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I have VAIO E series (SVE) laptop. While charger plugged in, touching VGA/HDMI/USB port metal casing gives me burning sensation. So, I checked it with voltmeter and I found that while plugged in there is presences of DC +1.0 V (w.r.t. ground). Same case with my friend's VAIO E Series (VPCEH). Service center says this is normal. But I do not believe them as it is not the case with Dell Inspiron.

This happens only when power adapter is plugged in.

Is there a way I can prevent this from happening?

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I can understand that one of the connection points is the metal on the ports, but what is the second connection point? 1v DC is not a lot of voltage potential, and with DC feeling it even with some amps usually starts higher than that? did you also check with the meter on AC? Again all we have is ONE connection point described in the question. –  Psycogeek Jun 27 '13 at 10:14
    
@SRbH The fact these connection do not have a shielding should be of a concern. The DC signal is normal. A screenshot of these connectors would be helpful. –  Ramhound Jun 27 '13 at 11:37
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What happens when you touch the port is called potential equalization. It usually happens when a device is connected to different circuits (wall charger and ethernet to hub powered from different outlets, screen to a 3rd outlet, usb-hub to another). Plus electrostatic charging when you walk over the carpet. –  ott-- Jun 27 '13 at 12:29
    
Sounds like a issue with grounding, perhaps just a single wire, but none the less may want somebody to take a look on the inside to make sure. –  user88311 Jun 27 '13 at 14:24
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One volt DC is essentially just "noise", and you would not be able to sense it. There could, in theory, be a higher AC voltage superimposed on the DC, but it seems unlikely it would be high enough to sense. –  Daniel R Hicks Jun 29 '13 at 17:38

1 Answer 1

This is an inherent issue with switch mode power supply circuits. Here's the full treatment of this issue...

Switch Mode Power Supplies

A laptop charger incorporates a switch-mode power supply, which is the only technology presently capable of providing the required power, wide range of a.c. inputs and small size/weight required. Instead of a bulky transformer running at 50/60Hz, these circuit rectify the a.c. supply to produce a d.c. voltage of 150V to 380V. A chopping circuit generates a far higher frequency a.c. which can pass through a much smaller transformer, and pulse control can be used to regulate the output voltage, which is rectified to provude the required d.c. output.

EMC Issues

One major issue designers face with switch mode power supplies is the generation of high frequency interference which must be limited by law. Rectifying mains a.c. produces interference as the rectifying diodes switch from conduction to blocking every half cycle. And the chopping circuit, operating at hundreds of volts, frequencies above 20kHz and very short switching times to reduce losses, contributes considerably to interference which can be conducted back down the a.c. wire, radiating as it goes, and passing to other devices which can be disturbed or fail.

To deal with these problems, designers incorporate filters using inductors and capacitors, to block much of the interference, and keep it within the power supply circuit itself. The most effective elements are capacitors: X-capacitors across the a.c. supply, and Y-capacitors between the transformer output and ground, or the transformer input and ground, depending on the source of the greatest interference. Because failure of these capacitors can cause fire or electric shock, they have stringent safety approvals. Other filter elements include lossy inductors, known as chokes, connected in series with the a.c. input to block high frequency currents from the a.c. cord.

Protective Earthing

There are two basic approaches to protecting users of a.c. powered products from electric shock.

Class 1 devices have 3 connections to the a.c. supply: Live; Neutral; and Earth. The Earth circuit is connected in the building to a copper rod buried in the ground and remains at a safe ground potential. In countries with a Neutral connection, (as opposed to L1 and L2), Neutral is connected to Earth at the building supply connection point. In the device, the Earth wire is connected to all exposed metal surfaces which the user can touch, keeping the user safe from electric shock. If a fault occurs, such as a live wire falls from a bad crimp connection and touches the inside of the metal case, a large fault current flows directly to Earth, tripping the supply fuse/breaker and keeping the user safe from shock.

Class 2 devices have only 2 connections to the a.c. supply: L1; and L2. They have no Earth connection. This makes connection easier, cables lighter and devices easier (and cheaper) to wire internally. But to protect users from shock in the event of a fault, two layers of insulation are required between live circuits and anything the user can come into contact with. (Double- or reinforced-insulation can comprise two separate layers of insulation, each rated for the worst case supply voltage, or a single layer twice as thick). The principle here is that even if one layer fails, the user will still be protected by the second insulation layer, or in the case of double thickness, it is extremely unlikely to fail.

Fixed household appliances made of metal are almost always Class 1, protected by an Earth connection. Portable devices are often Class 2, but can be Class 2 with a functional Earth.

Earth Leakage

Every earthed device has some a.c. current passing along the Earth wire. Tiny capacitances between wires, or between live wires and the earthed enclosure, allow these a.c. currents to flow. Currents from these stray capacitances are usually tiny, but can be more significant where there are transformers with earthed winding screens, reducing EMC emissions but increasing leakage capacitance and Earth leakage current.

For an earthed device, this leakage current is of no consequence to the user, except for two scenarios: 1) where the leakage current is large enough to activate a Residual Current Device (RCD) and trips the household supply; and 2) if for any reason the Earth connection becomes disconnected. In the second case, the leakage current cannot flow to ground through the Earth wire, but lies in wait for a user to touch the metal case and provide a path by causing a (usually) small electric shock.

Since the introduction of EMC legislation, designers face a difficult challenge: to balance the requirement to reduce EMC emissions, (often using Y-capacitors which increase earth leakage currents), and to maintain low leakage current levels which are safe and legal.

Earth Leakage in Class 2 devices

With no Earth wire to connect a Y-capacitor to, designers of Class 2 devices are often left with no option but to connect the capacitor to one arm of the rectified a.c. supply. While the rectified supply has a d.c. voltage across it, with respect to Earth it has half-wave rectified a.c. mains voltage. This causes an a.c. current to flow through the Y-capacitor, providing a leakage current path directly from the a.c. mains to the circuit output.

This is the cause of the shock you feel from the output of a 2-wire connected switched mode power supply, particularly if you also touch an earthed connection. I often feel a shock as I go to plug a video cable from my (Class 2) set-top-box into my (Class 1) TV. It's enough to make me jump, but not enough to cause a problem.

Safe Limits

In most countries, the acceptable limit for earth leakage current is 3.5mA, which is enough for a healthy person to feel but not enough to cause any injury. For medical devices, which can come into contact with very vulnerable people, the limit is 100uA.

Different Devices, Different Solutions

I suspect your Sony charger has a 2-pin figure-8 mains cord (a Class 2 IEC320 connector), which makes your charger, and any connected laptop, a Class 2 device. My Dell charger has a 3-pin cloverleaf connector, and includes an Earth connection which carries away any leakage current. This is probably why I have never had a shock of any kind from anything connected to my Dell.

Conclusion

A Class 2 device with a high (but legal) earth leakage current may give you enough of a tingle to make your eyes go wide, but not enough to cause you any harm, so long as you are not critically ill and require intensive care.

If you are still worried about it, get an electrician to test the earth leakage current and make sure it's safely below 3.5mA.

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The schematic here shows a laptop charger. The Y-capacitor causing most of the leakage current is shown as C11. The X-capacitor is C1. –  Billysugger Jun 29 '13 at 17:12

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