How do touchscreens work when a glass or plastic screen protector is on when glass and plastic are insulators?
In: Technology
The screen is already glass, it’s supposed to be an insulator.
Cellphones (well most) use capacitive touch screens. Capacitance is the build up of static electric charge. When you get shocked, that’s capacitance at play. When a conductor, that is you, being salty water, (but also a metal spoon or even a wet carrot) touch the one side of the glass, a small amount of charges from you rush to meet a small amount of charges from the conductors behind the screen to balance the charge there. They don’t mix, they just build up on either side of the insulation. The screen tells where you touched based on where these charges are pulled to. Static shock is the same thing, you build up charges by rubbing something. For this to work, you need to be insulated. If you were touching a metal fridge while rubbing your sock on carpet, you’d never built up a shock because there’s no barrier. You need an insulating barrier to actually build up static charge.
Adding a thin layer of insulator does nothing to this process. You need an insulator, so you and the conductor behind the screen don’t make contact. A thin layer of extra insulator does nothing. Adding a larger layer of insulator (a thick glove) ruins this as capacitance does fall off with distance.
Some resistance touch screen work differently, they respond to the actual force. These are the soft, platsic ones that you can feel depress a little when you touch them. These can detect anything, not just conductors, so long as they apply force. Some ATMs, older phones with a touch screen before they all moved to the iPhone style, the older Nintendo handheld systems have them. Most things have moved to capacitive even if it is a little more expensive, as it works better, feels better, and is more scratch resistant (though can shatter).
Short answer. Two variations of the technology: the screen either is sensitive to the magnetic/electrical field around your finger or it registers the shadow created by your finger. The protector just provides another layer of the same basic substance, not really an insulator.
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