Re: personnal X Ray machine
paulj, Thu Feb 07 2019, 07:33AM

My first question:

for the high voltage power supply, I've to use 75 kv for 30 ma, starting from an alternative source 230v 50hz

what power supply do you recommend?

-the classic transformer with rectifier (very expensive)
-the voltage multiplier (cockcroft walton multiplier for example)

Paul

Re: personnal X Ray machine
Sulaiman, Thu Feb 07 2019, 10:02AM

I know only a little in this area, but 30mA seems to me to be unnecessarily high?
Lower current would require longer exposure times
but would be 'safer' in terms of x-ray exposure rate
and reduce the cost (and possibly the hazard also) of the power supply,
and cooling of the x-ray tube would be much easier.
(75kV @ 30mA = 2250 Watts ... a lot of heat ! )

A voltage multiplier fed from a fairly high voltage ac source sounds do-able if currents are low,
75kV @ 30mA sounds expensive.

I would not try to diy a 75kV transformer as failure is likely,
the obvious choice is an x-ray transformer ;)

Re: personnal X Ray machine
Conundrum, Fri Feb 08 2019, 07:11AM

Same here: you do need to limit the current ideally with a dropper in the 1G ohm range.

Re: personnal X Ray machine
paulj, Sat Feb 09 2019, 06:59PM

hello,

thank you for your answers:
I can't find a transformer, they are very expensive and big.
The tube is old, I will not take any risk and limit the current.

So

I'm thinking of using a voltage Steve Ward multiplier :

the old design seems solid in time.

the only constraint, I want to stay on a frequency of 50 hz to adapt it to my safety system.

Re: personnal X Ray machine
Proud Mary, Mon Feb 11 2019, 08:39PM

(a) I think you will find this is an oil-cooled tube - not air-cooled.

(b) 75kV/30mA is the maximum rating.

(c) The anode current should be controlled by varying the heater current.

(d) I would suggest you set it up for operation at 35kV/1mA. Shielding costs will be much lower, and you will not have to worry too much about the anode heat capacity. Exposures will of course be much longer, but this is not usually a problem with inanimate, unmoving targets.

Re: personnal X Ray machine
paulj, Tue Feb 12 2019, 05:52AM

A) yes I know it is oil cooled, but the message refuses to be edited when I valid

b) ok

c) for the heating of the filament, to associate a cockroft walton (high voltage) and a conventional transformer without rectifier (heating filament), there is no risk to connect the common terminals of the two dipoles (destruction of the filament)?

d) Indeed, the objects will be inanimate, this power seems reasonable to me

Re: personnal X Ray machine
Proud Mary, Tue Feb 12 2019, 08:41PM

Setting the anode current needs accurate, incremental, control of the cathode heater current.

I have found that the most reliable way of controlling the heater current is by using simple old-fashioned technology using a 10 ohm 100W wire-wound ceramic variable resistor. I measure both heater voltage and heater current with moving coil meters. Of course, the variable resistor wastes a lot of power, and may get very hot (for which it is designed) but there are no vulnerable semiconductors to be destroyed by the voltage spikes that so often appear in high voltage apparatus.

There is no need for an expensive, giant power supply. Why not start off with 25kV DC from a TV line output transformer (LOPT) often called a 'flyback'.

I also monitor the anode voltage using an electrostatic voltmeter. These are not easy to find at a good price, but I think it essential to know what the actual anode voltage is at any heater current setting.

This is why: home made high voltage power supplies often have very poor voltage regulation, and as the current demand rises, the high tension voltage (HT) (haute puissance) falls, so the anode voltage is very often much less than the experimenter thinks it is.

A good LOPT/flyback will be able to deliver 25kV DC at 1-2 mA without the voltage falling very much. If your power input is 25W - 50W then you will have 250mW - 500mW of X-radiation, which is plenty to start your experiments.

Shielding: , the depth of penetration for a given photon energy is dependent upon the material density (atomic structure). The more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur and the radiation will lose its energy. Make sure you understand the half-value layer formula, so you can work out how thick your shielding needs to be. With a maximum photon energy of 25keV, you may find that layers of aluminium sheet will be sufficient.

Good luck!

Re: personnal X Ray machine
paulj, Wed Feb 13 2019, 06:20PM

thank you fier Mary

for the rheostat:

- Or I pass the 230 v directly in the dipole for limit the current, and therefore the heat.
- or lower the voltage a first time with a first transformer (which replaces the original autotransformer too dangerous) before the rheostat, then use a second transformer before the filament

so I think to manufacture a command deported with a servomotor not to handle it directly the rheostat by hand.

