Abstract: Based on the "Compilation of New Standards and Regulations for Mine Safety Monitoring," this article presents the first development of a novel power supply status sensor. This sensor uses the principle of capacitance detection to monitor the electric field state of the cable core to ground, thereby indirectly detecting the power supply status of the mine cable. It is independent of whether the load is operating or whether current flows through the cable; as long as the cable core is energized, it outputs the power supply status; conversely, it outputs the non-power supply status. This power supply status sensor has advantages such as simple structure, low cost, high reliability, high sensitivity, easy maintenance, and convenient replacement. Keywords: Power supply status, sensor, monitoring system, sensitive element, field-effect transistor Introduction Currently, several power supply status sensors have been developed both domestically and internationally. Most of these sensors are based on optical fibers, voltage transformers, or use Hall effect devices. Japan, Europe, and other countries have developed and applied fiber optic voltage sensors for coal mines. Domestically, Zhenjiang Zhongmei Electronics Co., Ltd. has developed the KGT8 power supply status sensor; the Changzhou Automation Research Institute of the China Coal Research Institute has developed the KGT23 intrinsically safe power supply status sensor for mining; Sanheng Automation Instrument Co., Ltd. has developed the KGT9 power supply status sensor for mining electromechanical equipment; and Changchun Dongmei High-Tech Development Co., Ltd. has developed the KJ19-31 power supply status sensor for electromechanical equipment. These power supply status sensors play a crucial role in coal mine safety production. However, existing power supply status sensors, aside from explosion-proof, dust-proof, and moisture-proof features, have not been further researched for the special conditions of underground coal mines. They suffer from problems such as difficulty in maintenance, low sensitivity, poor reliability, system complexity, and high cost, severely hindering their widespread application. Therefore, it is essential to address the problems of existing mine safety monitoring systems, such as functional complexity, poor reliability, high cost, inability to integrate well with existing safety production detection instruments, and improper use and maintenance. Furthermore, researching highly reliable, easy-to-maintain, and economical power supply status sensors tailored to the specific conditions of underground coal mines is of great significance. 1. Selection of Power Supply Status Sensor Scheme Based on the requirements of the mine monitoring system for power outage and power supply status detection, the methods for detecting power outage and power supply status can be divided into two types: direct contact detection and indirect non-contact detection. Direct detection refers to direct electrical connection with the load equipment, directly obtaining signals from the power supply network, such as using voltage transformers or current transformers to detect the presence or absence of signal output. Indirect detection refers to no direct electrical connection with the load equipment, such as electromagnetic induction, Hall effect, temperature measurement, magnetism measurement, photoelectric, and proximity (inductance) principles. Direct contact detection can be further divided into voltage transformers and voltage sensors based on the working principle of power supply status detection. Indirect non-contact detection can be further divided into fiber optic voltage sensors, electromagnetic induction voltage sensors, Hall effect voltage sensors, and capacitive voltage sensors based on the working principle of power supply status detection. The advantage of these direct contact voltage detection methods based on the principles of voltage transformers and current transformers is their simple equipment structure; the disadvantage is that they involve direct contact with the voltage being detected. However, for cables used underground, using this direct contact method to detect the cable's power supply status would require removing the cable's insulation, inevitably posing a safety hazard and violating the "Coal Mine Safety Regulations." Therefore, this voltage detection method is unsuitable for detecting the power supply status underground. Among indirect non-contact voltage sensors, fiber optic voltage sensors offer advantages such as strong anti-electromagnetic interference capabilities, resistance to harsh environments, good insulation performance, small size, light weight, and high sensitivity. However, their high cost, complex structure, and inconvenient maintenance limit their application in mine monitoring systems. Electromagnetic induction and Hall effect voltage sensors detect the presence or absence of current (or voltage) by detecting the presence or absence of a magnetic field; when no current exists, there is no magnetic field. For detecting the power supply status of underground cables, we need to detect both the presence and absence of current. Therefore, this type of voltage sensor is unsuitable for detecting the power supply status of underground cables. Based on the above analysis, this paper proposes a non-contact voltage detection method, which indirectly detects the cable's power supply status by detecting the presence or absence of an electric field between the cable core and ground using the capacitance detection principle. 2. Capacitance Principle Voltage Measurement Method Let's first analyze the distribution of the electric field around the underground cable. As shown in Figure 1, the three-phase cables are A, B, and C, with the current direction only for reference. From the structure of the three-phase cable and the theory of uniform transmission lines, it is known that parasitic capacitance exists between any two insulated conductors. Therefore, parasitic capacitance must also exist between the three-phase cables. The magnitude of this parasitic capacitance between transmission lines is inversely proportional to the distance between the cable cores and directly proportional to the conductor diameter and the dielectric constant of the insulation material, but independent of the material, ambient temperature, and frequency of the AC power transmission. This parasitic capacitance is equivalent to C0, and is usually very small. As shown in Figure 1, regardless of whether there is current in the three-phase cable, as long as there is voltage between the three-phase cables, charges will be generated on the insulation sheath of the three-phase cables, which will inevitably generate an electric field around the cable. The magnitude of the electric field depends on the voltage between the three-phase AC power and ground. Since it is a symmetrical three-phase alternating current, ideally, a zero electric field should be generated far away from the three-phase cable. However, a combined electric field caused by the three-phase alternating current will be generated very close to the cable. This combined electric field is an alternating field, and its frequency depends on the frequency of the three-phase alternating current. Thus, a fixed-value capacitor (capacitor C1 as shown in Figure 1) can be placed in this electric field between the three-phase cable and ground, generating a voltage Ux across capacitor C1. When there is an AC voltage between the three-phase cables, an AC voltage will also be generated across capacitor C1; when there is no AC voltage between the three-phase cables, no AC voltage will be generated across capacitor C1, i.e., Ux is zero. By detecting the presence or absence of the voltage Ux across capacitor C1, the power supply status of the underground cable can be indirectly detected, which is a simple and convenient solution. 