Best insulator for a C-W multiplier - Page 1
Forum section: electronics / projects

author: Neukyhm
author_rank: Contributor
timestamp: June 24, 2017, 08:43:44 pm
content: Hello forum, I'm here because I have to build a 70kv, 18 stages C-W multiplier with cathode grounded. I can't use liquid insulators so I don't know what to use, polyurethane, epoxy, nothing...What do you guys recommend?

author: floobydust
author_rank: Super Contributor
timestamp: June 24, 2017, 08:53:54 pm
content: I find this kind of thing is hard epoxy encapsulated. In air, I find a lot of losses from corona discharge because parts are close to each other.Potting something, well it's not repairable.What output current you are trying to achieve. 70kV/18 stages =3.9kV per stage, am I right. Seems high, as far as diode/cap ratings.

author: floobydust
author_rank: Super Contributor
timestamp: June 24, 2017, 10:17:46 pm
content: The 70kV output terminal is where the insulation is really needed. Usually a HV wire exits the potting compound - no screws, terminal blocks etc.For a commercial product, a potted pcb would be best.For hobby stuff, you can build this point-point wiring on perfboard in air, example:250kV HV multiplier.Image below fromHigh Voltage Shopof multiplier PCB with 15mm diodes

author: jbb
author_rank: Super Contributor
timestamp: June 25, 2017, 12:12:10 am
content: There are gel-type silicone compounds which might be suitable. They don't set hard and can be (messily) dug out for service if necessary.

author: macona
author_rank: Regular Contributor
timestamp: June 26, 2017, 09:11:00 am
content: Silicone is used on some multipliers. But a lot just use lots of air space.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 26, 2017, 11:03:38 am
content: What frequency of operation and capacitance per stage are you aiming for?With 18 stages and 2 mA load current, there will be a lot of voltage drop under load, and any stage beyond the tenth will likely not add much, for any practical input frequency and capacitor value. Practical multipliers often have less than 10 stages, I would aim for 6 or 8 maximum.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 27, 2017, 05:50:43 pm
content: One stage is two diodes plus two capacitors, so that would be a four stage multiplier, with two capacitors in series per stage. This is a much more reasonable number, and very typical for dental X-ray supplies, see the attached image.The main challenge with operating it in air is that high voltage diodes often don't have enough creepage distance across the package to stand off the rated voltage, so you need something other than air for insulation. If liquids are out, that leaves gases and solids. Gases are difficult to get (SF6) or have other dangers (butane), and sealing them is a pain. Silicone or epoxy can be an option, but they will not remove heat as well as oil so the available power is lower. How much power are you aiming for, and for how long? A dental x-ray tube can typically handle 500 watts for a few seconds, and 250 watts for a few tens of seconds, medical diagnostic tubes can handle more.Note that 350 - 600 kHz is a pretty high frequency for a multiplier like this, and diode recovery might be problematic, a more typical frequency is in the 50 - 200 kHz range, the multiplier board with the onboard transformer in my picture was made for 150 kHz operation.

author: KE5FX
author_rank: Super Contributor
timestamp: June 30, 2017, 09:30:41 pm
content: Quote from: Neukyhm on June 30, 2017, 08:13:34 pmIt's hard to believe that plain old air won't provide enough insulation at only 70 kV. That's not a very high voltage in the grand scheme of things.If you do use two-part epoxy, the secret is to avoid buying it by the milliliter as "epoxy." Instead, buy it by the quart/liter in home-improvement stores in the floor coating department. Same stuff, very different price.

author: floobydust
author_rank: Super Contributor
timestamp: June 30, 2017, 10:00:09 pm
content: Air can work if humidity is low, ozone does not build up and there is decent clearance on the 70kV.I find the sharp points on the trimmed leads has a lot of corona, not so much between CW stages, but end-to-end.This guy used a lunch sandwich boxbut filled it with oil....then went to 200kV

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2017, 03:15:45 am
content: Wire in air with good mechanical connections which are then soldered and allow no sharp points, coat exposed conductors with corona dope, and then vacuum pot the entire assembly in wax.

author: TimFox
author_rank: Super Contributor
timestamp: July 02, 2017, 05:27:34 pm
content: In general, with wax or resin potting, the trick is to avoid bubbles. From elementary electrostatics, the E-field inside an air bubble surrounded by bulk material with dielectric constantcwill bectimes higher than the field in the bulk material, and the dielectric strength of air is also less than that of a good insuilator. When the air breaks down, it can damage the material and possibly lead to a track of carbon from one electrode to another.

author: ConKbot
author_rank: Super Contributor
timestamp: July 03, 2017, 11:03:05 pm
content: I seem to remember from some HV mailing lists that LDPE (cut up and cleaned milk jugs) can be dissolved into wax to make something a bit more robust than plain paraffin wax (not sure what its called outside North America, where kerosene here is called paraffin there...) I can't seem to find any solid references at the moment though.EDIT:http://www.pupman.com/listarchives/2002/May/msg00378.htmlhttp://www.pupman.com/listarchives/2002/May/msg00527.htmlof course it was pupman mailing lists for something HV related.Looks like it can be either hot glue or polyethylene strips that you can add to the wax.