follvalsch

follvalsch:

Project INFRA #2

The second iteration of the coilgun project yielded in one awesome decision and one bad decision.

First the awesome decision: bronsen (who joined me in scaling up the coil gun) and I decided to get a proper high power capacitor. Specifically the Maxwell BCAP3000 that comes with a whopping 3000 F (note: no milli or micro here, proper Farads) capacitance.

It takes a “while” to charge it up (read: long minutes to few hours) depending on how much current you can provide.

However, the short circuit current of this beast is at around 10 kA thanks to a mere 0.28 mOhm ESR. Enough to use it for spot welding and melting of nails.

The bad decision we made was using 16 mm PVC tubing as a barrel.
The extra power we got from the huge capacitor was not enough to produce a magnetic field strong enough to get anywhere close the power we had with the small capacitor bank in the first iteration.

Also, with a capacitor of this size, we would not be able to simply shut down the magnetic field by waiting for the cap to discharge. It would simply take too long and produce way too much heat in the process.

So a means to actually switch such a high current was called for. The most powerful FET we found at Digikey was the IXYS IXTN600N04T2. It’s able to switch 40 V and up to 600 A. So we got a few :3

To measure the projectile speed and time the switching of the coil we decided to go for the bare bones solution using IR LEDs and suitable photo darlington detectors.

To attach the homebrew light barriers to the PVC tube, I quickly designed a clip-on holder in Inkscape and used the shackspace laser cutter to cut a prototype from birch plywood.

The light barrier turned out to be sub-par for the task. Especially the holes that had to be drilled into the tube were a bit tricky to manufacture. The most difficult part was sanding down the edges of the holes inside the PVC tube so that the projectile wont get stuck.

The next iteration will change two things:

  1. Replace the PVC tube with a borosilicate glas tube. It’s transparent so no holes have to be drilled.
  2. Replace the 16 mm sized tube with an 8 mm sized tube. This will result in a stronger magnetic field.
  3. Replace the homebrew light barrier with off the shelf 8 mm light barriers. No tricky alignment or mounting issues.
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