14AM
So, it appears that people have issues because the 5V regulator on the arduino mega burns out trying to dissipate 12V. So something I am going to try is just adding a 7805 directly to the RAMPS board and feed 5V power to the mega directly. So, instead of

+12V -- D1 -- Vin

I will have

+12V -- D1 -- 7805 -- AM-VCC

A TO-220 7805 should be able to dissipate the heat from 12V easily compared to the surface mount regulator on the arduino board.

I checked the mega schematics, and it appears to be able to handle being provided with an external regulated supply just fine, it regulates the 3.3V supply off of the 5V one and has a power mosfet protecting you from reverse driving the USB port.

Has anyone tried this? It would be nice to get this into RAMPS 1.4, having to power the arduino independently from the shield seems like a pain.

Re: Driving VCC directly.
July 19, 2011 11:09AM
Hmm.. doing the calculations, without a heat sink we can provide about 250mA, so we probably want a heat sink to at least be able to provide the 500mA it expects from the USB port.

Plus we will be powering the motor drivers directly from the main board which will be good. Though, I worry about us driving those PWM power MOSFETs directly from the avr pins, power mosfets can draw a lot of current on their gate and when switched on and off quickly that could be an issue. Are we sure we don't need something like a 4427 in there?

If nothing else, I think we need a 100-150 ohm resistor in series with the gates of those MOSFETs.
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