Smart ring magnetic Pogo pin connector charging solution

As the smallest category of wearable devices, the design of the charging system of smart rings faces the triple challenges of space limit, power consumption control and user experience. Traditional contact charging is difficult to adapt due to problems such as difficult alignment and easy wear, while the magnetic Cnomax Pogo pin connector solution has become the optimal solution under current technical conditions with the characteristics of "miniaturized structure + contactless docking". The solution needs to achieve stable power transmission and data interaction on the ring body with a diameter of less than 20mm and a thickness of only 3-5mm. Its design accuracy must reach 0.01mm level, and every structural detail carries the art of balancing function and experience.

Structural layout design under space constraints

The ring structure of the smart ring determines that the charging system must adopt the "distributed integration" design concept. The solution decomposes the charging module into three core components: a 2pin Pogo pin mother seat embedded in the inner side of the ring, a charging base with magnetic positioning, and a flexible cable connected to the power adapter. This modular layout keeps the size of each component within a strict range - the overall thickness of the mother seat is only 0.8mm, and the diameter of a single Pogo pin contact is 0.6mm, which is equivalent to the width of two grains of rice side by side, but it has to carry a charging current of 5V/100mA.
The selection of the contact position needs to take into account both ergonomics and structural strength. After 300 wear tests, the contacts were finally set at 10 o'clock and 2 o'clock on the inside of the ring, avoiding the knuckle activity area. The two contacts are symmetrically distributed at 120°, which not only ensures the force balance during charging, but also uses the arc of the ring structure to achieve natural guidance. The mother seat shell is stamped with titanium alloy, and a 0.4mm diameter through hole is opened on the 0.15mm thick side wall to accommodate the Pogo pin shaft. Its edge is rounded by 0.05mm to avoid the feeling of foreign body when wearing.
The design of the charging base also reflects spatial wisdom. The circular base with a diameter of 18mm integrates two alignment contacts, three NdFeB magnets and a micro indicator light. The magnet is made of N52 high-performance NdFeB material. The volume of a single piece is only 1.2mm×1.2mm×0.5mm, but it can generate sufficient adsorption force. The three magnets are arranged in an equilateral triangle, forming a magnetic circuit closed loop with the three iron sheets on the inside of the ring, ensuring that the positioning accuracy during docking is controlled within the range of ±0.3mm. The bottom of the base is designed with a 0.3mm deep annular groove to accommodate the lead-out end of the flexible cable, so that the overall height is controlled at 2.5mm, which can be placed stably on the desktop.
Parameter optimization of magnetic positioning system
The design of the magnetic structure is a precision engineering that balances the adsorption force and separation force. Through finite element simulation analysis, it was finally determined that the magnet spacing was 5mm and the surface magnetic field strength was controlled between 1200-1500 Gauss. This parameter range can ensure automatic guidance during docking (self-correction can be achieved within the range of 3mm) and ensure that users can easily separate with one hand. The separation force test shows that the optimal value is 1.8-2.2 Newtons - equivalent to the strength of picking up two one-yuan coins, which will neither fall off accidentally nor cause difficulty in removal.
The arrangement of the magnets adopts a "bipolar alternation" design, with adjacent magnets having opposite polarities (N-S-N) to form a closed magnetic circuit. This design concentrates the magnetic field in the docking area, and the leakage magnetic flux on the surface of the base is controlled below 50 Gauss, which is far lower than the impact threshold (100 Gauss) of medical devices such as pacemakers in the IEC 62311 standard, ensuring the safety of implanted devices in the human body. To verify the actual effect, the Cnomax company test team used a magnetic field strength meter to detect at a distance of 3cm from the base. The magnetic field strength has decayed to below 10 Gauss, reaching the safety level.
The guarantee of positioning accuracy also depends on the mechanical stop structure. A 0.1mm high annular boss is designed around the contact of the charging base, which forms a precise match with the groove on the inside of the ring. This dual positioning mechanism of "magnetic attraction + mechanical limit" stabilizes the contact pressure of the contact at 0.5-0.8 Newton, which not only avoids the elastic fatigue of the Pogo pin caused by excessive pressure, but also ensures sufficient conductive area. Through the measurement of the micrometer, the repeatability of this structure can reach 0.02mm, ensuring the consistency of the contact resistance of the contact during each docking (fluctuation range < 50mΩ).

Microscopic design of the core components of the Pogo pin

As the core channel for energy transmission, the microscopic structure of the Pogo pin determines the reliability of the entire system. It adopts a "three-part" structure: a gold-plated beryllium copper needle shaft with a diameter of 0.3mm, a 304 stainless steel spring and a brass needle tube. The top of the needle shaft is designed to be hemispherical with a curvature radius of 0.15mm, forming a point contact with the flat contact of the charging base. This design can automatically remove the surface oxide layer and maintain low contact resistance. The spring is wound with 304 stainless steel wire with a wire diameter of 0.08mm, a total of 5 turns, a free length of 1.2mm, and a spring force of 0.6 Newton when the compression stroke is 0.5mm, ensuring that the needle shaft and the contact point fit tightly.
Contact surface treatment is the key to resisting corrosion and wear. The "multi-layer electroplating" process is adopted: the bottom layer is a 5μm nickel layer to enhance adhesion, the middle layer is a 3μm copper layer to improve conductivity, and the surface layer is a 0.8μm hard gold (99.9% gold) layer to improve wear resistance. This plating combination enables the contact to withstand 5000 plug-in cycles (equivalent to 13 years of use with one charge per day), and the contact resistance is always kept below 30mΩ. The salt spray test shows that after 48 hours of neutral salt spray test, there is no obvious corrosion on the contact surface, and the conductivity attenuation rate is < 5%.
The gap control between the needle tube and the needle shaft can be called a precise match in the microscopic world. The 0.01mm gap not only ensures smooth extension and retraction of the needle shaft, but also forms a labyrinth seal structure. Together with the fluororubber seal ring on the surface of the mother seat, the entire charging interface reaches the IP67 protection level. In the test, the ring was completely immersed in 1 meter deep water for 30 minutes and then taken out. The charging function was not affected at all, which is crucial for daily use scenarios that often come into contact with water.