I don't have the money for an electrostatic voltmeter, I think to use a resistance divider between the anode and the ground, or to use a voltmeter class 2.5, the carbon resistances will make the case.

I do not know the mathematical formulas of armoring and my geiger counter is not compensated in energy for the measurement.

So I'm going to exaggerate the security. In addition to the lead, I would be several meters away.

I agree to start with a small TV power supply.

 I don't learned to use power electronics in next situation, that's why I would like to use a voltage multiplier without semiconductors

paulj

Re: personnal X Ray machine
Proud Mary, Wed Feb 13 2019, 11:45PM

I would suggest you plan to vary the cathode heater voltage across the range 1V - 5V. I use DC myself, as this means I can connect one side of the heater to Earth/Ground.

I would not feel happy if the heater supply was floating with respect to Earth/Ground.

It does not matter how exactly you monitor the anode voltage, just so long as you can see what voltage you have on the anode as you vary the cathode heater current.

Re: personnal X Ray machine
Conundrum, Thu Feb 14 2019, 06:28AM

Yes, this is the same problem I had.
Interesting idea: what about using a linear device like an LED array? Calibrate elsewhere and then measure it using optical fibre eg TOSLINK cable. Known length of cable should be pretty accurate.

Re: personnal X Ray machine
Proud Mary, Thu Feb 14 2019, 12:12PM

I don't understand your contribution, Andre - have you by chance posted it in the wrong thread? :)

Re: personnal X Ray machine
Conundrum, Fri Feb 15 2019, 04:59AM

This was to do with monitoring voltage and current remotely.

Re: personnal X Ray machine
paulj, Fri Feb 15 2019, 06:37AM

the idea of optical fiber is interesting, but for economic reasons, I will be content with vintage analog voltmeter with a simple resistance voltage divider and transmit coaxial cable.

Re: personnal X Ray machine
Proud Mary, Fri Feb 15 2019, 02:01PM


Take care that you do not have more current going into your voltage divider than into your tube! When your power supply can supply only a few milliamps this can be a serious issue!

In practice, you may find that the current drawn by the voltage divider has the effect of pulling down the voltage available to the anode, so do your Ohms Law calculations carefully before deciding what to do! :)

Re: personnal X Ray machine
paulj, Fri Feb 15 2019, 03:51PM

I take notes:)

Re: personnal X Ray machine
paulj, Wed Feb 27 2019, 07:57PM



first test with flyback transformer.

Re: personnal X Ray machine
johnf, Thu Feb 28 2019, 05:46AM

Ho boy
I hope you were no where near that
uSv/hr is not the correct range to be on uSv/min or uSv /sec would be much better --read quicker to a determination. I see that the output was still going up as you turned it off

CAREFUL
You do not want a suntan from that!!!

Re: personnal X Ray machine
Conundrum, Thu Feb 28 2019, 06:37AM

Indeed. Been there, done that. Also low energy X-rays are even more damaging.
There is a good reason why the window on some commercial tubes is made from beryllium, it is to block the useless X-rays below 9keV and changing the window eg for tungsten changes the pass energy.

Re: personnal X Ray machine
paulj, Thu Feb 28 2019, 07:11AM

oh yes, the translation!

I am 5 meters behind a block wall, and the opposite of the radioactive beam.

Re: personnal X Ray machine
paulj, Fri Mar 01 2019, 08:29PM

hello,

After reflection, and the first success at 170w, I will increase the power to reduce the exposure time that seems too long.

I will increase the voltage of the cathode, in order to have more penetrating and more energetic rays,
the first test was at 20 kv dc, I go to 40 kv dc.

Because of the high price, I continue to make the dump in order to find the necessary material to build a robust chassis, I hesitate to add a berylium window in order to block the energy "soft" rays and to improve the beam quality for photography.

my priority is safety, so here are the measuring instruments that I want:

- kv meter, and ampermeter for the cathode
-voltmeter and ampermeter for anticathode
- dosimeter (a geiger counter is not reliable)

Re: personnal X Ray machine
paulj, Tue Jan 14 2020, 07:44PM

hello, finish after 2 years !!

Re: personnal X Ray machine
paulj, Sat Jan 18 2020, 06:17PM

spec of exposition

measure with EPD mkii graded with cs137

voltage working
-40kv 8ma
-filament: 5v 3a


results at 3cm of tube without colimator (10s exposition )
-hp10 1 Sv/h
-hp07 13 Sv/h

measure on the side tube at 6m with berthold lb1210b: 200 cp/S