Before small cockroft walton multipliers became inexpensive, x-ray machines tended to use large iron transformers. These are the highest voltage transformers you’ll likely ever come across, with even a small one capable of turning 120V (or 240V in euroland) into 50,000V, 75,000V, 100,000V or even 320,000V. Needless to say, these things are awesome.
Unlike other transformers such as neon sign and microwave oven transformers, an XRT is not simply “plug and play”. This is because you can’t just buy an x-ray transformer all by itself. Instead you must buy part of an x-ray machine and pull it out of there yourself. X-ray transformers are located inside the “heads” of the x-ray machines; the part that the dentist puts next to your face when you get your picture taken. An x-ray head typically sells for about $100 used and they contain both the x-ray transformer and a nice x-ray tube. You could then sell the tube for about $60, or keep it for later experiments.
Tube heads are typically held together with either lots of screws, solder or epoxy so it takes quite a bit of work to get one open –unless you use a dremel like me. Take these heads apart outside because they’re filled with insulating oil. More recent tubeheads are filled with mineral oil, but some of the pre-1970s ones are filled with PCB oil. Contrary to popular belief, polychlorinated biphenyls are not deadly. They may be contaminated with dioxins though, especially if an electric arc has occurred in them. Dioxins are toxic, so use care.
Regardless of what type of oil is inside, save it for later use because it’s a wonderful insulating oil, much better than the stuff you can get at CVS.
Once you pull out the transformer you’ll have to identify its windings, but before you do that you need to know what to look for.
An XRT has 6 connections. The thickest of these connections will be the primary coil and these two wires get connected to the mains. Since an XRT is a center tapped transformer there will be two HV outputs and these are usually just balls of solder on the windings.
The ground of one HV secondary will be connected to the core, while the ground of the other secondary will be connected to a “mA” terminal on the x-ray head. To measure secondary current you may put a mA meter in between this wire and the core. If no measurement is needed you may solder the wire to the core.
X-ray transformers are not designed to be run in open air, and if you attempt to do just that expect them to die immediately from unwanted arcing. To prevent the destruction of your precious transformer you’ll need to put it under oil. Any oil will do, including mineral, vegetable, peanut or canola oil. If you decide to use an organic oil be warned that you’ll have to replace it every five months or it will go rancid and stink up your transformer. Because the oil I harvested from the x-ray head wasn’t enough to fill the container I made I decided to use canola oil for this transformer. Making an oil tight container is rather tricky if you have no clue what you are doing, so see my guide on doing that here if you want to make one.
The purpose of the oil is to provide better insulation for the transformer to prevent unwanted arcs. If the transformer has been out of oil for more than 20 minutes chances are air bubbles have formed between the windings; air bubbles that will cause insulation issues. Ideally one would pull a low vacuum on the transformer to get all the air out of the windings, but in the case that you don’t have a vacuum pump or refrigerator compressor there is an alternative, yet messier option. That option is putting your hands in the oil and massaging all the air out of the windings until you can squeeze out no more bubbles. It’s not as good as pulling a vacuum but 9 times out of 10 it will work.
X-ray transformers are not current limited; if you try to plug an XRT into the mains it’d be a near short circuit and draw currents in excess of 60A. Needless to say this is bad. Because of the high powers involved with an XRT a resistive ballast won’t be a good idea, mainly because the resistor would get insanely hot. An inductor is a much better option for ballasting XRTs, and the perfect inductor for the task is a shorted MOT. Simply put the MOT’s primary in series with the XRT’s primary and then short out the MOTs secondary. Viola, you now have a reliable ballast that doesn’t get hot and costs next to nothing.
XRTs pose can some rather unique hazards for those unfamiliar with them. Although a transformer might be rated at 70kV, this is actually it’s voltage under a 5-7mA load. When unloaded that XRT is capable of producing up to 200kV, and the electricity will gladly jump 8 inches through the air to bite you. This means a long, fiberglass chicken stick is *absolutely required* when you draw arcs from an XRT. A good rule of thumb is for every 100kV, make the stick 16 inches long.
It is advisable to put an over-voltage spark gap on the output of the x-ray transformer which is tuned to slightly above the nominal voltage of the transformer. This can protect the transformer from destroying itself if run unloaded at full voltage. Some, but not all XRTs are capable of withstanding no-load conditions without such a spark gap, so it is best to be cautious.
Now that we have safety out of the way, here is a video of an XRT arc. It goes BZZzzzz. ∎