X-rays intrigue me. Lots of radiation does. I appreciate the hazards.
So… There are these dental X-ray tubes for $190 on Ebay, just adorable little things. Great stocking stuffers. Just a 3 lead device, two wires for the filament (3.4 to 3.6 V) and a button on the other end of the glass for the anode. The high voltage can be 50 kV to 70 kV. Data sheet says it nominally handles 840 W of anode power for 1 second. Yikes! That’s for a 1 s on, 30 s off duty cycle.
I’m speculating that I could put high voltage on the tube and slowly ramp up the filament power until it starts radiating, continuously, but at much less than 840 W. Also, I could apply lower high voltage. I’m not sure what the 50 kV lower limit is about – are they saying not to try dental X-rays at lower power, or maybe the tube window won’t pass anything of lower energy, or what? Obvously you can make lower energy X-rays, like color TV tubes did inadvertently at something like 24 kV, or like Grenz rays for dermatology at something like 10 kV (though I bet this tube’s window wouldn’t pass them, I think you need beryllium foil or something for the window).
Well, I’m sure you’re aware of the lethal voltages involved, and ionizing radiation. I would normally advise people not to mess with that stuff. But I know you know a thing or two about electronics, which makes me feel better. Just… be super careful.
I’ve never personally dealt with X-ray tubes, but I think that at least part of the way that they work is the higher the voltage, the more efficiently they emit the X-rays. If the voltage is too low, it won’t be very efficient and you’ll burn out the tube from the waste heat. I’m not sure your idea of slowly ramping up the power is a good one.
But like I said, this is outside of my expertise. Just be careful.
Low filament voltage will cause the cathode to emit fewer electrons and there will be less anode current and less X-rays. At least, I think so. Perhaps I’m just trying to ask that, and perhaps find out what the relationship is like (does it fall suddenly below some threshold, or is this a good turndown method). That was my thought about throttling down the output with filament voltage.
Low anode voltage will lower the energy of impacting electrons, lower the photon energy of the continuum part of the spectrum, and (I think) lower the amplitude of the shorter wavelength emission lines.
But I don’t think the efficiency and burning up would be an issue. When they’re working right, these tubes are only something like 1% efficient anyway.
I looked into this many years ago when I found several of those shoe-fitting x-ray machines in an antique shop. In Ohio you cannot posses an x-ray device, even a non operating one, without a license.
The basic operating parameters of an X-ray system are pretty much as surmised.
Filament current is used to set the forward current through the tube, and thus the amount of X-rays produced. It is not very linear. The anode voltage sets the energy of the X-rays. Or at least the maximum energy of the Bremsstrahlung spectrum. For a general diagnostic system both are variable. Energy is set for best contrast for what is being imaged. For a dental machine there probably isn’t a great deal of variation needed.
Very low energy X-rays won’t make it out of the tube. So there is a minimum anode voltage needed before much happens. I remember once reading about use of a special tube with a lithium alloy window good for 15kv. It could imagine ridiculously thin materials - like fingerprints n plastic film.
Getting the heat out of the tube is always a problem.
That wasn’t the Amateur Scientist article I was thinking of. As an inquisitive child I was gifted a stack of old Scientific American magazines. The project I was most taken with was the linear particle accelerator. The vacuum pump was well beyond my resources, sadly (or, more probably, fortunately). They made the point that if you cranked the voltage up and used a suitable target, then x-ray generation was a distinct (and unsafe) result.
Yes, this is the kind of thing I’m looking to learn or hear confirmed. I wonder what that “not very linear” looks like.
I have book instructions for DIY X-rays somewhere. I’m not sure if it’s one or the other of the cited ones, or something else. But as observed, getting the vacuum is a bit of an issue.
It’s hard to compete with a professional grade, documented, new dental X-ray tube for $190!
The data sheet does confuse me in one regard. It says this tube is “center grounded”. But it’s a glass tube with only 3 connections, namely, the two ends of the filament (together constituting the cathode), and the anode. What is it that is nominally connected to ground?
My first interpretation is that the high voltage power supply is expected to be equal positive and negative voltages with respect to ground. Like the +15 and -15 rails of a power supply for linear ICs. This makes me think, what do they care? How would this impact the operation of an X-ray tube?
My second interpretation is that they mean halfway between the two filament leads, that is, the mean potential of the entire cathode is zero. I’d drive the filament from a transformer secondary having a center tap, and ground the center tap. That would make more sense, because, otherwise, delivering multiple amperes to a filament floating thousands of volts away from the chassis is going to require a handful of transformer with a huge insulation layer around its secondary.
Still not obvious, though. Crafter_Man, how would you interpret “center grounded” here???
I know next to nothing about X-ray tubes. But I did find three different configurations for connecting to a three-wire X-ray tube at the bottom of this page. (And the schematic on this page is the same as the “Cathode Grounding Connection” on the other page.) Pros and cons to each. Hopefully the documentation for the tube you want to acquire will show you the proper (or preferred) configuration.
Excellent interesting citations sir! I saved the first one with great interest.
My documentation doesn’t show a schematic. The first interpretation I spoke of would be what your first cite calls a “bipolar connection”. Your second cite’s schematic doesn’t show any chassis ground at all, does it?
Well, theoretically, you can tie any one point in that schematic to chassis ground and be O.K. But I suspect most designers would connect the negative side of the high voltage power supply (the side on the left that connects to the filament/cathode) to ground. Having said that, I can’t emphasize strongly enough how little I know about X-ray tubes, so take anything I say with a grain of salt.
