circuit Related Articles

IntroductionPrinted Circuit Board(PCB) is a board of most modern electronic devices that has lines and pads that connect various points together. Even if it is a small board, its manufacturing process is very cumbersome and exquisite. Here will introduce the PCB manufacturing process steps by steps with pictures and video.How is PCB made?The following are the detailed PCB producing processes:IntroductionStep 1. PCB CAD FileStep 2. Plate ProductionStep 3. PCB Inner LayersStep 4. Board Punching and CheckingStep 5. LaminationStep 6. DrillStep 7. Copper Chemical Precipitation on the HolesStep 8. PCB Outer LayersStep 9. Computer Control and Copper ElectroplatingStep 1. PCB CAD FileThe first step in PCB production is to organize and check the PCB layout. The PCB manufacturers get the CAD files from the PCB design company, and they will convert them into a unified format-Extended Gerber RS-274X or Gerber X2, because each CAD software has its own unique file format. Then the electronic engineers will check whether the PCB layout conforms to the manufacturing process, and whether there are any defects and other issues.Figure 1. PCB CAD FileWhen making a PCB at home, the PCB layout can be printed on paper with a laser printer, and then transferred to the copper clad laminate. During the printing process, because the printer is prone to lack of ink and breakpoints, it is necessary to manually fill up the ink with an oil-based pen. Figure 2. PCB Laser PrintingHowever, the factory generally uses photocopying to print the PCB layout on the film. If it is a multi-layer PCB, the layout film photocopied on each layer will be arranged in order.Figure 3. PCB Film Arranged in OrderThen the film will be punched with alignment holes. Alignment holes are very important, which is essential to align the materials of each layer of the PCB.Step 2. Plate ProductionClean the copper plate. If there is dust, it may cause the final circuit to be short-circuited or broken.Figure 4. Clean the Copper PlateThe figure below is an example of an 8-layer PCB, which is actually made up of 3 copper clad laminates plus 2 copper films, and then glued them together with prepregs. The production sequence is to start with the middle board (4th- and 5th-layer of circuits), continuously stack together, and then fix. The production of 4-layer PCB is similar, including one core board and two copper films.Figure 5. 8-layer PCB Plate DisplayStep 3. PCB Inner LayersFirst, make the two-layer circuit of the middle core board. After the copper clad laminate is cleaned, it will be covered with a photosensitive film on the surface. This film will solidify when exposed to light, forming a protective film on the copper foil.Figure 6. PCB CoreInsert the two-layer PCB layout film and the double-layer copper clad laminate into the upper PCB layout film to ensure that the upper and lower PCB layout films are stacked accurately.Figure 7. PCB Layout Film PlacingThe machine irradiates the photosensitive film on the copper foil with a UV lamp. The transparent film is cured under the light, and there is still no cured photosensitive film. The copper foil covered under the cured film is the required PCB layout, which is equivalent to the function of the laser printer ink of the manual PCB. In addition, the copper foil covered by the black film will be corroded away, and the cured transparent film will be preserved.Figure 8. Cured Photosensitive FilmClean the uncured photosensitive film with lye, and the required copper foil circuit will be covered by the cured film.Figure 9. Clean Uncured Photosensitive FilmThen use a strong base, such as NaOH, to etch away the unnecessary copper foil.Figure 10. Copper Foil EtchingTear off the cured photosensitive film to expose the copper foil of the required PCB layout.Figure 11. Tear Off the Cured Photosensitive FilmStep 4. Board Punching and CheckingThe core board has been successfully produced. Then punch alignment holes on it to facilitate with other materials.Figure 12. Punch Alignment Holes on PCBOnce the core board is pressed together with other layers, it cannot be modified. So PCB checking is very important. The machine will automatically compare with the PCB layout drawing to find out the error.Figure 13. PCB Layout Drawing ComparisonThe first two layers of PCB boards have been made.Step 5. LaminationA new raw material is introduced here called Prepreg, which is the adhesive among the core boards(PCB layers>4), as well as the core board and the outer copper foil, and it also plays a role in insulation.Figure 14. PCB Prepreg and CopperThe lower copper foil and the two layers of prepreg have been fixed in advance through the alignment hole and the lower iron plate, and then the finished core board is also placed in the alignment hole, and finally the two layers of prepreg, a layer of copper foil and a layer of pressure-bearing aluminum plate covers the core plate.Figure 15. Fixed PCB Prepreg and CopperIn order to improve work efficiency, this factory will stack three different PCB boards together before fixing them. The upper iron plate is magnetically attracted to facilitate alignment with the lower iron plate. After the two layers of iron plates are successfully aligned by inserting the alignment pins, the machine compresses