13. Power Cables

The following instructions work for the USB cables I used for my own cluster. If you get different cables, you’ll most likely need to find a different solution that works with the different cables.

USB Cables - All Colors

Before you start routing and cutting cables, think about how you want to route them. Had I thought mine out a little better, I would have arranged them so the color code of my top row was reversed from left to right. But what you mainly want is for your cables to cross each other as little as possible in the provided areas for cable management.

Cluster - Back Side Angled

Also, ideally, you should color code them the same way as your Ethernet cables, so that each Pi has its own unique 2-color combination of tie/cable.

For each row of Pis, do the following:

Step 1. Route a Row

Route a row of cables. I started with white, but I recommend routing black first and connecting it to the leftmost 2 ports on the 2 bottom rows of power cards. This will allow you to route it with the least possible amount of cable crossing.

For each cable:

  1. Mark the USB A end of the first cable with your chosen cable tie color and plug it into the power card in your chosen position.
  2. Consider if you want to repeat the same cable tie pattern across all cable colors. Then, route it loosely down to its corresponding Pi.
  3. Cut the cable an inch or so past the Pi, just as you did with network cables. Since you’re routing these cables loosely, you don’t have to be as exact with this. However, try to be as consistent as possible with how much extra you leave.

Now, remove all four cables. And keep the cut ends for the next step.

Step 2. Strip Your Wires

Note that you can use the wire that’s still connected to the micro-USB end of the cables for some wire stripping practice before moving on to the wires you intend to keep.

From the cut end of all of the cables you cut to length in the last step, remove roughly 12 mm (1/2″) of the jacket. Then, strip about 3 mm (1/8″) of the end of each wire to expose the bare metal at the end.

If you accidentally cut too far in when you’re cutting the jacket or wire insulation, you will need to trim the wire back a little further and try again. With practice, you can learn to strip wire this way reliably with just a sharp knife. Or, you may want to use a wire stripper for this part.

Step 3. Crack the Micro-USB End Adhesive

I found that you can generally crack the micro-USB end a little by hitting the side of it gently with something blunt. You need to tap it just hard enough to crack the adhesive bond between the two plastic pieces, but not so hard as to break it entirely. You don’t need to break the piece completely apart in this step; just tap it enough that you start to see the two parts of it separating a little.

Step 4. Separate the Plastic Pieces

Carefully use a utility knife and/or X-ACTO knife to separate the plastic pieces. You should easily be able to separate them by slipping the knife blade into the crack you made in the last step and widening it.

Be sure to keep them with the rest of the cable tip they came with, as sometimes the plastic and the connector from different cables won’t be compatible. If you’re unlucky, one-way incompatibilities can leave you stuck with one shroud/connector combination that doesn’t fit after you’ve just finished putting 3 cables back together with no difficulty.

Step 5. Remove Silicone Pieces

Use an X-ACTO knife to cut a slit in the silicone piece so it can be removed. Then, peel it from the end of the cable piece.

Step 6. Write Down Wire Color Code

Write down the wire color code that’s being used for the micro-USB end of your cables. Hopefully, it’s a standard color code, but there’s no guarantee that it will be. There’s also no guarantee that all of your cables will even have the same color code, so watch out for variations.

Step 7. Desolder the Tips

Your cables are very likely to have lead solder on the ends. If you care about your lead-free soldering station, use a cheap iron you don’t care about so much for this part.

First, get your soldering iron nice and hot. Then, use it to quickly¬†heat the solder joint attaching the wire to the connector. If you’re not quick about it, you’re likely to melt the plastic part of the the connector, which will likely ruin it.

Again, keep the parts from each micro-USB end together, so you can match them up again later when you’re putting them back together.

Step 8. Tin the Stripped Ends

Twist the individual ends of the wires you stripped earlier, so as to form a nice straight twist of strands on each end (as opposed to a frayed mess, which is what some of them probably look like). Then, apply a small amount of solder to the end of each wire. Be very quick about it, as the insulation on your wires is likely to melt if you leave the end in contact with the iron very long.

Step 9. Solder Micro-USB Ends Back On

Minding the color code(s) you recorded earlier, solder the tinned ends of the wires  back on to the micro-USB connector. Once again, be quick about soldering them and keep the rest of the parts with the same connector they were with originally.

Step 10. Glue Shrouds

For each cable, wrap the silicone piece back around the end of the jacket. Then, sandwich the connector between the parts of the plastic shroud. Check that the plastic parts fit properly as you have them placed. If they seem to fit well, clamp them lightly. Then, apply a very small amount of acrylic adhesive to the crack.

This adhesive should weld the two plastic pieces back together. However, take great care not to get any adhesive on your clamp, as many clamps have paint or plastic parts that this adhesive will melt.

After the end of the cable has been clamped for about 5 minutes, it should be cured well enough to remove from the clamp. Set it aside and let it cure for another 20 minutes or so before disturbing it further.

Step 11. Color Code Micro-USB End

Place the same color of zip tie on the Micro-USB end as you did on the USB A end of the cable. If you wrap the zip tie around the silicone piece you had to cut earlier when you removed the end, you can mostly cover the fact that it was ever cut.

Step 12. Retest Cables

Test each cables by trying to power a simple USB device. If a cable doesn’t work, cut one of your spare cables of the same color to the same length and make a new one from the spare cable. Keep the bad cable for parts until you have a full set of 40 cables. If you’re forced to try to fix a cable that you’ve previously shortened, I recommend that you first try replacing the micro-USB connector on it, as these seem to have thee highest rate of failure of all the parts of these cables.

Step 13. Reroute Cables

  1. This time, plug each micro-USB end into its associated Pi first.
  2. Zip tie the bundle of cables together next to the wire management comb.
  3. Twist the bundle enough that the cables curl upward (or downward) next to the micro-USB ends instead of sticking straight out. This will help decrease the risk of a conflict between the USB cables and the filters.
  4. Finish routing the cables back up to the power cards.
  5. After you’ve plugged in a row of cables, I recommend testing the whole row again to make sure they’re still working. You usually don’t lose a cable between the first and second test, but it can happen.

Follow the above procedure for all 40 cables. Once you’ve gotten comfortable with shortening these cables by doing a few rows, it doesn’t hurt to try doing a few rows at a time. As you can see in the picture below, I did my last 5 rows together. This worked out fine for me.

USB Power Cable Wiring

The cables will end up looking like the following once they’re routed.

Case Central Structure - Assembled Left Side Detail Cluster - Back Side Angled

12. Ethernet Cables 13. Power Cables 14. Final Assembly

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