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• Successful Experiment

  Dean Gouramanis • 01/25/2018 at 19:02 • 0 comments

  I swapped out the ceramic capacitors, and soldered an additional 100uF cap to th header pins. Then I repeated the experiment. Flawless! This small PCB driving such a large motor is an impressive sight. The device was perfectly stable throughout the current range. The motor was quiet turning at 10 RPM, with Vdecay set to 2.1 volts.
  

  With no fan attached the maximum current was 2.5 amps full scale. With a (very) small fan blowing on the heatsink I was able to take it up to 3.4 Amps.
  

  In most desktop 3D printers you are likely to run a NEMA17 motor between 1.0 and 1.5 Amps. The data shows you will have no problem doing this. No fan required.

  The device can accomidate larger motors if used with a fan. In this experiment I used a very small fan. The temperature was safe, but ideally could be lower. Next time I will get some data using a blower fan to show ideal conditions.
  

• cause of failure identified

  Dean Gouramanis • 12/31/2017 at 00:48 • 0 comments

  After the first prototype failed, I repeated the experiment. The second device failed in the exact same way. I measured a short across VM/GND and 30 ohms across VDD/GND.
  

  I felt discouraged, but the identical symptoms indicated a trend. After reading, thinking, and getting some help from the TI E2E forum, I believe the problem was identified. The device failed because of inadequate bulk capacitance on VM. (You can read up on bulk capicitance in section 9.1 of datasheet.) When capicitance on VM is too low, motor inductance can cause voltage spikes which damage the device.
  

  This is my best guess, because aside from this capicitor, most of the schematic matches the example provided by manufacturer.
  

  The reason I did not add more capicitance in the first place was because the PCB is so small. I looked carefully at the layout... I think one more capicitor can be squeezed. Also I can use different capicitors to bring the total capicitance from 20uf up to 66uf. This will increase the cost of the device a little, but It's worth it.
  

  My previous experience tells me 66uf will be enough. StepperStack had only 37uf on-board, and I have never seen one of those fail in the same way. Plus 3D printers usually have additional capicitance on the motherboard.

  Before spending more time and money on a new PCB prototype, I modified one of the V2 models with three 22uf ceramic caps. If this one survives stress testing, then we're in business.
  

  Although it was a discouraging and costly setback, I feel like real progress was made. There was really no way to learn the limits, without trying and failing first.
  

• Testing begins + KABLOOEY!

  Dean Gouramanis • 12/14/2017 at 04:13 • 0 comments

  Yesterday I started collecting thermal data. I was unable to complete testing after 6 hours. I let the temperature go too high (151°C) and the chip was damaged. Now I'm waiting for new parts in the mail. In the meantime here is a sample.
  

  Note how small the fan is.
  
  

  
  The sensor was fixed to the IC.
  

  And here is the one that killed it.
  

• New prototype tested

  Dean Gouramanis • 12/11/2017 at 06:24 • 0 comments

  The new prototype is working. I adjusted the current to 1 Amp and did a quick check with an IR thermometer. Temperature is around 35 C.

• New Prototype Ready

  Dean Gouramanis • 11/29/2017 at 20:29 • 0 comments

  I assembeled the new prototype and made a neat video showing the details. Next I can run some tests to see:
  1) What is the temperature at maximum power?
  

  2) Is it immune to power surges?

  Can anyone think of another interesting test? Please, share your thoughts and ideas.
  

  Note: Some parts of assembly were left out of the video, such as reflow and soldering the headers.
  

• reference schematic published

  Dean Gouramanis • 11/23/2017 at 10:05 • 0 comments

  I like Sparkfun's schematics so much, I used it as a template. Except TEM schematics are blue. Man that looks good! #CreativeCommons
  
  It was time to decide on a name. I thought about calling them "Ice Cube" drivers, because of the square shiny heatsink. For now I'm going with simply "TEM 8818 Driver".
  

  A couple of notes:
  

  1) The schematic specifies a 1.5K high-side resistor for Vref trimmer. That means the current can only be adjusted up to 2.6 amps. My prototype has an 866 ohm resistor, allowing for full-range adjustment.

  2) While the DRV8818 can opperate from 8-35 VDC, the TVS circuit is rated for 30 volts. So the TEM 8818 Driver can operate from 8-30 volt power supply.
  
  This is a good thing because we know the IC will be safe behind the TVS (in theory). Im looking foward to capturing some surge waveforms with the oscilloscope.
  

• heatsink prototype

  Dean Gouramanis • 11/18/2017 at 03:39 • 0 comments

  Yea this heatsink design is simple and fits nicely. Should be the coolest driver on the market. Literally.
  
  To make these heatsinks I started with an extruded aluminum heatsink profile. Heatsink USA has a great selection of sizes and affordable prices.
  http://www.heatsinkusa.com/0-601-wide-extruded-aluminum-heatsink/

  I diced the 4 foot bars into 0.8" pieces outside in the garage. Then I was able to machine the smaller pieces indoors without making too much mess. In this video I add the PowerPeg receptacle, and some recessions that fit the PCB.

  If youre wondering what machine is in the video, check it out here.
  
  

• Vref and Vdecay adjustment

  Dean Gouramanis • 11/15/2017 at 16:50 • 0 comments

  This video shows the trim pot configuration. The components are mounted on the underside of PCB, and are adjusted through a hole. Test pads on the top side alow for quick measurement of Vref and Vdecay for tuning.
  

  A unique problem with this driver is the wide range of adjustability. The trimmers rotate 270 degrees, and the current range is 0-3500mA. That means 1 degree of rotation equals 13mA. Dialing in the correct current requires a steady hand.
  

  To help I added resistors in series with the trimmers to narrow the range. (schematic) So this prototype is adjustable from 800mA - 3400mA, and the resolution was improved to 9.5mA/degree.
  

• ESD protection

  Dean Gouramanis • 11/13/2017 at 23:23 • 0 comments

  There will be a substancial ammount of precious metal on this board, between the thermal connector, heatsink, high-current headers, and PCB traces. That's the first thing I thought once I finished the CAD model. These drivers last a long time under normal circumstances, but they are notorious for EXPLODING if the cables come loose. Its not right to use such expensive parts unless it is built to last. Call me a hippie, but the idea of PowerPegs thrown in the trash made me cringe... I digress.
  
  Back EMF from the motor can cause permanent damage if the wires are accidently disconnected while the motor is powered. This happens sometimes with loose connectors on shaky 3D printers. So in the first prototype I experimented with a 30 volt TVS installed at the output. In theory this will prevent this sort of damage.
  

  The first prototype functioned normally. I did try yanking the cable on a NEMA17 motor, and no damage! Eventually I will do extensive testing , and capture some waveforms with the oscilloscpe. Stay tuned for that...
  

  After the first prototype success I thought the entire device should have ESD protection. The problem was: not enough space. Here is an image of the first prototype layout.

  As you can see not much room for five more diodes. Luckly there were three header pins not being used. So I decided to omit them from the layout to make room for more diodes.
  

  So now every pin on the device is protected from voltage surges. This includes step, direction, reset, enable, V+, A, -A, B and -B.
  
  

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