The iPhone chargers do use a transformer which isolates the 120 VAC input voltage from the 5 VDC output voltage and its lower 1 Amp current. The older chargers use a 120vac to 12vac transformer, a bridge rectifier to convert the 12vac to 12vdc, then use a 5v voltage regulator is used to limit the voltage to 5VDC. The isolation of the transformer prevents the shock hazard of house line voltage getting into the output.
However, the iPhone is quiet sophisticated. The iPhone power adapter is a switching power supply, where the input power is switched on and off about 70,000 times a second in order to get the exact output voltage required. Because of their design, switching power supplies are generally compact and efficient and generate little waste heat compared to simpler linear power supplies.
In more detail, the AC line power is first converted to high voltage DC by a diode bridge. The DC is switched on and off by a transistor controlled by a power supply controller IC. The chopped DC is fed into a flyback transformer which converts it into low voltage AC. The flyback transformer also isolates the chopped DC from the low voltage AC. This prevents the shock hazard of house line voltage getting into the output. Finally, this AC is converted into DC which is filtered to obtain smooth power free of interference, and this power is output through the USB jack. A feedback circuit measures the output voltage and sends a signal to the controller IC, which adjusts the switching frequency to obtain the desired voltage.
It is the knock-off counterfeit chargers that do the voltage drop with transformerless circuits using resisters, capacitors and diodes that are the problem. It is a circuit that is basically a voltage divider that uses a capacitor instead of a resistor to takes the 120 VAC from your wall and divides it down to whatever voltage you want. They may be cheap to build, but pose a shock hazard if a line voltage surge causes a critical capacitor to develop a short. Also, the wire that serves as the circuit’s ground reference is the same as your home’s neutral line (in contrast with the “hot” line). Now imagine mistakenly putting the plug in backwards. Ground is hot, and although the device works just fine because AC is symmetric, it becomes an electrocution hazard if you can come in contact with “ground” in a wet bath tub.
I think you nailed it. To get a burn mark it had to be 110V. The wall charger was not isolated. A non isolated charger is dangerous.
“It is the knock-off counterfeit chargers that do the voltage drop with transformerless circuits using resisters, capacitors and diodes that are the problem. It is a circuit that is basically a voltage divider that uses a capacitor instead of a resistor to takes the 120 VAC from your wall and divides it down to whatever voltage you want. They may be cheap to build, but pose a shock hazard if a line voltage surge causes a critical capacitor to develop a short.”
I don’t buy this part. It would not be cheaper to build, and it would have to be huge. At 60 HZ, it would take a 20uf capacitor to pass an amp of current. 22uf is the size of a run cap on a small blower motor, and there about $10 each.
Resistors would be more expensive and would have to dissipate about 100 watts or so.
“A feedback circuit measures the output voltage and sends a signal to the controller IC, which adjusts the switching frequency to obtain the desired voltage.”
No.
It controls the “on” time of the switching transistor. Longer on time means that more charge goes into the capacitors on the output side, thus higher voltage. Conversely, shorter on time means less energy and therefor lower voltage at the output. The frequency stays the same. Think Pulse Width Modulated circuit.
But none of this has a damn thing to do with the subject. It’s just a couple of techie guys swingin’ our dicks around.