A question in my notes says "Explain briefly why a modern X-ray tuve can be operated directly from the output of a step-up transformer."

I always presumed this was allowed as electrons only get emitted from the filament by means of thermionic emission and not the other way around and left it at that without thinking about it anymore but HOWCOME electrons cant travel back in the opposite direction thermionically as well when the AC waveform is in the opposite polarity? Could it be because the Target, say tungsten for the sake of argument doesnt thermionically emit as much electrons? Surely thermionic emission could still occur in the target as it gets quite hot.

 

Perhaps you should get the type number and look at the spec sheet on a "modern" X ray tube.

I have a few old X ray rectifiers that suggest that they used to work better with DC. If the polarity is wrong, no electrons will come from the target to the cathode (now anode). Certainly won't make for continuous illumination.

 

Hey beenthere, thanks for the reply. When you say: "If the polarity is wrong, no electrons will come from the target to the cathode (now anode)." howcome this cant happen?

 

An Xray tube is pretty much like every other vacuum tube. The electrons are accelerated harder, so they give off those high energy photons upon striking the target (plate).

Like other tubes, some energy is helpful in exciting the loosely bound electrons on the cathode. That's the job of the heater. No heater, very few free electrons available to fly to the plate under electrostatic influence - anything above about 25KV (yes tv CRT's give off long wave X rays).

It makes no sense to place HV AC across the X ray tube - are you sure your instructor was saying that, or did he mean that the filament had AC on it (presents no problem)?

 

The Xray tube is effectively a thermionic diode so if you drive it with AC will do its own rectification. There is no point in using a separate rectifier before the Xray tube and driving it with DC.

 

Yes beenthere, i am sure, in all my notes the Xray tube schematics are driven directly from a step up transformer without rectification. Also this question suggests much the same thing.

The Xray tube is effectively a thermionic diode so if you drive it with AC will do its own rectification. There is no point in using a separate rectifier before the Xray tube and driving it with DC.

Hi AlexR, i understand what you are saying and indeed thats what i thought as well but i dont understand HOW the target metal doenst thermionically emit electrons as well. Its a hot piece of metal just like the filamint so howcome this cant happen? If you dont mind me saying so you've just rephrased my question without actually answering it.

 

I would not have thought the the anode gets hot enough to emit electrons. As I remember the target is a lump of tungsten which is mounted in a copper block so the whole assembly has a fair amount of thermal mass. Also the tube is operated with a very low duty cycle (on for a couple of seconds then off for minutes while the technician fiddles about and changes the film).

 

Thanks Alex, i thought that was the reason, just wasnt sure if there were any other mechanism involved to prevent reverse currrent.

 

It really is a matter of tube basics, why the tube style rectifier works at all. It not just hot, the filament is glowing, which is causing a cloud of electrons in a vacuum to "boil" off.

So if a tube diode has 3 terminals, and a triode has 4 (minimum), is a light bulb a monode?

 

There may be some apple to oranges confusion here. My experience has been with a system doing small-angle scattering experiments. The X rays were generated by a tube with 60 KV across it, and a beam current of 60 ma. The target was water cooled. That was in a physics lab.

The ones the geology department use were a bit less intense - 40 KV @ 25 ma. All these were DC, with big tube rectifiers in a tank filled with mineral oil.

 

I would imagine that Xray scatter/diffraction involves long exposures so you would need the water cooling. Also you need a steady and stable Xray beam which is why the tube is run from DC.
Medical Xray machines however are usually air cooled (short exposures with very low duty cycle) and run from AC, it really doesn't matter whether the beam is steady or pulsed at mains frequency, all that matters is that there is enough of it to expose the film.

 

I just happened to notice this thread so I thought I would chime in here because this is my area of expertise (hense my screen name!). It is very common for manufacturers of Dental X-ray machines to power the tube directly from the secondary of the high voltage transformer. The other common method is providing DC to the tube via a switch-mode power supply driving a voltage multiplier circuit in the tube head. Here is a typical schematic diagram of an AC type of Dental X-ray tube head: http://www.airstreamcomm.net/~mike/SSW_Tubehead_Schematic.jpg

In this method of powering the tube, the tube operates in what's call "self-rectification" mode. The tube only conducts on every other have cycle of the AC sinewave when the anode is positive with respect to the cathode.

Also, notice in the diagram that there are two high voltage secondary windings. One goes to the anode, and the other goes to the cathode. The reason for doing that is so there is only 35KV (in a 70KV tube head) between any electrical component and the metal housing. Only the tube "sees" 70KV. Also, the low end of one of the windings is brough out of the tube head so that the tube current (mA) can be monitored.

By the way, I repair Dental X-ray tube heads for a living.