Isolated laptop power supply issues

I have earlier written about grounded laptop power supply problems. Other popular laptop power supply configuration used is floating output. There are power supplies without ground, for example some Apple laptops and many other brands. The power supplies where there is no ground on connection on the mains side are for sure built in this way. If your power supply mains side is grounded, then is possible power supply that has the output floating or grounded (measure to be sure what you have).


The galvanic isolation from mains power (live and neutral wires) is required for user safety and isolates the charger output from earth ground. Commercially approved laptop adapters that are supplied with 2-core mains flex have enhanced insulation, creepage distances and filtering that obviates the need for the earth.

This kind of isolated power supply avoids ground loop problems. Could easily think that ideally the laptop is intended to be floating, this means it has no connection to ground. Lack of ground has it’s benefits, but can also cause a whole different problems because there still exists capacitive coupling between the primary and secondary through the transformer.

Electric noise on audio recording article tells that ideally the laptop is intended to be floating, this means it has no connection to ground. In practice is pretty impossible to make a power supply that is completely floating, there is always more or less capacitance between mains side and low voltage side. It’s very common for power supply manufacturers to place a capacitance between the Mains side and ELV (Extra Low Voltage) side of the power adapter to meet Radiated Emissions requirements (EMC). This causes leakage current that can cause problems.

When connecting the laptop to peripheral equipment (such as a microphone, or amplifier output) some noise is introduced because the floating laptop ground being pulled down to real ground potential causes a voltage to be induced across the capacitor. This in effect causes the whole laptop to bounce up and down at 50/60Hz (the bounce can be from few volts to over 100V). How much the laptop potential potential varies depending on the power supply design. In some cases the way the power supply is plugged to wall can have considerable effect on the bounce (the Y capacitor can be wired between output and one of the mains connector pins, if that pin where the capacitor is connected to live you get largest leakage and of that gets connected to neutral you get almost no leakage).

When connecting a laptop with ungrounded power supply to some grounded equipment (PA mixer, amplifier, industrial equipment, etc.), damage to electronics is also possible if they are not well protected. I have personally damaged serial ports and audio connections due lack of grounding (that 100V charged to filter capacitor can be enough to kill not so well protected interfaces when signal wire gets connected before ground). Safest way to do the connection is to keep the laptop battery powered, do the connection and then add mains power supply last if you need that.


Dell laptops in electric shock shocker discovered a worrying new feature in some Dell laptops: if you touch them, you may get an electric shock. This discharge can vary in strength from a gentle tingle to a sudden jolt. Disturbingly, you could also be shocked when connecting printers, PDAs and other peripherals to the offending laptops. Dell’s Shocking Surprise: Laptops That Give You A Taste Of the Electric Chair article tells that Dell’s forums are full with users complaining that they have had shocking experiences while using certain notebook models. The pattern is the same: every Dell laptop with this problem that has a brushed-aluminum finish is likely to give you a taste of the electric chair. The reported models were XPS M1330 and XPS M1530, and their problem seems to be caused by the two-pronged connection between the main lead and the power adapter, which is not grounded because there is no available third pin. User’s don’t like to touch a thing that gives me tingling problem every day, every second. It is just not pleasant to feel that sensation. In can even be scary to some users.

The tingling sensation you are feeling, while not dangerous, can be disconcerting and is caused by the power supply even though the power supply is operating perfectly well. Inside the power supply the transformer which gives the low voltage output has a capacitive link back to the mains. This allows a very tiny current (generally less than 1mA) to flow through to the “ground” on the output. The voltage on the “ground” can be up to 100V or so based on my own measurements and XPS 1530. Scary power leakage to humans? discussion. If the entire system is properly grounded you should not experience any electrical shocks at all and there would be no voltage difference.

Designing low leakage current power supplies article tells that majority of AC/DC power supplies provide isolation from the high-voltage AC input to the low-voltage DC outputs. Safety standards specify both the strength of the isolation barrier. The maximum leakage current allowed to flow is based on the specific classification of the application.

In many applications a low leakage design is desirable. For example, when the laptop has metal parts that user touches under normal operation, chargers often must meet leakage-current specifications
set by the manufacturer that are below those specified by the applicable safety standard. This is to prevent users from feeling the “touch current” when they hold the computer when it is connected to mains power supply.

One of the biggest contributors to leakage current in AC/DC switching power supplies is the Y-capacitor—a safety agency rated capacitor that can be used to bridge the isolation barrier. It is used to return displacement currents (generated by the switching process) back to their source, preventing EMI. In general, the larger the value of the Y-capacitor, the lower the magnitude of the EMI that the supply generates and the higher will be the leakage current that flows across the isolation barrier.

For a two-wire (without a protective earth connection), universal input power supply with a floating output, rounding down to the next standard capacitor value gives a maximum Y capacitance of approximately 2.2nF. In information technology equipment (IEC60950) the allowed leakage to accessible parts not connected to protective ground is 0.25mA.


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  6. Tim Starling says:

    I was testing a new USB oscilloscope connected to an old laptop, and the exposed metal case of the scope gave me a small shock. Touching the scope probe to my finger or any large piece of metal showed a 50Hz signal of about ~20V P-P amplitude (with 10 MOhm probe impedance). Would connecting the USB ground to the earth of the AC mains be a reasonable action to take, to reduce interference and avoid damage to ESD-sensitive components?

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  15. ram says:

    is there always a leakage current flowing through all the sockets of motherboard (usb etc) .. while charging laptop(220 v).

    • tomi says:

      If you are using an isolated power supply, there is always some leakage current flowing through.

      If you have a grounded power supply connected to grounded outlet, with some designs you get rid of the leakage but get other problems.

  16. Tomi Engdahl says:

    Solving Laptop Noise and Ground Loop Audio Interference

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  23. Ilya says:

    There is a real problem with my new MacBook Pro – touch current. I’m going to tear-off that capacitor. Don’t care about EMI specs.

  24. Tomi Engdahl says:

    Fixing Faulty But Genuine Apple Power Adapters

    Being a miracle of modern manufacturing doesn’t mean Apple gets it right all the time; the UK adapter doesn’t use the ground plug, leading to the power supplies singing at 50 Hz when plugged in. [Gareth] had had enough of the poor design of his charger and decided to fix it.

    Solving 50Hz buzz/vibration/shock on Macbooks.

    Apple has definitely dropped the ball, almost everyone I know with a Macbook complains about the “buzzing” or “vibration” and even the occasional “pinching” or electric shock. I’m not sure how this happened, considering how well built and thought out their products are in general.

    Apple AC/DC adapters are meant to be internally earthed (isolated/floating output), but they also come with an earth connection and in many of the interchangeable plugs that come with Macbooks this is connected and there’s no buzzing issue.

    I verified what this was by using a multimeter to measure the frequency from the case to the ground, and it is indeed 50Hz. While this is not necessarily due to a lack of earthing per se (in theory these power supplies don’t need to be earthed), if the adapter is earthed the problem disappears, which is better than being shocked all the time.

    You could play it safe by always using the longer power cable

    This mod will ground the Macbook, and current will no longer use your body for grounding.

  25. Tomi Engdahl says:

    Sujit Gupta this looks isolated, but it still has capacitor between mains side low voltage side.
    “To suppress the high frequency common mode is is necessary 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.”
    “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”
    “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).”


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