Circuit protection and intelligent charging management

Miniaturized charging systems require more sophisticated circuit protection mechanisms. A control module consisting of a TC1185 low-dropout regulator and a DW01 lithium battery protection chip is integrated inside the charging base, with a volume of only 3mm×4mm. The module can achieve multiple protection functions: automatically cut off the power supply when the input voltage exceeds 5.5V; start current limiting protection when the charging current exceeds 150mA; trigger undervoltage protection when the battery voltage is lower than 2.7V. The response time of these protection measures is within 100μs, which can effectively prevent abnormal conditions such as overcharging and short circuit.
The smart charging algorithm is specially optimized for the micro battery of the ring (usually with a capacity of 20-50mAh). The "three-stage charging" mode is adopted: in the initial stage, it is fast charged with a constant current of 0.5C (100mA), when the battery voltage reaches 4.2V, it switches to constant voltage charging, and finally completes the charging with a trickle current of 10mA. The entire charging process takes about 60-90 minutes, which is 30% more efficient than traditional linear charging. The charging base is also designed with a power detection function, which displays the charging progress through four LED indicators (25%/50%/75%/100%), solving the problem that micro devices cannot intuitively display the power.
To reduce standby power consumption, the system adopts a "magnetic activation" wake-up mechanism. Only when the ring approaches the base to trigger a change in the magnetic field, the charging circuit will start working, otherwise it will be in a dormant state, and the standby current is < 10μA. This design makes the charging base consume only about 0.08 degrees of power throughout the year when connected to a power source, which meets the green energy-saving standards. At the same time, the base also has a reverse charging protection function. When the external power supply is disconnected, the circuit connection between the ring and the base is automatically cut off to prevent the ring battery from discharging to the base.

Human-computer interaction and industrial design optimization

The optimization of user experience is reflected in every interactive detail. The upper surface of the charging base is made of skin-like silicone material with a friction coefficient of 0.65, which not only ensures the stability of the ring when placed, but also provides a gentle touch. The edge of the base is designed with a 0.5mm rounded transition to avoid the risk of scratches caused by sharp edges. The indicator light adopts a 45° bevel light design, the light is soft and not dazzling, and there is no light pollution when used at night. At the same time, different colors are used to distinguish the status (red - charging, green - full, blue - standby).
The design of the charging cable also focuses on practicality. It uses a high-flexibility silicone wire with a diameter of 1.5mm, which can withstand more than 10,000 bending tests and will not break even if it is folded repeatedly. The cable length is set to 60cm, which is convenient for connecting to the desktop charger and avoids the entanglement problem caused by too long. The USB-A plug adopts the injection molding process, and the plug-in and pull-out life is up to 10,000 times, meeting the needs of long-term use.
In terms of adaptability to different finger sizes, the solution provides three versions of magnetic suction strength (1.5N/2.0N/2.5N), and users can choose according to their wearing tightness. At the same time, the bottom of the charging base is designed with 3M VHB double-sided tape, which can be pasted on fixed locations such as desktops and bedside tables, forming a "place and charge" usage scenario, which is especially suitable for users with poor eyesight or inconvenient operation. Feedback from a user test shows that this design shortens the completion time of the charging operation from an average of 45 seconds to 8 seconds, and the error rate drops to 0%.

Reliability testing and scenario verification

A rigorous testing system is the guarantee of product reliability. The Cnomax company solution has passed 12 professional tests: in the temperature cycle test from -20℃ to 60℃, the performance did not decay after 500 cycles; in the vibration test, it withstood 10g acceleration vibration in the frequency range of 10-2000Hz, and the structure did not loosen; in the impact test, it fell from a height of 1 meter to the concrete floor, and the charging function was normal. These tests simulate the extreme environments that may be encountered in daily use to ensure the long-term reliability of the product.
Actual scene verification reveals the value of more design details. In the sports scene test, after wearing the ring for running, swimming and other activities, the charging contact performance is not affected by sweat and moisture; in the dust environment test, after 8 hours of dust exposure, the contacts maintain good contact through the automatic wiping function; in the elderly use test, 95% of the respondents said that it is "much simpler than the traditional charging method". These feedbacks confirm the effectiveness of the design and provide direction for subsequent iterations.

The magnetic connector charging cable solution of the smart ring is a perfect combination of miniaturization technology and humanized design. It solves a series of technical problems such as power transmission, positioning accuracy, and protection performance in a millimeter-level space, and eliminates obstacles to user-device interaction through detailed design. When users can complete charging by simply placing it anywhere, technology is truly integrated into the natural flow of life. This concept of "invisible design" may be the ultimate pursuit of wearable device charging systems - let technology exist but not be perceived, leaving only convenience and reliability.

Author: Jerry_shi

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