Electronic – Laptop charger leaks AC while giving DC voltage at the same time. How


So, I got some laptop charger from my neighbor and I was thinking about to do something with it. Then one day I feel tiny shock from the output of the charger. I thought ac to dc filtering mechanism maybe shorted. Then I had tested other remaining charger to see if there was any AC current going on. And they all do the same. Is it normal to have flow some AC through DC ? Can I do something about it ? make it safer for work ? It does not feel like comfortable.

Best Answer

Is it normal to have flow some AC through DC

As surprising as this may sound, the answer is yes.

Most laptop chargers don't have an earth connection and to prevent (reduce really) the amount of high frequency switching noise on their DC output they use a Y capacitor connected from the DC output to the nearest thing to earth that can be found; that being the rectified DC bus on the high voltage side. See this for an example: -

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The size of the capacitor is usually about 1 or 2 nF and this inevitably can couple a proportion of the AC mains voltage to the DC output. So, which is worse? A noisy DC output full of high-frequency switching artefacts that can radiate or a little bit of AC voltage that is sourced through a 2 nF capacitor and causes you to feel a tingle?

In case you didn't understand the bit about high frequency noise, you just have to look at the culprit - the high-frequency power transformer - it has an inter-capacitance winding of about 100 pF and, the high-speed high-voltage switching on the primary inevitably superimposes some noise on the DC output. It's common-mode noise so it doesn't alter the voltage between the positive and negative DC outputs; it raises both of them up and down by several volts and can interfere with radios and needs quenching, hence the added capacitor (in the red box above).

Can I do something about it ?

You could find a good earth point and use capacitors from each DC output to the earth point - 100 nF would kill off the AC to sub 1 volt levels.

make it safer for work ?

Y capacitors should be used in the charger and these are deemed safe however, devices from unrecognized sources are not to be trusted.

A few words from here: -

If a Class-Y capacitor, also known as the "line to ground capacitor" or "the line bypass capacitor"—the capacitor placed between line and ground—fails short, this could lead to a fatal electric shock due to the loss of the ground connection. Class-Y safety capacitors are designed to fail open. A failure will cause your electronic device to be subjected to the noise and interference that the capacitor would normally filter out, but at least there will be no fatal electric shock hazard.