Disassembly: Without built-in WiFi and Bluetooth, how can shared chargers be connected?

With the exponential growth of mobile phone users, the sharing economy is no longer limited to the shared power banks that have long been commonplace. As an extension of shared power banks, shared chargers are also rapidly popularized in major hotels and guesthouses where people can rest for a long time, in order to adapt to the growing number of mobile phone users and provide them with charging convenience.

Recently, the author disassembled a shared charger and summarized the technical structure and design differences between the shared charger and our personal mobile phone charger as follows.

Design Differences between Shared Chargers and Personal Chargers

Due to different application scenarios, the primary consideration for shared chargers, whether in terms of appearance design or hardware circuits, is how to design them to effectively prevent theft. Therefore, the difference between shared chargers and personal chargers can be well distinguished from the appearance of the device alone.

Taking the shared charger disassembled this time as an example, in general, mobile phone chargers mostly use a two-stage structure in which the charging head and charging cable are separated and connected via USB-A or USB-C. However, for anti-theft considerations, the shared charger places a very abrupt power control module in the middle of the charging cable to control the power output at the end of the charging cable. It also adopts an integrated design in which the charging head and charging cable are directly connected to reduce the risk of device theft.

From the perspective of hardware design, the output power of this shared charger is only 5W, and it does not support any fast charging protocol, nor does it have a protocol feedback control circuit. Therefore, the hardware structure is very different from the high-power chargers we often use.

Considering the single functional structure, this shared charger adopts a single-panel design, and cooperates with the only PSR power switch chip on the motherboard to control the output power of the charger.

The reason why this shared charger adopts the PSR power design is that the author believes that "it is mainly due to cost control considerations." The full name of the PSR circuit is Primary Switching Regulator, which means feedback control of the circuit on the primary side of the transformer. The advantage of this circuit is that it can save the use of some components such as voltage regulators and optocouplers, simplify the difficulty of circuit design, and reduce material costs.

Its working principle is to control the output current by adjusting the turns ratio of the primary and secondary sides of the transformer, and then control the output voltage through the feedback resistor connected to the PSR power switch chip.

Power control module that does not support networking but the password keeps changing

The power control module of the shared charger is equivalent to a switch with a password on the output cable. The internal circuit consists of three parts: the main control chip, the MOS tube, and the control button. When the password and the password preset by the main control chip are entered, the main control chip will send a trigger signal to the MOS tube to turn on the MOS tube, so that the power supply can smoothly charge the mobile phone through the power control module.

The unlock password of the charger needs to be obtained by scanning the code, so many people think that this is a process of interconnecting a mobile phone with a shared charger. In fact, this is not the case. Through disassembly, it was found that the power control module does not have built-in IoT chips such as Bluetooth and WiFi. The reason why the password can be obtained by scanning the code is because the power module adopts an offline password solution, which is similar to the previous ofo's first-generation mechanical password lock model solution.

The offline password solution is to upload the unlock password to the cloud server in advance before the device leaves the factory. Users only need to scan the QR code on the shared charger to interact with the cloud server, and call out the unlock password based on the device identification code hidden in the QR code for users to unlock and charge. In

order to reduce the probability of the password being cracked, in general, there will be dozens of sets of passwords stored in the main control chip of the power control module, and these dozens of passwords will be sorted and used in a cycle.

For example, assuming that 30 sets of passwords are preset in the main control chip, then every 30 charging cycles is a cycle. Among these 30 sets of passwords, only one set of passwords corresponding to each charging cycle can unlock the device, and the remaining 29 sets of passwords are invalid passwords.

When the user charges by scanning the QR code on the shared charger, the server will provide the user with a password that matches the charging cycle. If the order is over, the shared charger will automatically enter the next charging cycle. Only after 30 charging cycles will the previously used password be reactivated, which can largely prevent the user from using the previous password to unlock the charger after the order is completed.

Of course, such a password unlocking solution does not represent all, and some older versions of shared chargers are still using the traditional single fixed unlocking password.

Conclusion

In general, this shared charger is not very high in terms of design difficulty or material cost. It is just a simple power control module added to the traditional 5V/1A power adapter, and the device is scanned and unlocked through the cloud server.