Courtesy of All About Circuits
USB has been around for the better part of 20 years and is now found on nearly every device that can connect to a modern computer. Despite its age and integration, it’s amazing that until recently, USB devices could only be powered or charged with either a computer USB port or a dedicated USB charger. Well, now you can buy wall sockets that have dedicated USB power ports so devices can be charged efficiently and not take up a socket space (which to people like myself is very important).
In this teardown, we’ll rip apart a twin USB wall socket to see how they’re made and how they function.
A quick note:
The items in this teardown were destroyed intentionally (except the screws) as these are mains powered devices and cannot be legally used once opened unless approved via a PAT test by a competent electrician (see BS7671 17th edition wiring regulations).
The Face of Things
The USB wall socket comes with two USB charge points at 2.1A each, a single socket outlet (this is a UK socket), a switch (for safe isolation), and two screw holes. UK sockets do have advantages over other socket types primarily due to their earth prong (the top hole of the three holes). The live and neutral conductors contain a flap that cannot be opened unless an earth prong is inserted which protects people from getting access to live parts (such as children armed with forks). The downside to the British system is the mains voltage which is 230V rms (whereas US is 110V), which makes it more dangerous (hence the circuit protective conductors).
The face of the socket
The back of the socket shows the three connections needed (Live – Brown, Neutral – Blue, and Earth – Green / Yellow). Interestingly, the screw connections are also connected to earth as they are considered exposed live parts. They are considered exposed live parts not because they are live but they are conductive parts that are in proximity to live wiring that under fault conditions could make contact. Therefore, if they make contact with live and are connected to earth, the RCD at the distribution board will detect the imbalance of current in the live / neutral wire pair and disconnect the supply.
The back of the socket in all its electrical beauty
Face – Off
Removing the metal earth bar across the screw fixings reveals the internal components in all their glory. A yellow transformer can be seen (most likely for bringing down the mains voltage for use with USB charging), as well as a large metal bus bar that connects the live pin to the switch. It’s vital that the live conductor of a system is the fused and switched line so that when they are triggered, the live conductor is made dead and safe. Remember that, in theory, the neutral conductor has a potential equal to earth so an electric shock from touching the neutral wire is unlikely.
The innards of the USB wall socket
Here is a close-up of the three prongs, the bus bar and switch housing (top left).
A well-made electrical outlet!
The Whole Circuit
Here we can see the whole circuit setup showing the USB socket PCB that houses two USB Type-A sockets and the power supply circuit shown on the right with a transformer and large electrolytic capacitors.
The complete circuit
The underside of the circuits show a heavy reliance on surface mount components. This is most likely due to the strict standards of the British Standards Institute which define the size and shape of socket outlets (in other words, not much room to play with!). Any device that does not meet the BSI standards is considered a deviation and therefore must be checked by a competent person (which individuals may not want to do).
The back of the circuits
The Power Circuit
The power circuit uses a few large components on the top of the PCB and many small surface mount components on the underside of the PCB. The transformer has the designation 58-000006-000 but this does not return any results using online datasheet search engines. But by understanding the application of the circuit, we can assume that it is a stepdown transformer that supplies a voltage close to 5V (possible between 7V and 9V), as well as providing isolation from the mains voltage.
The power circuit from the front
Here we can see a power MOSFET with the designation CL4N65D. While this does not return any results on databases, the part number 4N65D returns details about an N-channel enhanced mode power MOSFET. C may refer to china (origin of manufacture) and L an internal code to that factory.
The mystery MOSFET
The underside of the power circuit PCB shows many components which are likely to handle the conversion of the transformer output into the 5V @ 2.1A USB output. As there are no visible ICs on the top of the PCB (excluding the MOSFET), and that the bottom side components are surface mount it can be assumed that this circuit uses a switch mode power supply to provide USB power.
Side shot of the power PCB
Top view of the whole PCB
The close-up image here shows many resistors, capacitors and even a view ICs. Unfortunately, the ICs numbers were either impossible to read or non-existent, however, the IC on the bottom right of the image is a rectifier with the giveaway being the and – pin identifications as well as having four terminals.
The and – terminals on the IC (bottom left) show it’s a rectifier
The underside of the USB socket PCB shows an interesting configuration which uses an IC for each socket. These ICs are most likely Zener diodes that keep the D and D- terminals at a specific voltage to prevent damage to any connected USB device if a surge should occur. The power terminals for the USB sockets also show a large amounting of tinning which is to increase the current capability of the trace. A 2.1A trace would be rather large and with such little space available, tinning the trace is a viable alternative.
The USB socket PCB
The USB wall socket shows how technology is becoming ever more integrated as well as the inclusion of low power requirements directly into high voltage mains. Electronic devices always find a way of getting into everything, even a socket design that has remained untouched for the last 70 years!
One things for sure, when I move house next year I will be replacing my wall sockets with these USB sockets!