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Dental X-Ray head

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radhoo

Wed Jun 12 2013, 04:23PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Here is a dental x-ray head, exposing a connector with 4 pins:

Measuring them led to nothing, so I had to open the case, save the nice paraffin oil, check the wires, then put everything back together.

The high voltage transformer has a 220V primary and two secondaries: a HV one and the filament secondary
According to the numbering presented in the first picture, 1 & 2 is are the mains pins (220V) connected to the HV transformer primary.
Pin 3 is one of the filament connections, floating above ground. Placing a resistor between pin 3 and ground will limit the current fed to the filament, and so the temperature of the filament and the intensity of the x-ray output.
I have no idea what pin 4 is, but I assume it is a tap on the HV secondary used to measure the output. Something similar to this:

Everything was put back and case resoldered. Finally I applied a black paint layer, that looks good

After putting everything together and connecting the head as presented here, it appears to work, but the pin 4 arcs to ground:

The bulb was added between pin 3 and ground to limit the filament current and greatly reduce the x-rays output. A sensitive geiger dosimeter (RADEX 1706) would detect the emission only when placed in front of the outputting window.

To stop the arc, as a temporary solution , I placed a HV 1K resistor from pin 4 to ground as shown in the last picture.
The frontal window exposes an aluminum disc of about 0.8 mm width.

The question is: Does anyone recognize this head or has any info on what the pin 4 is?

radhoo

Thu Jun 13 2013, 10:13AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Anyone?

...

Thu Jun 13 2013, 04:38PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Are you sure that the pin is not simply the return for the high voltage coil? It seems logical that they would route it through the connector so that you could put in a resistor (1k is a good value) to measure the current flowing through the tube.

radhoo

Thu Jun 13 2013, 08:03PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

It is possible . I asked hoping someone would recognize this head model. For now it works as it is, and I don;t have plans for making a fancy circuit for it, as I would only need it on rare occasions , for "debugging" black boxes.

Wolfram

Sat Jun 15 2013, 07:31AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Pin 4 is definitely the return pin for one half of the secondary. It is used for measuring the tube current, like ... said.

radhoo

Sat Jun 15 2013, 01:40PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

So, very much like in the diagram I've posted? Here is the link again:

Radex 1706 :

Wolfram

Sat Jun 15 2013, 07:31PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Nice shot. Do you have some details on the type of screen and camera used, voltage, current, distance, iso and exposure time?

The circuit is most likely very similar to the attached image, minus the separate filament transformer. When x-ray tubes are used in self-rectified mode, the current sensing resistor needs to be in series with the secondary instead of between the center-tap and ground.

radhoo

Wed Jun 19 2013, 08:32AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Hi Anders,

Judging from the internals, I'd rather go for the attached diagram . Do you feel it looks like a common topology? Also the pics show the secondary as a single block, what do you think?

1371630761 1938 FT154842 Dental Xray Head

In my setup, Pin 3 is connected to ground for maximum filament temperature. Pin 4 is connected to ground , via a 1K resistor. Before the 1K resistor, pin4 was arcing to the case, like shown in the pictures above.

The above photo details are:
camera: nikon D90 + 18-55mm f3.5-56
settings: 8s exposure, f/6.3 , iso 640
postprocessed / software enhanced

image intensifier screen: kodak lanex fine

radhoo

Wed Jun 19 2013, 08:40AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Minor correction - the two filament wires can also be seen in the picture above:

One of the filament coil wires is connected to ground (not visible in the picture).

Fabio

Thu Jun 20 2013, 02:26PM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

This is a really nice object!

You can use it for build a really compact machine capable of both radiograpy (powerful shots for really short times only)and radioscopy (less powerful shots, but capable of few tens of seconds exposures), you only need a proper value resistor in series of pin 3 and a switch without any external transformer or any other bulk equipment!

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