Peculiar High Voltage Transformer

[Nathan Marshall]

I acquired a high voltage transformer on eBay a number of years ago, and I was going to see if it was usable for a fusor power supply. It was labeled as an x-ray transformer. It has two isolated secondary and primary coils. Each secondary has 28k ohm resistance. Neither coil is center tapped to the transformer core. The primary coils are isolated, but it came with them wired in series. Even still, the resistance is nearly a short. The thing is a beast weighing in at over 50 pounds. However, the primary wires are a tiny gauge. The multiplication ratio is extremely high. Running on about 8V AC input from another transformer, the arc jumped about 2 cm and the amperage was probably in the 5 ma range from my visual guess. Not that powerful of an arc, less than my 12kV 10ma oil burner ignition transformer. I wouldn't dare run it any higher than this out of oil! I was nervous even doing it this high. After sustaining this sort of an arc for about 20 seconds, the primary wires were already getting hot! Seems like a beefy transformer such as this should have heavier gauge input wires and be able to sustain a small arc for a while. Any guesses on what this transformer was actually intended to be used for? Seems a lot different than any x-ray transformer I've ever seen especially with no center tap attached to the core. Pictures of the transformer are attached below.

[Rich Feldman]

Welcome!

Standard advice 1:
don't abuse HV transformers (other than NST) by drawing arcs in air, if you intend to use them for more than arc play.
Especially if you are not proficient with ballasting.

Consider big x-ray transformers, like those used by chiropractors instead of dentists. Operating power might be > 10 kW instead of < 1 KW, but they are still designed to run for only a second or two out of every minute or two.

If it were my transformer, next step would be to measure primary resistance (within 10%), and determine how much primary current I would be comfortable with (for 1 second or 1 minute).
Then set up to measure primary current and secondary voltage, while varying primary voltage up to, say, 5 volts.
Chart I vs V up to your voltage or current comfort point, whichever comes first, while secondary windings are open / shorted / loaded with arc / loaded with resistor.

[Jerry Biehler]

I think you got that from me which I got from someone else here. its an x-ray transformer and you need to put it in a tank. It must be in oil, thats how it keeps cool and stops it from shorting out.

if I remember right its 100kv, 80ma. This is probably very short duty cycle at this voltage as x-ray exposure times are usually pretty short. So you will have to derate significantly for continuous use.

Primaries will not be center tapped, this is not a NST or a oil burner transformer.

[Maciek Szymanski]

With such low primary resistance it may be intended for pulse operation. I’ve seen some portable medical and industrial X-ray units operating like this. The capacitor bank is charged by the first stage small transformer. The second transformer is connected through a non-triggered spark gap. The cycle is repeated with 60 Hz mains cycle. The short pulses of high voltage drive the cold cathode x-ray tube for example ИМА: