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CatalogI IntroductionII Definition, Symbol and Labeling of Variable Resistor  2.1 Definition   2.1.1 What is Variable Resistance?   2.1.2 What is Variable Resistor?  2.2 Symbol  2.3 Labeling Method of Variable ResistorIII How The Variable Resistor WorksIV Features of Variable Resistor ShapeV Structure and Function of Variable Resistor  5.1 Basic Structure  5.2 Schematic Diagram of Two Variable Resistors  5.3 The Role of The Variable ResistorVI Types of Variable Resistors  6.1 Resistance box  6.2 Sliding Rheostat  6.3 Potentiometer  6.4 Specific Classification of Variable Resistors   6.4.1 Film Variable Resistor   6.4.2 Wire Wound Variable ResistorVII Typical Application Circuits of Variable Resistor  7.1 Variable Resistor Circuit in Transistor Bias Circuit  7.2 Stereo Balance Control Variable Resistor CircuitVIII Causes and Solutions of Variable Resistor Malfunctions  8.1 Causes of Variable Resistor Malfunctions8.2 Characteristics of Variable Resistor Malfunctions  8.3 Methods For Repairing Variable Resistor  8.4 Testing a Variable Resistor with a Multimeter   8.4.1 Method   8.4.2 PrecautionsIX Active Variable Resistors with Wide Range of Load ImpedanceX One Question Related to Variable Resistors  10.1 Question  10.2 AnswerXI FAQ I IntroductionA resistor is a current-limiting element. After the resistor is connected to the circuit, the resistance of the resistor is fixed. It generally has two pins, which can limit the current flowing through the branch connected to it. Those whose resistance cannot be changed are called fixed resistors, and those with variable resistance are called potentiometers or variable resistors.Setting Up A Variable Resistor, Rheostat, or Fixed ResistorII Definition, Symbol and Labeling of Variable Resistor2.1 Definition2.1.1 What is Variable Resistance?Variable resistance is a kind of resistance, which can play the role of resistance in electronic circuits. The difference from ordinary resistance is its resistance can be continuously changed within a certain range. In some cases where the resistance value is required to change but does not change frequently, a variable resistor can be used. 2.1.2 What is Variable Resistor？A variable resistor is an electronic component with adjustable resistance. It consists of a resistor and a rotating or sliding system. It is usually used in the circuit that needs to adjust the resistance frequently and plays the role of adjusting the voltage, adjusting the current, or controlling the signal. Its main parameters are basically the same as those of the fixed resistor. 2.2 SymbolThe symbol of the variable resistor is R and the unit is Ω. 2.3 Labeling Method of Variable Resistor(1) The variable resistor uses the direct standard method to indicate the nominal resistance value, that is, the nominal resistance value is directly marked on the variable resistor. In the case of high current applications, the variable resistor is also marked with the rated power parameter. In addition, the resistance value of small variable resistors is expressed in three digits, which is the same as that of resistors.(2) For variable resistors used in small-signal circuits, we generally only care about their nominal resistance and have no power requirements. III How The Variable Resistor WorksWhen a voltage is applied between two fixed electric shocks of the resistor body, the position of the contact on the resistor body is changed by rotating or sliding the system, and a position is formed between the movable contact and the fixed contact. Certainly related voltage. In other words, the resistor body of the variable resistor has two fixed ends. By manually adjusting the rotating shaft or sliding handle to change the position of the moving contact on the resistor body, the relationship between the moving contact and any fixed end is changed. The resistance value changes the magnitude of voltage and current. IV Features of Variable Resistor Shape(1) The volume of the variable resistor is larger than that of the general resistor, and at the same time, the variable resistor in the circuit is less, and it can be easily found in the circuit board.(2) There are three pins in the variable resistor, and they are different from each other. One is a moving pin and the other two are fixed. Generally, the two fixed pins can be used interchangeably, but the fixed and moving pins cannot be used interchangeably.(3) There is an adjustment port on the variable resistor. Use a flat-blade screwdriver to protrude into this adjustment port. Turn the screwdriver to change the position of the moving plate and adjust the resistance value.(4) The nominal resistance value can be seen on the variable resistor. This nominal resistance value refers to the resistance value between two fixed chip pins and is also a fixed chip pin and a moving chip pin. The maximum resistance value between.(5) The vertical variable resistor is mainly used in small-signal circuits. Its three pins are vertically downward and mounted vertically on the circuit board. The resistance adjustment port is in the horizontal direction.(6) Horizontal variable resistors are also used in small-signal circuits. Its three pins are at 90 ° to the resistance plane and are mounted vertically on the circuit board with the resistance adjustment port facing upward.(7) The variable resistance of the small plastic case is smaller and has a circular structure. Its three pins are down and the resistance adjustment port is up.(8) Variable resistors (wire-wound structure) for large power applications. The volume is large, and the moving blade can slide left and right to adjust the resistance. V Structure and Function of Variable Resistor5.1 Basic StructureThe variable resistor is chiefly composed of a moving piece, a carbon film body, and three pins. The three pins are two fixed pins (also called fixed pieces) and one moving piece pin. The moving piece of the variable resistor can be rotated left and right. When using a flat-blade screwdriver to reach into the adjustment port and rotate, the contacts on the moving piece can slide on the resistance piece. According to diverse uses, the resistance material of the variable resistor includes metal wire, metal sheet, carbon film, or conductive liquid. For currents of general magnitude, metal-type variable resistors are frequently used. When the current is slight, it is better to use a carbon film type. When the current is large, the electrolytic type is most suitable. 5.2 Schematic Diagram of Two Variable Resistors Figure3. Schematic Diagram of Two Variable Resistors5.3 The Role of The Variable Resistor(1) A variable resistor is an adjustable electronic component, which is composed of a resistor body and a sliding system. The variable resistor resistance is a resistor that can be adjusted for the current or change of the circuit In the case of circuit resistance, the light can be dimmed, and the motor can be controlled to start its speed. (2) The variable resistor mainly controls the current in the series circuit by changing its own resistance, thereby protecting some electrical components with requirements for the current. The variable resistor is generally used in circuits that do not require frequent adjustment, mainly To fix the same value for the resistor. VI Types of Variable Resistors6.1 Resistance BoxVariable resistors are divided into three types: resistance box, sliding rheostat, and potentiometer. The resistance box is a variable resistance device that uses a conversion device to change its resistance value. This conversion device usually adopts a decimal disc type (knob type) structure, and can also adopt a plug type and an end button type structure as required. The circuit of the resistance box can be divided into series lines and series-parallel lines. Compared with the sliding rheostat, the resistance box can continuously change the resistance in the connected circuit, while the sliding rheostat cannot display the resistance value of the connected circuit.Figure4. Resistance Box6.2 Sliding RheostatA sliding varistor is one of the commonly used devices in electricity. Its working principle is to change the resistance by changing the length of the resistance line in the circuit, thereby gradually changing the current in the circuit. The resistance wire of a sliding rheostat is generally a nickel-chromium alloy with a high melting point and a large resistance, and a metal rod is generally metal with low resistance. As a result, when the cross-sectional area of the resistor is constant, the longer the resistance wire, the greater the resistance; the shorter the resistance wire, the smaller the resistance.Figure5. Sliding Rheostat6.3 PotentiometerA potentiometer is a resistance element with three lead-out terminals whose resistance can be adjusted according to certain change law. A potentiometer usually consists of a resistor and a movable brush. When the brush moves along the resistor body, a resistance value or voltage having a certain relationship with the amount of displacement is obtained at the output end. The potentiometer can be used as a three-terminal element or a two-terminal element. The latter can be regarded as a variable resistor. Because its role in the circuit is to obtain an output voltage that has a certain relationship with the input voltage (external voltage), it is called a potentiometer.Figure6. Potentiometer6.4 Specific Classification of Variable ResistorsThe variable resistor can be divided into the film-type variable resistor and wire-wound variable resistor according to the material. 6.4.1 Film Variable ResistorMembrane variable resistors are usually composed of a resistor body (synthetic carbon film), a movable contact (a movable metal reed or a carbon contact), an adjustment part, and three pins (or solder pads). Two of the fixed pins are connected to both ends of the resistor body, and the other pin (center tap) is connected to the movable contact piece. You can change the resistance between the center tap and the two fixed pins by turning the adjustment part with a small flat-blade screwdriver and changing the contact position of the movable contact with the resistor. Membrane variable resistors are available in hermetic, semi-hermetic, and non-hermetic configurations. (1) Fully sealed film variable resistors are also called solid variable resistors. The resistor is made of carbon black, quartz powder, an organic binder and other materials, and then pressed into plastic or epoxy resin. The matrix of the material is polymerized by heating. The movable contacts use carbon contacts and the adjustment parts are made of plastic. The resistor body and the movable contact are sealed by a metal casing (there is an adjustment hole above the metal casing). Its advantage is that it has good dustproof performance and rarely has bad contact failure. (2) The manufacturing process of the resistor body of the semi-sealed film variable resistor and the resistor body of the fully sealed variable resistor is basically the same. The movable contact piece adopts a metal reed, and the outer plastic cover is sealed. When the plastic cover is rotated, the movable contact piece also rotates with it. This variable resistor is easy to adjust, but its dust resistance is not as good as a fully sealed film-type variable resistor. (3) Unsealed film variable resistors are also called chip tunable resistors. The resistor body is made of carbon black, graphite, quartz powder, an organic binder, etc. to form a suspension, which is coated on a glass fiberboard or glue. Made from wooden boards. The movable contact piece uses a metal reed, and the reed has an adjustment hole, and no separate adjustment component is provided. Its disadvantages are poor dust-proof performance, the contacts are susceptible to oxidation, and prone to failure due to poor contact with the synthetic carbon film. 6.4.2 Wire Wound Variable Resistor(1) High-power wire-wound varistor is also called sliding wire varistor, which is divided into axial ceramic tube-type wire-wound variable resistor and porcelain disc-type wire-wound variable resistor. It adopts an unsealed structure.(2) Low-power wire-wound variable resistors include round vertical wire-wound variable resistors, round horizontal wire-wound variable resistors, and square wire-wound variable resistors, all of which are fully sealed. Package structure.In addition, the variable resistor can be divided into a vertical variable resistor and a horizontal variable resistor according to the structure.Figure7. Wire Wound Variable ResistorVII Typical Application Circuits of Variable Resistor7.1 Variable Resistor Circuit in Transistor Bias CircuitThe figure below shows a variable-resistor voltage-dividing bias circuit. In the circuit, the transistor VT1 constitutes a high-frequency amplifier, and RP1, R1, and R2 constitute a voltage-dividing bias circuit. The output voltage of the voltage dividing circuit is determined by the resistance of three resistors, RP1, Rl, and R2. R1 and R2 are fixed resistors. The variable resistor RP1 is adjusted, and then the VT1 static operating current is adjusted. The amount of current determines whether VT1 can work in the best state. Figure8. Variable Resistance Voltage Divider Bias Circuit7.2 Stereo Balance Control Variable Resistor CircuitThe following figure shows the left and right channel gain balance adjustment circuits in the audio amplifier. RP1 in the circuit is a variable resistor in series with R1. Figure9. Left and Right Channel Gain Balance Adjustment Circuit in Audio AmplifierIn the audio circuit, for a two-channel amplifier, we need to strictly require that the left and right channel amplifiers have an equal gain (balanced), but the discreteness of the circuit components makes this impossible. In order to ensure that the gains of the left and right channel amplifiers are equal, left and right channel gain balance adjustment circuit needs to be provided, which is referred to as a stereo balance circuit. In the right channel circuit, the resistance of R2 is determined, so that the gain of the right channel amplifier is fixed. Taking the gain of the right channel amplifier as a reference, changing the resistance of RP1 so that the gain of the left channel amplifier is equal to the gain of the right channel amplifier can achieve the same gain of the left and right channel amplifiers. VIII Causes and Solutions of Variable Resistor Malfunctions8.1 Causes of Variable Resistor Malfunctions(1) The use time is long, causing oxidation.(2) The failure of the circuit caused the variable resistor to overcurrent and burned the carbon film. At this time, the burned trace of the variable resistor can also be seen from the appearance. 8.2 Characteristics of Variable Resistor Malfunctions (1) Damage to the carbon film of the variable resistorThe carbon film of the variable resistor is worn or burned. At this time, the contact between the moving piece and the carbon film is poor or cannot be contacted.(2) Poor contact between the moving piece of the variable resistor and the carbon film causes the contact resistance between the moving piece and the carbon film to increase.(3) The variable resistor pin is broken. 8.3 Methods For Repairing Variable Resistor (1) When the track of the contact of the moving blade on the carbon film is worn, the contact on the moving blade can be bent inward to change the original track of the contact of the moving blade.(2) The contacts of the moving blade are dirty. You can clean the contacts with pure alcohol.(3) There is a disconnection between one stator and the carbon film. At this time, if it is used as a variable resistor (not used as a potentiometer), this stator that is not disconnected can be used instead. Resistance value.(4) A pin is broken due to twisting. A lead can be welded with a hardwire as a pin.Figure10. Test a Variable Resistor8.4 Testing a Variable Resistor with a Multimeter8.4.1 MethodThe detection method of the variable resistor is basically the same as that of the resistor. The resistance between the primers is measured with an ohmic block. The measurement can be performed directly on the circuit board, or the variable resistor can be disconnected from the circuit. (1) Measure the nominal resistance of the variable resistor. The multimeter is placed in the proper range of the ohmic block. The two-meter bars are connected to the two fixed pin pins of the variable resistor. At this time, the measured resistance value should be equal to the nominal resistance value of the variable electrical accessory, otherwise, the variable resistance is explained. The device is damaged. (2) Measure the resistance between the moving resistor and the stator of the variable resistor. The multimeter is placed in the proper range of the ohmic block. One meter rod is connected to the fixed piece, and the other one is connected to the moving piece. In this measurement state, when the variable resistor moving piece is rotated, the needle is deflected and the resistance value increases from zero To the nominal value, or decrease from the nominal value to zero. 8.4.2 PrecautionsDue to the particularity of the variable resistor, the following issues should be noted during the detection process:(1) If the resistance between the moving piece and a fixed piece is 0Ω, at this time, you should see whether the moving piece has turned to the end of the fixed piece. To exclude the effects of external circuits). (2) If the resistance value between the moving piece and any certain piece is greater than the nominal resistance value, it means that the variable resistor has an open circuit fault. (3) In the measurement, if the measured resistance between a moving piece and a certain piece is less than the nominal resistance value, it does not mean that it is damaged, but you should look at the position of the moving piece, which is different from ordinary resistors. (4) When taking off the measurement, you can use the appropriate range of the multimeter's ohmic stop.