the space between the iron plates as much as possible, and then fixes them with nails.Figure 16. Fixed PCB LayersThe PCB boards clamped by the iron plates are placed on the holder, and then sent to the vacuum heat press for laminating. The high temperature can melt the epoxy resin in the prepreg and fix the core boards and copper foils together under pressure.Figure 17. PCB Layers LaminationAfter the lamination, remove the upper iron plate that presses the PCB. Then remove the pressure-bearing aluminum plate. The aluminum plate also plays the role of isolating different PCBs and ensuring the smoothness of the outer copper foil of the PCB. Finally the PCB taken out at this time will be covered by a layer of smooth copper foil.Figure 18. Remove the Upper Iron Plate and Aluminum PlateStep 6. DrillSo how to connect 4 layers of copper foils that are not in contact with each other in the PCB? First, make the through-hole through the PCB, and then metalize the hole wall to conduct electricity.Figure 19. PCB DrillPut a layer of aluminum plate on the punching machine, and then put the PCB on it. Since drilling is a relatively slow process, in order to improve efficiency, according to the number of layers of the PCB, 1 to 3 identical boards are stacked for drilling together. Finally, cover the uppermost PCB with a layer of aluminum plate. The upper and lower of aluminum plates are used to prevent the copper foil on the PCB from tearing when drilling.Figure 20. PCB DrillNext, you only need to select the correct drilling program on the computer, and the rest is done automatically by the drilling machine. The drill bit is driven by air pressure, and the maximum rotation speed can reach 150,000 revolutions per minute. Because such a high rotation speed is sufficient to ensure the smoothness of the hole wall.Figure 21. Drill ProgramThe replacement of the drill bit is also automatically completed by the machine according to the program. The smallest drill bit can reach a diameter of 100 microns, while the diameter of a human hair is 150 microns.Figure 22. Drill ReplaceIn the previous process, the molten epoxy was squeezed out of the PCB, so it needed to be cut off. Here the profiling milling machine cuts its periphery according to the correct XY coordinates of the PCB.Figure 23. Cuts PCB PeripheryStep 7. Copper Chemical Precipitation on the HolesSince almost all PCB designs use perforations to connect different layers of lines, a good connection requires a 25-micron copper film on the hole wall. The thickness of the copper film needs to be realized by electroplating, but the hole wall is composed of non-conductive epoxy resin and glass fiber board. So the first step is to deposit a layer of conductive material on the hole wall, and form a 1 micron copper film on the entire PCB surface by chemical deposition. The entire process such as chemical treatment and cleaning is controlled by the machine.Step 8. PCB Outer LayersNext, the PCB outer layer is transferred to the copper foil. The process is similar to the transfer principle of the previous PCB inner core board. The PCB layout is transferred to the copper foil by photocopying film and photosensitive film. The only difference is positive films will be used as boards.The transfer of the internal PCB layout described above uses the subtractive method, and the negative film is used as the board. The PCB is covered by the cured photosensitive film as a circuit, and the uncured film is cleaned. After the exposed copper foil is etched, the PCB layout circuit is protected by the cured film. The transfer of the outer PCB layout adopts the normal method, and the positive film is used as the board. The non-circuit area is covered by the cured photosensitive film on the PCB. After cleaning the uncured film, electroplating is performed. Where there is a film, it cannot be electroplated, and where there is no film, copper is plated first and then tin is plated. After the film is removed, alkaline etching is performed, and finally the tin is removed. So the circuit pattern remains on the board because it is protected by tin.Put the cleaned PCB on both sides of the copper foil into the laminating machine, and the photosensitive mold will be pressed onto the copper foil.Figure 24. LaminatorFix the printed PCB layout film of the upper and lower layers through the holes, and put the PCB board in the middle. Then, the photosensitive film under the light-transmitting film is cured by the irradiation of the UV lamp, which is the circuit that needs to be reserved.Figure 25. PCB Expose to the UV LightAfter cleaning off the unnecessary and uncured photosensitive film, inspect the PCB board.Figure 26. PCB CheckingClamp the PCB with clips, and electroplate the copper. As mentioned earlier, in order to ensure that the holes have sufficient conductivity, the copper film plated on the hole walls must have a thickness of 25 microns, so the entire system will be automatically controlled by the computer to ensure its accuracy.Figure 27. PCB Copper PlatingStep 9. Computer Control and Copper ElectroplatingAfter the copper film is electroplated, the computer gives instructions to electroplate a thin layer of tin. Then, check to ensure that the thickness of the plated copper and tin is correct.Figure 28. Electroplated Copper and Tin InspectionNext, a complete automated assembly line completes the etching process. Then, clean the cured photosensitive film on the PCB.Figure 29. Clean Cured Photosensitive FilmThen use a strong alkali to clean the unnecessary copper foil covered by it.Figure 30. Clean the Unnecessary Copper FoilFinally, use the tin stripping solution to strip the tin plating on the PCB layout copper foil. After cleaning, the 4-layer PCB layout is complete. Frequently Asked Questions about PCB Manufacturing Process1. Which are the techniques of PCB manufacturing?There are several PCB manufacturing methods that a PCB can be submitted to before reaching the final product. These methods include preparing the board's surface, placing components, soldering, cleaning, and inspection and testing. 