3. Sensor Hardware Circuit Design 3.1 Sensitive Element Circuit Principle According to the requirements of sensor circuit design, for sensors with high output impedance, the input impedance of the sensor circuit must be compatible with the output impedance. Therefore, only by using a field-effect transistor as the input stage can the design requirements be met. Therefore, this paper selects a FET (Field-Effect Transistor) to detect the electric field state, and its equivalent principle is shown in Figure 2. The principle of using a FET to detect the electric field state is not to directly measure the surface potential or charge, but to utilize the electric field generated by the charges distributed on the surface to induce charges on the probe, ultimately affecting the gate of the FET. From this principle, it can be seen that if the FET itself is close to the insulating film, charges will be induced on the gate, thus the FET can function as an electric field sensing element. In Figure 2, the conductive substrate is equivalent to the conductor of a cable; the insulator is equivalent to the insulating material of the cable sheath; and the measuring probe is equivalent to a capacitor. When there is a voltage on the conductive substrate, positive and negative charges will be induced on the upper and lower surfaces of the insulator. When the measuring probe approaches the upper surface of the insulator (which carries a positive charge), positive and negative charges will be induced at the two poles of the measuring probe respectively; that is, the pole closer to the positively charged upper surface of the insulator will carry a negative charge, and the other pole will carry a positive charge. In other words, a voltage Ux is generated between the two poles of the measuring probe. When the voltage Ux is greater than the gate-source threshold voltage VT (or turn-on voltage), with a small applied VDS, the drain current iD will increase rapidly as VDS rises. Simultaneously considering that the voltage Ux generated between the two poles of the measuring probe is a weak AC voltage signal, it should be amplified at the primary stage. The sensitive element circuit of the power supply state sensor circuit designed accordingly is shown in Figure 3. Generally, the R_GS of a field-effect transistor is greater than 10⁷ Ω. The K30A Y-2F junction N-channel field-effect transistor selected in this design has a maximum value R_GS = 3 × 10¹⁰ Ω. In Figure 3, the selection of resistors R1, R2, and RG is mainly based on the requirement of matching the input and output impedances. Therefore, resistor RG should be selected with a large resistance value, usually 1–10 MΩ. In this design, RG = 2 MΩ is selected, resistors R1 and R2 are 20 kΩ and 80 kΩ respectively, resistors RD = 10 kΩ, RS = 10 kΩ, RL = 100 kΩ, and the power supply VDD is 5 V. 3.2 Power Supply Status Sensor Circuit Design Since we need to detect the power supply status of the underground cable, that is, the presence or absence of the alternating electric field generated by the underground cable, this design uses the capacitance principle to detect the electric field status, generating a weak AC voltage signal at the output of the sensitive element. According to the design requirements of the sensor circuit, the output signal of the sensor circuit should be a switching voltage signal. Therefore, we need to perform detection, amplification, and signal conversion processing on the output signal of the sensitive element. The overall circuit is shown in Figure 4. As shown in Figure 4, the working process of the power supply status sensor is as follows: L7805 is a DC voltage regulator chip that provides the operating power for the sensor circuit; two LEDs, a red LED indicating the power supply status and a green LED indicating the power supply status. When the sensor is placed near the live cable being measured, a weak AC voltage signal is induced across the sensing capacitor C. This signal is amplified by a junction field-effect transistor (JFET) K30A, and the drain of the JFET outputs an AC voltage signal approximately five times the voltage across the sensing capacitor. Next, the AC voltage signal is converted into a unipolar signal by a detector circuit. This unipolar signal is still very weak and requires a second-stage amplification by a measurement operational amplifier (or instrumentation operational amplifier). By adjusting the adjustable resistor RW, a wide range of voltage gain can be obtained. Finally, the DC voltage signal passes through a switching circuit composed of two NPN transistors, outputting a TTL switching voltage signal. 4. Conclusion and Outlook Simulating the circuit shown in Figure 4 using EWB (Electronics Workbench) software, when an AC voltage signal greater than 0.1V is applied across the sensing capacitor, a high-level voltage signal greater than 3.6V is output from the transistor collector; when an AC voltage signal less than 0.1V is applied across the sensing capacitor, a low-level voltage signal less than 0.3V is output from the transistor collector. Similarly, in actual circuit experiments, adjusting the resistance value of the adjustable resistor RW yields a higher sensitivity than that simulated in EWB software. Therefore, this proves that the designed power supply status sensor circuit meets the design standards of the "Compilation of New Standards and Regulations for Mine Safety Monitoring and the Design and Selection Manual for Mine Safety Monitoring Systems". This paper develops a mine power supply status sensor based on relevant standards and regulations such as the 2001 edition of the "Coal Mine Safety Regulations" and the 2002 edition of the "Compilation of New Standards and Regulations for Mine Safety Monitoring and the Design and Selection Manual for Mine Safety Monitoring Systems". This sensor has the following advantages: ① High reliability; ② Simple equipment, low cost, and few required additional equipment; ③ High equipment sensitivity; ④ Long signal transmission distance; ⑤ Easy equipment maintenance and convenient replacement. Its outstanding feature is that this sensor is a non-contact detection method for the electric field state of the cable core to ground, which undoubtedly makes this power supply status sensor a promising candidate for application in mine monitoring systems.