High voltage Fusor grade x-former score (Temporary)

[Richard Hull]

I have always harped that the physical operation of a fusor has nothing to do with science, but, instead a form of art. Do not struggle to understand until you have operated one successfully for a while. The trainer wheel system on this bicycle is made of soft, unpredictable rubber. Over and over, I have written in FAQs, operation is an art. Science makes it do what it does always! In an effort to get the plasma going and stable, the science is always changing modalities as you go. It is incumbent on the operator to get a "feel" for how a system responds. Without full instrumentation it is a rough ride.

We are operating on a razor's edge in a fusor to do fusion. We are just working on the knife's edge between a dead short across a high voltage supply and a useless piece of pretty glow discharge. We seek to not blow out a high voltage power supply while raising the voltage in a near arc gas environment inorder to reach a point in the gas where fusion can occur. A daunting, confusing and seemingly impossible task for the first timer.

Full operational details are to be found in my several FAQs on the issue.

Richard Hull

[Dennis P Brown]

LOL; sorry Richard for asking (yes, I of course have read your FAQ's), but I just couldn't resist asking once I started playing with high voltage (HV) systems and realized just how flaky they can be. Learned some time ago that above 20 kV things, even at atmosphere and very low power, they kinda get strange compared to how things behave below that point.

More to the point, my new fusor power supply (PS) system, at times, just didn't make sense relative to what I was reading on outputs so I broke down the diode bridge in the PS - thinking one of the diode pairs was failing - and tested them all. They all passed; nice having the Van de Graaff (VdG) spray high voltage power supply - that is perfect for testing high voltage diodes (of course, I removed the caps from the unit!) Using this VdG 15 kV positive supply and using my high voltage probe/display (analog) readily proves whether a given HV diode is really working.

So I put the fusor system back together (made a few improvements like grounding the shield on the HV cable. Confirmed all system grounds.) I also hardwired in the milliamp meter in the fusor PS case and tested it with a 1 M-ohm load on the hot side of the PS. Ran the PS up to 28.5 kV and obtained the expected 10 milliamps output (the resistor didn't like the power) but just needed to check the performance of the bridge. I had removed the HV output cable from the system since it worries me on whether it can hold the potential safely.

I suspect that the output of my fusor supply isn't really maxed out at 28 kV (what the meter reads) but rather gets a good bit higher due to RMS (I keep overlooking this simple fact. For instance, my analog meter reads 10 kV on the VDG HV PS but with caps, reads the full 15 kV expected due to RMS.)

That would explain the HV power cable's impressive failure. Would not be too difficult to check the real voltage output that my fusor develops - I have a massive 50 kV cap (0.3 mf); I would install my HV meter on this cap and charge the cap to see the real max voltage of the system. Of course, this is an extremely deadly cap so I will have a good bleed resistor set across it (500 M-ohm and I also have a safe HV shorting probe/cable to confirm if a cap is discharged I really need to confirm the real high end so I can plan accordingly for safely feeding this HV into my fusor (still waiting for the new window for the fusor!) Also, yes, I know that once the plasma is conducting, the actual output voltage will be a good bit lower and more like the 28 kV.

An aside: I discovered the hard way that the 2 ft of small HV cable in my power supply (no caps at all, of course) gets a charge that hangs around for a while; surprising but harmless; still, I short that now before working on the diodes or output wire. The diodes really do work well holding that sub micro-amp of 28 kV for a while on the wire between the diodes and output resistor.

[Dennis P Brown]

Well, the unit made 33 kV (so yes, the voltage is higher than I was expecting and that makes cables very important) but I am learning that HV cables have quirky issues (really my lack of understanding them.) Shorts were occurring all the time (only lost a fuse once, and after that I used a de-rated fuse (3 amps) from that point on to be extra careful.) One needs a at least two inches clearance between the hot end and its own inner wrapping material - voltages above 26 kV really can travel along insulators to find a ground! And shrink wrap electrical tubing is not compatible at all with these fusor level HV cables (that insulating material really conducts above 25 kV); and found that fact out in a spectacular manner!

These HV cables are touchy and very unforgiving when trying to use them in a non-traditional manner. The FAQ really does not cover cutting/splicing such cables much less interfacing them to equipment. Having one end in an oil bath was one of the better things I did (in the fusor PS case) as well as placing the bridge under oil; but can't say I like these beasts, very much. Still, understand them a lot better now and realize that the inner components used in the cable aren't necessarily capable of handling the top end of the HV. But I really dread going about 25 kV now because it appears that everything acts like it is ready to short somewhere where I will least expect it and any such 'short' isn't ever a minor spark ...lol.

