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Registered Member #9711
Joined: Sat Jan 19 2013, 03:27PM
Location:
Posts: 65
I recently acquired a 50-60 KV x-ray head with the purpose of dismantling it and using the transformer to drive a mammography rotating anode tube. The purpose will be radiography of low density subjects such as insects and flowers. Anyway, I disassembled the head today and, after quite a bit of testing and checking, determined the correct terminals and voltages needed for them. I have a few questions:
1) The filament was non-adjustable before I opened it and already tied to ground. I separated it upon disassembly so that a resistor can now be placed in line for adjustment of current. However, there is a problem: since I am going to be driving the head transformer with a variac for no more than 20-30 KV output, I am concerned that even at minimal resistance, there won't be enough current getting into the filament. Is there any way I can drive the filament part of the transformer separately, say with a separate variac? How would I hook this up if so?
2) I initally forgot about this, but the head's HV output is in +/- form rather than simply positive or negative. Since the rotating anode tube is going to be separate from the transformer, I am getting a lot of arcing on the filament side of the tube-- the four filament terminals are just too close together. If I had run this DC, the filament would have been at ground, but not the case now. How can I stop the arcing at the filament side? I was thinking of lots of silicone around each terminal, but then I thought that I might be able to somehow place the entire tube in mineral oil with a hole for the output window, caulked around it so that when filled with oil it wouldn't leak. Any ideas?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Typical mammography tubes are designed to be run with the cathode grounded. Running it like this will solve both your problems, as it allows you to drive the cathode from a grounded adjustable regulator or supply, and it will eliminate any arcing at the cathode side.
Typical dental x-ray transformers have a center tapped secondary. You can run the tube from half of the secondary, but a much better solution is to use two diodes to make a full wave rectifier as shown here . For the diodes, you could use something like 5 of these in series , which would leave some safety margin.
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filament and negative HV x-ray transformer troubles
