Page 1 of 2

X-ray transformer issues

Posted: Wed Feb 13, 2019 2:59 am
by Joshua Turbyfill
I’m wiring my XRT and I’ve run into some issues. For one, it seems my diode strings are oriented in the opposite direction of what is shown in the FAQ's. This confuses me because they were producing dc output, and then when I disconnected the diode strings and just soldered two 20kv diodes in the opposite orientation shown by the HV FAQ #4a diagram it put out a slightly higher dc voltage. I've tested the XRT with less than 1v input and measured the output with two separate multimeters. They give different readings for everything (I’ve attached a chart below). Also, the guy who sold me the XRT claimed up to 120kv output with 0-90vac input. Thats 1333 volts out for every volt in, but when i measure with the better multimeter I get about half of that. I have no experience with any of this, so I’d really appreciate some guidance on this. Thanks.

Some more info to consider:
Measured resistance across primary ac inputs is .5 ohms. Across both secondaries is 140k ohms and 70k ohms from each secondary to the ground wire. These are all off from what Mr. Hull stated in the FAQs.
A while back I accidentally put too much voltage into the XRT in air and it arced from somewhere in or on the transformer to its plastic housing, where a flame remained for a few seconds.
Each secondary connected to a 100kv (5x 20kv cheap Chinese ebay diodes) diode string. Both strings then connect to an ohmite 50k ohm 100 watt ballast resistor. The two ac inputs & secondary ground are all connected to a variac.

The chart above shows several voltage measurements taken by the two meters (mm700 & mm300). The input voltages (.6 and .9) were measured by the better multimeter because the other one gives different readings on that too. However, they both measure a 9v battery and a wall outlet correctly.
5 C.jpg
I did it like this, but the faq's and say the diodes should be reversed. Do I need to reverse the strings?
6 C-min.jpg
I disconnected the diode strings and made a bridge with two 20kv diodes in the opposite orientation (negative end / the grey line towards the xrt). At .61 volts in, it put out 187 volts dc. When I had the diodes in the other orientation, they put out 160 volts dc. (According to the better multimeter)
7 R-min.jpg

Re: X-ray transformer issues

Posted: Wed Feb 13, 2019 6:01 am
by Richard Hull
The diodes in the photo are wired correctly to get a negative high voltage DC output for fusor work.

I hope you are going to place the transformer in oil as it will be severely taxed in air and arcing at or near its rated voltage will certainly destroy it. This assumes the flames you already have observed have not crippled it already.

Richard Hull

Re: X-ray transformer issues

Posted: Wed Feb 13, 2019 7:11 pm
by Joshua Turbyfill
Thanks Mr. Hull.

I'm about to put it in mineral oil.
I've got a 10ohm 10 watt resistor for current measurement, and I bought a polaris 40kv probe for the hv, but it apparently only reads to +40kv. Is there any way of making it read -40kv? If not I'll just make a divider with some cheap chinese hv 100 meg resistors (they're red and a bit short, not like the spiralled ones - but theyre cheap) or just measure how long the arcs are.
I think I've figured out the voltage readings. Someone said an XRT can output several times its rated output when not under load. So maybe the two multimeters have different input inpedance since one is autoranging and the other is manual and that is causing lower output voltages.

Re: X-ray transformer issues

Posted: Wed Feb 13, 2019 9:41 pm
by Richard Hull
A probe like you have will be accurate to +/- 2kv if you are lucky. Based on my past knowledge of this type of HV probe, to read negative voltages, just get to the rear of the meter movement and reverse the leads. Such probes are just a resistor in series with the meter movement. reversing the meter's leads make it negative hot reading. I would never hold such a probe, but attach it in some way to the power supply.

Richard Hull

Re: X-ray transformer issues

Posted: Wed Feb 13, 2019 9:59 pm
by Dennis P Brown
I have read here some people use a vacuum pump to remove air from the transformer winding/container before leaking in the required oil. In this manner the entire winding assembly gets filled with oil and no air is trapped. I have little experience with these devices so don't know if just submersion alone and sitting a few days is good enough to prevent arc over issues. Others might be able to offer more certain advice.

I too need to do this with a 70 kV x-former I have and was thinking to pull a vacuum first, then leaking in the oil to the case. A bit of work so if not necessary, might be worth avoiding.

Re: X-ray transformer issues

Posted: Thu Feb 14, 2019 2:58 am
by Rich Feldman
Dennis P Brown wrote:
Wed Feb 13, 2019 9:59 pm
was thinking to pull a vacuum first, then leaking in the oil to the case. A bit of work so if not necessary, might be worth avoiding.
Dennis, that would be doing it the professional way. When you see a pad-mounted transformer next to a building, sometimes with an oil-filled radiator on the side, the sheet steel box is reinforced to withstand full vacuum. On the Internet you can find installation manuals for such things. They say how much vacuum to pull, for how long, to remove air and moisture from windings (and often, many parts made from paper or wood). Before you admit any insulating oil.

My first XRT came from an ebay seller named Mike, who repaired and refurbished medical and dental heads. He told me a similar story about vacuum before oil, if you expect daily operation for years between service calls.

My yet-unused gallon of Diala oil was a gift from somebody named Carl on another nerd forum. He told me they would admit oil to evacuated transformer boxes through a spray nozzle, giving any air or moisture in the oil a final opportunity to escape.

OTOH, has many accounts from people who simply dunked their XRT in oil, followed by various combinations of vacuum, tipping and shaking, or letting it sit for a long time.
Some of them led to successful neutron production. Some end with destruction of a secondary winding by internal arcing. I bet it helps that used transformers were properly impregnated with oil at the factory, and it matters how long they have been out of oil on the way to a hobbyist application.

What is the minimum absolute pressure, and bubble expansion ratio, at a place under 3 inches of oil when there's a perfect vacuum above the oil? Even without hydrostatic pressure, expansion of tiny bubbles is limited by pressure from surface tension.

Here's a snip from How It's Made video about pole transformers. Dry can, with freshly baked coil and core assembly, about to go to the filling station. The presentation says they use vacuum, but is ambiguous about the order.

Re: X-ray transformer issues

Posted: Fri Feb 15, 2019 10:29 pm
by Joshua Turbyfill
Thank you, that fixed the probe.

There's a thread here that says rolling the filled tank around on a cart over bumpy ground would get most of the bubbles out. I'm going to try that. Its been out of oil for almost two years if not more, so i'll let it sit for a while and only ever run it at 1/3 to 1/2 output.

Re: X-ray transformer issues

Posted: Sat Feb 16, 2019 2:54 pm
by Dennis P Brown
Since surface tension and air being trapped deep in the coil are the main issues, simply "bumping" the item while it is under oil is not likely to remove these types of issues. Certainly it will help for bubbles on the x-former surface.

I would suggest, in lieu of vacuum, heating the oil and transformer up to 50 C and this would do far more. Besides helping to drive off water within the coil, small bubbles will expand, the viscosity of the oil will get lower, and the copper winding's will also expand all helping to force out air and make it easier for the oil to penetrate.

Others may weigh in with better advice but this is extremely simple to do and offers a better chance of success compared to just "bumping" or soaking alone.

As for your probe, I agree with Richard - don't hold it while measuring high voltage with a fusor grade power supply. I do use mine for my NST while holding it, but never with a fusor grade x-former nor with powerful voltage multipliers.

As to your arc over; I agree with you to use as low a voltage as possible. If a previously insulated wire now has a carbon path and/or burn thru, it is very likely to become conductive at the voltages it last arced at and likely, lower values as well. Oil will certainly help but as you suggested, use as low a voltage as possible.

Re: X-ray transformer issues

Posted: Thu Feb 21, 2019 1:13 am
by Joshua Turbyfill
Thanks for the advice, I'll do that then.

Yep I'm going to mount the probe somewhere and have a camera on it.

Re: X-ray transformer issues

Posted: Sun Mar 03, 2019 6:43 pm
by Joshua Turbyfill
I’ve thrown together a crude grid out of some 20ga nichrome wire and some spare glass tube. Do you all think this will work, or will there be arcing or other issues?

A single length of 40kv wire goes from the power supply up to the feedthrough. The wire does rest on the metal frame of the cart.
On the inside 20 gauge nichrome wire is wound around the stalk and goes to the grid. Its covered by glass tube.
The feedthrough stalk is less than 3/4 of an inch from the chamber walls. Will this arc?
The grid stands on three tubes of glass and rests on top of the turbo’s intake shield.