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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ruskie wrote ...
I tried sending them an email requesting data on the tube. I wasn't able to use their on-line form because I suspect this tube is obselete and no longer listed as a regular product.
If all else fails and becomes necessary, there is enough room in the shielded enclosure to allow me to place the tube within a container that would be oil filled. I've used small PVC junction boxes in the past for similar purposes and with the rubber seals they have for the lids, oil stays put. Only drawback I think is that I might have to increase the voltage and/or current to offset the small shielding effect of the PVC housing and oil.
The Toshiba-088S is still being installed in new production equipment - for example, 'Panoramic X-ray Unit For Oral Examination' variously branded by Sunshine Medical Equipment Co, and Shinova, from whose catalogues we learn that the tube has a focus of 0.8mm and maximum ratings of 90 kV/10 mA for 16 seconds when run under oil on a half-wave supply - i.e. in self-rectifying mode, when the tube will spend 50% of its 'on' time cooling down. The DC ratings will be considerably less.
I would send an email to Toshiba China X-ray Division to get your datasheet. Good luck!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ruskie wrote ...
Anyway, some of what you've said in this post has caused me to rethink several things. For one, I either read, or was under the impression, that running these tubes from DC was more efficient for the tubes and increased tube longevity because efficiency was better as the tube ran cooler. Please correct me if I'm wrong. I still have the original transformer for the tube. It's fairly robust though, being a 60 Hz device, and is something else I'd almost certainly have to place under oil. There was no diode present within the dismantled x-ray head originally, so I suspect this tube ran full wave AC(?).
A thermionic X-ray tube - one with a heated filament - is only a special case of a diode valve, so when AC is applied to it, it will function as a half-wave rectifier. It can only conduct (and emit X-rays) on alternate half cycles, and no current can flow through the tube for half the time. The other half of the time, when the tube is not conducting, the anode spot can dissipate heat and cool down. This is the point on the anode where the electrons impact. Nearly all their energy is converted into heat, so the tiny anode spot is subject to high local temperature, and if heat is not conducted away from it as fast as it builds up, the anode spot zone will quickly become pitted and cracked, producing poor quality images. Over time, vapourised tungsten will condense on the walls of the tube, rendering it conductive, so that flash overs occur. In practice, dental tubes are designed only to operate for short periods - a second or two - that is, the anode heat capacity of dental tubes is low, and periods of cooling down must be allowed between each conduction period. When using AC, the anode spot can cool on every other half cycle. Clearly, in DC operation this does not happen, but exposures must be correspondingly longer for the same quantity of X-ray energy to be delivered.
AC X-ray power supplies are still used and made, but the lower cost, weight, and compactness of SMPS X-ray tube supplies has relegated the old AC transformer power supply to a minor role, often in the developed world.
A major disadvantage of AC X-ray tube operation for many scientific purposes is that the X-ray output is both amplitude and frequency modulated at the line frequency - 50 or 60 Hz as the case may be.
My main interest is in ultra soft X-rays - so called Grenz rays - with energies below 5 keV, where most X-ray research is now taking place.
Anyway, Bill, I feel I've helped you as much as I can - your plans sound dangerous and haphazard by my standards - but I am sure the usual suspects will be in here like a shot to advise you on things outside their own experience!
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
All dental x-ray tubes that I have seen can take 100+ watts continously, provided that you have some way to get rid of the heat from the anode stem. 30W wouldn't require a very large heatsink, especially not if you have a fan blowing at it.
When mounting the tube, you don't want any metal in contact with the glass as this can distort the electrical fields in the glass and lead to breakdown. Most dental units either have the anode stem screwed into a rigid support or an insulating clamp that goes over the tube.
For a given peak voltage and x-ray output, a DC supply will lead to the lowest heat dissipation. Since the x-ray output of the tube goes up drastically with the voltage, the tube will only be operating optimally at the peak of the waveform with AC, so you need a higher anode current for the same x-ray output.
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