Electronics #1: Overall design

This is part #1 in a list of postings on the electronics of my CEVO. The full list can be found here. The topic of this list is overall design and a bit about power management.

I am basically a software guy, so my choices are probably more driven by the software/firmware side than by the hardware side. Also, I like to think in modules you can add together, but I am not really much for soldering. This, I think, have been quite influential in my choices.

A common choice for HEVO printers is the Duet WiFi, which is definitely very nice, but is also a monolithic and bit expensive solution where you are bound to a specific firmware choice. I am instead starting with choosing Klipper accompanied by OctoPrint. I like the approach of Klipper very much and it has good active development. OctoPrint I knows, and even though it is a bit of an ugly hodgepodge it has a lot of interesting plugins, and a generally good approach of how it should be to 3D print.

OctoPrint runs on a Raspberry Pi (or similar), and Klipper runs on most Arduino based controllers. The next choice is the which controller to use. I am going to use 6 steppers with an option for a 7th. This means I will have to use either an controller with an extension board or to use two controllers (which is an option with Klipper). Also I am going to use TMC2130 drivers for silent printing, for sensorless homing, and for tuning in software instead of with jumpers and screwdrivers. With this the most obvious choices seems to be either 2 RAMPS/Arduino, or 2 MKS GEN, or 1 RADDS/Due with extension board. With Klipper I don’t think I will have overall better performance with the 16bit Due than with 2 8bit Arduinos. So I am just going for the cheapest and most flexible choice, which is 2 RAMPS/Arduino (this will also keep the cost of each part under $12, which can keep me out of VAT and TAX in Denmark).

I am going to drive the heated bed, the hotend, and the TMC2130 drivers with 24V, while the Raspberry Pi and the Arduinos requires 5V. That leaves some choices for power management. For my printer I will need around 20A of 24V, requiring a fan, and just about 2A of 5V, good with no fan. The Raspberry Pi with OctoPrint works best with being powered all the time, while the 24V is only needed while actually printing. OctoPrint has a nice power controller plugin, that can automatically (or manually) turn the printer power on or off through a GCODE, and I am using that to control a simple 220V relay to turn the 24V power supply on and off. So a 5V power supply powers the Raspberry Pi, which powers the Arduinos over USB. The RAMPS boards are powered from the 24V power supply when OctoPrint has turned it on with the relay.

An alternative I did not really consider was using 220V for the heated bed, I had simple placed the order for my bed heater before thinking about this option. The benefit would have been being able to use a much smaller 24V power supply without a fan, but a downside would have been not being able to use PID but instead using bang-bang controlling of the heater. Sometimes it is OK that hard choices are made just by being a bit hasty ;-)

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