2. What is PCB design process?Step 1 – The DesignStep 2 – Printing the DesignStep 3 – Creating the SubstrateStep 4 – Printing the Inner LayersStep 5 – Ultraviolet LightStep 6 – Removing Unwanted CopperStep 7 – Inspection.Step 8 – Laminating the Layers 3. Which software is best for PCB design?Top 8 Best PCB Design Software of 2021PROTEL (Altium Designer)PADS (PowerPCB)ORCADAllegroEagle (Easily Applicable Graphical Layout Editor)KicadEasyEdaFritzing 4. What is a PCB layer?A PCB is defined as a number of copper layers in a well defined sequence. Copper layers of a PCB are usually just named layers or also called SIGNAL layer. However, to define the complete PCB, other layers are required. They are usually named by their functionality and position. 5. What are the components of a PCB?Some common PCB components include:Battery: provides the voltage to the circuit.Resistors: control the electric current as it passes through them. They’re colour coded to determine their value.LEDs: light emitting diode. Lights up when current flows through it, and will only allow current to flow in one direction.Transistor: amplifies charge.Capacitators: these are components which can harbour electrical charge.Inductor: stores charge and stops and change in current.Diode: allows current to pass in one direction only, blocking the other.Switches: can either allow current or block depending if they are closed or open.

IntroductionIn electronics, the diode has the unique characteristic of unidirectional conduction. The main functions are rectification, voltage stabilization, and detection. In addition, there are light-emitting diodes(LED) added with different materials for indication and illumination. In a diode circuit, current can only flow in from the anode and flow out of the cathode. According to different circuit requirements, there are many different types of diodes to choose from. Most of the early diodes were made of germanium single crystals. Later, with the solution of silicon materials and manufacturing processes, silicon tubes have been developed and popularized. Here's how to distinguish silicon(Si) diodes and germanium(Ge) diodes.CatalogIntroductionⅠ Circuit Properties: Si Tube vs Ge Tube1.1 Differences between Ge and Si Diode1.2 Differences between Ge and Si Transistor1.3 SummeryⅡ Common Diode Types UsesⅢ Common Transistor Models UsesⅣ New Development: SiC Schottky Diodes4.1 SiC Schottky Diode Basic4.2 SiC Tech Characteristics4.3 SiC Schottky Diode ApplicationsⅠ Circuit Properties: Si Tube vs Ge Tube1.1 Differences between Ge and Si DiodeThe circuit properties of Si diodes and Ge diodes are the same, and the manufacturing process is also the same. Due to the difference in materials, the thermal stability of Si diodes is good, and the thermal stability of Ge diodes is slightly poor.1) When the current is the same, the DC resistance of the Ge tube is smaller than that of the Si tube. However, as for AC resistance, the situation is opposite.2) According to experimental research, the Ge diode starts to have current at 0.2V in the forward direction, while the Si diode does not start to have current until 0.5V, that is to say, the initial voltages for the two to reach conduction are different.3) Under reverse voltage, the leakage current of silicon tube is much smaller than that of germanium tube. After the start of conduction, the Ge tube current increases slowly, and the Si tube current increases relatively quickly.4) The threshold voltage of silicon tube is higher than that of germanium tube, because the threshold current of silicon tube is much smaller than that of germanium tube. Generally, the threshold voltage of a silicon tube is about 0.5V~0.6V, and the threshold voltage of a germanium tube is about 0.1V~0.2V.5) Temperature changes have a greater impact on Ge diodes, but less on silicon diodes. Therefore, silicon tubes have better high temperature resistance than Ge tubes.6) Forward voltage required for diode conduction:Diode TypesForward Conduction VoltageSi0.7VGe0.15VIt can be seen from the above table that the forward voltage required for the silicon tube to be turned on is higher than that of the germanium tube, so the diode can be distinguished by knowing the forward voltage.In addition, there is a very direct method to measure your diode with the Ω barrier of a multimeter. As shown in the figure, the red pen (anode) of the multimeter is connected to the cathode of the diode, and the black pen (cathode) is connected to the anode of the diode. If the resistance