How to use a multimeter to measure the high voltage diode of a microwave oven

　　This article mainly introduces the high-voltage diode of microwave oven, and focuses on the detection and principle of high-voltage diode.

　　diode

　　Diode, (English: Diode), is an electronic component with two electrodes that only allows current to flow in one direction. Many uses are for its rectification function. Varicap diodes are used as electronic adjustable capacitors. The current directionality of most diodes is usually called the "rectifying" function. The most common function of a diode is to only allow current to pass in one direction (called forward bias) and block it in the reverse direction (called reverse bias). Therefore, a diode can be thought of as an electronic version of a check valve.

　　An early vacuum electron diode; it is an electronic device that can conduct current in one direction. There is a PN junction and two lead terminals inside the semiconductor diode. This electronic device has the conductivity of unidirectional current according to the direction of the applied voltage. Generally speaking, a crystal diode is a pn junction interface formed by sintering a p-type semiconductor and an n-type semiconductor. A space charge layer is formed on both sides of its interface to form a self-built electric field. When the applied voltage is zero, the diffusion current caused by the concentration difference of carriers on both sides of the pn junction and the drift current caused by the self-built electric field are equal and are in an electrical equilibrium state, which is also the diode characteristic under normal conditions.

　　Early diodes included "Cat's Whisker" crystals and vacuum tubes (called "Thermionic Valves" in the UK). Most common diodes today are made of semiconductor materials such as silicon or germanium.

　　How to use a multimeter to measure the high voltage diode of a microwave oven

　　The microwave oven high-voltage diode is measured with a pointer multimeter. The multimeter measurement function selects the resistance block, the gear selects the 10K block, and the test leads are connected to both ends of the high-voltage diode. The forward resistance is ≤150KΩ, and the reverse resistance tends to infinity. If the measured forward and reverse resistances are very small or both are not connected, the tube is broken down or broken. Due to the discreteness of semiconductor devices, it is normal for the measurement values ​​of high-voltage diodes of different models to be different. Since the high-voltage diode is composed of multiple diodes connected in series, the test voltage of the low-barrier of the pointer multimeter is low, so it cannot be measured with a low-barrier. The digital multimeter is limited by its measurement principle and is not easy to be used for the good and bad judgment measurement of high-voltage diodes.

　　High voltage diode detection

　　The conduction threshold voltage of high-voltage diodes is relatively high. If you use an ordinary multimeter with an internal battery voltage of 1.5V to measure its forward resistance, the measured resistance value may be very large, and the needle will not move most of the time, which makes it impossible to judge whether it is good or bad. Therefore, you should use the R×10k block of a multimeter with an internal battery greater than 6V, preferably 9~15V, to measure. The measured forward resistance is normally about 20-300kΩ; the reverse resistance is infinite. For example, when measured with a megohmmeter, the forward resistance is normally less than 2kΩ; the reverse resistance is infinite. If there is no such instrument, you can also use the R×1k block of an ordinary multimeter to measure, but you need to connect a 6~9V battery in series to a test pen before measuring. When connecting the battery in series, connect the red test pen to the positive pole of the battery, and the negative pole of the battery is used as the original red test pen for measurement. Do not short-circuit the two measuring ends when measuring, and do not measure high-voltage diodes or other components with known abnormal internal resistance (too small) to avoid damage to the needle by over-hitting; for insurance purposes, you can also connect a suitable current-limiting resistor in series before using it. The resistance value can be measured by referring to the value of a normal high-voltage diode measured by the same method. Of course, it is more convenient to judge based on general experience, and it is generally reliable. As long as the reverse resistance is infinite and the forward resistance is not infinite or very large (the needle deflects a certain angle, not a slight movement), it means that the diode is basically good. For some asymmetric protection diodes used in microwave ovens, the 10k range can be used for measurement, and the normal forward and reverse resistance should be infinite.

　　The high-voltage diodes of microwave ovens can generally be replaced with each other as long as their main characteristic parameters are the same or similar and their external dimensions do not affect installation.

