Testing rear window defroster grid

Question

Hello, I would like to develop a testing method to detect window defroster grid failures. Rear windows has 12 heating wires painted to the glass and cca. 2% are faulty pieces. I tried to measure the resistance, current and voltage drop of good parts and compare results with those where was at least 1 broken wire, but the difference was smaller than the tolerance specified by the supplier. (Other problem is that the resistance is greatly depending on the temperature) Now we are testing visually with a thermo foil but it takes more time and sometimes the operator mistakes or can forget to correctly check thermo foil. It is also very important to not hurt parts during the testing process, so best option would be contactless testing if possible... I'am thinking about to make a tool with 12 sensors, 1 sensor to every each heating wire to sense a flowing current in a wire. But I'am not sure that there is any sensor avaible in the market for such a purpose. Finally PLC have to check the outputs of every sensors, if all the 12 wires are good (current is flowing through them) gives visual signal or in case of failure sound alarm or red light flashing. Here is the specification of rear window defroster: Power: 167,37W ±10% Current: 12,87A ±1A Resistance: 1,0 Ohm -10%/+20% Wire thickness: 0,48mm In the attachment you can find technical drawing of the window. Any ideas and suggestions are welcome. Best regards, Ate 
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jw0752 · Answer

Hi Iansm, I had some time this evening and decided to breadboard and test the one Hall Effect device I have in my build stock: SS49E Here is a link to the Newark listing: http://www.newark.com/webapp/wcs/stores/servlet/Search?catalogId=15003&mfpn=SS49E&langId=-1&searchRef=SearchLookAhead&storeId=10194 Also here is a data sheet on the device: http://www.farnell.com/datasheets/1697810.pdf With a 5.2 volt supply to the Hall Effect sensor the output is approximately 2.6 volts. When a magnetic field that is passing through the sensor this voltage either increases above the 2.6 volts or decreases below the 2.6 volts depending on field polarity. The amount of of the increase or decrease depends on the strength of the magnetic field. Here is a snippet of a graph from the Data Sheet that shows this relationship. Click on the image to enlarge it. This came from the Data Sheet listed above. Here is the setup that I bread boarded to test this Hall Effect sensor under your specifications. The Hall Effect sensor is the small black transistor like device. I am powering it with a 5.2 volt supply which is a tad higher than shown in the data sheet graph. This has no effect except to raise the output to 2.6 volts when there is no magnetic field. The Yellow horizontal wire is hooked up to a separate power supply and a load so that I can put 1 amp through it, just as your window heater wires would experience under normal powered conditions. I experimented a while to see where I could get the best sensor reading on the current in the wire. I found that I was not able to detect the current in the wire unless the yellow wire was sitting right on top of the sensor. The magnetic field from 1 amp is quite weak and there was no ability to sense it with this sensor if the wire wasn't directly on top of the sensor. Here are the readings that I got on my meter. This is the output of the sensor when there is no current in the wire. This is the output from the SS49E Hall Effect sensor when the current in the wire is 1 Amp. This is only a change of 3 millivolts. I wish I had more experience with Hall Effect sensors so that I could recommend a specific sensor. One of the engineers on this site will probably have a good recommendation for you. This sensor, while it demonstrates proof of concept, would not be sensitive enough for your application. 3 mV is weak enough so that it would be difficult to discriminate it from noise in your test setting. John

Testing rear window defroster grid

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