of the tested diode is around 1kΩ, it is a germanium tube; if the resistance is 4~8kΩ, it is a silicon tube.Compared with germanium diodes, silicon diodes have higher voltage resistance, shorter response time, and stable performance. In most circuits, silicon tubes can replace germanium tubes, but its forward pressure drop is higher than that of germanium tubes. Therefore, in some specific environments, such as small signal detection circuits, germanium tubes are better.1.2 Differences between Ge and Si TransistorThe main difference is that the junction voltage drop is different, the forward voltage drop of the germanium tube is lower about 0.3V, and the silicon tube is higher about 0.7V. In addition, silicon materials are abundant and manufacturing processes are suitable for mass production, so they are widely used and become the protagonist of electronic devices.Germanium semiconductor materials have high electron mobility and are suitable for low-voltage and high-current devices, but the temperature characteristics of it is worse than that of silicon materials. The reverse leakage current of PN junctions is much larger than that of silicon materials. Therefore, silicon tubes have to be used in high-power devices and high back-pressure devices. The triode has two PN junctions. In terms of a PN junction, the forward voltage of the PN junction of the germanium tube is reduced to only 0.3V, while the silicon tube is 0.7V. The reverse withstand voltage germanium tube is very low, so it is easy to reverse breakdown. Therefore, the penetrating current of the Ge tube is relatively large, noise will be generated in the amplifying circuit, and it is easy to be damaged. 1.3 SummeryGermanium diodes was used a lot in early electronics, such as radios, but they have largely been replaced by silicon diodes. Because the structure of germanium crystals will be destroyed at higher temperature, while Silicon crystals are not easily damaged by excess heat. What's more, peak inverse voltage ratings of silicon diodes are greater than germanium diodes. As for price, Silicon material has low cost and its producibility of high quality silicon dioxide needed for impurity diffusion and surface passivation processes. Therefore, germanium tubes were only produced before the 1970s. Ⅱ Common Diode Types Uses① Zener DiodesThe Zener diode is also made of a PN structure. It is in the reverse breakdown state when working (the ordinary diode will be damaged in the reverse breakdown zone). When connected to the circuit, it should be reversed, that is, the anode of the Zener diode should be connected with the cathode of the voltage stabilizing circuit, so is the rest. The voltage stabilizing tube uses its reverse breakdown current to change in a wide range, the reverse breakdown voltage is basically unchanged, to achieve the purpose of voltage stabilization.② Light-emitting DiodesThe light-emitting diode emits light when it passes forward current, and has the performance of electro-optical conversion. The visible light includes red, yellow, green, blue, purple, etc. It is widely used in various electronic equipment as working status indicator.③ PhotodiodesThe reverse current of the photodiode increases with the increase of the light intensity. Its main feature is: the tube works in the reverse state, and the reverse current is proportional to the illuminance.④ Rectifier Diodes for AutomobilesThe working principle of the diode for automobile silicon rectifier generator is basically the same as that of other diodes, but the external structure is different from that of general diodes. It has one lead pole and the other pole is a shell. It is divided into two types: positive diode and negative diode. The terminal is the positive pole and the shell is the negative pole, while the leading end of the negative diode is the negative pole and the shell is the positive pole. In order to facilitate identification, the positive diode is usually coated with a red dot and the negative diode is coated with a black dot.⑤ Freewheeling DiodeFreewheeling diodes are common used in automobiles. In addition, fast recovery diode (a kind of semiconductor diode with good switching characteristics and short reverse recovery time) is mainly used for switching power devices (such as IGBT or MOSFET) of various power converters to play a freewheeling effect. Ⅲ Common Transistor Models UsesThe following table lists some commonly used transistor models and their main parameters, packages and alternative models.ModelBasic ParametersDescriptionPcVCBOVCEOVEBOhFEIC2SC2655(NPN, TO-92L)0.9W50V50V5V*2AHigh-speed switching tube, used for high-current PWM push-pull drive, complementary to the 2SA10202SC9013(NPN, TO-92)0.625W40V20V5V84-2020.5AComplementary to  the SS90122N5551(NPN, TO-92)0.63W180V160V6V30-2500.3AHigh voltage tube, complementary to the 2N5401KSP2222A(PN2222A)(NPN, TO-92)0.625W75V40V6V35-30006AUniversal switch type, complementary to the KSP2907AS8050(NPN, TO-92)0.625W40V25V5V50-3000.5AComplementary to the S8550BD681A(NPN, TO-12640W100V100V5V750-15004ACan drive large relays, Darlington tubeTIP41/A/B/C(NPN, TO-220)60W40~100V40~100V5V15-756AMiddle Power tubes, complementary to the TIP42, can be used for IGBT PWM driveMPSA44(NPN, TO-92)0.625W500V400V6V40-3000.3AHigh voltage