Wednesday, August 2, 2017

UMMD 3D Printer Electronics

Update: The smoothieboard has been replaced by a Duet Ethernet board.  The information below is the original stuff.

Major components of the electronics in UMMD are:

In a 3D printer, the extruder requires the most operator input for maintenance and repairs.  The electronics comes in a close second.  Most people building coreXY printers put the electronics in the bottom of the printer, utilizing some of the "dead space" under the bed.  That can be convenient if the printer sits on a table, but UMMD is tall and stands on the floor, just like me.  Putting electronics in the bottom would mean getting down on the floor every time I had to access the electronics.  I also intended to use an LCD control panel to print from SD cards, and LCD panels generally don't work with long cable runs, so the LCD panel has to be kept close to the electronics.

I wanted to keep the electronics easily accessible, and outside of the heated enclosure, and I hate bending over, so I planned to put the electronics on top of the printer from the start.  Most of the electrical connections to the printer have to be made on the XY stage, so keeping the electronics close by keeps cable runs shorter.  The electronics are mounted on the left side of the top of the printer, under a plastic basket used as a protective cover (it's ugly- I'm working on it!).

There are two buck converters in the system.  One provides a source of 12V to power some fans and some of the white LEDs used to light up the printer.  The other is used strictly for powering the UV LEDs.  It would have been less complicated if I didn't have the 12V parts in the system, but it is easier to find 12V LED light bars, and I already had a couple 12V fans, so I used a buck converter.

The other buck converter is used to power the UV LEDs, something not absolutely necessary but aesthetically pleasing when printing fluorescent filament.

You can see more details about the LED lighting in UMMD here.

UMMD has three electrical fuses, one for the main power supply (which probably has its own internal fuse), one for the bed heater circuit, and a spare that is going to be used for an enclosure heater circuit, not yet installed.  Electrical fuses protect against fires caused by wiring or electronic failures.

The bed heater is powered via an SSR that is driven by the controller board.  When SSRs fail, they fail shorted - i.e. on.  The controller normally keeps the bed temperature regulated, but if the controller fails, or the SSR fails, the bed heater could get dangerously hot, and possibly cause a fire.  I added a thermal cutoff (TCO) to the bed plate to protect against that possibility.  It would actually be better to have the TCO mounted on the heater because the heater is attached to the bed plate with adhesive which could let go.  I will probably move the TCO to the heater itself by cementing it with high temperature silicone.  That way if the heater comes off the bed plate, the TCO can still do its job.  Details of the bed and support here.

TCO clamped to the rear edge of the bed plate.  Don't do this.  Mount the TCO on the heater using high temperature silicone.


I used a TCO with a operating temperature of 184°C.  That allows the bed to operate normally up to about 160°C (for printing high temperature materials) without opening the TCO.

The rest of the wiring to motors, etc., is standard stuff you can see in the Smoothieboard documentation.



2 comments:

  1. Curious about power - aside from the lighting (which is cool as hell), why go for 24V? My board, motors, hot end and extruder all run 12V and my heater/bed is 115AC, I I'm most likely going to just get a 12V power supply, but I thought I'd get your opinion.

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    Replies
    1. Motor torque is proportional to the current in the windings. That's why you set the motor operating current to some target value in the drivers. The inductance of the motor windings causes the current to ramp up to the target value instead of turning on instantly. That's why you want to use low voltage, low inductance motors. As the current is ramping up, torque is ramping up. When the current pulses are really short, as they are when the motor is turning at high speed, the current may never reach the target value. If you use a higher voltage supply, the current and torque ramp up to the final value faster. That means you'll get more torque at higher speeds if the supply voltage is high.

      In practice, it may not matter, depending on how you operate your printer. If you want to push your printer to maximum speed and acceleration, you want higher voltage.

      A 12V, 40W hot-end heater will have resistance of 12V^2/40W=3.6 Ohms. If you use a cable/connector that has a total resistance of 0.5 Ohms, the total resistance of the heater circuit will be 4.1 Ohms. A 4.1 Ohm resistance will draw 12V/4.1 Ohms=2.93A from the 12V power supply. 2.93A going through the 3.6 Ohm heater will dissipate 2.93A^2 x 3.6 Ohms=30.9 W So your 40W heater only delivers about 31W at the end of the cable.

      If you have a 24V, 40W hot-end heater, its resistance will be 24V^2/40W= 14.4 Ohms. If you use the same cable and connector that add 0.5 Ohms resistance, the total resistance will be 14.4 + 0.5 Ohms=14.9 Ohms. The current from the 24V supply through that resistance will be 24V/14.9 Ohms=1.61A. Power dissipated by the heater will be 1.61A^2 x 14.4 Ohms=37.4 W.

      So the 40W heater will deliver closer to 40W with the higher voltage power supply. This is why electrical power is distributed at very high voltages and converted to lower voltage (using transformers) at the end of the line. It's also why we use AC power instead of DC- there's no transformer to step DC voltage up and down. That takes active circuits that are much less reliable and more expensive than transformers. Sorry Tom, Nick was right.

      Does it matter if the heater provides 31 W or 37 W? It depends on how you run your printer. If you want to maximize print speed, you'll want more power in the hot-end to ensure that it can keep melting filament as fast as the positioning mechanism needs. That means you want the higher voltage power supply (or you can tweak the 12V supply voltage up a bit, or just install a 50W heater).

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