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Recently I witnessed an external IBM laptor power supply that looked like a usual switched-mode power brick (rather small and lightweight for it more than 50 watts power) in plastic case but had a three-wires cable (phase+neutral+ground) between itself and the mains.

Seeing a three-wires cable used with a plastic case switched-mode supply is rather uncommon. Usually either the case is of metal and the cable is with three wires, or the case if of plastic and the cable is with two wires.

Looks like switched-mode power supplies do have galvanic separation. Also the unit had insulating plastic case, so it's impossible that a mains phase wire induces voltage onto the outer surface of the case should there be any kind of short.

What's the reason for a grounded cable in a switched mode power supply with an insulated plastic case?

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Below is a typical schematic of an AC/DC power supply EMI filter.

enter image description here

You can see that the X-capacitors (between line and neutral) plus the leakage inductance of the common-mode inductor give the differential-noise rejection, and the CM choke inductance combined with the Y-capacitors give the common-mode noise rejection.

I would also not be surprised if the output return is directly connected to earth.

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  • \$\begingroup\$ Usually I've seen the capacitors to ground on the line side of the balun, not the device side, but this still illustrates why the ground line is needed. I don't see a point to the input resistor though. \$\endgroup\$ – Olin Lathrop Nov 9 '11 at 22:41
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    \$\begingroup\$ Resistor is probably to discharge the 3 parallel capacitors if the unit is unplugged at Vpeak or near it. Depending on electronics or load involved it may be possible to have no load so caps stay charged. Touch input pins sand get a moderately unpleasant shock. I've actually seen (and felt) equipment where this happened. One example was a torch that charged offline using a series capacor transformerless power supply. Touch the mais pins soon after charging and ... !!! \$\endgroup\$ – Russell McMahon Nov 10 '11 at 2:26
  • \$\begingroup\$ Great, but is that EMI stuff necessary? I've seen may power bricks of more or less the same power and most of them accept a two-wired cable. \$\endgroup\$ – sharptooth Nov 10 '11 at 11:45
  • \$\begingroup\$ IIRC, US and Canadian safety standards dictate that any capacitors across the line have to discharge within 30 seconds of removal of the AC. The resistor provides this path. \$\endgroup\$ – Adam Lawrence Nov 10 '11 at 15:02
  • \$\begingroup\$ While this explanation is true for some devices I don't think it fits the OP's situation (see my answer) \$\endgroup\$ – Peter Green Jan 14 '16 at 12:05
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Switched mode power supplies use what is known as a "flyback converter" to provide voltage conversion and galvanic isolation. A core component of this converter is a high frequency transformer.

Practical transformers have some stray capacitance between primary and secondary windings. This capacitance interacts with the switching operation of the converter. If there is no other connection between input and output this will result in a high frequency voltage between the output and input.

This is really bad from an EMC perspective. The cables from the power brick are now essentially acting as an antenna transmitting the high frequency generated by the switching process.

To suppress the high frequency common mode is is nessacery to put capacitors between the input and output side of the power supply with a capacitance substantially higher than the capacitance in the flyback transformer. This effectively shorts out the high frequency and prevents it escaping from the device.

When desinging a class 2 (unearthed) PSU we have no choice but to connect these capacitors to the input "live" and/or "neutral". Since most of the world doesn't enforce polarity on unearthed sockets we have to assume that either or both of the "live" and "neutral" terminals may be at a sinificant voltage relative to earth and we usually end up with a symmetrical design as a "least bad option". That is why if you measure the output of a class 2 PSU relative to mains earth with a high impedance meter you will usually see around half the mains voltage.

That means on a class 2 PSU we have a difficult tradeoff between safety and EMC. Making the capacitors bigger improves EMC but also results in higher "touch current" (the current that will flow through someone or something who touches the output of the PSU and mains earth). This tradeoff becomes more problematic as the PSU gets bigger (and hence the stray capacitance in the transformer gets bigger).

On a class 1 (earthed) PSU we can use the mains earth as a barrier between input and output either by connecting the output to mains earth (as is common in desktop PSUs) or by using two capacitors, one from the output to mains earth and one from mains earth to the input (this is what most laptop power bricks do). This avoids the touch current problem while still providing a high frequency path to control EMC.

So why are laptop PSUs from major repuatable vendors class 1 nowadays when they didn't used to be? (and when cheap crap often still isn't) I don't know for sure but I expect it's a combination of.

  1. Even touch currents below the legal limits can be problematic. Some people are unusually sensitive to electricity and can feel currents below the legal limit. Some electronics can also be damaged by currents below the legal touch current limit during hotplugging.
  2. EMC regulations have got tighter over the years.
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Without a schematic it's hard to tell. However, the ground lead is most likely used by the EMI filter. Most likely there is a balun (common mode choke) on the power input before it goes to the rest of the circuit. This will raise the impedance of common mode signals, but that by itself won't attenuate them without some kind of load. That load will be a capacitor to ground on each of the two power leads on the outboard side of the balun.