-One rod is connected to the pin of the pad, and the other rod is connected to afoot. Then use a flat screwdriver to slowly rotate the pad in a clockwise or counterclockwise direction. At this time, the hands should continuously change from 0Ω to the nominal resistance. The same method is used to measure the change of wake value between another fixed film and a moving film. The measurement method and test result should be the same. In this way, the variable resistor is good, otherwise, the variable resistor is damaged.Figure11. Digital MultimeterIX Active Variable Resistors with Wide Range of Load ImpedancePower resistors, variable resistors, and other electronic loads are often used to test power supplies and voltage regulators, as shown in the following figure: Figure12. Active variable resistors with several orders of magnitude constant resistanceAlthough the function is the same as a mechanical potentiometer, it is based on an active device, which can provide a wide range of load resistance, high resistance adjustment resolution, and less heat than a mechanical potentiometer. Analyzing the circuit shown in the figure above, the voltage expressions of the non-inverting and inverting ends of the operational amplifier are: Figure13. FormulaThese two voltages are equal, so Figure14. FormulaThe whole circuit can be regarded as the resistance of the non-inverting terminal IN + and the inverting terminal IN-. The non-inverting and inverting equivalent resistances are constant and independent of the test voltage (VIN). RSENSE includes several series resistors that provide multiple orders of magnitude in impedance selection. For example, if 10Ω is required, the terminal is IN + and "B" near IN-1 (points A, C, and D are not connected). For high power loads, pay attention to the rated power of the sense resistor and nFET. The power supply of the operational amplifier can be a battery or any other DC power supply. Its maximum working current is only 20 μA. It is powered by a 9V battery. Under normal circumstances, the active load can be used for 1-2 years.X One Question Related to Variable Resistors10.1 QuestionVolume control regulator in a CD receiver, radio and amplifier also useA.transistorB.variable resistorC.thermistorD.fixed resistor10.2 AnswerB  XI FAQ1. What is a variable resistor do?A variable resistor gives the user more control over the resistance, as it allows for variance or for the resistor to be changed in order to meet the resistance requirements of the user. Changing the resistance as a user is actually very simple. 2. What are the two types of variable resistors?The different types of variable resistors include Potentiometer. Rheostat. Thermistor. 3. What is the difference between a variable resistor and a potentiometer?In the potentiometer, the resistance of the track remains the same as the wiper moves and only the potential on the wiper changes. In a variable resistor, the resistance of the track apparently changes as the wiper moves and short circuits more or less of the track resistance. 4. What is the advantage of a variable resistor?The advantage of variable resistors is that you have more control over the voltage. You can also adjust the amount of voltage flowing through a circuit. 5. What is the symbol of a variable resistor?A variable resistor also called an adjustable resistor, consists of two terminals, where one of the terminals is a sliding or moving contact often known as a wiper. The variable resistor IEC symbol is represented by a rectangular box and an arrow across (or above) it, like that shown in the figure below. 6. How do you identify a variable resistor?The variable resistor is represented by a zig-zag line and an arrow across (or above) it, like that shown in the figure below. 7. How many types of variable resistors are there?Variable resistors can be categorized into three types: Potentiometers. Rheostats. Digital potentiometers. 8. Is LDR a variable resistor?An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits. 9. Do variable resistors have polarity?Resistors are blind to the polarity in a circuit. Thus, you don't have to worry about installing them backward. Current can pass equally through a resistor in either direction. 10. Why is a variable resistor needed in a circuit?Simply put, a variable resistor is able to have its electrical resistance adjusted. These devices are used when working with electrical circuitry because they help to control voltage and/or currents. They specifically work with voltage and currents that are a part of the circuit. 

Ⅰ Working Principle1.1 TerminologyA diode is a two-terminal electronic device characterized by unidirectional conductivity—it allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction. Historically, diodes are divided into vacuum tube diodes (formerly called electron diodes) and semiconductor diodes (crystalline diodes). Due to the high heat loss, large size, and lower efficiency of vacuum tubes, semiconductor diodes are the standard in modern electronics.The fundamental principle of a modern diode relies on the PN junction. Adding leads and a protective package to this PN junction creates the discrete component we know as a diode.A semiconductor diode consists of a PN junction formed by joining a P-type semiconductor and an N-type semiconductor. A depletion region (space charge layer) forms at the interface, creating a self-built electric field. In the absence of applied voltage, the diffusion current (caused by the difference in carrier concentration) and the drift current (caused by the internal electric field) balance each other out, resulting in a state of electrical equilibrium.Forward Bias: When a forward voltage is applied, the external electric field opposes the self-built field. This lowers the barrier, causing the diffusion current of carriers to increase significantly, resulting in a forward current (conduction).Reverse Bias: When a reverse voltage is applied, the external field reinforces the self-built field. This widens the depletion region and prevents majority carriers from crossing. Only a tiny "reverse saturation current" flows (leakage), which remains roughly constant over a specific voltage range.Breakdown: When the reverse voltage exceeds a critical threshold, the electric field strength in the depletion layer becomes high enough to trigger a multiplication of carriers. This generates a large number of electron-hole pairs, causing a sharp increase in reverse current. This is known as the breakdown phenomenon. It is worth noting that reverse breakdown is categorized into two types: Zener breakdown (in highly doped junctions at lower voltages) and Avalanche breakdown (at higher voltages). Figure 1. P-type Semiconductor and N-type Semiconductor 1.2 PN JunctionA PN junction is the boundary interface between two types of semiconductor materials: P-type and N-type. The "P" (Positive) region contains an excess of holes, while the "N" (Negative) region contains an excess of free electrons. Due to the concentration gradient, free electrons from the N region diffuse into the P region, and holes from the P region diffuse into the N region. This movement creates the depletion region at the junction.Metal leads are connected to these regions to form terminals: the lead connected to the P-region is the Anode (positive pole), and the lead connected to the N-region is the Cathode (negative pole).1.2.1 Doping PrincipleP-type formation: Intrinsic semiconductors (pure silicon) are doped with trivalent impurities (Group III elements), such as Boron. A Boron atom has only three valence electrons. When it forms covalent bonds with surrounding silicon atoms (which have four electrons), a "hole" (a lack of an electron) is created in the lattice. This hole can accept an electron, effectively making the Boron atom a static negative ion. In P-type material, holes are the majority carriers.N-type formation: Similarly, when intrinsic silicon is doped with pentavalent impurities (Group V elements), such as Phosphorus, the impurity atoms form covalent bonds with silicon. Since Phosphorus has five valence electrons, one excess electron is left free to move. In N-type material, free electrons are the majority carriers. Figure 2. PN Junction StructureWhen these two regions meet, the diffusion of electrons and holes across the boundary disrupts the electrical neutrality near the junction, creating an electric field that eventually stops further diffusion, establishing equilibrium.1.2.2 Feature: Unidirectional ConductivityWhen forward voltage is applied (Anode positive, Cathode negative), the external field pushes holes and electrons toward the junction. This narrows the depletion region and neutralizes the internal electric field. Once the voltage exceeds the threshold voltage (typically ~0.7V for Silicon, ~0.3V for Germanium), the diode conducts current with very low resistance.1.2.3 Supplementary NoteForward Bias: Current flows easily; the diode acts like a closed switch (low impedance).Reverse Bias: Current is blocked; the diode acts like an open switch (high impedance). Ⅱ Diode ApplicationsDiodes are ubiquitous in electronics. From simple power conversion to complex signal processing, they protect circuits, regulate voltage, and enable logic functions. Understanding the diode is the first step to mastering electronics.Function of a Diode in Circuit Design2.1 Main FunctionsDiodes serve four primary roles in modern circuitry:(1) Switching Circuit (Current Steering)In digital logic and computing, diodes utilize their unidirectional conductivity to act as automatic switches. They ensure current flows only when specific conditions are met (like in AND/OR logic gates). Switching diodes (like the 1N4148) are optimized for speed, offering much faster response times than mechanical switches and preventing damage from reverse currents.(2) Limiter/Clipper Circuit (Signal Control)Limiter circuits (or clippers) use diodes to restrict the voltage amplitude of a signal. By placing diodes in parallel with the signal path, any voltage exceeding the diode's forward drop (plus any series reference voltage) is shunted to ground. This is essential for protecting sensitive inputs on microcontrollers or audio equipment from signal spikes.(3) Regulator Circuit (Voltage Stabilization)Zener diodes are the key component here. Unlike standard diodes, Zeners are designed to operate in the reverse breakdown region reliably. If the voltage across a Zener exceeds its "Zener Voltage" (Vz), it conducts heavily, clamping the voltage at that level. This makes them perfect for creating simple voltage references or low-power regulators.(4) Varactor Circuit (Tuning and Frequency Control)Varactor diodes (or Varicaps) act as voltage-controlled capacitors. When reverse-biased, the width of the depletion layer changes with voltage, which changes the junction capacitance. These are widely used in Voltage Controlled Oscillators (VCOs) for tuning radios, TVs, and mobile phones, as well as in frequency modulation (FM) circuits. 2.2 Typical Diode ApplicationsLight-emitting diode (LED)Figure 3. Light-emitting DiodeLEDs emit light when electrons recombine with holes at the PN junction, releasing energy in the form of photons. They have revolutionized lighting due to their safety, high efficiency, durability, and fast response time.Key Applications:1. Consumer Electronics: Backlights for LCD TVs, computer monitors, and smartphone screens.2. Automotive: Used in headlights, brake lights, and turn signals. Their fast switching speed improves safety (brake lights trigger faster than incandescent bulbs), and their longevity reduces maintenance.3. Industrial & Mining: Due to their robustness and efficiency, LEDs are replacing traditional lamps in harsh environments like underground mining.4. Urban Lighting: Replacing high-voltage, fragile neon tubes with LED strips for signage and architectural lighting reduces energy costs and fire risks.Zener diodeZener diodes maintain a constant voltage across their terminals when reverse-biased, even as current fluctuates. They are categorized by their breakdown voltage (e.g., 3.3V, 5.1V, 12V). They can be connected in series to achieve higher regulated voltages. Figure 4. Zener Diode CircuitRectifier diodeRectifier diodes allow current to flow only in one direction, converting Alternating Current (AC) into pulsating Direct Current (DC). This is the fundamental component of power supplies. Figure 5. Full Wave Rectifier CircuitLow Frequency (Mains): For standard 50Hz/60Hz rectification, the 1N400x or 1N540x series are standard. Key parameters are Maximum Rectified Current (Io) and Peak Inverse Voltage (PIV).High Frequency: In Switching Mode Power Supplies (SMPS), standard rectifiers are too slow. Fast Recovery Diodes (FRD) or Schottky diodes are required to handle high switching frequencies efficiently.Detector diodeDetector diodes (often Germanium or Schottky point-contact diodes) possess high detection efficiency and low junction capacitance. They are used to demodulate Amplitude Modulated (AM) signals in radios, extracting the audio signal from the carrier wave.  Figure 6. Detector Diode CircuitSchottky diodeA Schottky diode uses a metal-semiconductor junction rather than a P-N junction. This gives it two distinct advantages: 1. Low Forward Voltage Drop: Typically 0.15V to 0.45V (compared to 0.7V for Silicon), which reduces power loss and heat. 2. High Speed: Zero reverse recovery time makes them ideal for high-frequency switching power supplies, inverters, and motor drivers.Switching diodeDesigned specifically for rapid on/off operations. In the circuit below, VD1 acts as a switch to control the charging path of capacitor C2. Figure 7. Switching Diode CircuitFast recovery diode (FRD)FRDs are PN junction diodes doped to have a significantly reduced Reverse Recovery Time (trr). While a standard rectifier might take microseconds to stop conducting when voltage reverses, an FRD stops in nanoseconds. This is critical in modern power electronics like inverters and PWM controllers to prevent short-circuit currents. Update for 2025: In high-power applications, Silicon Carbide (SiC) diodes are increasingly replacing traditional silicon FRDs due to their ability to handle higher voltages and temperatures with almost zero switching loss.Transient voltage suppressor (TVS)Transient Voltage Suppressors (TVS) are specialized avalanche diodes designed to absorb high-energy spikes. They are the primary defense against ESD (Electrostatic Discharge) and voltage surges in sensitive electronics. Figure 8. Diode Circuit Symbols Ⅲ One Question Related to Diode Functions and Going Further3.1 QuestionWhy do we use diodes in a circuit?3.2 AnswerThe primary function is to serve as an electronic "check valve" or "one-way street" for electricity. This enables: 1. Rectification: Converting AC power (wall outlet) to DC power (batteries/electronics). 2. Protection: Blocking reverse polarity (if you put a battery in backward) or clamping high-voltage spikes (TVS). 3. Signal Manipulation: Demodulating radio signals or creating logic gates. 4. Reference: Providing a stable voltage reference (Zener). Ⅳ Diode Distributors RecommendationWhether you are sourcing standard rectifiers or advanced SiC power diodes, reliability is key. Here are some recommended sources for diode components:Mouser Electronics (Global Distributor)onsemi (Leading Manufacturer)KYNIX Semiconductor (Electronic Component Distributor)Digi-Key Electronics (Global Distributor) Frequently Asked Questions about Diode Function1. What is a diode used for?Its most common function is to allow electric current to pass in one direction (forward direction) while blocking it in the opposite direction (reverse direction). This is used for rectification, protection, and signal isolation. 2. What is the main function of a PN junction diode?It controls the flow of electrons. By manipulating the PN junction bias, it acts as a switch that is either ON (conducting) or OFF (insulating), depending on the direction of voltage applied. 3. What is the function of a rectifier diode?Rectifier diodes are specifically built to handle the conversion of AC (Alternating Current) to DC (Direct Current). They are robust enough to handle the high currents found in power supply units. 4. Do diodes output AC or DC?Diodes do not generate power. However, when an AC source is fed into a diode, the output is pulsating DC. The diode blocks the negative half of the AC cycle, leaving only the positive flow. 5. What is the function of a Zener diode?Zener diodes are used for voltage regulation. Unlike standard diodes, they are designed to conduct in reverse at a specific breakdown voltage (Vz). They are used to stabilize voltage rails and protect circuits from over-voltage surges. 6. What is the difference between a diode and a rectifier?"Diode" is the broad name for the component type (a two-terminal device). "Rectifier" is a function or a specific type of diode designed for power conversion. All rectifiers are diodes, but not all diodes are rectifiers (e.g., LEDs, Zener, and Varactors are diodes but are not used as rectifiers).

