I am curious if anyone has taken one apart. I am wondering what is the typical construction these days? Is it a cheap inline resistor to drop the 12V down to 5V, or, do they use DC-DC converters? I have an old one that I paid $10 for from Walmart 2 years ago. It gets warm when a phone is plugged in. I leave it plugged in inside of the center console all the time. I am just curious if it is drawing power other than to power the little red LED, when no load is plugged in.
It can't be just a resistor, it has to be have some sort of voltage regulator inside. A typical series voltage regulator IC costs several cents. It should have a couple capacitors with it, but the lowest cost producers might leave them out. The IC will draw some bias current all the time, but the amount varies greatly depending on the design. The cheapest chargers will always be series regulator style, not DC-DC style, and will get much warmer.
Are you talking about a zener diode regulator like this? This makes sense. The reversed bias current is probably pretty small, if R1 is large enough, in steady state with no load. Thanks for the reply and I learned something new, I am an EE but have no idea how these things are commercially made.
^^ That is the right concept, but a cheap 3-terminal regulator IC can do it much better, just as cheaply, with fewer joints to solder. Effectively, the IC is in the same package as the transistor, with the zener and resistors synthesized by additional transistors and incorporated on the same die. The actual schematic is more complex than shown here, and performs better, but essentially costs the same because the die size is what drives the real cost.
Cheap inline resistor, ha. USB power is a whole lot more complex than that, and also more complex than a 3-terminal IC regulator, although some chargers are simpler. See the wikipedia definition. A "charging port" might supply up to 500 mA (there's a new charging standard for mobile phones); but a true computer USB port may only allow as little as 100 mA unless the attached device requests (via a digital interface) more power (this can be why you get slower charging from some USB ports). Note that in my Gen-3, the 12V power outlets shut down when the car is off. I don't use them for charging, but I have charged my phone off the USB connector (ipod connector) for the entertainment system.
Yes, a proper USB power port will be considerably more complex. But I don't expect the cheapest charger or power adapters to conform to USB standards.
No, at least not the full USB standard, but maybe at least the charging standard, though since this is a recent standard you can't even count on that. I'm sure cheap chargers are considerably simpler, and may not be very power-efficient either (resulting in the heat the first poster mentioned), but they should be at least complex enough to regulate the voltage properly, which a simple resistor (or voltage divider resistors) will never do. But the complex standard does mean you won't get the same charging current in every case.
The typical construction is a step-down DC/DC converter with output current limitation. It is today's most effective way, which didn't produce to much heat. I disassembled the lighter socket USB charger as shown below. It is 1A vesion, where USB pins 2-3 are shorted to instruct the connected device to use full charge current.