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  • \$\begingroup\$ Whoever downvoted this, please explain what you think is wrong. I read this over and still think it is at least one possible valid explanation. \$\endgroup\$ – Olin Lathrop Jan 14 '16 at 22:00
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Have you ever had a "nip" when touching the low voltage output of a modern power pack?
This is annoying and is potentially equipment destroying.
The reason is that the system described in the question has been implemented but not properly used,

Madmanguram's diagram and comment should be noted.

Madmanguram has provided an excellent illustration.
Note the comment re output return also being grounded. This IS sometimes done and, when it is, is an utter disaster when the ground lead is not grounded eg a 2 wire cord is used.

Local ground = capacitor centre tap is now at half mains wrt true ground. ie about 115 V on a 230VAC system. The whole supplied equipment floats at half mains above ground. The two caps are typically 0.001 uF each so impedance is as of 2 caps in parallel .
Z~= 2/(2.Pi.f.c) or about 5 megohm giving leakage currents of about 10 to 20 uA. This does not sound like much but produces annoying "bites" on fingers etc when touching Vout while ones body is grounded - due to voltage level - and happily charges up stray capacitance to have enough energy to blow things up - which definitely does happen.

The solution is to ground the ground lead.. BUT

Worst is when manufacturers connect centre tap to output negative and then do not make allowance for using a ground conductor. You get half mains floated equipment and no easy way to fix it. A nasty result which needs running or using a ground connection outside the power cord.

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Yes, the power adapter is fully isolated, but the device that is powered by it may have exposed conducting parts, that may carry a dangerous voltage in case of malfunction. Or, may carry a low but annoying voltage due to normal leakage currents. Galvanic isolation cannot completely avoid capacitive leakage currents.

(Actually it can, with a grounded screen between the windings, e.g. for surgical devices, but obviously this needs the ground wire.)

I don't understand why other answers pay so much attention to the inner workings of the switched mode power adapter. Evidently, every design features galvanic isolation. Before, a 50 Hz (US: 60 Hz) two-winding transformer. Nowadays the transformer works at a much higher frequency, and is accordingly smaller and ligher, but that is not the point.

Note that the ground lead is just an optional thing. It only does any good if using a grounded wall outlet. It does nothing on an ungrounded wall outlet. Ungrounded wall outlets should only be used where you won't be instantly killed when touching live voltage, such as a living room with a wooden floor instead of a concrete floor. But nowadays I am seeing grounded outlets virtually everywhere.

Note also that the outlet earth may not completely eliminate the annoying small voltage on your device. That ground is designed for safety, to blow the fuse before you get electrocuted, but not for guaranteeing zero volt. The ground wire resistance, and also inductance, may still be significant. For example, I often experienced 'tickling' voltages when handling the VGA cable on the 17 inch CRT monitors, even on a grounded outlet, probably because of the capacitive leakage from the internal 10.000 volt for the tube. (17 inch? those monitors were so big, expensive and heavy. Now we have cheap lightweight 23 inch, 27 inch, UHD, .... )

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  • \$\begingroup\$ You are most certainly not experiencing the high voltage (10kV or more) acceleration voltage from the tube leaking back through the VGA pins. That would destroy the monitor in short order. You've got some kind of leakage from line voltage to the grounding on the shell of the VGA cable going on. \$\endgroup\$ – JRE 2 days ago
  • \$\begingroup\$ @JRE If you feel any 'tickling', it will certainly not be 'low' voltage of 5 V or so. Thousants of volt is more likely. Easy to get by walking over a carpet. In the CRT, there are high voltages, and capacitive coupling is everywhere, so ... Yes, static electricity may damage electronics, if not properly designed. But here the question is the purpose of the ground wire, and my point is that the ground wire does not always protect against 'tickling' voltages. In fact, the jargon usually is 'grounding' for safety, and 'bonding' for getting rid of static electricity. \$\endgroup\$ – Roland 2 days ago
  • \$\begingroup\$ I didn't say it was from 5V. I said it was leakage from the line voltage (120VAC or 240VAC depending on your location.) \$\endgroup\$ – JRE 2 days ago
  • \$\begingroup\$ If you touch the screen of a CRT and hear and feel a slight "zap" then that could be from the acceleration voltage. There should never be a path inside the monitor that will lead to the acceleration voltage being on any externally accessible conductor. \$\endgroup\$ – JRE 2 days ago
  • \$\begingroup\$ @JRE There was definitely an annoying voltage on the VGA connector of several A-brand (like Sony) CRT monitors. Indeed, "should" not be, but it apparently is hard to avoid any capacitive coupling, especially if high voltages are around. I believe that annoying voltage is otherwise harmless; not enough energy to kill me, and I never saw damage occur to the PC or monitor. Conclusion can only be that using grounded cables and outlets for PC and monitor do not always eliminate annoying voltages of pretty high values. \$\endgroup\$ – Roland yesterday

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