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x-ray device with floating cathode, connection problems.

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Subzero01

Sat Dec 26 2015, 12:10AM

Registered Member #55277 Joined: Tue Jun 16 2015, 11:40PM
Location:
Posts: 2

Hi,

Can anyone help me with connection of x-ray tube where cathode is floating one, meaning HV pover supply is +70/-70 kV and filament is not at ground (0V).
I have some pics and schematics of previous models that may help. The colour one is the actual HV supply.
My problem is that filament is at -70kV, and I am more used to have it at 0V. So how would I connect it? Battery, or separate trafo........?
Thx.

1451087556 55277 FT0 20150607 115704 Hdr

1451087556 55277 FT0 20150607 115723 Hdr

GrantX

Sun Dec 27 2015, 07:39AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

You could use either a battery or transformer, but they will need to be isolated from earth and/or mains with very good insulation. In the case of a transformer, the low-voltage filament coil will need to be encased in very thick epoxy, or an oil-filled case with large bushings, in order to survive 70kV without arcing. These high voltage isolating transformers are occasionally seen on eBay and similar sites, but they are very heavy and expensive compared to a regular mains-voltage transformer of the same capacity.

A battery would be much easier. You could use an affordable remote-operated relay, or even just some kind of timer circuit, so you don't have to bring your hand anywhere near the deadly HV supply. The battery and it's circuitry could then be mounted on a cheap HV standoff made out of normal PVC plumbing parts. A small fraction of the cost and mass of an isolation transformer. You may need to use some kind of voltage regulation to keep the filament happy and extend the lifespan of both the tube and battery.

Hopefully that helps a bit.

hboy007

Sun Dec 27 2015, 11:38AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Unfortunately much of the magic is hidden in the "I/T" symbol next to T2 which should be an analog filament controller adjusting the filament voltage to obtain a stable HV current.
I took apart an XRB100

which is a 100kV grounded anode type unit. I recall seeing one or two flyback transformers and also a bipolar cascade.

For small heater currents you could try using Qi charger parts with a 3-4mm glass plate in between, that will give you 5V 1A at ~100kV isolation, you'll only have to combine that with an off the shelf tracking POL DCDC converter and get a PWM signal across optically.

If you like to keep it in the same style the rest of the HV generator is, I'd suggest getting some thick HV wire and planar E cores with no air gap to build a transformer for a royer converter - based design.

macona

Sat Jan 02 2016, 01:24AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

You could use two permanent magnet motors coupled with a plastic or fiberglass shaft to provide the isolation and power the filament that way.

klugesmith

Sat Jan 02 2016, 04:06AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I did the motor-generator thing more than 5 years ago, and Macona might remember our discussion about it on a different forum.
Universal motor with speed control (in blender) and brushed PM motor (in cordless drill).
Repeated here for your interest:
---------------
Just for fun, I thought I'd try a motor-generator with HV-insulating drive shaft. That method was used for HV DC power transmission starting in the 19th century (even then, more than 1 megawatt from series-connected, physically insulated HV generators).

First glimpse of calibration chart:

Verb  Filament_voltage 
 ----- ----- 
 Stir  1.9 
 Whip  2.3 
 Blend 2.9

The blender speed control has more range when there's a significant mechanical load, and I wasn't equipped to put a fan on the drive shaft. So there's a 1-ohm shunt resistor across generator, as well as a 1.8-ohm R in series with x-ray tube filament.

This was an after-dinner project, but the upper shaft connection was inadequate & its design was changed the next day. No blender parts were damaged, but the cordless drill has made its last hole and driven its last screw.

The only cutting tool used to mount drill onto blender was a hacksaw. I love serendipity in mechanical fitting! With the drill's chuck and gearbox removed, the 2-piece body is a good fit around nominal 1-1/4" PVC pipe, augmented by 1 layer of electrical tape. A PVC coupling at the bottom perfectly fits the ID of blender jar's base gasket. A PVC adapter bushing has flange that snugly fits ID of the blender's jar's rubber lid.
Drive shaft is 9 inch length of 1/4 inch fiberglass rod, which is stiff enough to handle the maximum RPM without center support.

A couple weeks later, I used the filament osterizer with a different Coolidge tube and NST to locate the sweet spot in a pen dosimeter. Without the floating supply, I had been limited to half of the NST voltage. Resulting x-ray flux apparently was too soft to penetrate the aluminum dosimeter tube, but gave a lively response from a Geiger counter.

Subzero01

Sat Jan 02 2016, 11:30PM

Registered Member #55277 Joined: Tue Jun 16 2015, 11:40PM
Location:
Posts: 2

Thank's for all tips and ideas. I think I'll go with battery because of space and looks. Everything will be controlled by Arduino, like timer, camera shutter, fillament, cooling fans, pumps etc, with me far away.
Tube is from bagage screener ment to work all day long, so there is a lot of power.
Will post pics when all is finnished....in two months I guess.

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