tubeKSP2907A(PNP, TO-92)0.625W-60V-60V-5V50-3000.6ACommon-type, complementary to the KSP2222AS8550(PNP, TO-92)0.625W-40V-25V-5V50-3000.5AComplementary to the S80502SA1020(PNP, TO-92L)0.9W-50V-50V-5V40-2402AHigh-speed switch tube, complementary to the 2SC26552SA684(PNP, TO-92L)1W-60V-50V-5V50-3401ADarlington tubeTIP42/A/B/C(PNP, TO-220)65W-40~-100V-40~-100V-5V15-756AMiddle power tubes, complementary to the TIP41, can be used for IGBT PWM driveKSP94(PNP, TO-92)0.625W-400V-400V-6V40-3000.3AHigh voltage tubeⅣ New Development: SiC Schottky Diodes4.1 SiC Schottky Diode BasicSchottky diodes, also known as hot carrier diodes, form a Schottky barrier through metal and semiconductor contacts to achieve rectification. Compared with ordinary PN junction diodes, the reverse recovery inertia of it is very low. Therefore, Schottky diodes are suitable for high-frequency rectification or high-speed switching.Silicon carbide (SiC) is a high-performance semiconductor material, so SiC Schottky diode have advantages of higher energy efficiency, higher power density, smaller size and higher reliability. It can be used in power electronics to break the limit of silicon, and becomes the preferred device for new energy and power electronics.4.2 SiC Tech CharacteristicsSiC is a compound semiconductor composed of silicon and carbide. It provides a number of advantages over silicon. The band gap of SiC is 2.8 times that of silicon (wide band gap), reaching 3.09 eV. Its insulation breakdown field strength is 5.3 times that of silicon, up to 3.2MV/cm, and its thermal conductivity is 3.3 times that of silicon, about 49w/cm·k. Like silicon semiconductor materials, it can be made into junction devices, field-effect devices, and special Schottky diodes. Here is SiC characteristics:1) Silicon carbide single-carrier devices have a thin drift region and low on-state resistance, about 100-300 times smaller than silicon devices. Due to the small on-resistance, the forward loss of the silicon carbide power device is small.2) The silicon carbide power device has a high breakdown voltage due to its high breakdown electric field. For example, the voltage of commercial silicon Schottky diode is less than 300V, while the breakdown voltage of the first commercial SiC Schottky diode has reached 600V.3) SiC has higher thermal conductivity.4) SiC devices can work at higher temperatures, while the maximum operating temperature of Si devices is only at 150ºC.5) SiC has high resistance to radiation.6) The forward and reverse characteristics of SiC power devices vary little with temperature and time, and their reliability is good.7) SiC devices have good reverse recovery characteristics, with low reverse recovery current and switching loss.8) SiC devices can reduce the volume of power devices and circuit losses.4.3 SiC Schottky Diode ApplicationsSiC Schottky diodes can be widely used in medium and high power fields such as switching power supplies, power factor correction (PFC) circuits, uninterruptible power supplies (UPS), photovoltaic inverters, etc., which can significantly reduce circuit losses and improve circuit operating frequency.Substituting SiC SBD(Schottky barrier diodes) for the original silicon FRD(fast recovery diodes) in the PFC circuit can make the circuit work above 300kHz, and the efficiency remains basically unchanged, while the efficiency of the circuit using silicon FRD above 100kHz drops sharply. As the operating frequency increases, the volume of passive components such as inductors decreases correspondingly, and the volume of the entire circuit board decreases by more than 30%. Frequently Asked Questions about Silicon and Germanium Diodes1. Why silicon diode is better than germanium diode?The structure of Germanium crystals will be destroyed at higher temperature. However, Silicon crystals are not easily damaged by excess heat. Peak Inverse Voltage ratings of Silicon diodes are greater than Germanium diodes. Si is less expensive due to the greater abundance of element. 2. How can I tell if I have a Ge or Si diode?You can easily distinguish Silicon and Germanium Diodes. Silicon diodes should read approx 0.7V and Germanium diodes should read 0.3V. A little difficult to distinguish Schottky diodes though. They should show approx 0.2V which is close to 0.3V. 3. What are the differences between silicon and germanium semiconductor?The key difference between silicon and germanium is that the Germanium has d electrons, but Silicon does not have any d electrons. Silicon and germanium, are both in the same group (group 14) of the periodic table. Hence, they have four electrons in the outer energy level. 4. What is difference between silicon and germanium?Silicon and germanium have four valence electrons but at the given temperature germanium will have more free electrons and higher conductivity than silicon. Silicon is more widely used in the electronic device than germanium since it can be used at a higher temperature. 5. What is the difference between silicon and germanium transistors?To answer you actual question, there are two significant differences between silicon and germanium transistors: germanium has a low melting point and germanium transistors have much less tolerance for high temperatures. The forward voltage drop of a germanium junction is lower than for silicon.

