How to switch HV between AC and DC? - Page 1
Forum section: electronics / projects

author: Plow
author_rank: Newbie
timestamp: January 22, 2024, 07:16:18 am
content: I'd like to build a power supply that will be able to be switched between AC and DC. I'd like to avoid changing leads around and since I think it's very difficult to switch the HV side, short of a massive HV relay, is it best to just use 2 transformers and switch the primary side? This is what I'm planning atm, as attached.Now the left AC side should be about 30kV, and the DC side on right about 150kV.What I don't know is how the DC side will effect the AC side since its effectively shorted through the AC transformers secondary. Is this a problem and how can it be fixed? Will a capacitor in series between the AC transformer and output terminal solve this issue? If so what sort of capacitor?

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 22, 2024, 07:57:58 am
content: Have you ever worked with voltages that high?I can tell you that you definitively will need oil baths/tanks to do anything at 150kV

author: Andy Chee
author_rank: Super Contributor
timestamp: January 22, 2024, 08:04:02 am
content: Quote from: Plow on January 22, 2024, 07:16:18 amTheoretically speaking, a DC blocking capacitor would indeed prevent DC flowing back into your AC transformer.Practically speaking, a DC blocking capacitor capable of withstanding 150kV is not easy. You're probably looking at vacuum capacitors at a minimumhttp://ur4ll.net/#caps3Several caps in series would give you the required voltage rating, bearing in mind that series connected capacitors also reduce capacitance, the reduced capacitance may affect your supply depending on the frequency of your AC.Easier just to disconnect the unused supply and change leads.

author: Plow
author_rank: Newbie
timestamp: January 22, 2024, 08:36:37 am
content: Quote from: PartialDischarge on January 22, 2024, 07:57:58 amI was planning on potting the entire thing in a big trough of resin.Quote from: PartialDischarge on January 22, 2024, 07:57:58 amAny problem with the ceramic 50kv 2200pf caps that are quite prevalent on ebay etc.?https://www.ebay.com.au/itm/194340544877The frequency is 25kHz.Is the DC side ok? I feel like it'd just half wave short through the multiplier.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 22, 2024, 10:09:43 am
content: Quote from: Plow on January 22, 2024, 08:36:37 amI assume the multiplier is a Cockroft-Walton voltage multiplier ladder. In which case, are the ladder "rungs" capacitors or diodes? To block the AC, the ladder rungs should be capacitors.It's hard to tell from the photos, but I don't think those ebay caps are rated for 50kV, especially not continuous 50kV.

author: Plow
author_rank: Newbie
timestamp: January 22, 2024, 10:53:02 am
content: Can you please elaborate on the capacitor "rungs", afaik there is only one way of making a CW multiplier with diode "rungs".I do have one of the 50kv capacitors they are quite chunky and obviously designed to be smothered in insulation as 50kV would arc straight past it. I was going to use the axial ones in the CW multiplier. Here's a pic of one sitting on one of the transformer I was going to use. Two of those in series to make up 30kV.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 22, 2024, 12:06:28 pm
content: Actually I probably made that more confusing than it had to be.Basically you need to finish your ladder with another diode, in order to stop HV AC from your second supply, traveling back into the ladder via the capacitors.

author: Plow
author_rank: Newbie
timestamp: January 22, 2024, 02:44:22 pm
content: Quote from: Andy Chee on January 22, 2024, 12:06:28 pmI've redrawn it with where I think you mean to put the diode in red, but wouldn't the other half cycle short through the diodes?

author: CosteC
author_rank: Regular Contributor
timestamp: January 23, 2024, 09:01:52 am
content: There is number of oil, SF6, vacuum insulates switches which can do what you need, however, none is:SmallLightCheapEasy to getMost of such devices can handle many kW of power, not mere uA, mA levels you need.You could try to overload some HV relay and see how it performs as your short circuit currents are super low anyway. Not suitable for industry use, but for personal lab it may work.