Image of Voltage Multiplier for a NST

[Dennis P Brown]

This is a full phase voltage multiplier for a center tapped Neon Sign Transformer (NST.) The transformer is rated for 15 kV (or 7.5 kV since center tapped) and 60 ma (lucky to also get half that under a real load.) This is a single stage (3 caps: 0.01 Mfd, 40 kV; and four 25 kV 1 amp diodes.) The system measured out just under 15 kV.

[Richard Hull]

I love those old Plastic Capacitor brand oil filled caps! For DC work, they are tough to beat and, now, tough to find at hamfests. I have about 50 or more here in the lab, all bought over the years at hamfests. If you can find them at hamfests now, they are usually smaller values and lower high voltages under 5kv. They used to come out of old HV power supplies that hams would scrap out and then sell the inards at hamfests. Gone are those days!

Issues I have seen with a rare one of these babies..........The glass to solder seals occasionally spring a leak usually due to mounting stresses over the years in OEM gear. If wiped carefully and quickly with acetone, a hot gun and electronic solder can re-seal provided you are quick about it.

Nice multiplier!

Richard Hull

[Dennis P Brown]

So the second stage of my new voltage multiplier (VM; by the way, this is full phase) is complete; performance was about 27 kV and the humidity being high, the corona discharge on the unit was significant, as well. I will use this higher voltage system for my Van de Graaff (VdG) positive emitter electrode. Hopefully. the VdG will have sufficient current using this power supply to be useful for my accelerator deuteron gun.

As I noted in the other topic section, this is a rather large system compared to just using "door knob" caps. I was trying to provide as much storage cap for these two stages as possible but I only have four of these large caps - this might draw my final voltage down since these caps will, most likely, not operate as fast as just the lower sized door knob caps; however, at my NST operating frequency of just 60 Hz I'd think this wouldn't be much of an issue. Also, the second unit of this system can be placed under oil (will have too if more stages are tried.) The strips have room for more stages. Since the diodes are rather large (can handle 2 amps at 20 kV) this is rather necessary. Smaller diodes would be more compact but these high current diodes are just too inexpensive to pass up and they offer advantages for future, more high current applications, if I decide to go that route.

The first stage was designed to be independent of the follow on stages - more for ease of operation, than anything else; also, this design will allow for a high voltage positive (or negative) power source for diode testing in the future that can easily be disconnected from the follow-on stages with little trouble. This first stage unit can also be placed under oil for enhanced operation.

I have nine more door knob caps (but need ten more diodes) to create three more stages (if needed.) I will assembly these extra units only if the VdG under performs by not producing enough current to allow the voltage to maintain the electric field for the accelerator's operation. Then I guess I will try but others have suggested and do away with the VdG and just use the VM for high voltage. Will have to wait for dryer weather before trying that experiment; if it works, I guess I will also finish the humidity control system for the room so the VdG will not have issues on humid days.

[Richard Hull]

Those diodes have the appearance of old damper diodes from color TV sets. Typically 5-10kv rating @ 100- 500 ma.

Richard Hull

[Dennis P Brown]

These are certainly Chinese copies of HV diodes; they claim to be rated to 20 kV and hold up well to the NST: which I would think, doesn't exceed 10 kV in my chain. So, you may very well be correct and they are labeled incorrectly ... since my system handles these (so far), appear's to work fine even as I ramp to max. voltage. However, I could test one against a real load of 15 kV (RMS would be close to the limit) and see if it goes "poof". Think it might ...

[Dennis P Brown]

Next up, I guess, is assemble, install and test stage three (50 kV or so?) of this voltage multiplier. This exceeds my high voltage probe's measurement level (45 kV) so a voltage divider will be needed (if I want to measure the full range.) Also, due to severe corona problems at these high voltages I guess a full test will most likely require this system be placed under oil.

I realized that the wire to connectors on the second stage really needed to be tinned to reduce corona issues. Also, from experience, I know power systems get very flaky above 20/25 kV so oil will be needed for further testing.

Turns out a paper I read discussed the use of synthetic automotive oil for high voltage systems immersion. They tested a number of oils and found these similar in performance to most other HV oils (the anti-corrosion components in these oils were actually helpful in improving the overall dielectric properties.) I have tried one of these with some success in the past so I may use that type again with this system.

[Dennis P Brown]

Well, added another stage to the voltage multiplier and the performance is falling - considering the transformer, not surprising (NST, 60 Hz.) Apparently, the max voltage is well below the max reading on the HV probe so was able to measure the performance of the three stages. The system achieved 40/41 kV and manage to short along a nylon screw and into the floor. The current is a bit higher than I expected and caused a powerful, "continuous" arc (as in close to 60 Hz.) The system still provides a good bit of power so I am very careful and use a safety grounding rod on all caps before working on it. Not sure if it is worth two more stages to get over 100 kV - might do it but not sure - the current is higher than I expected. The corona on the last cap sounds like an angry bumble bee but unless I place it under oil, that is also expected (and will affect the voltage by drawing off current; hence lowering the final voltage.)

The latest pics show the full system and some close ups including the voltage probe topping out at about 41kV. Another double would maybe reach 70 kV or so and yet another would be lucky to reach/exceed 120 kV. Might try that with the smaller diodes that I have along with the door knob caps I also have. Just to see if 120 kV is possible. Of course, would need an arc length to prove that since low end performance is not linear. That is, a reading on the HV probe of say, 40 kV with the varic at 30% would not translate to 120 kV+ at full load.

A few quick comments: I used very large diodes simply because they were so cheap; also, made wring the system a bit safer in order to prevent arc's (which fry smaller diodes - this system arcs and I have no lost diodes (so far...lol.).) However, the overall size is getting out of hand so I really need to use small diodes from here on out for testing higher multiplier values. If it works, maybe rewire using better diodes and place it fully under oil. This is a positive system but with the current wiring, would be trivial to reverse the polarity - one advantage to this design, at least (no soldered joints so diodes can quickly be reversed if desired.)

Of course, could replace the NST with a fly-back and up both the current's system voltage a great deal and also increase the max power. Will consider this only if the VdG is a wash. Again, a line of work to be considered.