Friday, 20 January 2017

Touchscreen stop working while mobile plugged into a charging point in the train. Why ?

Mobile plugged into a charging point provided in the train, the battery started to charge up but the touchscreen stop working, if unplug it touchscreen will start working again. Why ?

Mobile adapters are almost always 'universal AC input', that is, they can work with input voltages anywhere from 85V-265V (RMS). So it likely has nothing to do with the fact that the train gives a lower supply of 110V. The issue has to do with conducted EMI because of the switching in the adapter. 

How capacitive touchscreen works: Basically, you have a transmitter and receiver electrode separated by a junction. A mutual capacitance links the electrodes, and the charge movement into Rx electrode is monitored to calculate this capacitance. When a finger is placed on the screen, it affects the fringing fields and hence causes a change in the mutual capacitance, hence causing a change in charge movement across junction. These changes in capacitance are measured across the matrix of Tx-Rx electrodes, to form a touch map.

How the adapter (charger) for phones is built: There is no earth connection in these chargers. It is isolated for safety, using a transformer. While the transformers physically isolates the secondary from the primary, there is a capacitance formed across the isolation between the windings. The topology of the charger is called as the flyback converter. Apart from the switching waveform, there is a great deal of high frequency resonant LC ringing in this topology. This switching EMI is coupled across the isolation because of the coupling capacitance (a capacitor couples high frequency very well). Since this appears as a common mode noise on the secondary side (i.e. it exists on both the DC power line and DC ground), there is no complete circuit path for this conducted EMI, so normally it causes no issues.

Now when you touch with your finger, there is a coupling capacitance formed from the display, through your body to 'earth'. The conducted EMI now has a complete circuit. Charge flow occurs from the Tx-Rx junction to the finger, and so the finger changes the measured capacitance. Not only this, the finger also has mutual capacitances linking to other Rx electrodes, which can cause charge flow changes at points where the finger isn't actually touching. The entire touch map gets screwed up, and the touchscreen behaves erratically.

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