I think I'd like to install one of these on my printer:  http://www.elefu.com/index.php?route=product/product&product_id=51 .

I think I’d like to install one of these on my printer: http://www.elefu.com/index.php?route=product/product&product_id=51 . The Melzi is ok, but the expandability and reviews of this controller have been positive, and it still uses screw-terminals (I agree with nophead’s conviction that these are necessary.)
I doubt I would be able to compile and load @nop_head 's Mendel 90 firmware onto it. Thoughts?

There shouldn’t be that much to running any firmware on it - the RA comes with Marlin that has an adjusted pins.h (as expected) and some config tweaks. Both are pretty easily transferable to other firmwares and something you’d need to do anyways when switching boards.

So I’d take the Marlin that it comes with, and put in the various critical values from the Mendel90 firmware? I guess a list of said values to modify is what I’m after. As a MechE, digging around in the code isn’t quite in my wheelhouse, although I do have a firm grasp of a majority of the relevant values.

As a Mendel90 + Marlin user myself, the most critical things you’ll need to change on a firmware that’s already configured for a “normal” printer and this specific board are the endstop position (and inversion), axis travel, speeds/accelerations and PID values of your hotend. All of which you can most likely just copy-paste from your current config.
What is the 90’s default firmware? Sprinter and Marlin use nearly-compatible configs, other firmwares might have some settings called differently.

Does anybody know the dimensions of this board. It looks like it might be too long to fit on a Mendel90.

Also it doesn’t allow all of the ATX PSU wires to be connected, which is no good if you use a cheap PSU with thin wires.

It looks like it just lets you plug an ATX PSU in directly, which seems a lot easier then tying multiple wires together and trying to fit them into screw terminals. As for the dimensions, I don’t mind mounting it on it’s own supports, not necessarily attached to the printer. I do wish they included the dimensions on the site though

Yeah, the lack of documentation and general non-openness make the RA look like a dead-end.
A screw clamp has a 5.08mm pitch, so counting them, i’d estimate it at about 250x120mm, probably mighty tight on a regular 90.

@Anthony_White1 It might be easier to connect half the wires but you need the yellow and back wires from the drive connectors as well to get enough current. Not to mention the dummy loads, which a lot of people leave off and then wonder why their bed does not get hot enough or the PSU switches off.

I definitely wired up the dummy loads, I understand electrical engineering well enough to comprehend their necessity. Are you saying the drive molex 5 and 12 volt lines are necessary to connect together becaus eof how much power is available on those rails, or because of the level of current through the # of conductors?

On the cheap PSUs I use the issue is the wire gauge is too small for the current on the 12V rail. Not only do they get warm if you don’t use them all, but the voltage is less and a small reduction has a big effect on the bed because the effect is squared.

Also the ground noise on the USB gets less the more black wires are connected.

I’ve never thrown my current meter on the 12 volt line, what kind of amps does the Mendel 90 pull nominally/peak?

I haven’t either but it is probably about 16A when both heaters and all the motors are on.

I know from earlier measurements that the heated bed draws between 12 and 10A, depending on its temperature, so 16A total sounds about right.
One way or another, I think that the PSU should not be a component to cheap out over. 350W 80+ PSUs are just 35€ nowadays - the premium over a cheap “400W” one will save you nerves in the short run and lower your power bill in the long run.

I agree. A good way to tell if it will be a good power supply for the task is to look at the available amperage on the 12volt rail, a required spec to disclose on ATX PSUs

I’ve seen some interest in using the 6 pin 12v rail intended for graphics cards, instead of the 4 pin rail intended for the cpu.

@Stephanie_A either one should be able to power a normal printer , but as nophead put it - the more, the merrier.
Personally, i have both the 8 pin P4/CPU connector and all four of the 8 pin PCIe/graphics card connectors hooked up - mainly because i can, but also because i’m running two heated beds from it. That 850W / 80+ silver power supply was on sale, by the way.

@nop_head @Thomas_Sanladerer The board dimensions are 204mm x 90mm - some further information in Issue 2 of the RepRap Magazine - http://reprapmagazine.com/ A nice board, the serial link to the LCD display is handy.

Slightly shorter than a Melzi then, so it should fit.

I think I’m going to take the plunge, thanks for everyone’s input!