I IntroductionTwo adjacent conductors are sandwiched by a layer of a non-conductive insulating medium to form a capacitor. Capacitors are one of the most commonly used electronic components. They play an important role in circuits like tuning, bypassing, coupling, and filtering. For example, they are often used in the tuning circuit of the transistor radio, coupling circuit and bypass circuit of the color TV. This article mainly introduces how to properly use multimeters to test capacitors and aluminum electrolytic capacitors (solid state capacitor), including detailed operating steps, working principles, notice, and explaining some fundamental knowledge about capacitors. We also have a related post about how to check start capacitors you may be interested in. Don't miss it! How to Test Capacitors with a Digital MultimeterCatalogI IntroductionII Definition of CapacitorIII The Reasons and Effects of Testing Capacitors and Withstand Voltage Performance  3.1 Why Should We Measure the Capacitance of A Capacitor?  3.2 Why Should Capacitors Undergo A Withstand Voltage Test?IV The Difference of Capacitors with Different Capacity in Test  4.1 Small-capacity Capacitor Test  4.2 Large-capacity Capacitor Test  4.3 Supercapacitor TestV How to Test Capacitors with A Multimeter?  5.1 Direct Test with A Capacitor  5.2 Test with Resistance File  5.3 Test with Voltage File  5.4 Test with Buzzer  5.5 Use a Digital Multimeter to Measure Capacitance Greater Than 20μFVI How to Detect Capacitors in Aluminum Capacitors  6.1 Appearance Physical Inspection  6.2 Capacity and Loss Test  6.3 Ripple Voltage Test  6.4 Leakage Current Test  6.5 Explosion Test  6.6 Temperature TestVII Considerations for Capacitor TestingVIII One Question Related to Testing Capacitor  8.1 Question  8.2 AnswerⅨ Frequently Asked Questions about How to Test a CapacitorII Definition of CapacitorCapacitors comprise components that store electricity and electrical energy (potential energy). A conductor is surrounded by another conductor, or the electric field lines emitted by one conductor all terminate in the conduction system of the other conductor, called a capacitor. This is a short introduction of capacitor. Under what circumstances do you need to test the capacitors, that's when you have capacitor uncertainty in use. So let's analyze it here. III The Reasons and Effects of Testing Capacitors and Withstand Voltage Performance3.1 Why Should We Measure the Capacitance of A Capacitor?The purpose of measuring the capacitance value of a capacitor in a general sense of electricity is to check the change of its capacitance value. By comparing the measured value with the value on the nameplate, you can judge whether the internal wiring is correct and whether the insulation has deteriorated because of moisture, whether the component has broken down, and whether oil leakage has caused the capacitance to decrease. So be careful during the substantial operation. 3.2 Why Should Capacitors Undergo A Withstand Voltage Test?The withstand voltage test refers to the test of the capability of withstanding voltage of various electrical devices and structures. The process of applying a high voltage to an insulating material or an insulating structure without damaging the performance of the insulating material is considered a withstand voltage test. Broadly speaking, the primary purpose of the capability of withstanding voltage test is to check the ability of the insulation to withstand working voltage or overvoltage, and then to check whether the insulation performance of the product equipment meets safety standards capability of withstanding voltage test is to check the ability of the insulation to withstand working voltage or overvoltage, and then to check whether the insulation performance of the product equipment meets safety standards.Figure1. Capacitor TestingIV The Difference of Capacitors with Different Capacity in Test4.1 Small-capacity Capacitor TestThe capacitance of a small-capacity capacitor is generally below 1 UF because the capacity is too minor, the charging phenomenon is unobvious, and the angle of the hand to the right is not large when measuring. Therefore, it is generally impossible to estimate its capacitance with a multimeter, but only to detect whether it has leakage or breakdown. Under normal conditions, the resistance value of both ends of the multimeter R × 10 k should be infinite. If the certain resistance value is measured or the resistance value is close to 0, it means that the capacitor has leaked electricity or has been damaged by a breakdown.Related recommendation: How to Test Ceramic Disc Capacitor 4.2 Large-capacity Capacitor TestLarge capacity can generally be tested by 1K-10K, see the sweep of the meter during charging, and the resistance value indicated by the last meter. The closer to the left, the better. If the resistance is too small, it cannot be used. 4.3 Supercapacitor TestThe method of measuring supercapacitors is completely different from other types of capacitors. Supercapacitors have exceptionally large capacitance values that cannot be measured directly by standard equipment. Ordinary methods for testing the capacitance of these capacitors are by charging the supercapacitors at the rated voltage and discharging the supercapacitors by a constant current load.Figure2. Different CapacitorsV How to Test Capacitors with A Multimeter?5.1 Direct Test with A CapacitorSome digital multimeters have the function of measuring capacitance, and their ranges are divided into five ranges of 2,000p, 20n, 200n, 2μ and 20μ. When measuring, you can directly insert the two pins of the discharged capacitor into the Cx jack on the meter board and select the appropriate range to read the display data. 2,000p file, suitable for measuring capacitance less than 2000pF; 20n file, suitable for measuring the capacitance between 2000pF and 20nF; 200n file, suitable for measuring the capacitance between 20nF and 200nF; 2μ file, suitable for measuring between 200nF and 2μF Capacitance; 20μ range, suitable for measuring the capacitance between 2μF and 20μF. Experience has shown that some types of digital multimeters (like DT890B +) allow a considerable error when measuring small-capacity capacitors below 50pF, and there is almost no reference value for measuring capacitance below 20pF. At this time, the small value capacitance can be measured by the series method. Method: First find a capacitor of about 220pF, use a digital multimeter to measure its actual capacity C1, and then connect the small capacitor to be tested in parallel to measure its total capacity C2. The difference between the two (C1-C2) is subsequently the capacity of small capacitors under test.It is extremely accurate to measure the small capacitance of 1 ~ 20pF with this method.Figure3. How to Test a Capacitor with a Multimeter5.2 Test with Resistance FileThe practice has proved the charging process of capacitors can also be observed by using a digital multimeter, which actually reflects the change of charging voltage in discrete digital quantities. Assuming that the digital multimeter's measurement rate is n times/second, in the process of observing the charging of the capacitor, you can see n readings that are independent of each other and increase sequentially. According to this display characteristic of the digital multimeter, it is possible to detect the quality of the capacitor and estimate the size of the capacitance. The following describes the method of detecting the capacitor using the resistance meter of a digital multimeter, which is of practical value for instruments without a capacitor. This method is suitable for measuring large-capacitance capacitors from 0.1 μF to several thousand microfarads. 5.2.1 Operation Method of MeasurementAs shown in Figure 4, set the digital multimeter to the appropriate resistance level. The red and black test leads respectively to touch the two poles of the capacitor Cx under test. At this time, the displayed value will gradually increase from "000" until the display Overflow symbol "1."If"000" is consistently displayed, it means the capacitor is short-circuited internally; if it is constantly displayed, the internal poles of the capacitor may be open-circuited, or the selected resistance level may be inappropriate. When checking electrolytic capacitors, pay attention to the red test lead (positive charge) is connected to the positive electrode of the capacitor, and the black test lead is connected to the negative electrode of the capacitor.Figure4. Digital Multimeter 5.2.2 Measurement PrincipleFigure5 shows the measurement principle of measuring capacitors with resistance files. During the measurement, the positive power source charges, the capacitor Cx to be measured through the standard resistor R0. At the moment when charging starts, Vc = 0, so “000” is displayed. As Vc gradually increases, the displayed value increases. When Vc = 2VR, the meter starts to display the overflow symbol "1." The charging time t is the time required for the displayed value to alter from "000" to overflow. This time interval can be measured with a quartz meter.Figure5. Principle of Measurement 5.2.3 Measured Data Using DT830 Digital Multimeter to Estimate CapacitanceThe principle of selecting the resistance range is: when the capacitance is small, a high resistance should be selected, and when the capacitance is large, a low resistance should be selected. If you use a high-resistance range to estimate a large-capacity capacitor, the measurement time will last a long time because the charging process is very slow. If you use a low-resistance range to check a small-capacity capacitor, the meter will always show an overflow because the charging time is extremely short, and you cannot see the change. 5.3 Test with Voltage FileDetecting capacitors with the DC multimeter of a digital multimeter is actually an indirect measurement method. This method can measure small-capacitance capacitors from 220pF to 1μF, and can accurately measure the capacitor leakage current.5.3.1 Measurement Methods and PrinciplesThe measurement circuit is shown in Figure6. E is an external 1.5V dry battery. Set the digital multimeter to the DC 2V range, connect the red test lead to one electrode of the capacitor Cx under test, and the black test lead to the battery negative. The input resistance of the 2V range is RIN = 10MΩ. After the power is turned on, battery E charges Cx via RIN and starts to establish voltage Vc. The relationship between Vc and charging time t isFigure6. Wiring Diagram of Measuring Capacitor with Voltage Block Here, because the voltage across RIN is the instrument input voltage VIN, so RIN actually has the function of a sampling resistor. obviously,VIN (t) = E-Vc (t) = Eexp (-t / RINCx) (5-2)Figure7 is the change curve of the input voltage VIN (t) and the charging voltage Vc (t) on the capacitor under test. It can be seen from the figure that the change process of VIN (t) and Vc (t) is just the opposite. The curve of VIN (t) decreases with time, while Vc (t) increases with time. Although the meter shows the change process of VIN- (t), it indirectly reflects the charging process of the capacitor Cx under test. During the test, if Cx is open (no capacity), the displayed value will always be “000”. If Cx is internally short-circuited, the displayed value will always be the battery voltage E and will not change with time.Figure7. Change Curve of VIN (t) and Vc (t) Equation (5-2) shows that when the circuit is turned on, t = 0, VIN = E, the initial display value of the digital multimeter is the battery voltage, and then as Vc (t) increases, VIN (t) gradually decreases. Until VIN = 0V, the Cx charging process ends, at this timeVcx (t) = EUsing digital multimeter voltage level detection capacitor, not only can check small-capacitance capacitors from 220pF to 1μF, but also measure the capacitor leakage current. Let the leakage current of the capacitor being measured be ID, and the stable value displayed by the meter at the end is VD (the unit is V), thenFigure8. Equation (5-3) 5.3.2 ExamplesExample 1:The measured capacitance is a 1μF / 160V fixed capacitor, using the 2VDC range of the DT830 digital multimeter (RIN = 10MΩ). Connect the circuit according to Figure6. Initially, the meter displayed 1.543V, and then the displayed value gradually decreased. After about 2 minutes, the displayed value stabilized at 0.003V. Find the leakage current of the capacitor under test.Figure9. Equation The leakage current of the capacitor under test is only 0.3nA, indicating good quality.Example 2:The capacitor under test is a 0.022μF / 63V polyester capacitor. The measurement method is the same as in Example 1. Due to the small capacity of this capacitor, VIN (t) decreases rapidly during measurement, and after about 3 seconds, the displayed value decreases to 0.002V. Substituting this value into equation (5-3), the leakage current was calculated to be 0.2nA. 5.3.3 Notes(1) Before measurement, the two pins of the capacitor should be short-circuited and discharged, otherwise, the change process of the reading may not be observed.(2) Do not touch the capacitor electrode with both hands during the measurement to avoid meter jumping.(3) During the measurement, the value of VIN (t) changes exponentially, and decreases rapidly at the beginning. With the increase of time, the decline rate will become slower and slower. When the capacitance of the capacitor Cx under test is less than a few thousand picofarads, because VIN (t) initially drops too quickly, and the meter's measurement rate is too low to reflect the original voltage value, the initial display value of the meter is lower than the battery Voltage E.(4) When the measured capacitor Cx is greater than 1 μF, in order to shorten the measurement time, a resistance file can be used for measurement. However, when the capacitance of the capacitor under test is less than 200pF, it is difficult to observe the charging process because the change in the reading is very short. 5.4 Test with BuzzerUsing the buzzer file of the digital multimeter, you can quickly check the quality of the electrolytic capacitor. The measurement method is shown in Figure10. Set the digital multimeter to the buzzer position, and use two test leads to contact the two pins of the capacitor Cx under test. A short beep sound should be heard, the sound will stop, and the overflow symbol "1" will be displayed. Then, measure the two test leads again, and the buzzer should sound again, and the overflow symbol “1” will be displayed at last, which indicates that the electrolytic capacitor under test is basically normal. At this time, you can dial to 20MΩ or 200MΩ high resistance to measure the leakage resistance of the capacitor to determine its quality.Figure10. Wiring Diagram For Testing Electrolytic Capacitor with Buzzer The principle of the above measurement process is: At the beginning of the test, the charging current of the instrument to Cx is large, which is equivalent to the path, so the buzzer sounds. As the voltage across the capacitor continues to increase, the charging current rapidly decreases, and finally, the buzzer stops sounding. During the test, if the buzzer keeps sounding, it means that the internal of the electrolytic capacitor has been short-circuited. If the buzzer keeps sounding and the meter always shows "1" when the meter pen is repeatedly measured, it means that the capacitor under test is open or the capacity disappears. 