Product Introduction: Microwave Oven Capacitor is a non self-healing power capacitor used in a 50Hz/60Hz power supply microwave oven, connected to the main power circuit of a magnetron, to stabilize the current of the magnetron. It has the following properties that are different from other capacitors: ① high rated voltage. The rated voltage of capacitors used in microwave ovens is generally between 2kV and 3kV, which is significantly higher than other similar components; ② There is a discharge resistor inside, which is set up for the capacitor to release a large amount of charge after cutting off the operating voltage. Therefore, it has the characteristics of high resistance and parallel connection with the capacitor. The requirement for it is to reduce the voltage at both ends of the capacitor to below 50V within 1 minute of cutting off the operating voltage, and to withstand various tests with the capacitor. In addition, its resistance value should also consider the effect of superimposed DC voltage; ③ When designing the structure and circuit of the entire machine (microwave oven) and capacitor, full attention should be paid to improving the explosion-proof ability: for example, a fuse that can cut off the power supply within 3 seconds of capacitor breakdown should be installed in the entire machine; In terms of capacitor structure, the packaging of the cover adopts the "bending edge" process, and the internal circuit adopts the "hanging wire" method, which causes the "bending edge" to stretch and straighten when the capacitor shell expands for some reason (which is often a precursor to explosion). The "hanging wire" is forced to break the internal wire, causing the capacitor to open circuit, avoiding further expansion and thus preventing explosion. Figure 1 shows the structural schematic diagram of a microwave oven capacitor. Based on the above reasons, there are special instrument and method requirements for certain safety testing of microwave oven capacitors. Next, we will introduce the measurement of capacitance and loss tangent. The microwave capacitor is a type of high-voltage capacitor, so a Xilin bridge should be used for capacity and loss measurement (Figure 2). The essence of the bridge method is to use comparison and gradual approximation methods to ensure that no current flows through the ammeter in the end. The process of gradual approximation is the process of continuously adjusting the resistance values on different digits. After calculation, these resistance values can be proportional to the capacitance values on different digits, so the size of the capacitance can be directly read from the panel. The two high-voltage bridge arms of the Xilin bridge are respectively composed of the test sample zx (cx represents the capacitance of the measured capacitor, rx represents the AC equivalent resistance value) and a lossless standard capacitor cn. The two low-voltage bridge arms are respectively composed of an uninductive resistor r3 and an uninductive resistor r4 in parallel with c4. The admittance of each bridge arm is: yx=1/rx+j ω Cx, y3=1/r3, y4=1/r4+j ω C4, yn=j ω Cn. When adjusting r3 and c4, the bridge balance should meet the following conditions: yxy4=y3yn (1), i.e.: (1/rx+j ω Cx) (1/r4+j ω C4)=(1/r3) j ω CN (2) Solve this equation, where the real and imaginary parts are equal, with: tan δ= 1/（ ω CxRx)= ω C4r4 cx=cn/(1+tan ²δ)] R4/r3 when tan δ& Lt; When the error tolerance is not greater than 1%, the above equation can be rewritten as: cx=cnr4/r3 (3). Additionally, due to the use of high-voltage Xilin bridge for power frequency voltage ω= 2 π f=100 π; At the same time, r4 is taken as 103/π (Ω), where there is: tan δ=ω C4r4=kc4 × The unit of c4 in equation 106 is f, measured in uf, and the above equation is changed to tan δ= Kc4, where k=1f-1, so, tan δ Direct reading can be achieved by adjusting c4. Tan δ Defined as the tangent of the loss angle, which is the reciprocal of q (quality factor). Since cn, r4, and r3 are all fixed and known, cx can be calculated. At this point, the measurement of capacitance and loss tangent has been completed. The impedance of the high-voltage arm of the high-voltage Xilin bridge is much larger than the corresponding low-voltage arm, so the test sample and standard capacitor are placed on the high-voltage arm for shielding protection. The shielding wire is connected from its low-voltage end to the low-voltage arm, and the method of adjusting r3 and c4 on the low-voltage arm is very safe. Figure 2 Xilin Electric Bridge