IntroductionAs one type of relays, reed relays use an electromagnetic coil to control one or more flexible ferromagnetic metal reed switches directly. It is composed only by the contacts themselves that triggers the conduction in the secondary circuit. Reed relay switches can be used as magnetic proximity switches or relays, which are smaller in size, higher in speed and longer in working life than general mechanical switches. Here, let's explore this special relay.What is a Reed Relay? Characteristics IntroductionCatalogIntroductionⅠ A Special Relay: Reed RelayⅡ Structure Comparison: Relay vs Reed RelayⅢ Reed Relay CharacteristicsⅣ What is a Reed Relay Used For?Ⅰ A Special Relay: Reed RelayIn electronic circuits, relays are the switches which close and open the circuits contacts electronically as well as electromechanically. When the relay coil is energized or de-energized, it plays the role of automatic regulation, safety protection and circuit conversion. Relays are available with two main types. As everyone knows, relays are divided into mechanical relays and semiconductor relays.Figure 1. Mechanical Relay vs Semiconductor RelayOn the insulation detection circuit of BMS, we often use a device called PhotoMOS, which is used to switch the Wheatstone bridge. Actually, it is a semiconductor relay.Figure 2. PhotoMOS AQV258For example, AQV258 is a model we often choose.PhotoMOS is a semiconductor relay with LED as input and MOSFET as output, but we are not mainly discussing PhotoMOS today.In recent years, another device has appeared on the insulation detection circuit, which is called Reed Relay Switch, Reed Switch, Reed Relays, etc. It is also used for the switch of the Wheatstone bridge arm. Ⅱ Structure Comparison: Relay vs Reed RelayReed relay is a kind of mechanical relays, but its structure is different from the traditional electromagnetic relay. The traditional electromagnetic relay is an electrical relay that uses the suction force generated by the circuit in the input circuit between the electromagnet core and the armature to work. This kind of relay is large in size, slow in action, and limited in life, but it is reliable. With the development of electronic appliances in the direction of miniaturization, new requirements have been put forward for relays. In this case, reed relays can meet the needs of this development in many aspects.Figure 3. Electromagnetic Relay StructureThe reed relay is composed of two parts: a reed switch and a coil. Compared with other types of relays, it is more compact and has higher energy efficiency. The following describes its structure and principle in detail.As shown in the figure below, the magnetic reed switch consists of two magnetic reeds, which are sealed inside a glass tube. The two magnetic reeds have a contact overlap area at the middle end of the glass tube, and has a small gap. The inside of the glass tube is filled with inert gas, such as nitrogen, to prevent oxidation. In order to improve the strength of electrical breakdown, the inside of the glass tube can also be evacuated.Figure 4. Reed Relay StructureThe magnetic reed body is made of nickel-iron alloy, and the surface of the contact overlap area of the two magnetic reeds is plated with hard metals such as rhodium or ruthenium to play a role in wear resistance. Placing the magnetic reed switch in a magnetic field, when the two magnetic reeds are magnetized into opposite polarities, they generate attractive force, and the contact overcomes the elastic force to connect. Similarly, when the magnetic field disappears, the contact is pulled away without magnetic force.In order to allow the reed switch to be closed or opened by human control, the coil is wound on the surface of the glass tube, and the coil is energized, thus forming an electromagnet, which generates a magnetic field for the reed switch to act.Figure 5. Relay CoilTherefore, as a circuit switch device controlled by a magnetic field signal, the reed switch can be used as a sensor for counting and limiting (in the security system, it is mainly used for the production of door and window magnets). It is widely used in various communication devices. In practical applications, permanent magnets are usually used to control whether the two metal pieces are connected or not, so they are also called magnetic relay.What’s more, the more turns the coil has, the less current reed relay switch needs to work. The relationship between the number of turns and the working voltage is restricted by the following factors: the wire diameter of the coil, the number of turns, the resistivity of the conductor, and the size of the reed contact.In theory, the working principle of the reed relay is like a switch: when the permanent magnet is close to the reed switch or the magnetic field formed by the coil winding on the reed switch magnetizes the reed, the contact part of the reed will be attracted by the magnetic force. When the attractive force is greater than the elastic force of the reed, the normally open