I have a Microsoft Surface Pro tablet that I really like. However, when I slide my fingers across the back of the tablet (made of metal, maybe aluminum?), I get a weird vibrating/buzzing sensation. At first, I thought it may be caused by the cooling fans or something, but then I realized that a) it only happens when the tablet is charging and b) when I unplug the cable connecting the power supply to the outlet, rotate the little figure-eight plug and reconnect it to the power supply, it goes away! What’s going on??
That shouldn’t happen. It sounds like your charger is either some cheap crap or has failed in some way. The wall power has a neutral and hot wire. If one of those is being coupled to the case of your tablet, then you may well feel a buzz when it’s the hot wire and not when you flip the plug and the neutral is connected.
Whether it’s a shock risk is hard to say without more tests. It may just be a capacitive coupling, which case a buzz is all you’ll get. But it’s entirely possible that the whole casing is live and the only reason you haven’t gotten a serious shock yet is because you aren’t grounded.
I would throw away the charger immediately and get a new one. Ideally, a Microsoft branded one from a reputable shop.
Thank you for your comprehensive response! The charger I’m using is the original one the device came with. It sure has been dropped once or twice, but I’m not aware of any serious damage to it. Nevertheless, I’ll get a new one and see if the problem goes away.
As you seem knowledgeable about this, can you indulge a possibly naive question? I thought that AC current reverses 50 or 60 times per second. Wouldn’t that mean that the hot wire becomes neutral and vice versa all the time?
You basically ‘can’t’ get shocked by a tablet or its USB charger outside some truly remote freak accident circumstances.
Oh, and another thing I just remembered: the buzzing sensation stops when I stop moving my fingers across the back, even though they’re still touching the metal. Once I move them again, it comes back.
This is caused the the “Common-mode” noise suppression capacitors in the power supply.
You ARE being electrocuted, but the amount of current is so small it’s negligible.
It’s not a USB charger, it has a proprietary plug.
If it’s the original charger, I expect it to be relatively safe. Knockoff chargers often take shortcuts on the internal circuitry. The shortcuts may not be apparent until something else goes wrong. Reputable chargers tend to have multiple layers of safety and so there is probably not an immediate shock risk. Not impossible, but less likely.
The neutral wire is tied to ground at the electrical box, and should always be at zero volts relative to ground. You’re right that the hot wire reverses direction and goes through 0 volts, but most of the time it’s at a higher voltage.
The reason for the separate neutral wire is for safety. Ideally, there’s an equal amount of current that flows through the hot and neutral wires. No current should flow through the ground. High power devices (more than tablets) usually have the casing connected to ground, so if there’s a fault that makes the casing hot, then it’ll also be connected to ground and trip the breaker. Or the ground fault interrupter, if you have one.
Well, shocked, not electrocuted. Though I’ll admit that battle is lost.
It does happen. But it’s only the cheap knockoffs that can fail in a way that is a serious shock hazard. Reputable ones will fail safely.
I’m still surprised that the charger apparently feeds 110V (well, 230V in my case, but you get my point) to the tablet!? I thought the whole point of the charger was to lower the voltage. I shouldn’t be able to feel 5V or 12V or whatever, right?
The voltage is lowered–but the pins on the figure-8 plug you mentioned are at the full 120/240V. It then goes through the box and is lowered to probably 16-20 V. The two voltages should be totally isolated, and on a good charger they will be. But with a cheap charger or a bad enough failure, they can be connected and deliver a shock.
As beowulff said, it’s probably the common-mode capacitors that are on the fritz. If they fail, they can allow transmitting a tiny amount of current. Enough for a buzz, but not a serious shock (except in the case of knockoffs–in that case, all bets are off).
I see what you mean, sorry about that. Still distantly unlikely
Voltage is always the difference in potential between two points. The voltage difference between the two ungrounded terminals of a US socket will be roughly 120V, and 230V in most of the rest of the world. The voltage difference between the output terminals will be whatever the specification says (5V, 12V, 20V, etc).
Where it gets interesting is when you talk about the voltage between the output terminals and the AC input side neutral or ground. It would be entirely possible to have, say, a 20V DC power supply where the DC negative terminal was tied to the hot AC input terminal, and the DC positive terminal was always 20V DC above that. The device wouldn’t care at all, but you’d get shocked if you managed to touch either DC output terminal and ground.
Ideally power supplies would have their DC output totally isolated from the AC input so that the above couldn’t happen, even in a fault. However, isolating transformers are heavy, expensive, and inefficient. Switching power supplies are small, light, and efficient but have the risk of allowing some AC power to be exposed on the output terminals (especially if using an ungrounded plug).
Supplies from reputable vendors may leak a tiny little bit of AC current to the DC terminals, which you might be able to feel but it will be limited to a level well below anything dangerous. It wouldn’t pass Underwriters Laboratories (UL) testing (or your local equivalent) if it didn’t. Knockoff power supplies have no such guarantees and may indeed be deadly if you touch the output terminals. A dozen USB chargers in the lab: Apple is very good, but not quite the best is a good read that covers some of this.
I have found that Apple power supplies tend to cause MacBooks to “buzz” when lightly touched with fingertips, but only when using the charger with the 2-prong AC adapter. If you use the grounded adapter, they don’t do it, which implies there there is a tiny amount of leakage from the secondary side to ground.
The Microsoft Surface Pro charger is not grounded and should charge at 65 Watts. It doesn’t have cooling fans or any other moving parts inside.
I think you are getting a small static charge from the metal casing. Maybe you are brushing it against something (nylon?) that is charging it up.
By the way, this is a large part of why some devices try to enforce using original chargers and batteries. Depending on the setup there can varying levels of checks. Medical devices, for instance, may be more…adamant… about using proper sources.
It allows us to know that those safety features are there, and thus not doing bad things to the device or user.
Yea, that was a good read. And here is a teardown of a counterfeit charger. The author noted a couple problems: 1) there was only 1 mm of creepage between the primary (“hot”) side and secondary (isolated) side, and 2) the transformer had only one layer of insulating tape between the primary and secondary windings. He didn’t measure leakage current, but it wouldn’t surprise me if it was greater than the safety limit (which I think is 0.5 mA).
No he ain’t. Electrocuted means DEAD. Electricity + Executed = Electrocuted. The word itself was coined for it.
You say the buzz appears/disappears when you reverse the figure 8 plug. Does it also toggle when you reverse the connector between the charger and the Surface? In any case, why don’t you simply keep it plugged in the way nothing happens? You can mark the cord and charger in some way so that you always plug it in the same way. I’ve not noticed this on my Surface, but I always use it propped up and never touch the back.