Noobie in Need - Powering Raspberry Pi

[warrmr]

Howdy Guys,

I am in the process of acquiring a 3D printer but as like most printers they are not networked so you have to have a computer teathered to them for the duration of the print and for some people this is not practical.

What I would like to do is use a Raspberry Pi as the "computer" to drive the printer, So far I have found out that the driverboard on the printer has a serial port I just need to test that they are live otherwise I will need to tweak the firmware.

To be able to plug the RPi into the driverboard I will need a logic shifter which I understand you can make out of some N type Mosfets and resistors. So I am planning on making it a small PCB that will sit ontop of the GPIO header of the RPi. I would like to have this board power the RPi to save me trying to source and solder a Micro USB plug onto a DC converter.

SO I have 2 ideas and am not sure which one would be the best for a total noobie and would be the safest.

1) Tap the 12V feed directly from the printer PSU and build a small switching regulator with a similar design to the one on the Driverboard
2) Purchase a 12V - 5V DC -DC converter and have the 5V feed soldered onto my shifter/power board

With both options I will need to provide my own power protection circuit as by putting 5V into the GPIO header you are bypassing the protection circuit on the RPi.  I have put together a quick sketch in paint to show the protection circuit that I was thinking of but im not sure how safe it would be and if I need any caps in there before the +5V out.

I am hoping the experts can help or advise on some reading material that is suitable for a total noobie.

[ptricks]

So you need 2 things, a RS-232 level shifter for 3.3V and a 5V supply that can supply 700ma.
You will need to check the printers serial port and see if it is using 5V logic or 15V logic.
Level shifter is easy, just get a Maxim chip like the Max3222 lineup , cost about $3 max.  You can do it with discrete parts but when the alternative is $3 and you are done why bother.

The 5V supply I would not source from the printer. Normally you can power a micro off an items existing supply, but usually the power needed isn't much a few ma. This is 700ma though and that equates to an extra 3.5W that the printer supply may not be designed to handle.
I would use a 5V wall wart adapter, cost about $5.

If you think the power supply of the printer can provide the extra power then the easiest route is one of the switching converters based on the LM25xx series. They are available on amazon, ebay for about $5.
http://www.amazon.com/LM2596-Converter-Module-Supply-1-23V-30V/dp/B008BHAOQO/ref=sr_1_3?i

[bingo600]

You can't beat this price for the RS232 adapter

http://www.ebay.com/itm/Serial-Port-Mini-RS232-to-TTL-Converter-Adaptor-Module-Board-MAX3232-115200bps-/251103811631


This one might be beatable (but still cheap)
http://www.ebay.com/itm/2-PCS-MAX3232-RS232-Serial-Port-To-TTL-Converter-Module-DB9-Connector-w-Cables-/190850616787

/Bin

[amyk]

The 5V supply I would not source from the printer. Normally you can power a micro off an items existing supply, but usually the power needed isn't much a few ma. This is 700ma though and that equates to an extra 3.5W that the printer supply may not be designed to handle.
I would use a 5V wall wart adapter, cost about $5.

If you think the power supply of the printer can provide the extra power then the easiest route is one of the switching converters based on the LM25xx series. They are available on amazon, ebay for about $5.
http://www.amazon.com/LM2596-Converter-Module-Supply-1-23V-30V/dp/B008BHAOQO/ref=sr_1_3?i

Check the rating on the printer's PSU first, but I doubt an extra 3.5W on a device that's consuming 100W+ is going to overload it; if it does, then they're cutting it too close to the margin. Nor do I think the RPi will draw the full 3.5W - some Googling around shows people getting around 2.x W max, and this is with the SoC running at full speed. Good software will keep the CPU idle most of the time, reducing average power consumption below that.

[warrmr]

Wow, im starting to love this website already! Thanks for all of the replys I will try to respond to all of the questions and sugguestions.

The level shifter I was going to use for my prototype and then once I have everything working on breadboard I am going to get a nice little PCB made that should just clip onto the GPIO of the RPi and a a ribbon cable up to the Driver board.

www.adafruit.com/products/757

The driver board is using 5V logic and the RPi uses 3.3V logic.

I was looking at pulling teh +5v from the driver board on the printer but the switching regulator looks to be limited to 300 or 600mA as it is controlled by a LM2841 so drawing the power from the driver board is a no go.

So my idea was to tap into the back of the power switch and grab 12V from there and either use one of these and solder the 5V feed onto the PCB that I am going to create but I will need a protection curcuit on the board I am going to make as the GPIO lines feed directly into the Pi and bypass all of the fuses and filter caps that it would go through if you power it from the micro USB.




The other option was to copy the switching regulator design from the driver board and implement it on my PCB that will clip to the Pi that way you can take the 12V feed directly from the Power switch and solder direct to my PCB.

I will put something together in Eagle tonight and post it up here so you have a better Idea of what I am trying to achieve.

I have attached the switching regulator design from the driver board and also a photo of one of the Beta Printers, I am still waiting for mine to arrive but it should be here by the end of July.


As for overloading the PSU I dont think it will be to much of an issue as the spec advises that you will get a 60Watt version with the printer I have ordered and a 120Watt version with the version with the heated bed.

So worst case is I could upgrade the PSU to one that comes with a heated bed.

[warrmr]

Ok I have put this together in Eagle,

The level shifter part should be good to go but im still not sure of the power supply as inputting +5v into the GPIO pins needs some form of overvoltage/overcurrent protection as the GPIO pins bypass all of the built in protection on the Pi. The part I am not sure of is if I should bother with the Diode and if I need to put any caps in there.

The whole idea of wanting to make this myself is so that I have a nice neat PCB that will sit onto of the RPi and interface both devices, sure I could buy a level shifter then rig them up on protoboard but it will look horrible in a printer with a clear case.

The other option is to build a Power converter into my board and just draw the 12V and direct from the printer and then drop it down to 5V for the Pi.