Any one know what this is?

That board's been a useful source of bits for you - you've had practically everything off of there other than the passives. You must be a real expert in desoldering by now.

To my shame, I can't even work out where the device was on the board.

The board is evidently mostly a power supply - the 10A fuses and large choke top-right are where the power came in. Transformer T2, bottom left is the bridge from high voltage to low voltage. You can see the secondary to primary feedback along the bottom edge with a guard track - I assume J1 was an optocoupler. The rest in between is a discrete SMPS. Presumably, C30 and C31 were two very nice high voltage electrolytics off the board. You can see it's high voltage by the careful layout and clearances. It's not very easy to say what the top-left (the low voltage side) was doing, unless you tell us the part numbers of the chips you removed.

It might be that the part was a safety measure, as DAB suggests [a reasonable guess without knowing more about the system], but I don't think it's as simple as shutting down if the temperature exceeds 130C. If you were going to shut the device down on the basis of the temperature, I think you'd choose something a lot lower than 130C. Since the board is a power supply, I would imagine it's more likely that it's being used either as an electronic fuse or a way of getting a delay of a few seconds at startup, perhaps to control in-rush or to delay switching until the electrolytics were fully charged.

Hi Jon

I really wish you lived a little closer as we would have a good time analyzing some of these boards I take apart. I believe that the PTC was in the position marked R164 and it was in series with R159 which I have no way to put a value on at this point. The traces are very small so it was a low current circuit. The board is, as you say a power supply board. In this case a specialized power supply that is designed to supply controlled current to the filament of an X-Ray tube. This has to be controlled as it determines the mA current of the electron beam striking the tungsten target and subsequently the brightness of the x-ray beam. The board also supplies a controlled AC voltage to a high voltage transformer to produce the potential between the X-Ray tube anode and cathode. The High Voltage transformer in this case was enclosed in the same housing that has the X-Ray Tube. This is commonly called the x-ray tube head. In this case the x-ray machine was a panoramic machine designed to take a picture of all the teeth in the mouth by sweeping the x-ray beam around the back of the head and shooting through the skull from the back past the teeth and producing shadows of the teeth on a screen and film that is synchronized to the rotation and in front of the face. The other function of the board was timing as there has to be a short period of filament voltage to allow it to warm up before the high voltage is applied to the Anode and Cathode. Then the rotation of the head and film have to be timed to match the time the x-rays are emitted. I find that there is so much to learn from these boards beyond the value of their salvaged parts. While I lack your skills I often take the time to study the circuits and look up the data sheets on the ICs prior to demolition. Once in a while I find an interesting circuit arrangement and try to duplicate it on a bread board in order to understand it better. Thanks for your input.

John

Hi Jon,

My educated guess was based upon a big issue about these machines overexposing patients when they came out.

The EU in particular mandated automated circuits to "limit" the exposure automatically.

Using that knowledge I deduced that this simple solution was an easy and inexpensive way for the manufacturer to pass the requirement.

Based upon John's measurements, it seemed like a logical function for a device that normally did not belong in a power supply.

DAB