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Input connector pinout, cathode glow

A project log for Xray Head Teardown

Reverse engineering a dental xray head

drhatchdrhatch 12/22/2014 at 23:570 Comments

As mentioned in a previous log, the input connector is a 6-pin molex plus an earthing wire (ground wire).

A key objective is to define the pinout of the connector of the xray head.

To help figure out the function for each pin, it is useful to have some idea of what the xray head needs.

Let's start with the xray tube itself. It is a simple device, with only four pins.

Inside the xray tube there is a heater filament (the cathode). The function of the cathode is to boil off electrons that will be accelerated toward the anode. So we can expect that the input connector will have two pins that connect to the xray tube's cathode.

Near the cathode is a focusing cup. This is often set to ground (return for the cathode current) but I was thinking perhaps it would be accessible via the main connector.

The other part of an xray tube is the anode. This is the target for the electrons. The anode is held at a very high voltage, around 70kVDC. This is created by an internal high-voltage transformer, so we can expect that there will be two pins, maybe three, that connect to the primary of the high voltage transformer.

Also available via the connector could be a feedback pin to monitor the high voltage output. That pin would provide information about the high voltage but at a lower voltage or current (or both).

CONNECTOR PINOUT

After disassembling the head as discussed in the previous post, I determined the pinout to be:

Pins 1 and 2: Primary of the HV transformer.

Pins 3 and 4: Cathode of the xray tube.

Pin 5: Connects through a series 30K resistor to the focusing cup.

Pin 6: After opening the unit and examining the HV transformer, I found an isolated secondary winding. One side of that winding connects to this pin. The other side of this winding connects to Pin 4 (cathode return). I believe that this winding will output a voltage proportionate to the output of the high voltage windings. Further experimentation is needed.

MAKING THE CATHODE GLOW

I thought it would be nice to make the cathode glow. If for no other reason, it would verify that the heater in the tube in this unit is functional.

To do this, and prior to fully disassembling the unit as described in my previous post, I opened up the front of the case to have visual access to the tube.

I applied a low voltage (~1 VDC) to the cathode pins, 3 and 4, and looked for the glow of the filament.

Success.

I then sealed up the front, and went back to disassembling the unit from the back, as discussed in the previous post.

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