contact will attract and close; when the magnetic force is reduced to a certain extent, the contact is opened by the elastic force of the reed.Figure 6. Reed Relay Magnetic FieldThe lifespan of reed switches is shorter than that of semiconductor switches, but much longer than general electromagnetic relays. The following figure shows the operating parameters of a certain type of reed switch, which can be switched on and off millions of times.Figure 7. Life Test DataThe reason for using the reed relay in the bridge method is that it has higher insulation and withstand voltage regardless of the load end or input/output. Ⅲ Reed Relay Characteristics1) High reliabilityThe failure rate of general relays is set at 50ppm, so in order to meet this requirement, the quality level of reed switches is much higher than this standard. So far, there has not been any electromechanical device that can achieve this level of quality.2) High securityReed relay switch has excellent insulation properties. Its insulation resistance can reach up to 1015Ω, that is, its leakage current only has 10-15A. Such extremely low leakage levels are widely used in medical electronic equipment, such as probes inserted into the human body or cardiac pacemakers, because these equipment require no leakage current close to the heart, even if the current is microampere or submicroampere, which will change the electrical properties of key parts of the heart.3) High adaptabilityThe reed sensor is sealed. Because the switch components is airtightly sealed in an inert gas, it will not contact the external environment, so it can work in almost any environment without being sensitive to humidity and not affected by the outside world. At the same time, the reed switch has no special requirements for the ambient temperature and has a wide temperature adaptability. The typical working temperature range can be from -50℃ to +150℃, and no special additional conditions, restrictions or costs are required.4) Long lifespanBecause the reed switch is a wear-free component and does not use a sliding component, there will be no worn metals, ensuring practically unlimited mechanical service life. At the same time, because the contact made of inert precious metal rhodium has a high melting point, it can reduce the loss of arc power generation on the contact surface, so it is more resistant to wear and can maintain a longer working life. General reed switches can work up to a million times under low-level loads (less than 5V and lower at 10mA).5) Small sizeThe reed relay switch can be installed in a limited space and is very suitable for miniaturized equipment.6) Superior electrical performanceReed relay switches have a series of excellent electrical properties. For example, their contacts have extremely low on-resistance when they are turned on, typically as low as 50mΩ. In addition, their direct switching signals can range from a few nanovolts to thousands of volts, and the current ranges from micro An to An, etc.7) High speed operationSince each movable component has only a very small weight, the operating speed is very high, making the reed switch a part that can be used in transistor circuits or integrated circuits.Ⅳ What is a Reed Relay Used For?Reed relay switches have been widely used in many fields of automatic control systems, such as machinery, automobiles, electronics, electric power, petroleum, chemical industry, office automation, communications and other engineering. For example, in the military field, high-voltage reed relays are related to national defense components for modern weapons and equipment. Frequently Asked Questions about Reed Relay Basics and Uses1. What is a reed relay used for?Reed Relays are ideally used for switching applications requiring low and stable contact resistance, low capacitance, high insulation resistance, long life and small size. 2. What is the difference between relay and reed relay?A relay switch is composed of an electromagnet that actuates a high power switch made of two metallic contacts. ... A reed switch is composed only by the contacts themselves that triggers the conduction in the secondary circuit. 3. Why is reed relay used in a switching circuit?The reed relay consists of a switch with magnetic contacts that move under the influence of an external magnetic or the induced field from its solenoid. They have faster switching speed compared to the electromechanical ones but their switching current and voltage is lower mainly because of its contacts thickness. 4. What is required to operate a reed relay in a switching circuit?The coil surrounds the reed switch and generates the axial magnetic field needed to close the reed contacts. 5. Where is a reed switch used?Reed switches are used in fluid level sensors for brake fluid reservoirs and to monitor motor oil levels. They are also used in speed sensors for engine control and power steering. Automatic door locks, air bags, parking brakes, seat, door, and hood proximity sensors also utilize reed switches.

