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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, I am planning on building a 60kV DC multiplier for a future xray tube I hope to win on ebay. I need some capacitors and thought someone here may have some lying around they may want to sell. My plans are to run a flyback at about 10kV AC and then use a couple stages to get to 60kV.
Im not sure how many I need, but Im thinking enough for three stages. Im thinking 30kV capacitors? 2*input voltage times # stages equals output voltage? Is this correct?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hiya Matt,
rounding things up to real world values, you are right to say you will need caps of 30kV rating. Working on a basis of 20kHz, you will need six caps of 50nF, and six diodes, for an EHT output of 60kV at 1mA or so?.
With a half-wave multiplier, it's best to triple C in the first stage, and double C in the second to keep the ripple down.
May I suggest that you find out what capacitors are available, that are suitable, and which you can afford to buy, and then decide upon Vin which will be the least of your problems.
Doorknobs are good, but are not necessary if you submerge the multiplier in a suitable dielectric oil (e.g. sunflower seed oil). Doorknobs are built the way they are to make the longest possible breakdown path through surface tracking and through the air.
Is 60kV a figure that you have just plucked out of the air? Let's suppose your anode is of Mo (as very likely it is) You'll see from the graph there's not a lot of point in driving the tube with more than 20kV, just for a bit of extra Bremsstrahlung (braking radiation) You'll see from the graph there is nothing whatever to be gained from increasing kVp from 40 to 60.
If your anode is made of nickel, another likely contender, you'll see that again there is nothing to be gained by increasing the anode voltage from 40 to 60kVp.
Lastly, if you confine yourself to not more than 30kVp your shielding costs will be dramatically smaller.
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
a quick note: most CRT TVs use not only a flyback, but a multiplier to increase the voltage, and most TV repair shops have one or two lying around (that is actually how I get my flybacks etc) - you might look into that as a possible alternative, though you would not have as much fun as building your own
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, I did sort of pull 60kV out of my head. The tube I want is rated for 90kV. I assumed higher voltages would bring brighter images? The mini focus tubes are nice, but it seems that just as you get a nice bright image you also exceed the voltage and it flashes over. I figured that getting a real x ray tube and running it at much higher voltage would produce a much brighter image?
I do have one of those TV triplers. So is it safe to assume that 30kV would be more than adequate for a normal x ray tube? If so, why are some tubes run at such high voltages?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
EEYORE wrote ...
Hello, I did sort of pull 60kV out of my head. The tube I want is rated for 90kV. I assumed higher voltages would bring brighter images? The mini focus tubes are nice, but it seems that just as you get a nice bright image you also exceed the voltage and it flashes over. I figured that getting a real x ray tube and running it at much higher voltage would produce a much brighter image?
I do have one of those TV triplers. So is it safe to assume that 30kV would be more than adequate for a normal x ray tube? If so, why are some tubes run at such high voltages?
Thanks! Matt
With X-ray tubes you have to pin down what you want the X-rays for, and then choose a tube that meets those needs. And then only can you decide what sort of power supply is needed for the job in hand.
In human X-radiography the shortest possible exposure will (generally) make for the sharpest images, just as a brief exposure of an ordinary camera pointed at a moving target will provide a sharper picture than a long exposure. Moreover, high energy X-rays will tend to pass through the body with little interaction, while low energy rays - so-called 'soft' X-rays - are likely to be absorbed in whole or in part. (This is why soft X-rays are the most hazardous)
At a distance of 100cm at 30kVp and 1mA tube current, you will receive a fatal dose of 6.6 Sv in one hour. If we increase anode volts to 90kVp, at the same tube current and distance, the dose rate goes up to 17.6 Sv/hr - rather you than me! At 30kVp/1ma 1mm of lead shielding will reduce the dose rate at 100cm to 5.78E-007 uSv/hr, but at 90kVp/1mA, 1mm of lead will only reduce the dose rate to 1.2 Sv/hr. Given that 3 Sv is the LD50/30 dose (the dose at which 50% of luckless laboratory mice will die within 30 days of exposure) you'd have mucho big bills for shielding.
There's no answer to your question about a 'normal' X-ray tube. There are tiny microfocus tubes running on 10kVp or less with only a few uV through them, and there are huge water- cooled tubes with rotating anodes designed to look through steel castings and great blocks of concrete. There are tubes with exotic anodes like germanium, and transmission target tubes with a beryllium window at one end, and there are tubes designed to run with a grounded anode, and so on.
If your tube keeps flashing over, I should put it in an oil bath - ordinary kitchen sunflower seed oil will do fine, and will absorb very little of your beam. The oil bath will also reduce the chance of glass electrolysis, a common cause of tube failure.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, Thanks for the tips! I do not plan to have any more human exposure. I also dont have a particular target in mind, just general x-ray "fun". I have alot of lead sheeting that I have gotten from numerous uranium rocks that I will put to good use also. So from what I understand, higher voltage will penetrate deeper, and higher current will cause more x-rays? (Greater dose).
Since I dont want to x-ray steel beams, Im guessing that keeping current low will be a good idea. Say, 50-100uA? As to the mini focus tube arcing, it is internal arcing that is the problem. I suspect it may have sustained some damage as well. This is just a guess. Thus the desire for a real x-ray tube.
I will be basically limited to what kinda deals I can get on ebay as far as tubes go. I dont want to spend much either. I really have no idea what is a good deal or not, but ive seen some small tubes go for 50-$60.00. These are used of course, with no real guarantee they will survive more than an hour. The one I plan to snipe is from a dental x-ray head rated 90kV and 5mA max. I would keep the current very low though.
Im guessing it may be more complicated than this, right? Im guessing that you can just get away with under powering a tube? BTW-What happend to that hand? Yikes! Matt
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
We can divide X-ray tubes into two broad classes. Those that produce electrons from a heated filament and those that have a 'cold' cathode which produce electrons by field emission, such as 'flash' X-ray sources.
Of the heated type, there are those where Ik (cathode current) is controlled by varying If (filament current) , and those which have a constant filament current, but have a grid which can control the current as in a basic triode valve. Some tubes will have both a grid and a focusing 'cup' which improves resolution by attenuating the 'wings' radiating from the focal spot.
There are also two further classes in which we can place tubes - those where the anode is at EHT +ve potential, and the cathode is earthed, and those tubes where the anode is held at ground potential, and the filament/cathode at EHT -ve.
While the grounded anode type is inherently safer from electric shock hazards etc, and make for easy heat sinking, it puts you in the position of having to provide 'floating' supplies for filament and grid bias/focusing cup that can stand off the whole EHT voltage.
As for internal flashover, some tubes can recover from it, and others can not.
The real bad stuff here isn't the hand --- it's the possble genetic damage inside his body that could get transferred along someday to an offspring. Kids with bad deformities. Or cancers.
If this is really your hand and not a joke, I would drop this experimentation like a hot potato Way too dangerous for amateurs.
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