5.5 Use a Digital Multimeter to Measure Capacitance Greater Than 20μFFor common digital multimeters, the maximum measurement value of the capacitance file is 20 μF, which sometimes cannot meet the measurement requirements. For this reason, the following simple method can be used to measure the capacitance of more than 20μF with the capacitance file of the digital multimeter, and the maximum capacitance of several thousand microfarads can be measured. When using this method to measure large-capacitance capacitors, there is no need to make any changes to the original digital multimeter circuit. The measurement principle of this method is based on the formula C string = C1C2 / (C1 + C2) of two capacitors in series. Since two capacitors with different capacities are connected in series, the total capacity after the series connection is smaller than that of the capacitor with the smaller capacity. Therefore, if the capacity of the capacitor to be measured exceeds 20 μF, only one capacitor with a capacity of less than 20 μF is used. In series with it, you can measure directly on the digital multimeter. According to the formula of two capacitors in series, it is easy to derive C1 = C2C string / (C2-C string). Using this formula, the capacitance value of the measured capacitor can be calculated. Here is a test example to illustrate the specific method of using this formula. The component under test is an electrolytic capacitor with a nominal capacity of 220 μF, and is set to C1. Select an electrolytic capacitor with a nominal value of 10μF as C2, use a digital multimeter 20μF capacitor to measure the actual value of this capacitor as 9.5μF, and connect the two capacitors in series to measure the C string as 9.09μF. Substituting C2 = 9.5 μF and C string = 9.09 μF into the formula, thenC1 = C2C string / (C2-C string) = 9.5 9.09 / (9.5-9.09) ≈211 (μF)Figure11. Digital MultimeterNote: No matter how much the capacity of C2 is selected, a capacitor with a larger capacity must be selected under the premise of less than 20μF, and C2 in the formula should be substituted into the actual measured value instead of the nominal value, which can reduce errors. The two capacitors are connected in series and measured with a digital multimeter. Due to the capacitance error and measurement error of the capacitor itself, as long as the actual measured value is close to the calculated value, the capacitor C1 to be measured is considered good. capacity. In theory, this method can measure the capacitance of any capacity, but if the capacity of the capacitor under test is too large, the error will increase. The error is proportional to the size of the capacitor to be measured.Do you want to know about other tools to test capacitors? You can Three Measuring Tools to Test Capacitors. VI How to Test Aluminum Electrolytic Capacitors6.1 Appearance Physical Inspection(1) First check whether the capacitor under test has a formal "Product Specification", which includes the product name, specifications, installation dimensions, process requirements, technical parameters, and supplier name, address and contact information to ensure this. Batch products are provided by regular manufacturers. The logo on the capacitor should include the trademark, working voltage, standard capacitance, polarity, and operating temperature range. (2) Refer to the process parameters in the “Product Specification” and observe whether the appearance, color, and material of the capacitor are consistent with the process indicators marked on it. (3) Use a vernier caliper to confirm the installation size of the capacitor to ensure that the diameter, height, and diameter and spacing of the lead-out terminal are within the tolerance of the product process, and the external dimensions must meet the company's selection requirements. (4) Check the appearance of the capacitor to ensure its appearance is neat, without obvious deformation, breakage, cracks, spots, dirt, rust, etc., and its marking is clear, firm, correct and complete. (5) Check the lead-out terminals to ensure that their terminals are straight, free from oxidation, rust, and have no effect on their conductive properties and that the lead-out terminals are free of distortion, deformation, and mechanical damage that affects insertion and removal. (6) Check that the production date marked on the electrolytic capacitor should not exceed six months, and make a record.Figure12. Aluminum Electrolytic Capacitor6.2 Capacity and Loss Test(1) Use the electric bridge to test whether the actual capacity is consistent with the nominal capacity (the electrolytic capacitor generally has an error range of ± 20%). The loss tangent value tanθ (that is, the D value) is in compliance with the standard. (2) How to use the Zen tech bridge tester: After connecting the power supply correctly, press the "POWER" key to turn on the tester's working voltage; press the "LCR" key to select the test type (L: Inductance, C: Capacitance, R: resistance). (3) Press the "UP" and "DOWN" keys to select the test range (μF, nF, pF) and press the "FREQ" key to select the test frequency (100HZ,(120HZ, 1KHZ) can choose the required test frequency according to the technical parameters provided by the manufacturer, the test in this article selects "100HZ". (4) Press "SERIES" (parallel) and "PARALLEL" (parallel) to select the connection mode for the test, small capacitance (less than 10μF)To use parallel mode, use large mode (10μF and above) in series mode. (5) After the setting is completed, connect the bridge test ports ("LOW" and "HIGH") to the two ends of the capacitor, and use the label paper to record the capacity value and loss value on the display respectively. And attach the label paper to the corresponding capacitor for subsequent analysis. 6.3 Ripple Voltage Test(1) Connect the circuit as shown below, and connect the capacitor to be tested to the adjustable DC power supply (note that the positive and negative poles are not connected reversely). Connect the positive electrode of the oscilloscope probe with a non-inductive capacitor (1μF 1200V.DC) in series to the positive electrode of the capacitor to be tested.Figure13. Circuit of Ripple Voltage Test (2) For the setting of the oscilloscope, it must be set to the DC test position first, and the fine adjustment knob of the oscilloscope voltage must be locked. (3) During the test, the DC voltage should be slowly increased to the rated voltage with a voltage regulator, and the changes displayed by the oscilloscope should be closely monitored. The correct range should be selected to ensure that the voltage can be accurately read from the oscilloscope waveform. (4) Take the ripple waveform with the camera, and record the range and division of the oscilloscope with label paper (that is, calculate the ripple voltage and paste it on the corresponding capacitor for subsequent analysis and comparison. (5) After the recording is completed, disconnect the DC power supply, discharge the capacitor under test and the non-inductive capacitor with the bulb load, and then remove the capacitor under test from the test bench. 6.4 Leakage Current Test6.4.1 Indirect Measurement Method OneConnect as shown below. Connect a 1K resistor in series with the capacitor under test and connect it to a DC adjustable power supply. Use an oscilloscope probe to connect to both ends of the resistor. Indirectly calculate the leakage current of the capacitor to be measured by sampling the voltage signal across the resistor. Operating essentials and precautions: After the circuit is connected, adjust the DC adjustable power supply to the rated voltage of the capacitor. After the circuit is equilibrated for two minutes, read the voltage value across the resistor. When reading the oscilloscope, the voltage trimming knob should be locked. Record the maximum value of the voltage waveform as the voltage value and divide it by the resistance value to obtain the value of the leakage current. The current is too large and the resistor is burned out. After the test, the capacitor should be discharged and then removed to avoid accidents.Figure14. Circuit 6.4.2 Indirect Measurement Method TwoConnect the wiring as shown in the figure, and add an air switch in series between the capacitor and the DC power supply. First close S1 and S2 respectively, and adjust the voltage regulator to the rated voltage to charge the capacitor for two minutes.Figure15. Circuit After that, both S1 and S2 are disconnected. At this time, the adjustable power supply is at the rated value. Do not move. Add a milliamp meter between S1 and S2, as shown in the figure below: S1 and S2 are both closed, and the leakage current can be directly read through the milliamp meter after one minute of stabilization.Figure16. Circuit 6.4.3 PrecautionsRemember not to connect the milliamp meter to the line directly when the capacitor is not charged, because the initial charging current is large, the milliamp meter will be burned out by accident. In the disassembly process, first discharge the capacitor with the bulb load. When discharging, remove the milliamp meter first, and ensure that the discharge current does not pass the test resistor to prevent damage to the test resistor and the millimeter meter.6.4.4 Leakage Current at 1.2UnAdjust the DC voltage to 1.2 times the rated voltage of the electrolytic capacitor, measure its leakage current again and compare different samples. 6.5 Explosion Test6.5.1 DC TestApply reverse DC voltage to the capacitor under test, slowly adjust the adjustable DC voltage, and observe the current closely with a clamp meter.The DC power setting is generally not more than 30V. The current value is set according to the size of the capacitor as follows:When the capacitor diameter is 6mm ≤ 22.4mm, the current cannot exceed 1A; when the capacitor diameter is> 22.4mm, the current cannot exceed 10A. 6.5.2 Observe The Surface Temperature of The CapacitorDuring the experiment, use a thermometer to closely observe the surface temperature of the capacitor (the sensing contact of the thermometer can be wrapped around the capacitor with tape). Note that the initial current is very small and almost zero. When the temperature of the capacitor rises (about 35-40 ° C) The current is significantly increased. At this time, close observation should be made. When the current reaches or approaches 10A, the voltage should be lowered to ensure that the current is controlled within 10A. 6.5.3 Capacitor Safety ValveWithin 30 minutes after the start of the test, the capacitor safety valve should be opened. If the capacitor fuse is open, the power should be cut off immediately (the electrolytic capacitor of 350V 6800F will automatically open under the following conditions, the current is about 8A, the surface temperature is about 45-60 ° C.), If the current is close to 10A and the fuse is still 30 minutes later, If it is not turned on, this function is missing.Figure17. DC Digital Voltmeter6.6 Temperature TestThe capacity of a capacitor will change due to different ambient temperatures. In general, the capacity will increase as the temperature rises. The temperature test is to test the change of capacitance after equilibration under the set temperature. 6.6.1 High-Temperature Test(1) Connect two small wires to the lead-out terminal of the capacitor to be tested respectively, and test the capacity of the two lead terminals at normal temperature, and label them for record.(2) Put the capacitor into the high and low temperature alternating humidity and heat test box, and leave the leads outside the test box to test the capacitance.(3) Turn on the test box switch button, click "Temperature Setting" on the screen, set the temperature to 100 ° C, and click "Run" to start the test box.(4) Test the capacity again about 2 hours after the temperature reaches 100 ° C, and calculate the percentage change in capacity (the initial measurement of the difference). 6.6.2 Low-Temperature Test(1) Put the capacitor to be tested into the test box (be careful not to use capacitors that have been tested at high temperatures, except for special needs).(2) Turn on the test box switch button, click "temperature setting" on the screen, set the temperature to -25 ° C, and click "run".(3) Test the capacity again about 2 hours after the temperature reaches -25 ° C, and calculate the percentage change in capacity (the initial measurement of the difference). 6.6.3 PrecautionsThe test should pay close attention to whether there is any obvious change in the capacitor. If serious conditions such as cracking of the capacitor surface and opening of the safety valve occur, the test box should be stopped immediately. During the test, the operating procedures of the test box should be strictly followed, and the door of the test box should not be opened at will. At the end of the high temperature test, the capacitor can only be taken out after the temperature inside the test box has dropped to prevent accidents such as burns.Figure18. CapacitorsVII Considerations for Capacitor Testing(1) When measuring with a multimeter, select the gear according to the rated voltage of the capacitor. For example, the capacitor voltage commonly used in electronic equipment is low, only a few volts to dozens of volts. If the multimeter RX10k is used for measurement, the battery voltage in the meter is 12 ～ 22.5V, which is likely to cause capacitor breakdown. Therefore, the RXlk file should be used. measuring.(2) For the capacitor just removed from the line, be sure to discharge the capacitor before measurement to prevent the residual charge in the capacitor from being discharged to the meter and damage the meter.(3) For capacitors with high working voltage and large capacity, the capacitors should be sufficiently discharged, and the operator should have protective measures to prevent electric shocks during discharge. VIII One Question Related to Testing Capacitor8.1 QuestionWhat should we do when checking a capacitor with an ohm meter？8.2 AnswerTo remove the capacitor from the circuit. It's usually easy to remove a start or run capacitor – you simply unhook it from its harness and disconnect the wires. However, be careful to avoid touching the capacitor terminals. If the capacitor isn't dead, it might have a full charge, and if so, you could get a serious shock.  Ⅸ Frequently Asked Questions about How to Test a Capacitor1. How do you check if a capacitor is bad with a multimeter?Use the multimeter and read the voltage on the capacitor leads. The voltage should read near 9 volts. The voltage will discharge rapidly to 0V because the capacitor is discharging through the multimeter. If the capacitor will not retain that voltage, it is defective and should be replaced. 2. How do you test a capacitor at home?Set your voltmeter to read DC voltage (if it's capable of reading both AC and DC). Connect the voltmeter leads to the capacitor. Connect the positive(red) lead to the positive (longer) terminal and the negative (black) lead to the negative (shorter) terminal. Note the initial voltage reading. 3. How to test capacitor using multimeter? 4. Can you test capacitor on board?You just cannot test a bad capacitor inside or outside a circuit board by measuring its capacitance value with a capacitor meter or a multimeter. ... When the capacitor is outside the board, sometimes a bad capacitor may give you a proper capacitance value on the multimeter or capacitor meter. 5. What is the best capacitor tester?Best Capacitance Meter Review:Signstek MESR-100 V2 Auto Ranging in Circuit ESR LCR Meter CapacitorELIKE Digital Capacitor Tester 0.1pF to 20mFHoneytek A6013l Capacitor TesterMESR-100 circuit tester, KKMOON mesr-100 capacitor testerMultimeter Digital Capacitance Meter Capacitor Tester 0.1Pf to 2000uFExcelvan M6013 Digital Auto Ranging Capacitance Meter Capacitor TesterDigital Capacitance Meter Professional Capacitor 0.1Pf – 20000Uf 6. How do you test a capacitor with a cheap multimeter? 7. How many ohms should a capacitor have?1,000 ohmsSet it to its highest ohm (Ω) setting, at least 1 kΩ (1,000 ohms). At this setting, the meter generates a small current when you connect the meter leads to the capacitor terminals. 8. What is the capacitor symbol on a multimeter?Most digital multimeters use a symbol similar to –|(– to signify capacitance. Move the dial to that symbol. If several symbols share that spot on the dial, you may need to press a button to cycle between them until the capacitance symbol appears on the screen. 9. What if a capacitor reads high?It is reading as if there is a short circuit across it. If we read a very high resistance across the capacitor (several MΩ), this is a sign that the capacitor likely is defective as well. It is reading as if there is an open circuit across the capacitor. ... But not 0Ω or several MΩ. 10. What is the first step in testing a capacitor?The first and most simple is to inspect the capacitor. If it appears “blotted” or swelled, it is a safe bet that it is bad. It is good practice to go ahead and perform the following test even though it is swelled. Make a sketch of the wires connected to the capacitor and note the colors or numbers that identify them.

