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FAQ - X-ray transformers

Posted: Wed Dec 04, 2013 8:14 pm
by Richard Hull
The x-ray transformer of yesteryear is the ideal fusor power source. By yesteryear I mean really old like 30 years or more old. They weigh from 50lbs to 200lbs They are found at the following locations.

E-bay - Rare
Hamfests - extremely rare
Dentists offices - closeted old spare.
Big cities - at X-ray repair facilities.

Remember they must be rather heavy iron core units and all are in oil. If you have to pay for one, make sure it works otherwise you will be the proud owner of a super massive door stop.

Such transformers, if very old, will have tube rectifiers which will most likely be blown (filaments bad) There are no replacements for these old kenetrons. Most x-ray supplies are set up as negative hot, which is ideal for fusor work. You will be forced to replace the tubes with modern high voltage rectifiers. (Another special quest in and of itself.)

Most all older x-ray tranformers have a center tapped secondary that comes out to a screw lug that must be tied to ground through a current meter in order to work. You will have to find this key lug amidst what is often a field of screw lugs on top of the transformer case. Most all of this work means untanking an oily mess, letting it drain a bit and replacing the rectifiers as well as metering out the primary and secondaries, finding which lead goes to which external screw post. Sound tough? It can be a daunting task for someone with zero electrical knowledge. Thus, the limiting factor here is often age and experience or lack thereof.

Rarely, one half of the secondary is open or bad. You can still get full voltage though by using a single rectifier off the good half of the secondary. The resulting rectification will be half wave (60 hz) and not the full wave 120hz. Still, very servicable. The primary of most all x-ray transformers demands 220 volt AC of the type normally only found in air conditioner outlets. Normal 120 volt iron core units are very rare and very old. It is even more rare to find a single coil secondary, but one of its ends will still be grounded.

Individual cases may vary, of course and adaptive and creative thinking by an adroit electrical minded person can often prevail where the average neophyte would fail.

One great advantage of the x-ray transformer is the output is easily taken from 0 to full high voltage using a simple variac, auto-transformer, on the primary input circuit.

This has been a broad overview with little key specifics, but should act as more of a warning than a how too. You are working with high voltages that can kill here and a lot of internal rewiring may be required with a special outlet or even a special electrical cable run from you breaker box. make sure you are up to the task before plunking down a lot of dough.

Richard Hull

Re: FAQ - X-ray transformers

Posted: Wed Oct 15, 2014 12:21 am
by Charles Vorbach
How would one go about testing an x-ray transformer to make sure it works?

Re: FAQ - X-ray transformers

Posted: Wed Oct 15, 2014 5:49 am
by Richard Hull
Untank the device, let the oil drain back into the still full tank. Ohmeter the HV secondary usually three leads in older units. Two outside winding leads and a center tapped lead. You should read a good, solid high ohms reading of from 15k-60k ohm across the entire secondary of the classic 60hz system and half that from the center tap to each of the outside leads.

The primary resistance should be low 5-50 ohms at most.

Ricahrd Hull

Re: FAQ - X-ray transformers

Posted: Fri Aug 19, 2016 7:17 pm
by Duncan Wilkie
X-ray transformers are more common than suggested. Transformers cannot be easily found by themselves, but you can easily find one on eBay in the form of an X-ray head. Plus, you get a nifty X-ray tube too.

Re: FAQ - X-ray transformers

Posted: Sat Aug 20, 2016 8:06 am
by Richard Hull
Dental x-ray transformers of the more modern type are, generally, not satisfactory, unless you are really good at electronics. Such transformers are of two types.

1. High frequency switching type - Usually, these have a ferrite core either potted or in oil and demand a special electronic supply of high frequency AC.

2. Standard laminated iron core of small frame to fit in the oil filled head.

As if the special high frequency types aren't already special enough, these smaller x-ray "heads" are...... oddly..... designed for modern dental x-ray work.

Ever been to the dentist? Ever had a dental x-ray? They pull the head over to you after putting the film in your mouth and then they go away and you hear a 0.5 second to 1 second high frequency pitch. That was the head making your x-ray. a 1 second total on time. Now did the dentist ever yank you from your chair and have another person sit for the same x-ray 1 minute after your x-ray and then a third person and then a fourth......A line outside the door to get x-rays in the chair you were in???? Of course not! No. They yanked the films and leave you all alone and a few minutes later they tell you what the issue, if any, is with your teeth and then take another half hour working on you. This long paragraph should tell you something.

These little heads and little transformers are normally expected to be used for 1 single, 1 second x-ray per hour or less!! X-ray currents might be 20 ma or less.

A fusor....A real fusor.... will demand one hour or more continuous on-times at 20ma or more. (Not 1 second per hour!) The little modern Dental units and their transformers were never designed nor manufactured to supply continuous power. In other words, it is something they just cannot do!

So, the modern heads with that free x-ray tube riding along inside is just not up to the task. Most folks coming here and reading this FAQ can't reconcile the need for a 50-150 pound tanked transformer of old, when there are a real profusion of these cute little dental heads on e-bay. Why?... It is due to a lack of electrical/electronic knowledge and background.

In the old days x-ray units were made for delivering voltage and current to a tube and this meant a heavy mass of iron in a 18 inch cube solid steel tank with a giant umbilicus cable to the head which contained an x-ray tube only! Modern circuitry that could deliver the same juice and current hadn't arrived on the scene. For the first time, smaller transformers could be made to do the job of that old monster due to that 1 second on and 1 hour off need of the dentist. However only that old monster will serve the continuous need of a fusion reactor...A fusor.

In short, once high energies at high voltages were available via switching supplies, It was recognized that 400 x-ray watts for 1 sec every hour could easily be handled by this modern methods with no real iron mass demanded in the old linear x-ray supplies.

Richard Hull