Safer Coil Power Supply

3 08 2012


All Photos (1/2)
All Photos (2/2)

The capacitor bank which powers the coils is very lethal; when fully charged, it dumps more than a megawatt into the coils. Needless to say, If I were to accidentally complete the circuit through my body, it would rearrrange my insides. To decrease the likelihood of such an occorence, I’ve done my best to idiot-proof it. Here’s how it looked before:

Pretty easy to thoughtlessly reach in and touch the dangerous parts.

So first, I took a broken shipping pallet that someone across the street was throwing away, and made a box out of it

The box fits nicely on the crate that we’ve been using as a controls stand


I also left the front partially open so they we could get at the controls

With this setup, the only thing you can touch is wood, plastic controls, and the grounded metal face of the box, so it would be pretty hard to blow your hand off.

The next step was making the HVDC out wires safer. (the fat green and yellow ones in the first pic) The way we had it, the only way to dissconnect them was to reach into the supply and unscrew the ring terminlas to which they were connected. Not great. To fix this, I installed a high current outlet onto the back of the box and connected some shorter wires.

The other hole is where the cord which plugs into the wall goes

Took the wires we had and connected them to a matching high current plug

Assembled (and labeled):


So now it’s pretty safe. In fact, it’s so safe that getting into the power supply at all istoo much of a pain. We don’t want to have to move this box every time we need to reset the breaker. To make access easier, I put hinges on the bottom of it (see above), and chains on the side, so it opens like a tool box.

MakerBot mailed me an iron-on patch with some supplies I ordered, so I taped it on for good luck. (top right)

Look’s pretty badass, kinda like a ten-cylinder car engine.



Here’s one more safety feature I added:

If you open the “hood”, the power cord is automatically pulled out. This way, it’s impossible to open the power supply while it’s plugged in.

The coil supply is idiot-proof, but isn’t so locked down that minor repairs and upgrades are too much of a hassle. This is one more step towards a streamlined, repeatable, experiment procedure.

Domenick Bauer




8 responses

4 08 2012
Remy Dyer

I like it.
But never say anything is “idiot proof”. Nature will just produce a better idiot.

Passive safety and interlocks like this simple one with the power cord are good though. It’s simple, and therefore less likely to fail. It’s not perfect – disconnecting the power may keep the cap bank charged.

I’d add a mains powered relay, to connect a “dump” load when the power is cut. But not much more – it’s easy to go overboard with “safety” features. Maybe also a big resistor + red LED to the cap bank, so that it’s obvious when it’s got more than 120V or so on it.

Less than that (if “ripple free” DC) is considered ultra-low voltage and while not considered “safe to touch” (nothing ever is), it rates at least “you may feel it, but it probably won’t kill you on contact”. For AC the threshold is 50V. Well, at least in .au, anyway.

5 08 2012
Mark Suppes

Remy, this capacitor bank has permanently attached bleed resistors. The capacitors will always discharge after 20 minutes or so. Also there is a permanently attached voltage meter so you know if the caps are charged.

8 08 2012
Remy Dyer

Ok Mark, so I’d scrawl something like “Possible High Voltage Inside – Check here before opening” and draw an arrow pointing to the meter. Also draw on the meter where is “safe to open below here”.
I’d consider that warning enough.

Also shouldn’t be able to open the box without some kind of tool or key. Maybe just one screw to hold it shut – but this is good enough to stop / slow down a curious child, so it’s as far as the safety standards I work with require. (just in case that child can’t read the warnings).

Of course that child shouldn’t be there unsupervised in the first place, but it’s a useful scenario to do hazops with.

Stapling a page with the circuit diagram whiteboard photo printed on it to the inside of the lid would be a nice hint for future reference. Much like having well-commented source code. I’d also label cell-phone photos of the actual wiring in GIMP and attach prints.

AS/NZS 4000 electrical safety standards require the circuit diagram inside the box, along with labeled wires and double-insulation everywhere inside to at least finger-proofedness (IP20).

Quick, cheap work that is still sufficiently safe, compliant and good enough is the goal. I can’t guarantee that I’ve covered every (Aussie electricals law) requirement, but probably the main point is that having an idea of what the requirements even are is a kind of insurance in itself.

It’s worth checking the actual safety standard/legislation. You should be able to walk into a public library and ask a librarian for help finding them. Those things aren’t as scary as one might think, I’ve found. The Legislation usually is just “thou shalt use standard #x”, and the standards themselves are not quite international, but are almost always adopted widely. A lot of them are almost copied verbatim.

You’re doing extremely well. Keep it up!

You might also want to look at pg39 of “EMC2-0491-03 Electron Recirculation.pdf” for Bussard’s electron transit time calculations, if you haven’t already.

Keep in mind though that a lot of those EMC2-xxxx-yy reports are from early on, and some have errors that were later caught. The patent (US#4826646), the Valencia paper and the video are probably the best references.

7 08 2012


7 08 2012

Glad to see the attention to safety! Even in university labs you see some really dangerous contraptions, especially with high voltage capacitors and high power lasers without interlocks or housings or anything. It’s easy to be careless when you are working feverishly on something exciting.

11 11 2012
Joe Leikhim

20 minutes HV bleed down sounds scary. You should check out an ARRL handbook and read about HV safety techniques for HAM radio linear amplifier power supplies. Most have a gravity activated cam made of aluminum that shorts the capacitors to ground when the cover is lifted.

Nice work however, on your fusion project.

5 02 2013
Philip Williams

Diggin’ that woodshed aesthetic!

11 03 2013
Storm Copper

I too am digging that woodshed aesthetic. The best part of this is almost all the material was recycled from an old wooden pallett!

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