I have a HP laptop , and I've noticed a very very faint tingle / shock /vibration when I lightly run my fingertips over the the plastic caseing surounding the keyboard and touch pad. This is not there when the laptop is unpluged from the charger . The charger delivers 19Vdc . The caseing is plastic so how is this posible? The tingle is very very faint but definietly there when charging , and gone when unpluged.