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After installing one of these heated desk mats on my cold work desk, I've noticed some erratic behavior on touchscreen devices I develop software for when they're laid down on the mat. Most noticeable on iphones, when the heat is on, part of the touch grid becomes unresponsive and sometimes receives ghost touches. It happens regardless of device temperature.

I'm posting here in EE, because I have a suspicion the cause might be some sort of electromagnetic interference. I'm happy to move it elsewhere.

There seems to be a cause-effect relationship, because it's happening on multiple devices from different manufacturers. What is going on here?

Here's a video demonstrating the effect: https://youtu.be/0jz8HaytlEs

Touch Test with EMI Video

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    \$\begingroup\$ EMI for sure! It's probably radiated due to the geometry but if you want to try it you can test to wind the cable though a ferrite choke several times or have a clamp on ferrite. Is the touchscreen and heated mat connected/share ground? \$\endgroup\$ – winny Jan 4 '17 at 15:05
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    \$\begingroup\$ Could be the capacitive sensing itself or just "back feeding" cable conducted EMI to the point where the capacitive sensing electronics behind the screen gets distubed. \$\endgroup\$ – winny Jan 4 '17 at 15:09
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    \$\begingroup\$ @winny: You wouldn't win a lawsuit; there's already a disclaimer on the web page: "-To prevent overheating and damage, digital devices should not be placed on WarmSpot." \$\endgroup\$ – Dave Tweed Jan 4 '17 at 15:35
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    \$\begingroup\$ @Dave You still need to comply with EN/CISPR 55022 radiated emissions. \$\endgroup\$ – winny Jan 4 '17 at 15:45
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    \$\begingroup\$ @winny: Those emissions are not measured at <1 cm distance. I'd bet that the mat passes the compliance tests just fine. \$\endgroup\$ – Dave Tweed Jan 4 '17 at 15:50
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The problems you are describing are typical of EMI disturbance. Your tablet/phone with touch screen is a typical class B device which must accept any radio disturbance (up to a level) but may misbehave or stop functioning properly when a disturbance is applied.

Your heat mat is most likely PWMed to regulate the temperature and if the implementation isn't good in this regard, the dV/dt and dI/dt flanks are high with the load acting like a large antenna, you have an EMI problem. EDIT: With your updated information that the problem still exists even when switched off but plugged in tells us that the power supply is the root cause of your problem. It will increase with load. You can try using clamp-on ferrites or of the wiring is detachable, get a toroid core and wind several turns around it to form a common mode filter. If the power supply is something simple, like a 12 V "dumb" supply and all control is done is the mat itself, you can try a different (approved) power supply. This will also increase you chances if suing the manufacturer who most likely bought and bundled a power supply with the mat and you would not believe how many no name Chinese manufacturers of non-EMI compliant manufacturers they have to choose between. CM filters

Your phone/tablet will be subjected to higher and higher field strength the closer to get to the mat and at some point something will misbehave. Exactly what will "give" first it difficult to say but something physically large, like the screen falls into the high probability category.

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You have described the problem as being particular to the touch-grid. While the EMI theory proposed is certainly valid, it could also be due to a capacitive effect as mentioned in your comments. A simple experiment to try would be attaching the mat surface to earth ground. This is commonly done for ESD work mats to allow static charge to slowly dissipate. The earth ground terminal is accessed through any mains wall plate screw, using a ring terminal connection at one end of an insulated wire. The other end is usually riveted to the ESD mat, but in this case do not puncture your heat pad! An alligator clip to the surface should work fine. It's a bit of a shot in the dark but easy to implement, assuming that you're comfortable removing the wall plate, connecting wires, etc..

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Capacitive touchscreens are sensitive to radiated electrical noise. They need to sense pico-Colulombs of charge, so any electrical interference can cause problems. Interference from your heat mat could easily cause touchscreen malfunctions (bad performance, phantom touches, erratic behavior).

I'm most familiar with touchscreen problems from my charger investigations. Cheap chargers generate a lot of noise - both radiated and in their output voltage, and this can cause touchscreen malfunctions. If your phone's touchscreen doesn't work while charging, you probably have a dangerous, cheap charger.

If the wall wart for your heat mat contains a switching power supply, that could be what is generating the interference. And then the wire in the mat will be acting as a huge antenna. If you have an oscilloscope, try waving the probe near the mat and see what you get.

For a detailed discussion of capacitive touchscreens and their noise problems, see Noise Wars: Projected capacitance strikes back by John Carey of Cypress Semiconductor. It explains in detail how cheap power supplies leave out components that would reduce the noise.

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