Extruder fan controller

I brought my makerbot to the Reading Friends and Family Day to show at the Reading Robockets booth. I’ve been having some issues with the extruder skipping, and I think part of the problem is that the stepper motor that does the extrusion is overheating a little.

MIT Swapfest was also this past weekend, and I got a number of small useful items, like a few of these fans for $1 each (including the finger guard). these fans are LOUD and move a lot of air.
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One thing that I liked about the fans is that they are the four wire variety which means they can have speed controlled with a PWM signal applied to the blue wire. The base frequency is around 25 kHz, and the duty cycle controls speed (from 30% to 100%).

I did a bit of research online, and came across a neat ieda for a duty cycle controller using a 555 and an op-amp. The speed input to his circuit is a potentiometer, but I decided to use an LM34 which gives a nice 10 mV/°F output signal where 0 V is 0°.

Given the parts on hand and the timing requirements, I used 10nF for Trigger Cap, 2.1K between trigger and discharge, and 1K between discharge and 5V. This provides a 25 kHz base frequency with a roughly 50% duty. Since the 555 charges and discharges from 1/3 to 2/3 of the power input, I had to scale these values to be the temperature range I wanted. A simple 100k/100k voltage divider brought the charge and discharge voltages down to ~800mV and ~1600mV, which is 80°F and 160°F.

I also took the opportunity to use some 50mil grid SMD protoboard I recently picked up. I modified the DIPs  to be surface mounted, and used SMD components everywhere I could. I only has 10nF caps in the lentil variety.

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The TO-92 package LM34 hangs off the back to be in contact with the motor. The motor came with a bracket, which I haven’t removed, and the hole spacing just barely worked for the fan mount. Here is the board attached with a spare motor/bracket combo I had available.

imageFor testing, I used the side of the barrel of my soldering iron to heat the chip up, and a few drops of isopropanol to quickly cool it off. With the LM 34 at room temperature, the fan still idles around 30% speed, which is much nicer on the ears.

Once everything is mounted to the makerbot, I’ll post a video of it working, and I’ll try to work up a proper schematic.

 

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