Controlling Power Supply and powering Mega Vin

Hi all, I have yet another PS_On Question!

My cooling fans are always on, and I tend to run prints while I'm away at work, so I want to kill the power supply at the end of my prints.

I want to power my Mega off the +5V Standby line from the PS, and have the mega switch the main 12V lines on the PS. The On/Off rocker switch S1 on the control panel switches the Mega/LCD/PS Off.

The 5V line from the PS is regulated, but I'd prefer to run it through the Megas regulator before going on to the mega board, just in case I accidentally connect a +12V to it / short it / whatever, so I want to use the Vin pin rather than the 5V Pin.
Firstly, am I right in thinking the +5V pin on the Mega is on the board side of the voltage regulator, whereas the Vin pin is regulated down to 5V and then outputted onto the 5V line?

My proposed wiring diagram is here:


So I'd turn on the Rocker to power the Mega, then navigate through the Menus on the LCD Screen to turn on the Power Supply, at which time the PS goes on, and the +12V lines going into the RAMPS would go high, turning on the Fans and Print bed lighting etc.

Then at the end of the print, I'll use a G4 S180 command to wait 3 minutes for the hot end to cool before the fans and lighting switch off again (I don't want to get heat soaking back up the hot end and giving me extrusion problems on start-up)

I'm thinking if D1 is on my board I can leave it in, since I'm wiring onto the forward side of the diode anyway, so my +5V won't back-feed the RAMPS.
Alternatively, I can put the +5V switched via S1 into a barrel connector and put that into the Mega power jack, which will go into the regulator and then power the board? This may be a more elegant solution than soldering a line under my stepper drivers onto D1.

Will i kill my board (again ) wiring it like this? And will powering up the mega give the LCD power and allow me to navigate the menus and switch PS_ON ?

Thanks in advance guys

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KealanOCarroll
Firstly, am I right in thinking the +5V pin on the Mega is on the board side of the voltage regulator, whereas the Vin pin is regulated down to 5V and then outputted onto the 5V line?
...
I'm thinking if D1 is on my board I can leave it in, since I'm wiring onto the forward side of the diode anyway, so my +5V won't back-feed the RAMPS.
Alternatively, I can put the +5V switched via S1 into a barrel connector and put that into the Mega power jack, which will go into the regulator and then power the board? This may be a more elegant solution than soldering a line under my stepper drivers onto D1.

Yes. The Vin pin is the same pin that power would be supplied via the 12V-5A power input to the RAMPS board. You need to remove D1 however as you'd be backfeeding 12V via the 5VSB line when your power supply is on. The barrel connector is how I power mine via 5VSB.

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My proposed wiring diagram is here:

s210.photobucket.com

Does the XBox power supply need the PS_ON wire to go HIGH? ATX power supplies require it to go low/grounded. Just double checking.

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And will powering up the mega give the LCD power and allow me to navigate the menus and switch PS_ON ?

It should.

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cdru


Yes. The Vin pin is the same pin that power would be supplied via the 12V-5A power input to the RAMPS board. You need to remove D1 however as you'd be backfeeding 12V via the 5VSB line when your power supply is on. The barrel connector is how I power mine via 5VSB.

Does the XBox power supply need the PS_ON wire to go HIGH? ATX power supplies require it to go low/grounded. Just double checking.

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KealanOCarroll
And will powering up the mega give the LCD power and allow me to navigate the menus and switch PS_ON ?

It should.

Cheers I hadn't thought of backfeeding the Standby line after the PS came on at all...I was only thinking one way! I reckon I'll use the barrell connector too, it should be a tidier job and make it much easier to isolate power

I believe it is reversed from the ATX, there's an option in Marlin Config.h to define the the PS_ON as inverting or not, one option is commented ATX and the other XBox

Edited 1 time(s). Last edit at 03/24/2015 11:17AM by KealanOCarroll.

Just to clarify, does the D1 Diode have to be removed for powering the Mega via the barrel connector ?

It may be missing from my board as is, I haven't checked...id like to be sure to be sure though

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KealanOCarroll
Just to clarify, does the D1 Diode have to be removed for powering the Mega via the barrel connector ?

No. The MEGA board has a diode in line with the barrel connector that will prevent backfeeding.

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It may be missing from my board as is, I haven't checked...id like to be sure to be sure though

Most boards should have it. It's located right on the dividing line between the X and Y stepper driver headers, just below the capacitor for the y stepper.

Why not feed the 5V from the PS via a Schottky diode (such as 1N5817) to the 5V (e.g. VCC pin on RAMPS) of the Arduino Mega? This way the voltage drop would only be about 0.3V compared to the almost 1V loss if you go through the linear 5V regulator of the Mega.

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enif
Why not feed the 5V from the PS via a Schottky diode (such as 1N5817) to the 5V (e.g. VCC pin on RAMPS) of the Arduino Mega? This way the voltage drop would only be about 0.3V compared to the almost 1V loss if you go through the linear 5V regulator of the Mega.

Because I'm afraid I'll either short it or connect a 12V line to it and fry my Mega...I want to have a regulator in between my line in and the 5V line on the board for safety.
I'm a bit inclined to rush into changing something and end up doing damage

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enif
Why not feed the 5V from the PS via a Schottky diode (such as 1N5817) to the 5V (e.g. VCC pin on RAMPS) of the Arduino Mega? This way the voltage drop would only be about 0.3V compared to the almost 1V loss if you go through the linear 5V regulator of the Mega.

Warning for anyone who does this that you're tying the input power directly to the 5V rail that runs the microcontroller. You're bypassing the polyfuse that the USB connection uses, and the current limit of the regulator that also has thermal protection. You could easily draw more current than what the board normally would allow. Not saying you can't or shouldn't, just that there are risks.