I IntroductionThe light sensor is developed based on the photoelectric effect principle of semiconductors. It can be used to detect the intensity of ambient light, and it can also be used to detect the difference in light between different colored surfaces. Users can make projects that interact with light with it, such as smart dimming lights, a laser communication system or something more awesome.Light Sensor Using Arduino and LDR | Arduino Light SensorCatalogI IntroductionII Definition  2.1 What is a Sensor?  2.2 Definition of the Light SensorIII Spectrum and Photometric Physical Quantity  3.1 Spectrum  3.2 Photometric Physical Quantities  3.3 MID Display's Perception of Backlight Brightness Under Different IlluminationIV How the Light Sensor WorksV Types and Characteristics of Light Sensors  5.1 Photodiode Type  5.2 Photoresistor TypeVI Applications of Light Sensors  6.1 Types of Light Sensors in Application  6.2 Typical Applications  6.3 Practical Application CasesVII The Circuit Diagram of a Light Sensor  7.1 Model Introduction  7.2 Appearance and Size  7.3 Application  7.4 Functional Framework Diagram  7.5 Application CircuitVIII Programming Guide  8.1 mBlock Programming  8.2 Arduino Programming  8.3 SchematicIX A Related Question about Light Sensor  9.1 Question  9.2 AnswerⅩ FAQII Definition2.1 What is a Sensor?In a broad sense, a sensor is a sensor that converts a measurement into a signal that can be perceived or quantified. In a narrow sense, a device that senses the measurement and converts it into an output signal of the same or another nature according to a certain law. The sensor is generally composed of a sensor element, a conversion element, a measurement circuit, and an auxiliary power source. The sensor element and the conversion element may be combined into one, and some sensors do not require an auxiliary power source.2.2 Definition of the Light SensorThe light sensor usually refers to a device that can sensitively sense the light energy of ultraviolet light to infrared light and convert the light energy into an electrical signal. The light sensor is a kind of sensing device, which is mainly composed of light-sensitive elements. It is mainly divided into four categories: ambient light sensor, infrared light sensor, sunlight sensor, and ultraviolet light sensor. It is mainly used in the field of changing body electronics applications and intelligent lighting systems. Modern electrical measurement technology is becoming more and more mature. Due to its advantages such as high accuracy and easy microcomputer connection for automatic real-time processing, it has been widely used in the measurement of electrical and non-electrical quantities.  However, the electrical measurement method is susceptible to interference. In the AC measurement, the frequency response is not wide enough and there are certain requirements on the withstand voltage and insulation. Today, the rapid development of laser technology has been able to solve the above problems.Figure1. Light SensorIII Spectrum and Photometric Physical Quantity3.1 SpectrumThe spectrum is a pattern in which monochromatic light, which is dispersed by the dispersive system (such as a prism and a grating), is sequentially arranged according to the size of the wavelength (or frequency). The largest part of the visible spectrum is the visible part of the electromagnetic spectrum of the human eye. Electromagnetic radiation in this wavelength range is called visible light. The spectrum does not include all the colors that the human brain can distinguish, such as brown and pink.Figure2. Spectrum3.2 Photometric Physical Quantities3.2.1 Light Intensity(I/Intensity)(1) Definition: the intensity of light emitted by a monochromatic light source (frequency 540 × 1012 Hz, wavelength 555nm) in a unit solid angle in a given direction (radiation intensity in this direction is 1/683 watts per spherical degree) .(2) Unit: cd (Candela)(3) Luminous intensity of common light sources:●  Sun, 2.8E27 cd●  Highlight flashlight, 10000 cd●  5mm super bright LED, 15 cd 3.2.2 Luminous Flux(F/Flux)(1) Definition: The energy emitted by a point light source or a non-point light source in a unit time. Among them, the visual person (radiation flux that humans can feel) is called luminous flux.(2) Unit: Lm (lumens)(3) Efficiency of common light sources (lumens / watt, Lm / W)● Incandescent, 15● White LED, 20● fluorescent lamp, 50● The sun, 94● Sodium lamp, 120 3.2.3 E/Illuminance(1) Definition: Luminous flux irradiated onto a unit area.(2) Unit: Lx / Lux (1), 1 (Lx) = 1 Lm / m2.(3) Common Illumination (Lx):● Direct sunlight (noon), 110,000● Overcast day, 1000● Inside the mall, 500● Cloudy room with window, 100● Under normal room lighting, 100● Full moon, 0.2 3.2.4 L / Luminance(1) Definition: The intensity of light emitted by the unit light source area in the normal direction and within the unit solid angle.(2) Unit: nt (nits), 1 (nt) = 1 cd / m2.(3) Brightness of common luminous body (nt):● Solar surface, 2,000,000,000● Incandescent filament, 10,000,000● White paper under the sun, 30,000● Brightness that human eyes can get used to, 3,000● The human eye can better distinguish the brightness of the color, 1● No moon night sky, 0.00013.3 MID Display's Perception of Backlight Brightness Under Different IlluminationFigure3. Ambient Illumination-LUXIV How the Light Sensor WorksThe light sensor actually works according to the principle of the photoelectric effect. The so-called photoelectric effect refers to the phenomenon that certain special substances can convert light energy into electrical energy after absorbing light. The photoelectric effect can be divided into two types: an external photoelectric effect and an internal photoelectric effect. The external photoelectric effect refers to the fact that under light irradiation, electrons can be emitted from the inside of the material to generate electricity. The photocell and photomultiplier are originals based on the external photoelectric effect.  Correspondingly, the internal photoelectric effect occurs inside the substance. When light is irradiated onto the substance, the resistivity inside the substance is changed, thereby generating electromotive force. Photoelectric elements such as photoresistors and photovoltaic cells are made based on the internal photoelectric effect. Take the light sensor on the mobile phone as an example:The light sensor in a mobile phone should actually be an ambient light sensor, which is mainly composed of two parts, a light projector, and a light receiver. The white dot next to the front camera acts as a lens that focuses the light in the environment and transmits it to the receiver via the projector. According to the photoelectric effect, the light receiver can convert various light signals into corresponding electrical signals, and then further process them into various switching and control actions to realize the sensitivity adjustment of the mobile phone. An infrared cut-off film is often attached to the chip of the ambient light sensor to eliminate the interference of infrared light so that our electronic devices such as mobile phones and laptops can accurately detect the visible light intensity in the environment. When the display consumes too much power, the light sensor can also automatically reduce the screen brightness to extend the operating time of the battery. Figure4. Light Sensor in PhoneV Types and Characteristics of Light Sensors5.1 Photodiode TypePhotodiodes and semiconductor diodes are similar in structure, and their die is a PN junction with photosensitive characteristics, which has unidirectional conductivity, so a reverse voltage needs to be added when working. When there is no light, there is a small saturation reverse leakage current, that is, a dark current, at which time the photodiode is turned off. When exposed to light, the saturation reverse leakage current greatly increases, forming a photocurrent, which changes with the intensity of the incident light. When light irradiates the PN junction, an electron-hole pair can be generated in the PN junction, which increases the density of minority carriers. These carriers drift under the reverse voltage, causing the reverse current to increase. So you can use the light intensity to change the current in the circuit. It is turned off when there is no light and turned on when there is light. Features:(1) High sensitivity can reduce the influence of stray light(2) Photodiode (photodiode) is a photoelectric conversion device, which can convert the received light into a current change(3) The working mode of the photodiode (photodiode) is to increase the reverse voltage or not increase the voltage. When a reverse bias is applied to it, the reverse current in the tube will change with the intensity of the light. The greater the light intensity, the greater the reverse current.Figure5. Photodiode5.2 Photoresistor Type(1) PrincipleIt works based on the semiconductor photoelectric effect. The photoresistor is non-polar and is purely a resistive element. It can be applied with DC voltage or AC voltage.(2) Working characteristics of the photoresistor: When the light is on, the resistance is small; when the light is off, the resistance is large. The stronger the light, the smaller the resistance; when the light stops, the resistance returns to its original value.(3) Spectral range: from ultraviolet to infrared.(4) Features:● The internal photoelectric effect has nothing to do with the electrode (only related to the photodiode), that is, a DC power supply can be used.● Sensitivity is related to the semiconductor material and the wavelength of the incident light● Epoxy resin package, high reliability, small size, high sensitivity, fast response speed, and good spectral characteristics.Figure6. PhotoresistorVI Applications of Light Sensors6.1 Types of Light Sensors in Application(1) Ambient light sensorThe ambient light sensor can sense the surrounding light conditions and tell the processing chip to automatically adjust the backlight brightness of the display to reduce the power consumption of the product. On the other hand, the ambient light sensor helps the display provide a soft picture. When the ambient brightness is high, the LCD monitor using the ambient light sensor will automatically adjust to high brightness. When the external environment is dark, the display will be adjusted to low brightness to achieve automatic brightness adjustment. (2) Infrared light sensorThe infrared light sensor uses a charged thermopile and a scandium bromide iodide (KRS-5) window to sense wavelengths from 580 to 40,000 nm. The sensor can be used to measure a range of phenomena, including infrared radiation from the palm of your hand. (3) Sunlight sensorSolar sensor. It can recognize horizontal and vertical 360 degrees. The location of the sun, identification, cloudy, cloudy, semi-cloudy, sunny and evening during the day. Tracking bearing identification. Identification circuit processing and server drive. A digital chip is used to complete the processing of the above information. It can serve a variety of ordinary motors, stepper motors. The power consumption of the whole machine is 3mA, and the chip working voltage is 5V.  International advanced solar tracking equipment uses computer data theory, which requires data and settings for the latitude and longitude of the earth. The circuit principle and equipment technology are complicated. Intelligent sun tracker uses recognition theory technology, simple circuit and few components, no theory of latitude, longitude and data information. There is no need to consider the route that the sun runs through the year. From which direction the sun rises and from which direction it falls, it can accurately identify the position where the sun rises and falls. If he is placed on a walking car or boat, the tracker can face the sun no matter where he goes. (4) UV light sensorThe UV light sensor uses a filter to measure the UV light band (315nm-400nm). Remove the filter, the sensor can sense visible light at the same time. The sensor includes a UV filter, a sight, and a sensor handle. Figure7. Types of Light Sensors6.2 Typical ApplicationsBacklight adjustment: TV, computer monitor, LCD backlight, mobile phone, digital camera, MP4, PDA, GPS;Energy-saving control: outdoor advertising machines, induction lighting appliances, toys; instruments and meters: instruments and industrial controls for measuring light intensity;Environmentally friendly replacement: Replace traditional photoresistors, photodiodes, phototransistors6.3 Practical Application Cases6.3.1 Changing Body Electronics Applications(1) Ambient light detectionIn body electronics applications, ambient light sensors are used to adjust the backlight intensity of the dashboard, as well as the LCD backlight intensity in navigation systems (GPS), temperature control, and DVD screens. This is especially important for displays like BMW's iDrive and Prius' Multi-Info. For example, when daylight becomes dim and dark, the dashboard backlight will be adjusted to varying degrees to achieve the best visibility and reduce the glare that may be caused to the driver. Using these sensors eliminates the problem of turning on the headlights during the day, and the display automatically adjusts brightness. The key function of the ambient light sensor is to use the sensitivity visible wavelength of 380nm ~ 780nm to replicate the sensitivity of the human eye. (2) Tunnel detectionTunnel detection requires the input of two sensors. The first sensor has a wider field of view "looking up" and a relatively long average moving period, which prevents the lights from turning on and off. The second sensor has a narrower field of view "looking forward" and a relatively short average moving time. This allows the tunnel sensor to respond quickly to sudden changes in daylight, turn on the car's headlights, and adjust the display's backlight brightness when entering the tunnel. Forward-facing sensors eliminate the need to turn lights on and off when entering under a bridge or a tree covering the sun. In these cases, the sensor will still "see" the light ahead. When entering the tunnel, the signal from the tunnel sensor will drop, while the signal from the wide-field sensor will remain high; the headlights of the car will be turned on. When exiting the tunnel, the signal from the tunnel sensor will increase and the signal from the wide field of view sensor will decrease; the headlights of the vehicle will be turned off. With different average moving periods, the controller makes a clear distinction. 6.3.2 Intelligent Lighting SystemTo improve the comfort of the working environment, the lighting control system adopts a light sensor to automatically control the lighting equipment according to the illuminance of the current environment, so that the illuminance is controlled within a comfortable range. In traditional lighting control systems, ordinary light sensors are often combined with A / D converters (ADCs). Because the light signal detected by the light sensor contains both visible light components and infrared light components, the infrared light is filtered to detect the light sensor detection results.VII The Circuit Diagram of a Light Sensor7.1 Model IntroductionThe light sensor shown below is a low-cost I2C digital light sensor (ALS), which can convert light intensity into a digital output signal that can directly interface with I2C, providing a wide dynamic range from 0.01lux to 64K lux The linear response is very suitable for applications under high ambient brightness.Figure8. Model7.2 Appearance and SizeFigure9. Appearance and Size of the model7.3 Application(1) Back-lighting Control in mobile / portable devices(2) Touch Panel Control in mobile / portable devices7.4 Functional Framework DiagramFigure10. Functional Framework Diagram7.5 Application CircuitFigure11. Application CircuitVIII Programming GuideThe programming described below is based on the Me light sensor developed based on the photoelectric effect principle in semiconductors.8.1 mBlock ProgrammingThe light sensor module supports the mBlock programming environment. The following is a brief description of the module instructions:Figure12. Programming GuideHere is an example of how to use mBlock to control a light sensor moduleWhen the LED receives the light, M-Panda will move left and right and say I love sunshine; Cover the LED light, M-Panda will stop moving and say I love night. The results are as follows:Figure13. Result8.2 Arduino ProgrammingIf you write a program using Arduino, you should call the library Makeblock-Library-master to control the Me Light Sensor. This program instructs Me Light Sensor to read the current light intensity through Arduino programming.Figure14. Arduino ProgrammingFunction list of light sensor:Figure15. Function List of Me Light Sensor8.3 SchematicFigure16. SchematicIX A Related Question about Light Sensor9.1 QuestionHow to combine these 2 circuits together so that during complete darkness on the LDR, the LED would turn on instantly and when light falls on the LDR there would be around a 1 or 2-second delay before completely shutting off?The circuit would be running on a 5V DC power supply and powering an LED array.How to combine them together? Figure17.Circuit1Figure18. Circuit29.2 AnswerIn the 555 circuit the capacitor controls the wait time, if the capacitor is short-circuited the circuit will wat forever.In the LDR circuit the transistor acts like a switch but unfortunately it's switching to ground but the capacitor in the 555 circuit is connected to +9VTo resolve this I swapped the parts in the 555 circuit upside down to have the capacitor to ground. Then it was simple to I merge the two circuits.Figure19. AnswerIn the dark R1 turns Q1 on the keesp C1 duscharged so 555 output will be high.when there is light the LDR turns Q1 off and C1 charges , once it gets enough charge the 555 output goes low.We could have instead built the upside-down version of the LDR circuit using a BC557 transitor (or other similar PNP type) instead of the BC547 NPN transistor and merged that with the original 555 circuit.