No Doorknobs Needed For This Nitrogen Laser Build

Sometimes the decision to tackle a project or not can boil down to sourcing parts. Not everything is as close as a Digi-Key or Mouser order, and relying on the availability of surplus parts from eBay or other such markets can be difficult. Knowing if and when a substitute will work for an exotic part can sometimes be a project all on its own.

Building lasers is a great example of this, and [Les Wright] recently looked at substitutes for hard-to-find “doorknob” capacitors for his transversely excited atmospheric lasers. We took at his homebrew TEA lasers recently, which rely on a high voltage supply and very rapid switching to get nitrogen gas to lase. His design uses surplus doorknob caps, big chunky parts rated for very high voltages but also with very low parasitic inductance, which makes them perfect for the triggering circuit.

[Les] tried to substitute cheaper and easier-to-find ceramic power caps with radial wire leads rather than threaded lugs. With a nominal 40-kV rating, one would expect these chunky blue caps to tolerate the 17-kV power supply, but as he suspected, the distance between the leads was short enough to result in flashover arcing. Turning down the pressure in the spark gap chamber helped reduce the flashover and prove that these caps won’t spoil the carefully engineered inductive properties of the trigger. Check out the video below for more details.

Thanks to [Les] for following up on this and making sure everyone can replicate his designs. That’s one of the things we love about this community — true hackers always try to find a way around problems, even when it’s just finding alternates for unobtanium parts.

15 thoughts on “No Doorknobs Needed For This Nitrogen Laser Build

  1. Why not snip one wire off two “wire ended capacitors”, and file the protective coating down in the middle and solder two together. It would not be double the voltage rating unless you epoxy coated them but it should handle more voltage. My guess would be that as long as the silver coating is intact from the filing off the coating you should be able to get a good low inductance solder joint in the middle. I’m thinking of two pancakes (capacitors) with maple syrup (solder) in the middle.

    1. Yep, for sure! I am going to have another go at de-potting these things.

      Someone suggested in the YouTube comments dissolving the epoxy in Formic Acid and Acetone, so I will give it a go once the holidays are out of the way.


    1. They can turn up at good prices on eBay, depends how much time you have to wait, but also a lot of gouging goes on with shipping there too. Yeah, export restrictions are a pain as well, but if you are on a tight budget, wire ended caps will certainly do the job!

  2. It came to my attention recently that PET (For which mylar is a name) is actually quite a good insulator, and I was pondering construction of some high voltage caps with sodapop bottles and cans. I think however, one needs to try to dehydrate it as much as possible.

      1. Nice, thanks, I might have a use for one of those. I was pondering whether diode lasers had enough juice for something that’s a proof of concept that relies on a dozen tweaky lashups, so might take more time and patience than I’ve got.

  3. Material Dielectric strength (kV/inch)
    Air 20 to 75
    Paraffin Wax 200 to 300
    Transformer Oil 400

    Probably easest to put the cap in a small container. Pour moltern wax to cover the cap. Make sure the leads coming out have good separation.

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