[Richard Hull]

Just did a new FAQ on this subject.

viewtopic.php?f=29&t=10950&p=72514#p72514

Richard Hull

[Dennis P Brown]

Thank you Richard for that FAQ; yes, rather an important issue since getting high voltage to a device isn't knowledge that is 'out there' nor commonly experienced! Good FAQ, too. Especially when ebay HV Spellmans, etc are available generally without cables. As I have been discovering, even with a fair amount of exposure with HV (mostly up to 15 kV,) that experience just isn't enough to prepare one for the really strange electrical world of above 20 kV and especially with the high currents, which really test cables (and lets not even think about now the world that changes even more above 75 kV!)

Finding out that synthetic motor oil is as good and may be slightly superior to many other oils for HV work was nice and made this easier building my power supply system. I will say, if I were ever to "splice" a HV cable, I would strongly consider oil. One area I was really caught off guard is getting the real 'max' voltage since many HV meters don't measure the peck but closer to a high average. Nor that these voltages like to run on the surface of insulators (nothing like humid air to provide a nice conductive surface) and compromising a connection. Humidity of summer is a bad time for HV work; even with AC.

[Dennis P Brown]

Well, I decided that instead of using a divider on the high voltage secondary (I started to do this but really had issues - and how a voltmeter has issues in the DC high end ...lol) I just tapped into the x-former's primary instead. Just needed a diode (the meter only reads DC, positive voltage) and a 3.1 Meg resistor to go with the 10 turn pot and my digital display tracks the HV perfectly. That is, I do have a HV probe but being only able to read positive values, it is a pain to setup safely and totally unsafe to leave as a permanent meter. However, did demonstrate the required voltage ranges to tune the display. Very linear over the entire scale - from 5 kV to 32 kV.

While I realize that there may be issues tapping the primary rather than the secondary when the fusor plasma arcs or fails or any number of things, this voltmeter's only purpose for me is to read a voltage for steady state situations. Once the variac is set, I'm not too concerned with display variation compared to any 'real' secondary voltage until the fursor reaches the steady state - especially since, using the higher end range voltages, current is the only real issue. And another thanks to Mr. Reite for the connecting the x-former's positive HV output to the milli-amp meter. Works great (especially since that is a required ground point for the diode bridge.)

Decided to wait to install the master "on/off" switch until I can borrow a panel punch. Drilling a square hole isn't too easy! Getting impatient for the new glass chamber window. Can't do anything until that arrives and is drilled for an electrode. (Currently, there is an on/off switch to enable the HV x-former to come on.)

Did get a small piece of uranium to better test the Geiger/scintillation counters as well as to cross reference them against each other and test various voltage settings for the units.

[Dennis P Brown]

Finish all the safety work relative to the fusor power supply (that I can readily think of... for instance, have really gone over and over grounding issues - for the x-former, bridge, cables, fusor, enclosure, fusor HV, and even the AC line current.): so added a master power switch w/light, better cable protection features, warning stickers and a plastic shield cover over the top of the HV section when the cover is open (the rear door has both a warning sticker and key lock. Still have to finish the business (fusor) end of the high voltage cable. Have a ceramic section for that and will install that so the "hot end" isn't easy to contact.

Obviously, I am far more concerned with the high voltage hazard than any other; I have multiple x-ray detectors (some calibrated) but with my steel chamber, it is the electric that is hardest safety issue to address with any confidence.

[Dennis P Brown]

Installed and tested the ceramic end adapter for the high voltage cable (see below.) The resistive tester rig (unlike my first iteration) can be adjusted to handling longer resistors. The new end cable adapter system held the full voltage nicely. This system works well for the fusor but being 2 .3 M-ohm, does keep the current draw rather low - see last photo.

Getting impatient (like building a special rig for testing the cable - yes, useful for general fusor tests to have a resistive load device - speaking of which, there was an entire thread on this subject but really, I'm getting too far afield, here) to test the fusor - hopefully, the person ships the new window early this week and I get it in time to drill it, add the electrode and test the power supply in the few micron range by this weekend (with deuterium gas for fun. No way yet to measure neutrons.)