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.

 

The tingling is a very low current AC from the charger. The charger isn’t grounded/earthed so there is a potential between the neutral line coming in to it and the earth ground which is the level of your potential. Sometimes reversing the plug in the outlet can solve it (if the plug is not polarized).

Adding an earth ground will work, but that’s a bit ugly.

If the case is plastic, it must be conductive plastic—which makes sense for the required RFI shielding. Take a DMM and measure the resistance across the case.

 

It's probably a metallic plastic cover and the reason for it is in the psu they use a suppression capacitor from the negative side of the output to the negative side of the input, and your body is providing an earth path leakage.

 

Thanks for those two insightful answers . I've just tried testing the case using two pointed probes on a multimeter , I get infinity even when the probes are pressed hard into the plastic and are a quarter of a mm apart! so that means higher resistance than 20MOhms (across 1/4mm) . But as you say the plastic must be conductive to some degree. The current I'm percieving must be ridiculously small!

 

Yes, plastic cases usually have very high but non-infinite resistance. The plastic case is unlikely to have anything to do with the "typical tingle".

 

It might just be capacitive leakage. If you have a capacitance meter, try measuring between your finger and the AC adapter plug (with it unplugged from the power source). Calculate the impedance at 60 Hz.

 

Capacitance can have an effect. The well insulated secondary leads of a neon sign transformer certainly feel fuzzy when held loosely, although no shock is felt. The electrostatic field is felt right thru insulation.

 

It is very common for switching power supplies to have conducted electromagentic interference filters with capacitors from neutral and line to ground (or both lines to ground). I have noticed that the capacitors often use the largest capacitor that will not exceed the maximum chassis to ground leakage current specified in the relevant safety standard.

 

A typical expensive multimeter will still have an input resistance of less than 50K ohms, and so any low-value capacitance coupled voltage will not register. To detect the electrostatic leakage use a battery powered DVM, set for a low AC voltage range. Connect one input lead to a piece of conductive foil held against the case of the power supply, and rout the other input lead straight away from the supply, and then hold the conductive end of that lead, standing away from the device being checked. You will see o few volts indicated on the display, no doubt. So there will be a bit of current flowing but not enough to feel.

A caution, though: do this experiment in an area with a non-conductive floor, not standing barefoot on a cement floor.