Prusa X carriage, LM8UU with LEDs and fan

by jridley, published

Prusa X carriage, LM8UU with LEDs and fan by jridley Sep 8, 2012
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This X carriage starts with the LM8UU holders I designed to be very simple and adds a fan bracket that tilts and also a pair of 5mm LED holders.

The fan bracket is optional and can go on either side (or both if you want to go nuts on that). The LED size can be varied in the SCAD as well.


The STL file is set up for SAE 6-32 screws but the SCAD can easily be changed for other sized hardware.

(Note that the belt attachment point on the photo of the bottom of the carriage is incorrect, it was from my first print where I screwed up the direction of the belt)

Insert two 5mm LEDs (you probably want white). Solder together the long lead from one to the short lead from the other (putting them in series), then solder a 220 ohm resistor in series with one of the unused leads on one of the LEDs (it doesn't matter which). Solder wires leading to 12 volt power to the remaining lead of the resistor and the lead from the other LED. If the LEDs do not light, reverse + and - on the power. If they still don't light, you probably hooked up one of the LEDs backwards - reverse one of them and try again.

Press the LM8UU bearings in firmly. Insert a small zip tie into each retaining channel, then tighten and snip off the tail.

Install as you would any X carriage. It is designed to have the belt come off the bottom of the pulley and bearing and lay on the top surface with the belt teeth up. On one side, use one of the toothed belt clamps and a pair of screws to fasten the belt. On the other side, use the belt tightener with the channel facing down towards the belt, insert a screw and a nut through the tightener and into the belt ram. Wrap the belt over the ram and over the top of the tightener (engaging the belt teeth on top of the tighener) and top it all off with the smooth cap. Tighten everything up and tension the belt as desired.

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Made this one for my Prusa i2 project ;-)

24mm width is to narrow, could not fit the LM8UU bearings (otherwise pliers needed!)
So, edit the file: LM8UU_holder_ziptie.scad and adjust:

LM8UU_length = 24; -TO- LM8UU_length = 24.4;
Now they fit nicely!

Also...Greg's wade extruder v3 is M4 and NOT M3!, i miss the option to seperate the WADE holes from the GT2 belt holes..., so i've added this option:

edit the file: x-carriage-ziptie.scad, add line:

screw_thread_diameter_wade = 4.5; -AFTER- screw_thread_diameter = 3.5;

in the function "module groovemount_holes()", change line:

cylinder(r=screw_thread_diameter/2, h=base_z+groovemount_elevation, $fn=20); -TO-
cylinder(r=screw_thread_diameter_wade/2, h=base_z+groovemount_elevation, $fn=20);

Good idea the LEDs, I'll add some to mine then :) Blue and White LED's have about 3.6 volts forward dop so you could run four of them in series without a resistor - chart here: http://www.oksolar.com/led/led_color_chart.htmhttp://www.oksolar.com/led/led...

Actually you can't do four. You need to calculate a current limiting resistor using the remaining voltage across the resistor. With two LEDs you have 7.2 volts forward, so you have 4.8 volts remaining across the resistor. To put the current across the whole circuit at 20 mA, you make the resistor 4.8/0.02 = 240 ohms (or thereabouts).

With three LEDs you'd only have 1.2 volts left across the resistor so you'd need to make it 1.2/0.02 or 60 ohms. With 4 LEDs in series there's nothing left out of 12 volts to calculate the limiting resistor value with and you just can't do it.

You could run four LEDs, 2 circuits of 2 LEDs each w
ith a 220 ohm resistor.

I tried to do three in a triangle arrangement but I couldn't cram them in there without putting one behind the smooth rod where it wouldn't do any good. I suppose I could try putting them really tight in a line.

What I'm getting at in the end is that the resistor is not there to limit voltage, it's there to limit current. Having a series of LEDs that have a forward voltage that adds up to your supply voltage doesn't mean you don't need a resistor.

If you take an LED with a 3.6 volt requirement and give it 3.6 volts but don't limit the current, it'll overheat and fry.

That's not my experience. I put LEDs in series all the time without current limiting resistors. I don't get where this myth comes from.

It isn't a myth - the impedance of an LED drops very rapidly as you approach the rated threshold voltage. If you are below that voltage (either by putting multiple LEDs in series or dropping the supply voltage) then you can still run LEDs without a limit resistor (because the impedance is still high). LEDs aren't a simple on/off - they behave like a diode from nanoamps to amps, and you get light out of them across a range of currents. The rated voltage you see on an LED datasheet is coupled with a rated current. But running without a resistor to limit current means not getting the most out of your LED at the least (because you are relying on the LED impedance to keep the current below the rated value), and risking frying your LEDs at the worst (if you are too close to the threshold voltage).

Thanks for the explanation!