My Laptop Bit Me - The Shocking Truth

Apple aren't the only company with products that can feel tingly or buzzy to a light touch, it is all due to the widespread use of switched-mode power supplies (SMPS) because they are lighter, smaller and more efficient than traditional transformer based supplies.

To simplify, an SMPS essentially takes the AC input and turns it on and off very quickly (between 10,000 and 20,000 times per second), with the output voltage being determined by the duty cycle - the amount of time it is on for each flipflop. The shorter it is on, the lower the voltage. This is fed into regulators and capacitors to produce a stable AC voltage, which is then converted into DC.

The switch circuits "float", bouncing from positive to negative & back, and to guide its timing the switch circuits (there's always more than one) base their timing on the input AC voltage frequency (in Australia, 50 Hz), which makes them very picky about the right phase the mains supply is. In order to 'float', the switch needs a baseline, so it relies on the common ground (sometimes called common rail) shared by the entire power supply.

The issue of "tingly" cases can be traced back to a mandatory pair of capacitors which bridge the Active and Neutral inputs to the common rail. Switch-mode supplies create a lot of spurious interference frequencies, and to prevent these from passing back into the mains supply (and cause havoc to other devices) 'decoupling' capacitors form a crude but effective low pass filter which shunts these unwanted noise-frequencies to the common ground.

When the device's common ground is connected to an earth, it is maintained at zero potential, and the EMI gets dumped. When the device's common ground is not connected to an earth, the common ground (which often includes the case, if it is metal) is energised to half the mains AC voltage, but at a very very high impedance. There's not enough current to harm you, nor the electronics inside the device being powered by the SMPS.

So what causes the tingling sensation? If you touch the case (and thus, common ground) you, dear human, become the earth -- current flows from the case into you, and through you into the atmosphere. Sure, its high resistance, but it is a conductive path that was not there before. Whilst the current levels flowing through you into the air borders on submicroscopic, you're being exposed to 120 volts. It's current that kills, not volts. If you touch a floating common ground lightly enough, and the atmospheric conditions are right, you'll feel each rise of the 50 Hz mains as the resistance between the tiny contact point of your skin's surface and the electrically-driven pressure (touch) nerve endings changes.

In short (ha) you should always use a triple-prong lead -- one with an earth pin and conductor -- between the power point and the SMPS, and plug it into a power point with a good earth. If your laptop's power supply only uses a two-pin 'donut' mains cable, you should invest in an aftermarket "universal laptop power supply". Alas, for MacBook Pro owners this is not an option thanks to their use of the custom 'MagSafe' power connector; users of Apple's laptops should make a point of using the full length mains cable that was shipped with their sleek metal slabs instead of the simple 'duck-head' snap in adaptor. Even though the MagSafe power supply itself is not connected to the mains earth, there is just enough inductance leakage for the floating charge to dissipate through the plastic sheath of the mains cable.

It's worth noting that the fuzzy-electronics effect can be felt if you use an earthed power point but the power point's Active and Neutral are wired back to front. Older houses and bodgy-job wiring can have them swapped, and for many decades it didn't matter. If you know your power supply has an earth pin and should be ground but you still get tingles and zaps, you have some serious mains wiring issue and should get a qualified electrician in to investigate as soon as possible.

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