　　Detection of high voltage diode in microwave oven

　　The circuit power of a microwave oven is relatively high when in use, generally more than one kilowatt, and the requirements for the microwave oven high-voltage diode are also relatively high. The symbol of the diode is D, which has positive and negative poles. Electromechanical control type microwave ovens only have high-voltage diodes, with the same symbol. For all microwave ovens in current homes, there must be a high-voltage diode that provides an anode for the microwave oven. It is dedicated, just like the high-voltage silicon stack on the commonly used black and white TV. Once damaged, the microwave oven high-voltage diode of the same model must be replaced. When replacing the microwave oven high-voltage diode, you must pay attention to whether the size of the microwave oven high-voltage diode is appropriate.

　　There are many reasons for the damage of microwave oven high-voltage diodes during use. During use, microwave oven high-voltage diodes and capacitors together form the rectification circuit of microwave ovens. It can be said that the damage of microwave oven high-voltage diodes is the most important reason for microwave ovens not to work properly. The reasons for the damage of microwave oven high-voltage diodes are that during the use of microwave ovens, due to unstable voltage, overvoltage burns when working under peak voltage. To measure the quality of the diode, use the resistance range of the multimeter, disconnect the circuit and measure it separately. Because the red stick of the multimeter is connected to the negative pole of the battery inside and outside the meter. Therefore, the red stick is connected to the negative pole of the diode, and the black stick is connected to the positive pole of the diode to conduct (the pointer turns to low ohms). For ordinary diodes, the forward conduction is 4-5k ohms, and the reverse resistance is more than a few m ohms. The larger the better.

　　The high-voltage diode here works in a 4000v circuit, taking into account the peak and margin, and the withstand voltage requirement is higher. This microwave oven high-voltage diode is sold in stores. The negative pole has a ring that can be connected to the bottom plate, and the positive pole has a pin that can be inserted into the high-voltage capacitor. It is easy to use. The high-voltage diode is actually made up of several diodes in series, and the internal resistance is relatively high. The forward resistance is about 100k ohms, and the reverse resistance is "infinite". If the high-voltage diode breaks down, the high-voltage fuse will burn out. If the high-voltage diode burns out internally, there will only be AC ​​high voltage, no DC high voltage.

　　Detection method of high voltage capacitor of microwave oven

　　At present, pointer multimeter is commonly used for high-voltage capacitor detection. Set the meter to R×10k and connect the test leads to the two poles of the high-voltage capacitor. The needle first swings to about lookΩ and then gradually returns to 10MΩ. In addition, the insulation resistance between the two poles of the high-voltage capacitor and the shell should be ∞.

　　However, the battery voltage of the pointer multimeter is too low (even for the higher-end 500-type meter, the voltage is only 15V in the R×10k range).

　　The high voltage capacitor can only be charged to a little more than 10V. It is too difficult to use it to detect high voltage components, and some hidden dangers may not be detected. Therefore, the author proposes to use a megohmmeter for detection, which is reliable and effective.

　　1. Clip the alligator clips of the megohmmeter to the two electrodes of the high-voltage capacitor, as shown in the figure above.

　　Shake it quickly once, and note that you only need to shake it once and stop immediately. You don’t need to shake it more quickly. Then you can see that the megohmmeter pointer starts to deflect from the natural stop position (usually in the middle of the dial) to the “O” MΩ end (to the left), indicating that the high-voltage capacitor begins to charge. The megohmmeter pointer continues to deflect to the end, that is, below the “0” mark.

　　2. The megohmmeter pointer stops below the "0" line and then turns to the "∞" MΩ end (to the right), indicating that the high-voltage capacitor starts to charge after discharging. The megohmmeter pointer deflects to the other end. If the capacity of the high-voltage capacitor is completely normal, the megohmmeter pointer will definitely stay at this extreme point (charging) for more than 15 seconds.

　　3. After charging to the peak, the high-voltage capacitor begins to discharge again, and the megger pointer slowly returns to its insulation resistance value from the "∞" MΩ end super-limit point. The entire charging and discharging process of the high-voltage capacitor is shown in the figure below.

　　4. If the "∞" MΩ end exceeds the limit of the peak charge state, use a screwdriver to short-circuit the two poles of the high-voltage capacitor, you can see sparks and hear a "pop" discharge sound. If you also connect a digital voltmeter to the megohmmeter, you can find that the megohmmeter can generate a voltage of nearly 150V with just one quick shake.