Ⅹ FAQ1. How is a relay added to a light sensor circuit?Presumably, your light sensor will be generating a variable voltage signal in response to how much light is hitting it, and you want to trip a relay when this light is above (or possibly below) a threshold. One way to do this is with a comparator circuit, which will compare two voltages and output a high or low depending on which one is higher. You then compare the signal from the light sensor to a reference voltage that you can set with a potentiometer and generate a high or low output signal from that. You can also use a microcontroller and read the signal from the light sensor with an analog input pin. This is more complex but useful if you want to implement features like hysteresis in the comparison. Now, the logic level signal can’t drive a relay coil directly, so you will need to use a transistor to switch the relay coil current. Which transistor to use will depend on the voltages involved and the amount of current you need to switch, but it’ll be a small signal transistor of some kind. You also need a current limiting resistor on the gate, possibly a pull-down on the gate as well, and a flyback diode across the relay coil. 2. What is a light sensor?Light sensors respond to changes in infrared light to detect motion or proximity to another object. Proximity sensors help robotic machines navigate obstacles and avoid bumping into objects. They are also used for devices in vehicles that sound an alarm when the vehicle is close to bumping into an object. 3. What are the disadvantages of a light sensor?Following are the disadvantages of Light sensor :• LDRs are highly inaccurate with high response time (about 10s or 100s of milliseconds).• Resistance varies continuously (analog) in photoresistors and is rugged in nature.• Photodiodes are temperature sensitive and are uni-directional, unlike photoresistors. 4. What does a light sensor do?Light sensors are electronic devices that indicate the intensity of daylight or artificial light. They convert light energy to electrical signal output. Light sensors have several uses in industrial and everyday consumer applications. 5. Where are light sensors used?Light sensors have a lot of uses. The most common use in our daily lives is in cell phones and tablets. Most portable personal electronics now have ambient light sensors used to adjust brightness. 6. How many types of light sensors are there?By using LDR as a circuit, we can calibrate the changes in its resistance to measure the intensity of Light. There are two other Light Sensors (or Photo Sensors) that are often used in complex electronic system design. They are Photo Diode and Photo Transistor. All these are Analog Sensors. 7. How long does a light sensor last?Long Duration Settings – In most cases, your motion detector light should only stay on for 20 to 30 seconds after it's triggered. However, you can manipulate the settings so it will stay on longer. For example, many lights come with settings ranging from a few seconds to an hour or more. 8. Is a light sensor analog or digital?Analog sensors that are used for detecting the amount of light striking the sensors are called light sensors. These analog light sensors are again classified into various types such as photo-resistor, Cadmium Sulfide (CdS), and, photocell. 9. What is a light sensor in a phone?Ambient-light sensors (ALS) are widely used in smartphones to provide information about ambient-light levels, in support of the backlight LED power circuit. 10. How do you wire a light sensor to an outside light?Connect one black wire on the photocell to the black wire that comes from the building. Be sure to twist the exposed copper wire so that it forms a tight connection. Connect the second black wire on the photocell to the black wire on your light fixture, making sure that the copper wire is twisted together completely.  

IntroductionLED is a new type of light source that has entered the application field in the past ten years, and it has been used in the medical field for about 6 years, showing excellent therapeutic effects in such a short period of time. LED is a cold light source, which can well moisturize the skin while treating diseases. Besides, since high-power LEDs have strong light intensity and a good effect on the deep layer of the human skin, they are widely used in various departments in hospitals. The LED light sources currently used in the medical field include red, blue and purple. The efficacy of LED in disinfection and sterilization, wound healing, wound treatment, inflammation elimination, edema reduction, and photodynamic tumor treatment has been fully affirmed in the medical field. Can LED Light Improve Your Skin?CatalogIntroductionCatalogI What is LED Light Therapy? Is it Safe?  1.1 LED  1.2 LED Light Therapy  1.3 Safety&Side EffectsII LED Light Therapy Colors: Which do you Need?  2.1 Does LED Light Therapy Actually Work?  2.2 How to Choose the LED Light Therapy Colors?III Specific Uses and Benefits of LED Light TherapyIV Procedure of Performing a LED Light Therapy  4.1 Perform a LED Light Therapy at a Professional’s Office  4.2 How to use At-home Devices?V LED Light Face Mask  5.1 Does LED Light Face Mask Work?  5.2 Are LED Face Masks Safe?VI Frequently Asked QuestionsI What is LED Light Therapy? Is it Safe?1.1 LEDLED is the abbreviation of Lighting emitting diode, which is a light-emitting diode, which is made of compounds containing gallium (Ga), arsenic (As), phosphorus (P), nitrogen (N), etc. Semiconductor. When electrons and holes are recombined, they can radiate visible light, so they can be used to make light-emitting diodes. The gallium arsenide diode emits red light, the gallium phosphide diode emits green light, the silicon carbide diode emits yellow light, and the gallium nitride diode emits blue light. Light-emitting diodes appeared as early as 1962. In the early days, it could only emit low-light red light, and later developed other monochromatic light versions. The light that can be emitted today has reached visible light, infrared light and ultraviolet light, and the light level has increased to a considerable level Degree.Figure1. LED 1.2 LED Light TherapyFor many years, scientists have studied how the sun's rays, including so-called burning rays of the sun, or ultraviolet B radiation, ultraviolet A rays, or UVA, affect the skin. Only recently have we started to talk about the effects of visible light on the skin — not necessarily LED light, but visible light in general," says Dr. Buzney, assistant professor of dermatology at Harvard Medical School. LED lights have been around since the 1960s, but have only recently been used as a skin treatment that uses varying wavelengths of light, including red, purple, green and blue. Different wavelengths of the visible light spectrum correspond to different colors of LED light and penetrate the skin to different depths. Depending on how deeply they penetrate, LED lights are thought to have different biological effects.1.3 Safety&Side EffectsAccording to Harvard Health Publishing, for the most part, these LED light therapies appear to be relatively safe, at least in the short term. The FDA has approved some products for home use. LED skin devices don't have a lot of power, so they're unlikely to burn our skin. However, it is important to shield your eyes from the light while using them. In addition, a recent study named Phototherapy with Light Emitting Diodes indicates that a device using LEDs with frequencies of 415nm (blue), 633nm (red), and 830nm (infrared) has demonstrated significant results for the treatment of medical conditions, including mild-to-moderate acne vulgaris, wound healing, psoriasis, squamous cell carcinoma in situ (Bowen’s disease), basal cell carcinoma, actinic keratosis, and cosmetic applications.  Although photodynamic therapy with the photosensitizer 5-aminolevulinic acid might cause stinging and burning, phototherapy is free of adverse events. We determined that phototherapy using LEDs is beneficial for a range of medical and aesthetic conditions encountered in the dermatology practice. This treatment displays an excellent safety profile. The above content is extracted from the journal directly, Let’s put it simple:● In the short term, LED light therapies are relatively safe, but you have to care about the device's potential to damage the eyes, especially for people with underlying eye conditions or those who are taking medication that makes the eyes more sensitive to light.● Light Therapy doesn't use UV light so there's no risk of tanning whatsoever. Therefore, you do not have to worry for regular use.● LED light therapy doesn’t cause burns compared to other anti-aging treatments such as chemical peels, dermabrasion, and laser therapy. In general, side effects are rare, but there may be(1) increased inflammation(2) Redness(3) Rashes(4) TendernessYou should not use LED therapy if having taken certain medications, such as isotretinoin (Accutane), for acne or use topical treatments that cause sensitivity to sunlight. People with skin conditions should speak to a dermatologist before using LED light therapy.Figure2. LED Light TherapyII LED Light Therapy Colors: Which do you Need?2.1 Does LED Light Therapy Actually Work?The light therapy effect of LEDs is based on the wavelengths of different spectra. Red light LEDs of 633nm to 660nm can promote collagen proliferation of the skin and even lighten fine lines and dark spots. Near-infrared LEDs with a wavelength of 780nm to 1200nm can be used Anti-inflammatory and analgesic. The blue LED and the green LED can also be used in the medical beauty market, which are respectively suitable for preventing the growth of acne and brightening, helping to absorb nutrients in skin care products, and improving the effect of allergic skin. Laura Ferguson, founder of The Light Salon, a company focused on LED beauty, said that LED light therapy has many benefits, different effects at different wavelengths, such as anti-wrinkle or blemish, and the light will not have a band that will damage the skin Intrusive, painless light therapy, it even has a mood-relieving effect. Ferguson said that LED can also solve the problem of skin sensitivity. After light therapy, the newly stimulated cells will have a protective layer to help prevent sunburn from being exposed to strong sunlight. LED light therapy is not like a solarium, so you don't have to worry about melanin precipitation. However, the current price of LED medical beauty products is still high. In the future, the price is expected to decline as the technology of LED medical products matures. At that time, consumers' choices and needs will be more and more diversified. Figure3. Four Types of LED Light2.2 How to Choose the LED Light Therapy Colors?LED light stimulates fibroblasts, which can produce collagen, thereby improving skin elasticity, scarring and enhancing skin metabolism. It can also make use of the organism's absorption of different visible light wavelengths to stimulate the mitochondria and glands to produce more energy and extend the life cycle of cells. LED light can be used in all parts of the human body, whether it is psychological or physical, scalp or toes, internal medicine or surgery, as long as the light reaches the place, it can be used for effective treatment. After laser and pulsed light, LED light therapy has become a new light therapy. (1) Red lightThe red light with a wavelength of 635nm has the characteristics of high purity, strong light source, and uniform energy density. Red light has bactericidal, repairing and pain-removing effects, which can increase cell activity and promote cell metabolism. (2) Blue LightBlue light with a wavelength of 405, 415nm has a strong bactericidal effect, which can quickly inhibit inflammation. During the formation of acne, it is mainly caused by Propionibacterium, and blue light can cause no damage to skin tissue, effectively destroy this bacteria. (3) Purple LightThe violet light with a wavelength of 313, 410nm has the advantages of sterilizing, purifying the skin, activating cells, and promoting protein and collagen synthesis. It has a good effect on specific dermatitis, vitiligo, scleroderma and so on. (4) Green Light (560nm)Natural and soft light color, which has the effect of neutralizing and stabilizing nerves, can improve anxiety or depression; regulate skin gland function, effectively dredge lymph and improve edema, improve oily skin, acne, etc. Figure4. Function of Diffrent Light III Specific Uses and Benefits of LED Light Therapy(1) PhotorejuvenationPhoton skin rejuvenation technology is defined as non-exfoliation skin rejuvenation treatment using low-energy density under continuous high pulse light. At present, it has become one of the main methods to improve skin photoaging. This technology can significantly improve skin wrinkles and texture.  Rough, irregular pigmentation and enlarged pores.Such phenomena have been recognized by many technical professionals and beauty applicants. The characteristic histological changes of skin photoaging are elastic fibrosis and collagen fiber maturation disorder in the dermal matrix, which leads to skin relaxation and wrinkles. The study found that visible to near-infrared LED light penetrated the epidermis of the skin and reached the dermal layer of the skin, and promoted the regeneration and rearrangement of elastic fibers and collagen fibers through photothermal and photochemical effects, thereby reducing skin wrinkles and increasing elasticity. (2) Prevent or treat pigmentation after inflammationPigmentation of skin after inflammation is a common and difficult to avoid phenomenon in skin physical and chemical cosmetology, and it is especially easy to appear in Asian people. For example, in order to reduce the degree of skin pigmentation after laser treatment in the clinic, generally avoid the season of excessive ultraviolet rays, ask patients to avoid sun, apply sunscreen, etc., but it is still difficult to completely avoid pigmentation after inflammation. Recent studies have found that 660nm LED light can prevent or even treat skin pigmentation that occurs after this inflammation, which will be a new research hotspot in the skin and beauty industry. (3) Promote wound healingIt can be seen that LED light of various wavelengths in the near infrared can promote the growth of epithelial cells after trauma, and promote wound healing. At the same time, it also has a good therapeutic effect on the healing of chronic ulcers in the lower extremities of diabetic patients. (4) Reduce inflammationA series of studies have shown that LED has anti-inflammatory effects. Research has found that 635nm LED light can inhibit the release of the inflammatory mediator prostaglandin E2 (PGE2) by the fibroblasts of the gum, thereby reducing the inflammatory response of the gum. Before using pulsed dye laser to treat skin photoaging, if LED light source is used to irradiate in advance, it can reduce the discomfort of skin erythema, swelling and pain caused by dye laser. The use of LED light sources in advance of radiation therapy for breast cancer patients can reduce the side effects of radiotherapy. (5) Scar preventionKeloid keloids is a skin disorder that affects beauty and is difficult to treat clinically. It is caused by excessive proliferation of connective tissue after skin damage. Patients often have a scarring constitution. It started clinically as a small, hard red pimples, which slowly increased, producing round, oval, or irregular scars, higher than the skin surface, extending outward in the shape of a crab foot, with smooth and shiny skin, which may be accompanied by Pain, itching, etc. The clinical treatment is difficult and the effect is not ideal. Studies have found