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I was reading about capacitive touch screens. I understood that your hand picks some current flowing on screen. That sounds alarming.

Is it safe to use a capacitive touch screen, since it picks small amount of current from screen?

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Since there is an insulating layer on top of the conductive layer (making one side of the capacitor), there is no real current flowing from the screen to your finger. When you put your finger on the screen, you are adding the missing plate of the capacitor, thus completing the screen capacitor. The current flowing through your finger is not really coming from the screen.

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Capacitor values are about 1 pF - 1 fF, and the voltage switch is in the range 10-100V (usually 12, 24 or 48V) in micro seconds ~ 1 MHz signal. So, using the below formula for capacitor:

\$ i = C \frac{dV}{dt}\$

we have a max transient current of

\$ i = 1.10^{-12} . \frac{100}{1.10^{-6}}\$ = 100 uA

100 uA is safe for your body.

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    \$\begingroup\$ IMO, the finger capacitance may in the order of picofarads and not so constant, so 1pF may not be the worst case. \$\endgroup\$ – diverger Nov 26 '14 at 11:12
  • \$\begingroup\$ @diverger I don't know if we must use the finger capacitance or the ITO plate in the capacitance value. Since the 2 plates have not same characteristics. Any idea? Anyway, I wanted to give an order of magnitude. \$\endgroup\$ – RawBean Nov 26 '14 at 13:45
  • \$\begingroup\$ "When you put your finger on the screen, you are adding the missing plate of the capacitor, thus completing the screen capacitor." - Which causes charge to move, and thus current to flow. To be honest, I find your assertion that this is "not real current" puzzling. If you stand on a fully insulating surface and touch a live wire, you're also just completing a capacitor (with the earth, in this case). This will sting noticeably; is this also "not real current"? \$\endgroup\$ – marcelm Aug 5 at 15:43
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There has to be a flow of about 1mA before any sensation registers, 10mA before paralysis occurs, and 100mA before there are any cardiac effects.

Capacitive touchscreens use a current of 100uA or less. There is no chance of danger unless you are a microbe.

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