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I myself use chargers all the time, of course. But I am not sure why am I not dead yet.

Let me state the obvious: when we use a charger we are connecting a conductive cable to an electric socket which has a pretty high voltage; in Europe the standard for homes is $220$ Volts. If you stop and ponder this situation, from an abstract point of view, it seems very unsafe.

But in practice I know that electric plugs are specifically designed to mitigate the risks; for example Tom Scott made a brilliant video about the safety features of British plugs. But what about the other end of the cable?

There are a lot of different types of ports for chargers: USB, USB-C, ecc. And people, myself included, touch the port, the metal conductive end of the cable by mistake all the time, while the charger is connected to the electric socket! Why am I not dead? What are the safety feature in place to ensure we don't electrocute ourselves? And how do these safety feature work?

Remember: we are touching a piece of metal connected directly to an high voltage, high enough to produce electric arcs, so the fact that the manufacturer felt safe leaving the conductive end exposed baffles me. What is going on here?


Notice that I am not interested in the specific way a charger is made, I am interested in comprehend the physics behind chargers and how can it be that electricity doesn't flow through me when I touch the end of the charger, from a physical point of view.

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    $\begingroup$ Remember: we are touching a piece of metal connected directly to an high voltage - How is that? You are touching the metal connected to a low voltage which is the output of the circuitry stepping it down $\endgroup$
    – Eugene Sh.
    Commented Dec 18, 2020 at 18:07
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    $\begingroup$ See en.wikipedia.org/wiki/Galvanic_isolation. $\endgroup$
    – gandalf61
    Commented Dec 18, 2020 at 19:02

4 Answers 4

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Between the plug and the USB-side of the cable there is the "box" of the charger which contains a transformer i.e. an electrical component which converts the (e.g.) 220V from the socket to a much less dangerous 5V (usually).

So, whatever happens, never try do see what is inside the "box" element of the charger while it is connected !

Of course, in case of shortcuts, faulty parts etc. both sides of a charge can become dangerous, but it usually the socket-side which causes the issue.

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  • $\begingroup$ that transformer electrically isolates the mains voltage from the (lower) output voltage, so that it is impossible for you to get hit with the mains voltage by touching either terminal of the output. $\endgroup$ Commented Dec 18, 2020 at 18:16
  • $\begingroup$ The transformer most likely does not convert 220V to whatever, else it would be much larger! $\endgroup$ Commented Jan 11, 2021 at 16:03
  • $\begingroup$ The input is 220V and the output is 5V so.. there is a transformer in there. It operates at high frequency and thus can be smaller but the point that it physically separates the high voltage input to the low voltage (safe) one is still valid I guess $\endgroup$
    – JalfredP
    Commented Jan 12, 2021 at 20:14
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A typical charger looks like this (the part marked with a red frame):

enter image description here
(image from ElectroSchematics - simple 12 volt charger

The functional parts of this charger are

  1. a transformer (transforming from 230 V AC down to 12 V AC)
  2. a bridge rectifier (rectifying 12 V AC to 12 V DC)
  3. a capacitor (for converting pulsed DC to true DC)
  4. an LED (acting as a status display)

The important part for safety is the transformer. You see, the dangerous high voltage (230 V) is not delivered by the charger. Only the harmless low voltage (12 V) comes out of the charger, and you can touch this without risk.

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  • $\begingroup$ That's certainly not how modern transformers for electronic devices work! They drive the transformer with much higher frequency, so they involve electronic on the high-voltage side! $\endgroup$ Commented Jan 11, 2021 at 16:04
  • $\begingroup$ @NorbertSchuch Any modern household device will not come with a transformer at all. Switching regulators do the job in 99.9% of the applications. $\endgroup$
    – DarioP
    Commented Jan 11, 2021 at 20:16
  • $\begingroup$ @DarioP "Switching regulators do the job" -- Sure. But what do switching regulators contain? They must have some way of galvanic decoupling (unless there is no chance of touching anything on the secondary side). A quick search suggests that it is either done through a very small transformer, driven at a very high frequency, or capacitively (though the latter is apparently not permitted everywhere). You have to cut off the current between the two sides somehow. $\endgroup$ Commented Jan 11, 2021 at 20:30
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For sure, your phone would also die if you connected it directly to $220$ Volts; but you don't. Between the socket and the wire, there is usually an electric transformer (the small boxy thing) which decreases the voltage to a much safer value (this value is sometimes even written on the charger, take a look)

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Chargers usually have a label that shows their input voltage and output voltage (and maximum output current). For a smartphone charger, which typically ends in a USB connection, which means an output of $5 V$. Not a big voltage. The circuitry inside a charger rectifies the alternated current from your power socket (because smartphones and other electronic works with direct current) and reduces its voltage.

To complement the answer, what kills you is not the high voltage. It is the current supplied by the source. And modern chargers can output some decent amount of current ( $> 1A$). According to Wikipedia values as small as $30 mA$ passing through your chest can kill you because it messes with your heart.

So, one may wonder why these do not kill people. The answer is that this is the maximum amount of current it can supply. The exact amount that will be supplied will depend on the resistance in the circuit. So, if you close the circuit with your body, that would its resistance. Now, the resistance of the human body skin can be as high as $100 kΩ$. This gives you a current of only $0.05 mA$. That is not enough to kill you. And that is the reason high voltages are considered dangerous. If it was $5000 V$, the current would be $50 mV$.

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  • $\begingroup$ This is beyond the point of the question. It is the potential relative to ground, not the output voltage, which potentially matters. $\endgroup$ Commented Jan 11, 2021 at 16:05
  • $\begingroup$ You are right that relative potentials are what matter. However, the output voltage listed in the output of chargers are already relative to the ground. Besides, you missed my point about currents. You may be subject of a potential difference of thousands of volts, but the circuit is able to discharge only a tiny current. This will not kill you. This, by the way, is what happens on plasma balls. $\endgroup$
    – WilhelmM
    Commented Feb 5, 2021 at 13:16

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