that LED can significantly improve the patient's pain, itching and other discomfort, make the scar flat, and have the advantages of non-invasive. (6) Other functionsIn addition, LED can also be used as a non-ultraviolet light therapy instrument, used in photodynamic therapy, hair loss treatment, skin damage reduction after ultraviolet irradiation, and so on. In short, LED as a new type of light source has been gradually applied to dermatology. With the continuous innovation of LED lamps and the study of the biological effect of LED on medicine, the application of LED in dermatology will have unlimited prospects. . At the same time, LEDs have higher security and can be used more widely as home medical equipment.Figure5. LED Light Therapy DevicesIV Procedure of Performing a LED Light Therapy4.1 Perform a LED Light Therapy at a Professional’s OfficeWhen getting an LED treatment, you really don’t have to do much but lie in a comfy bed with your face positioned directly under panels that emanated different colored lights. Generally, LED panels will be placed a few inches away from your face after microneedling or microdermabrasion. Each session lasts approximately 15-20 minutes. At first, it feels warm, and many people report it is really like the feeling of relaxation.4.2 How to use At-home Devices?Using at-home light therapy devices is like working out by yourself and things done in the office are like working out with a trainer. Both are good. But you’re not going to get as intense of a treatment at home. This means at-home LED devices may be more convenient, but they may be less effective than professional treatments. When using an at-home device, it is important to follow the manufacturer’s instructions. These devices typically come in the form of a mask that a person applies to the face for several minutes or a wand that they use on the skin. LED light therapy is suitable for use on any body part, including the face, hands, neck, and chest.Following treatment, no recovery time is necessary.Does LED Light Therapy Really Work? Can You Do it At HOME?V LED Light Face Mask5.1 Does LED Light Face Mask Work?The LED mask was invented by John Tsagaris, a Chinese medicine doctor. It is understood that John has a degree in human biological sciences from Chinese Medicine, and also holds a graduate diploma in skin treatment and beauty care. LED blue light can play a bactericidal and anti-inflammatory effect to improve the surface skin, and has a good effect on the treatment of acne and rosacea. LED red light can promote the growth of skin collagen, for deep skin beauty care. The working principle of LED mask is similar to photosynthesis. It treats skin folds from the depth by changing the energy of LED red light. It can not only calm the skin, but also prevent the growth of bacteria inside the skin. The use of LED mask masks not only does not have a claustrophobic feeling, but the LED red light delivered to the skin in large quantities can make people feel comfortable. Just wear the LED mask for 25 minutes every day, and the LED red light can gradually improve your skin. According to a survey report from the United States, LED masks have a significant effect on eliminating wrinkles. Another report showed that 90% of people believe that LED mask can reduce skin aging, make the skin more delicate and smooth, and greatly improve the crow's feet, red blood, and melanin deposition in the corners of the eyes.Figure6. Depth of Light Energy Penetration5.2 Are LED Face Masks Safe?Actually, it is the same concern as whether LED light therapy is safe. There’s no doubt that one of the most important aspects of LED phototherapy devices is their safety. Phototherapy with Light Emitting Diodes also points out that LEDs are nonablative and nonthermal, and when used alone (i.e., without topical photosensitizers in PDT applications) do not cause damage to the epidermis or dermal tissue.  There are no adverse events associated with the use of these devices and little to no downtime for the patient. When LED phototherapy is used alone, patients do not experience redness, peeling, blistering, swelling, or pain. In fact, patients can have a treatment during their lunch hour and return to work immediately afterwards. While home use devices have been available for several years, there are many differences between those devices and those specifically designed for use by physicians. The home use devices necessarily deliver significantly less power and typically do not have light panel arrays large enough to treat the entire face at once, for example. As they often are hand-held, it might be cumbersome, time-consuming, and impractical to treat the entire face in a single session.  In contrast with the medical LED units and their protocols, home use devices have not been validated by controlled clinical studies published in peer-reviewed journals. In some cases, home units may be used adjunctively with dermatologist-provided treatment to address specific areas of concern, but they are dissimilar enough from the medical-grade units to not be considered an alternative to these tested technologies.Figure7. LED Face MasksVI Frequently Asked Questions1. Does LED light therapy actually work?LED light therapy appears to be a safe treatment for several skin conditions, including acne, skin aging, skin wounds, and other problems. Research indicates that this therapy offers promising results, although people should not expect a 100% improvement. 2. What does LED light therapy do?LED light therapy is now used by some aestheticians to help regenerate the skin from aging. It's also used for acne. Your healthcare provider uses red or blue light frequencies based on the skincare concern. Red is primarily used for anti-aging, while blue is used for acne treatment. 3. Does LED light reduce wrinkles?LED light therapy can stimulate collagen production, which reduces fine lines and wrinkles, as well as eliminate acne-causing bacteria, which improves skin clarity. 4. Can you overdo LED light therapy?Light therapy cannot be overdone for most people. If you notice any extraordinary results, stop treatment, and contact your physician. For the best results, choose the right device style and LED color, and use it as directed. 5. Does red light therapy tighten loose skin?Amber light stimulates collagen and elastin. Red light is most commonly used to promote circulation. White light penetrates the deepest and works to tighten and reduce inflammation. Blue light kills bacteria. 6. Are LED lights bad for your eyes?New findings confirm earlier concerns that "exposure to an intense and powerful [LED] light is 'photo-toxic' and can lead to irreversible loss of retinal cells and diminished sharpness of vision," the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) warned in a statement. 7. Are led masks effective?The research behind LED masks is centered on the lights used, and if you're going on those findings, LED masks can be beneficial to your skin. 8. What are the side effects of red light therapy?Red light therapy is considered safe and painless. However, there have been reports of burns and blistering from using RLT units. A few people developed burns after falling asleep with the unit in place, while others experienced burns due to broken wires or device corrosion. 9. Is red light therapy the same as laser therapy?In clinical practice, low-level laser (LLT) therapy involves exposing tissues to red and near infrared (NI) light, which are lower in energy than the lasers used in surgery. 10. How long does it take for light therapy to work?Light therapy can start to improve symptoms within just a few days. In some cases, though, it can take two or more weeks.

CategoryⅠ IntroductionⅡ Development Background  2.1 Limitations of Microwave Oscillators  2.2 Origin of OEOⅢ Working Principle of OEO  3.1 The basic structure of OEO  3.2 Principle-based improvement directionⅣ Operating Characteristics of OEO  4.1 Advantage Performance  4.2 Disadvantage PerformanceⅤ Application of Optoelectronic Oscillator  5.1 Light Pulse Output  5.2 Clock ExtractionⅥ SummaryⅦ FAQ Ⅰ IntroductionThe optoelectronic oscillator (OEO) represents the first practical microwave oscillator that uses optical energy storage elements to generate signals with high spectral purity in the frequency range of several hundred MHz to more than 100 GHz. Many light wave energy storage components, such as fiber Fabry-Perot resonators, fiber ring resonators, optical micro disc resonators, etc. can be used to form OEO. It is a long fiber loop. The use of optical resonators can greatly reduce the size of OEO. Especially the optical microdisk resonator, which is a key component of integrating OEO in a single chip.  Figure1. Opto-Isolator OscillatorⅡ Development Background2.1 Limitations of Microwave OscillatorsGenerally speaking, the quality of the microwave signal generated by the microwave oscillator depends on the energy storage performance of the oscillation cavity. To produce high-quality microwave signals, a high-Q and low-loss energy storage unit is required. Current microwave oscillators are mostly based on electronics (such as dielectric oscillators) and acoustic (such as crystal oscillators) energy storage elements. When these components operate at frequencies above GHz, the energy storage characteristics will drop sharply, and the phase noise and spectral purity of the high-frequency microwaves produced will be poor. 2.2 Origin of OEOIn 1996, XSYao and L. Maleki of the California Institute of Technology Jet Power Laboratory developed a microwave oscillator based on a photonic energy storage unit during the use of photonics technology to improve the performance of a microwave system. This oscillator was named optoelectronic oscillator (OEO). Compared with microwave oscillators based on electronics and acoustic energy storage units, optoelectronic oscillators can generate high-purity microwave or millimeter-wave signals from several MHz to hundreds of GHz, and the Q value of their energy storage elements is as high as 1010, which generates high-frequency signals. The phase noise is as low as -163dBc / Hz at a frequency offset of 10kHz, and has both optical and electrical outputs. It is a very ideal high-performance microwave oscillator and is expected to be widely used in the future. Ⅲ Working Principle of OEO3.1 The basic structure of OEOThe basic structure of the optoelectronic oscillator is shown in Figure 2. It is a positive feedback loop composed of laser, electro-optic modulator, high Q optical energy storage unit (such as a certain length of optical fiber), photodetector, bandpass filter, microwave amplifier, phase shifter and microwave coupler. The energy of the oscillation comes from the injected light in front of the electro-optic modulator. After the injected light is modulated by the electro-optic modulator, it becomes an optical signal carrying a specific frequency. This optical signal is converted into an electrical signal by a photodetector, amplified, and then band-pass filtered. The filter filters out a specific frequency, part of which is used for output, and part of which is fed back into the microwave input port of electro-optic modulation to complete a cycle. After continuous cycling, a stable oscillation is finally formed. Since the optical oscillator uses a high-Q optical energy storage unit such as a low-loss long fiber, the output signal has extremely low phase noise. Figure2. Basic Structure of OEO3.2 Principle-based improvement directionIn addition, the loss in the optical energy storage unit such as optical fiber does not change with the change of microwave frequency, so theoretically the performance of the output signal of the optoelectronic oscillator will not deteriorate with increasing frequency. After nearly two decades of continuous exploration, the research on opto-electronic oscillators has made rapid progress. In the United States, opto-electronic oscillators have been successfully applied in cutting-edge technologies such as drones as high-quality local oscillators.  Nevertheless, in order to obtain a wider range of applications, optoelectronic oscillators need to be continuously improved in terms of performance and stability. Current research on optoelectronic oscillators is mainly focused on reducing phase noise, improving side mode suppression ratio, improving frequency stability, expanding output frequency, improving frequency tuning performance, miniaturization and multi-frequency oscillation, etc.Details are as follows:  (1) Phase NoiseThe phase noise of the output signal of the optoelectronic oscillator mainly comes from the thermal noise, scattered noise, and relative intensity noise of active devices such as lasers, photodetectors, and amplifiers. Phase noise can be reduced by optimizing the structure of microwave photonic links and the way the devices work. In experiments by D. Eliyahu and some others that produced extremely low phase noise (-163 dBc / Hz @ 6kHz) signals, a high power Nd: YAG laser with low relative intensity noise and an array amplifier with low phase noise were used. P.S.Devgan et al. Used low-biased Mach-Zehnder modulators and optical amplifiers to achieve an all-optical gain optoelectronic oscillator.  Compared with optoelectronic oscillators using electric amplifiers, the phase noise of this solution has been improved by 10dB. In addition, the use of high-power photodetectors can effectively reduce white noise, while the use of photodetector arrays to receive signals can effectively reduce the effects of flicker noise.Figure3. Phase Noise Modulation(2) Side Mode SuppressionIn order to obtain microwave output with low phase noise, the resonator of the photo-electric oscillator must have a very high Q value (Q = 2πfτ, f is the center frequency, and τ is the energy decay time), that is, a very large energy decay time is required. A larger τ can be obtained by increasing the fiber length, but as the fiber length increases, the longitudinal mode spacing (Δf = 1 / τ) in the cavity of the photo-electric oscillator decreases（As low as several tens of kHz）, in order to effectively suppress the non-oscillation mode and select a single oscillation frequency, a relatively narrow microwave band-pass filter is required. ①Dual-loop optoelectric oscillatorOne way to suppress side modes is to use a dual-loop optoelectronic oscillator. Two optical fiber loops of different lengths are formed in the cavity of the photo-electric oscillator. Only modes that satisfy the conditions for selecting the two loops at the same time can start oscillation. By selecting appropriate loop lengths, single-mode vibration can be achieved. The dual-loop optoelectronic oscillator scheme can be divided into an optical-domain coupled dual-loop structure and an optical-domain coupled dual-loop structure.  