Any one know what this is?

Question

Disassembling a nice Siemens Circuit board from an X-Ray machine with a early 1990s manufacturer date. Among the salvaged parts was this little fellow:

The blue case had the numbers J29 P120 and Siemens symbol. It looked like the case could be opened so I inserted a knife blade and opened it. Here is what it looked like inside. I set the insides up on the cover in the second shot to get a better picture.

The device has 6.4K Ohms of resistance.

This is just my curiosity asking. Since the machine has European Design and other components I thought someone from there might recognize it.

Thanks

John

shabaz · Accepted Answer

Hi John,

Interesting find! I've never seen such a device, but I did see a similar thing on ebay:

PTC MINI MIVAR SIEMENS J29 P190 2PIN PEZZI 5 | eBay

Apparently a 'switching thermistor' which as it gets warmer rapidly drops to very low resistance. Interesting packaging, I've no idea why it is like this. Maybe it changes shape as it gets warm and needs the springy clips.

jw0752 · Answer

Hi Shabaz,

That was fast. Tomorrow I will run some temperature vs resistance tests on it to see how it reacts. Nice to see the exact thing listed on ebay. Either there is still some relevant application or they made a ton of them back in 1990 and still haven't sold them all.

John

jc2048 · Answer

The passive section of Siemens became EPCOS and that then became a part of TDK. Here's a datasheet from their website https://en.tdk.eu/inf/55/db/PTC/PTC_Switching_Plastic_case_230V.pdf

jw0752 · Answer

Tonight I had the chance to run a crude experiment to see for myself what are the Temperature vs Resistance characteristics of this PTC. I used the following test set up.

The blue box is the control for the heat gun. I can produce temperatures between room temperature and 340 C. The Fluke is monitoring the resistance of the PTC device and the Red meter is a thermal couple thermometer of which I only used the bottom probe. The PTC is clipped to the Thermal Couple so that they have nearly the same temperature. The aluminum foil to to protect my plastic probe clips from melting. Here is a close up of the PTC and the Thermal Couple poking through the foil.

Here are the readings that I got from my test of this PTC:

Temperature Resistance

Degrees C Ohms

---------------- ---------------

20 6.8 K

25 6.6 K

30 6.2 K

35 5.9 K

45 5.5 K

55 4.9 K

65 4.5 K

75 4.5 K

85 5.4 K

95 10.7 K

105 36.5 K

115 1.9 M

125 13.0 M

135 50.0 M

I graphed this but I had to shift the resistance scale logarithmically and it was too much of a mess to post but you can get the idea. While my measurements did not match the data sheet exactly it is probably due to my not matching the current of the manufacturers test conditions. The resistance of the PTC acts like a NTC thermistor from the 20 C room temperature to about 60 C and then it turns PTC and increases rapidly to 50 Megohms at 135 C. Thanks again to everyone who took the to put me on the right track to identifying this part.

John

jc2048 · Answer

An easy way to do log graphs is to paste the data into a spreadsheet and get the software to do it for you. I put your values into the LibreOffice spreadsheet and got this

The shape is similar to the datasheet but, as you say, your curve going up is happening about 30C lower than theirs.

jw0752 · Answer

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

DAB · Answer

Hi John,

I think the trend is correct.

The device was probably used to prevent overexposure.

I think you will find that as the device runs, the temperature increases rapidly. This simple device would detect the temperature change and throttle the current running to the x-ray projector so the patient would not be injured.

DAB

Any one know what this is?

Top Replies