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paulj

Mon Feb 04 2019, 08:17AM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

Hello,

for create film photography, I'm going to build an x-ray machine.

the tube: 2BDM4-75

-75 kv
-2kw
-air cooling

I am aware of the risk with radiation,

For the safety, I have an industrial power supply 9 kw + two Variac 0-230v with emergency stop, circuit breakers and protections against electrocutions.
Â
I haven't yet designed the electrical scheme high voltage
I take any advice to improve the performance without using the already old tube (1961).
Who has experience on these constructions here ?

Paul

paulj

Thu Feb 07 2019, 07:33AM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

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

Sulaiman

Thu Feb 07 2019, 10:02AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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 ;)

Conundrum

Fri Feb 08 2019, 07:11AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

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

paulj

Sat Feb 09 2019, 06:59PM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

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.

Proud Mary

Mon Feb 11 2019, 08:39PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

(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.

paulj

Tue Feb 12 2019, 05:52AM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

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

Proud Mary

Tue Feb 12 2019, 08:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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!

paulj

Wed Feb 13 2019, 06:20PM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

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.

Â 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

Proud Mary

Wed Feb 13 2019, 11:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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.

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