This research group proposed a dual-loop optoelectronic oscillator based on polarization modulation and polarization division multiplexing. The polarization beam splitter not only realizes the conversion of polarization modulation to intensity modulation, but also realizes that the incident light wave is divided into two orthogonal polarization states to form a double loop. The side-mode rejection ratio of the 10GHz signal generated by this solution reached 78dB. Compared with the electric-domain coupled dual-loop scheme, the optical-domain coupled scheme requires only one photodetector. Optical domain coupling dual loop schemes can also be implemented using wavelength division multiplexing technology.Figure4. A Dual-loop Optoelectronic Oscillator②Coupled optoelectronic oscillatorAnother method to suppress side modes is to use a coupled optoelectronic oscillator (COEO). The coupled optoelectronic oscillator includes two parts: an actively mode-locked laser loop and an optical feedback loop. The active mode-locked fiber laser loop can effectively increase the Q value of the oscillator. Therefore, a shorter fiber length can be used to obtain low phase noise. This research group used a non-pumped erbium-doped fiber to achieve a 10.7GHz stable coupled photo-electric oscillator with a phase noise below -120dBc / Hz @ 10kHz. (3) Frequency StabilityThe factors that affect the frequency stability of the optoelectronic oscillator are mainly two aspects:  ①The high-Q components in the system (including long optical fibers and narrow-band electrical filters) are susceptible to changes in the environment, and the output frequency is changed to cause the output frequency. Instability, especially the change of equivalent cavity length caused by environmental factors such as temperature. ②Because the filters used in optoelectronic oscillators usually have a relatively large passband range, they are within the gain bandwidth of the loop. There will be many side molds. One of these side modes may obtain sufficient gain during the change of cavity length to replace the original starting frequency, resulting in unstable starting frequency. In addition, the bias point drift problem of common electro-optic modulators will also affect the stability of the output frequency, but isolating the optoelectronic oscillator from the environment or using a temperature control device can reduce the impact of environmental changes on the system. For example, in experiments of XSYao, the optoelectronic oscillator was placed in a foam-filled box to isolate the influence brought by vibration. The active phase-locked loop circuit control is used to lock the oscillation signal of the optoelectronic oscillator to an external reference source, which can also effectively improve the frequency stability of the optoelectronic oscillator.Figure5. Frequency Stability(4) Working FrequencyTheoretically, the optoelectronic oscillator can generate signals from several MHz to hundreds of GHz, and the phase noise has nothing to do with frequency, but the high-frequency millimeter wave optoelectronic oscillator is difficult to realize. This is mainly due to the use of microwave devices such as photoelectric modulators, microwave couplers, microwave phase shifters, microwave amplifiers, and microwave transmission lines in optoelectronic oscillators, whose operating frequency is limited by electronic bottlenecks. Although there have been recent reports of high-frequency microwave or millimeter-wave devices, these devices are generally expensive, consume large power, and have poor performance. In response to the above problems, M. Shin et al. Used the LiNbO3 Mach-Zehnder modulator's half-wave voltage to the proportional relationship between the wavelength to achieve the simultaneous generation of 10GHz fundamental frequency and 20GHz octave signal. (5) TunabilityIn order to generate a broadband adjustable microwave signal, the optoelectronic oscillator needs to use a broadband adjustable high Q filter, which can be a tunable electrical filter, an optical filter, or a microwave photon filter. Limited by the electronic bottleneck, the tuning range of the output signal of the optoelectronic oscillator using a tunable electrical filter is limited. Optoelectronic oscillators based on microwave photonic filters usually have a large tuning range.Figure6. Schematic of The Tunable Opto-electronic Oscillator(6) Miniaturization ResearchOptoelectronic oscillators usually include laser sources, intensity modulators, long fiber delay lines, photodetectors, electrical amplifiers, electrical phase shifters, electrical bandpass filters, and other electrical or optical devices. These discrete electrical and optical components make the optoelectronic oscillator bulky and cause large power losses. By using high-Q optical resonators (such as whispering wall mode resonators) to replace fiber lengths of several kilometers, the size of the energy storage unit of a photo-electric oscillator can be significantly reduced. (7) Multi-frequency OscillationOptoelectronic oscillators usually only produce a pure single frequency signal. In applications such as wideband channelized receivers and multi-band radars, signals of multiple frequencies are required. In 2012, F. Kong et al. Used a birefringence characteristic of a phase-shifted Bragg grating to implement a dual-frequency optoelectronic oscillator. The disadvantage of this solution is that it can only generate signals of two frequencies, and the system is very sensitive to the environment. If a multi-frequency optoelectronic oscillator based on a single-phase modulator and a multi-wavelength light source are used, a single-passband tunable microwave photon filter can be formed on each optical carrier. By increasing the number of optical carriers, it will be easy to obtain more channels of different frequency signal output. Ⅳ Operating Characteristics of OEO4.1 Advantage PerformanceOptoelectronic oscillator is generally a positive feedback loop composed of light source, intensity modulator, filter and photodetector (PD). It takes advantage of the low loss characteristics of modulators and optical fibers to turn continuous light into stable, clean spectrum RF/microwave signals. The continuous light emitted by the laser is transmitted to the photodetector through the optical fiber after passing through the electro-optic modulator. The photodetector converts the light into an electrical signal and enters the frequency selection, amplification, and feedback modulation device.  During this process, the active device will generate noise disturbances of different frequencies. These disturbances are filtered by the filter at the output to the desired frequency and used to feedback and control the electro-optic modulator. The amplifier in the loop provides gain, and after several cycles of the signal, a stable oscillation can be established, and its oscillation frequency is mainly determined by the passband characteristics of the filter. 4.2 Disadvantage PerformanceAlthough the performance of the optoelectronic oscillator is outstanding, its system composition also determines some of its shortcomings. First of all, in order to obtain a high Q signal output, a long fiber is generally used in the cavity. At this time, the length of the cavity also determines the interval between the oscillation modes. The longer the cavity, the smaller the mode interval. In theory, a sufficiently narrow filter can be used to filter out unwanted modes, but it is quite difficult to obtain the device.  Secondly, in terms of the phase noise of the signal, the relative intensity noise of the light source, the photodetector and the electric amplifier will all affects the phase noise of the resulting microwave signal. Excessive bandwidth of filters and amplifiers will also reduce the signal-to-noise ratio in the passband range and affect the quality of the oscillation frequency.  Finally, because the loop is mainly composed of optical fibers, its cavity length is easily affected by environmental conditions and stress. The change causes the change of the fundamental frequency of the oscillation to cause the output frequency to drift or hop. In addition, the long optical fiber occupies a relatively large volume, which causes obstacles to the miniaturization and integration of the entire optoelectronic oscillator system. Solving the above problems is some of the key work for the final practical use of optoelectronic oscillators. Figure7. Cristal Oscillator Ⅴ Application of Optoelectronic OscillatorThe basic function of the optoelectronic oscillator is to generate high-quality optical and electrical microwave signals, but after being updated, it has also derived some new applications. In these applications, the electrical output of the photo-oscillator basically keeps the microwave signal output, but some changes occur in the light output part.5.1 Light Pulse OutputIn 1997 and 2000, X. Steve Yao and others successively analyzed and demonstrated the hybrid structure (COEO) of the optical resonator and optical oscillator loop provided by SOA to generate electric microwave signals and light pulses. This solution is similar to a regenerative mode-locked laser.  The main difference is that the photoelectric loop of COEO needs to be oscillated, and the final output mode is constrained by the selection of the two loops. In 2007, Ertan Salik demonstrated a COEO structure based on erbium-doped fiber amplifier (EDFA) to provide optical path gain, and obtained a 9.4 GHz microwave signal with ultra-low phase noise of -150 dBc / Hz (at a frequency offset of 10 to 100 kHz). Output and light pulse output with only 2 fs jitter. The optical pulse output mechanism of this structure is based on a fiber mode-locked laser. Therefore, in order to obtain high-performance output, there are high requirements on the design of the cavity length stabilization, dispersion control, and polarization maintenance of the optical cavity. Another feasible solution is to generate light pulses by changing the photoelectric modulation characteristics in the optoelectronic oscillator loop. In 2003, Jacob Lasri et al. Used electro-absorption modulator (EAM) to replace Mach-Zehnder intensity modulation (MZM) in the traditional scheme. By controlling the bias of EAM, a narrow modulation transmission window was obtained. Electric microwave signal and light pulse output. If a multi-wavelength light source is used in the light source part, this structure can also conveniently generate multi-wavelength light pulses. The structure of this scheme is relatively simple, but EAM generally has a large insertion loss, and the resulting pulse width is also wide.Figure8. Electro Absorption ModulatorIn addition, using a semiconductor laser operating under gain switching conditions or using a large-signal direct-modulation as the light source of the photo-electric oscillator, it is possible to obtain an electric microwave signal and an optical pulse output without requiring an additional modulator.5.2 Clock ExtractionBecause the structure of the optoelectronic oscillator has the function of frequency selection and amplification feedback, no matter whether the optical or electrical signal is injected into the optoelectronic oscillator, its clock signal (or frequency-divided clock) can be changed as long as it falls within the passband of the filter. The output can be recovered after locking and regeneration.  The maximum recoverable clock frequency is determined by the center frequency of the filter in the loop and the bandwidth of the modulator and the photodetector. X. Steve Yao et al.  Later, Caiyu Loun and others analyzed the extraction scheme of the frequency-divided clock based on the optoelectronic oscillator in 2002. By using the output electrical signal of the optoelectronic oscillator as a trigger signal to observe the injected optical signal on an oscilloscope, the electrical signal at this time can be determined. Whether the output is a divided clock of the injected signal, and experimentally verified the divided clock extraction under the condition of 10 Gb / s injected signal. In 2005, Hidemi Tsuchida and his partners demonstrated a frequency-divided clock extraction experiment with an injected signal rate of 40 Gb / s and 160 Gb / s. Figure9. Clock RecoveryIt should be noted that this method also provides a new idea for clock extraction of non-return-to-zero (NRZ) signals. In theory, there is no obvious clock component for NRZ signals to be extracted, but as long as the frequency selection of the optical oscillator filter is carefully adjusted. The clock signal of the injected NRZ code signal can be found and generated by the window. Li Huo et al. proposed the clock of the injected 10 Gb / s NRZ code signal, and obtained the converted zero (RZ) at the same time in the optical output part of the optoelectronic oscillator. The EAM-based optoelectronic oscillator can also complete the clock recovery of the RZ code signal. In the experiments demonstrated by Jaoob Lasri et al., In order to obtain the optical clock pulse signal at the same time, a DC light with a wavelength different from the wavelength of the injected signal light was added. Since the power change of the injected signal light will form a periodic switching window on the EAM and transfer the clock information to the simultaneously injected DC light, the wavelength of this DC light is selected by the optical filter to complete the Oscillation can generate an electrical clock signal and simultaneously obtain an optical clock pulse at that wavelength. It should be said that in addition to optical and electrical microwave sources, pulse sources and clock extraction systems, there are other applications, such as generating dual-frequency signals, inserting encoders to form multi-function signal generators, and so on. However, various applications are based on the feature that the photo-electric oscillator structure can automatically generate stable low-phase noise microwave signals. Therefore, as long as it focuses on various fields that require high-quality microwave signals, many new applications can be developed. Ⅵ SummaryIt can be seen that as a high-quality optical and electrical microwave signal generator, the optoelectronic oscillator has great advantages and wide application prospects. Various unique application methods also lay the foundation for the multifunctionalization of the optoelectronic oscillator.  However, it is undeniable that the current optoelectronic oscillator is still mainly in the laboratory research stage. There is still a period of time before it can be practically applied in the national economic construction and the development of national defense science and technology. Its main constraints focus on how to make the optoelectronic oscillator system into a compact, integrated, and compact frequency control system. The realization of these requirements depends on the development and manufacturing process of new photonic microwave devices and corresponding active devices.  Although there are no direct targets for optoelectronic oscillators, recent literature reports show some opportunities. For example, utc-pd (uni-traveling -Carrier Photodiode) in optoelectronic detection can receive high optical power and have high power electrical signal output, which can reduce or avoid the use of electric amplifiers in optoelectronic oscillators. The development of integrated semiconductor laser and modulator technology makes it possible to miniaturize the light source and feedback modulation of the photoelectric oscillator. The high Q value photonic filter with semiconductor structure is helpful to realize the system integration and tunability of optoelectronic oscillator. It is believed that with the gradual maturity of these technologies, the optotoelectric oscillator will be applied in practice and play its due contribution.  Ⅶ FAQ1. What do you mean by optoelectronic devices?Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers or instruments that use such devices in their operation. ... Optoelectronics is based on the quantum mechanical effects of light on electronic materials, especially semiconductors, sometimes in the presence of electric fields. 2. What are optoelectronic devices give example?Examples of optoelectronic devices are: laser diodes, superluminescent diodes and light-emitting diodes (LEDs), converting electrical energy to light. photodetectors (e.g. photodiodes and phototransistors), converting optical signals into electrical currents. 3. What is the working principle of optoelectronic devices?Optoelectronic devices are primarily transducers i.e. they can convert one energy form to another. These devices produce light by expending electrical energy. They can also detect light and transform light signals into electrical signals for processing by a computer. 4. What are Optoelectronics used for?Optoelectronic devices refer to components used to detect or emit electromagnetic radiation, typically in the visible and near-infrared (NIR) regions of the electromagnetic spectrum. Each of these functions exploits the photoelectric effect of materials, also known as light-matter interaction. 5. Is LDR an optoelectronic device?There are two types of optoelectronic devices. These are Photoconductive devices and Photovoltaic devices. Photoconductive devices detect variations in light intensity to activate or inhibit electronic circuits. LDR, Photodiodes and Phototransistors fall in this category. 6. What are optoelectronic junction devices?Optoelectronic junction devices are p-n junction devices in which, carriers are generated by photons. Photodiodes, light-emitting diodes (LEDs) and solar cells are examples of optoelectronic devices. A photodiode is a device that is used to detect optical signals. 7. Which substance has optoelectronic property?Unlike the majority of electronic devices, which are silicon-based, optoelectronic devices are predominantly made using III–V semiconductor compounds such as GaAs, InP, GaN, and GaSb, and their alloys due to their direct bandgap. 8. Who discovered optoelectronics?Three Bell Laboratories scientists, William Shockley, John Bardeen, and Walter Brattain, demonstrated the first transistor-based on point-contact germanium (Ge) device. On the other hand, the semiconductor laser was discovered 15 years later in 1962. 9. Is solar cell an optoelectronic device?Solar Cell is another example of an Optoelectronic device based on the p-n junction, and the operating mechanism of a solar cell is essentially the same as that of Photodiode in that, a p-n junction is illuminated by light and the photogenerated carriers are separated by the built-in electric field across the p-n junction. 10. What are optoelectronic devices Name any two optoelectronic devices?Examples of optoelectronic devices include telecommunication laser, blue laser, optical fiber, LED traffic lights, photo diodes and solar cells. The majority of the optoelectronic devices (direct conversion between electrons and photons) are LEDs, laser diodes, photo diodes and solar cells.