In the industrial control circuit board design of a coffee grinder, overload protection circuitry is a core component to prevent motor burnout. Motor overload is usually caused by clogged grinding chambers, excessively hard beans, or mechanical transmission failures, leading to a sharp rise in current. If the power is not cut off in time, the motor windings will be permanently damaged due to overheating. Therefore, the overload protection circuit must possess characteristics of fast response, accurate judgment, and reliable operation, and must work in conjunction with the motor drive circuit, control chip, and other modules to form a complete protection system.
Current detection is the foundation of overload protection. Common methods include the sampling resistor method and the Hall sensor method. The sampling resistor method converts current changes into voltage signals by connecting a low-resistance resistor in series in the motor circuit, which are then amplified and sent to the control chip. This method is low-cost and simple to implement, but resistor power consumption and heat dissipation must be considered. The Hall sensor method utilizes the principle of magnetic field induction to detect current non-contactly, offering advantages such as good isolation and strong anti-interference capabilities, making it particularly suitable for high-power motor scenarios. Both methods require comparing the detected signal with a preset threshold to determine whether to trigger protection action.
Threshold setting needs to comprehensively consider the motor's rated current and starting characteristics. During motor startup, the instantaneous current can reach several times the rated value. If the threshold is too low, it will lead to false protection; if the threshold is too high, the overload current cannot be cut off in time. Therefore, a reasonable threshold needs to be determined experimentally, typically 1.5 to 2 times the rated current. Furthermore, the protection circuit must have a delay function to avoid frequent shutdowns due to brief motor overloads. The delay time can be implemented using an RC circuit or a digital timer, ensuring that the protection action is triggered only after the current has continuously exceeded the threshold for a certain period.
The execution method of the protection action directly affects motor safety. Common solutions include power cut-off and frequency reduction operation. Power cut-off achieves rapid shutdown by controlling a relay or MOSFET to disconnect the motor circuit, but the impact of mechanical inertia on the equipment must be considered. Frequency reduction operation reduces the motor drive signal frequency, lowering the speed and torque, reducing load pressure, and is suitable for reversible overload scenarios. In practical design, both methods can be combined: first, attempt recovery by reducing the frequency; if the overload persists, then completely cut off the power. In addition, the protection action needs to feed back signals to the control chip to record fault information or trigger an alarm.
Thermal design is crucial for ensuring the long-term stability of the protection circuit. During overload, both the motor and the protection circuit generate heat. Poor heat dissipation can lead to component performance degradation or secondary damage. Therefore, the circuit board needs a proper layout, separating power components from sensitive components and adding thermal pads or heat dissipation pads. Additionally, a temperature sensor can be installed near the motor housing or circuit board to monitor temperature changes in real time. When the temperature exceeds the safety threshold, the protection mechanism should be triggered even if the current is not overloaded, providing double protection.
Interference suppression design improves the reliability of the protection circuit. During coffee grinder operation, the motor's start-stop and bean crushing processes generate electromagnetic interference and mechanical vibration, which may affect the accuracy of the current detection signal. Therefore, a filter circuit, such as an RC low-pass filter or ferrite bead, needs to be added at the signal input to suppress high-frequency noise. Simultaneously, the length of the detection signal line should be shortened during layout, avoiding parallel routing with the power lines to reduce coupling interference. Furthermore, decoupling capacitors should be added to the control chip's power supply to ensure stable power supply.
Testing and verification are the final steps in ensuring the effectiveness of the protection circuit. Simulated overload scenarios, such as artificially blocking the grinding chamber or increasing the load, are needed to test the response speed and accuracy of the protection circuit. Simultaneously, the stability of the protection circuit under harsh environments such as high temperature and high humidity must be tested to ensure its long-term reliable operation. Furthermore, the impact of protection actions on motor lifespan must be verified to prevent premature motor failure due to frequent protection activation. Through comprehensive testing, circuit parameters can be optimized, protection performance improved, and the ultimate goal of preventing motor burnout can be achieved.
