Testing rear window defroster grid

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

Hi John,

Thank you so much for your helpful support, really appreciate!

I hope it will be the right concept, "just" need to find more sensitive sensor. Maybe it would be more sensitive with AC power or I can lower the voltage a little.   Unfortunately I have no other idea, what could be fast and contactless. I am little bit sceptic about thermo cameras, there could be many confounding factors, I think.

Hi Iansm,

Your idea to use AC voltage may have merit. It would provide a 6 mV PP signal voltage (using the SS49E) that could be amplified with an OpAmp and then drive some sort of display.

Just for the fun of it I took my sensitive thermistor instrument out and turned on my rear window defroster in my car. Within a minute I could see the heat gradients rise and fall as I pulled the thermistor across the embedded wires in the window. Unfortunately this effect diminished as the window heated up towards an equilibrium temperature.

I will continue to think about this problem and I will re-post if any other ideas come up.

John

Hi Ben,

I think that Iansm is trying to do Quality Control in a small factory setting.

John

That would be correct, this is not a one off repair, solutions would be far simpler of that where the case

Hi Iansm,

I borrowed a Fluke 80T-IR Infrared Temperature probe from my sons shop to see how accurately I could read the temperatures of the individual wires in my rear window. The 80T-IR will plug into any handheld DMM. It outputs the temperature to the multimeter at 1 mV per degree. The scale is selectable C or F by a switch in the battery compartment. I was able to easily read the temperature of each of the wires in the window as they heated up. This temperature probe is 22 years old and they are still available on EBay for $60.00 and up. The more modern probes are probably more sensitive and accurate. A person could easily check each of the 12 wires in the window in the time it would take to position a test jig that would check all 12 simultaneously. Something to think about.

John

Just to add to your thoughts....

If the voltage/current was regulated, any broken wires would mean it falls below spec.

Would this change of 1/12th of the resistance show up.

The other thought was using something like a tuned circuit over each wire.

If the wire is intact, it will detune it like a shorted turn.

Mark

Hello,

Thank you so much for your time and efforts.

I uploaded a video to show you how we are testing heating wires currently. Visually it is totally OK, fast and accurate, thermal paper almost instantly changes its color. I have to extend our PLC controlled Poka-yoke system with Go/no-go testing to automatically block broken windows from further processing. With a regulated power supply the difference between 12 and 11 wires is very small. I measured current consumption using a non regulated power supply and the difference was clearer, 5-8% less Amps but this is still much smaller than the tolerance defined by the manufacturer. So I am afraid that 2 weeks later for example from the next delivery all parts could be evaluated as a broken ones. That's why I'm trying to detect wires separately. Than summarize outputs into one digital go/no-go signal.

Thermal camera I think would be very problematic, can catch fingerprints, or glass can be accidentally preheated, in this case the contrast will be insufficient. We are using infrared thermometers, pyrometers and heat switches in other departments in the factory but I thought that is difficult to measure glass surface, and these kind of sensors detects bigger area not just 0,5mm thickness.

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RE: Testing rear window defroster grid

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Sticking with my suggestion of Hall effect sensors and Johns excelent testing of such, but thinking about the low signal to noise ratio, one option would be to significantly increase the current... AKA use a pulse rather than a continious current of say 100Amps

LESS HEATING

MORE MAGNETIC STRENGTH

EASY TO DO

so have a capacitor charged upto 60V say, or simply have a 60V psu but the cap ensures there will be no long term high current flowing. Use a fet/thyristor to quickly discharge across the heater and at that point all the HALL sensors should have a massivly bigger field to detect, this could all be controlled via the PLC but the output of the hall sensors may need to be latched if the pulse is too quick for the PLC to capture (It may have edge detection inputs as an option ?? )

I can not think why this would not work, it should do no harm to the heater / screen and is quick and simple and should resolve the issue with the small signal john has been seeing