IntroductionA clamper circuit is an electronic circuit that shifts the DC level of a signal to a desired level without changing the shape of the applied waveform. Unlike clipper circuits that cut or limit portions of a signal, clampers preserve the entire waveform while repositioning it vertically on the voltage axis. This is achieved by fixing a specific part of the pulse signal (such as the positive or negative peak) at a specified voltage value while maintaining the original waveform shape unchanged.What is a Clamper Circuit?Ⅰ Clamper Circuit ApplicationsClamper circuits are widely used in various electronic systems and display devices. Key applications include:Television Systems: Clamper circuits restore the DC component of video signals and maintain the synchronization pulse at a fixed voltage level, ensuring stable image positioning and proper sync signal separation.Oscilloscopes and Test Equipment: Used to stabilize waveform display by fixing reference levels, preventing image drift caused by varying scanning speeds or DC component loss.Radar and Sonar Systems: Employed to maintain consistent signal levels for accurate detection and ranging.Amplifier Protection: Protects sensitive amplifier input stages from excessive DC offset voltages.Power Supply Circuits: Helps in voltage regulation and transient suppression.Communication Systems: Restores DC levels in signal transmission and reception circuits.Digital Logic Circuits: Provides voltage level shifting between different logic families.A basic clamper circuit comprises a capacitor, a diode, and a resistor. More sophisticated designs may include additional components such as bias voltage sources. In the following sections, we will explore different types of diode clamper circuits and compare their characteristics and performance.Ⅱ Diode Clamper Circuit2.1 Why Use Diode Clamper Circuits?While diode clipper circuits limit or cut the amplitude of waveforms, many applications require preserving the complete waveform while shifting its DC level. Clamper circuits fulfill this requirement by shifting the signal vertically to position its peak value at a desired level without distorting the original waveform shape.A diode clamper circuit utilizes the relatively stable forward voltage drop of the diode (typically 0.6-0.7V for silicon diodes or 0.2-0.3V for Schottky diodes) and its low reverse leakage current characteristics. These properties enable the circuit to clamp the potential at specific points and maintain the peak or trough of periodically changing waveforms at predetermined DC levels.Dual-Diode Clamper Protection: In protection applications, two diodes connected in reverse parallel configuration provide bidirectional clamping. Only one diode conducts at any given time while the other remains in the off state. This arrangement limits both positive and negative voltage excursions to approximately ±0.6V (for silicon diodes), effectively protecting sensitive circuit components from overvoltage conditions and electrostatic discharge (ESD).2.2 Diode Clamper Circuit TypesDiode clamper circuits are classified into two main categories: positive clampers and negative clampers. Each category includes both simple (unbiased) and biased variants.✅ Diode Positive ClamperOperating Principle:Positive Half Cycle: The diode is reverse-biased (OFF), acting as an open circuit. The capacitor charges to the peak input voltage Vi through the load resistor.Negative Half Cycle: The diode becomes forward-biased (ON), acting as a short circuit. The capacitor maintains its charge, and the output voltage Vo ≈ 0V (or slightly positive due to diode forward voltage drop).According to Kirchhoff's voltage law, the output waveform can be calculated for both positive and negative cycle conditions.(1) Simple Positive Clamper (Unbiased)Figure 1. Simple Positive Clamper CircuitOperation:When Vi is in the negative half cycle: D → ON, capacitor C charges to voltage V (negative on left plate, positive on right plate), Vo ≈ 0V.When Vi is in the positive half cycle: D → OFF, Vo = VC + Vi = 2V (assuming input amplitude is V).(2) Biased Positive ClamperFigure 2. Biased Positive Clamper CircuitSimple Method to Determine Output Waveform:The reference point of the output waveform on the voltage axis is determined by the bias voltage V1.The diode orientation determines the direction of waveform shift. If the diode points upward , the waveform shifts upward; if it points downward , the waveform shifts downward.After determining the reference point and direction, sketch the original waveform on the output coordinate axis using the reference point as the baseline to obtain the clamped output waveform.↪️ Diode Positive Clamper Circuits Comparison:Figure 3. Positive Clamper Circuits Comparison✅ Negative Clamper Circuit(1) Simple Negative Clamper (Unbiased)Figure 4. Simple Negative Clamper CircuitOperation:When Vi is in the positive half cycle: D → ON, capacitor C charges to voltage V (positive on left plate, negative on right plate), Vo ≈ 0V.When Vi is in the negative half cycle: D → OFF, Vo = -(VC + |Vi|) = -2V (assuming input amplitude is V).(2) Biased Negative ClamperFigure 5. Biased Negative Clamper CircuitOperation:When Vi is in the positive half cycle: Diode D → ON, capacitor C charges to voltage V (positive on left plate, negative on right plate), Vo = +V1 or -V1 (depending on bias polarity).When Vi is in the negative half cycle: Diode D → OFF. With a sufficiently large RC time constant, Vo = VC + Vi (negative half cycle) ≈ -2V + bias voltage.↪️ Diode Negative Clamper Circuits Comparison:Figure 6. Negative Clamper Circuits Comparison✅ Key Design ConsiderationsDiode Orientation: The direction of the diode determines whether the waveform shifts upward (positive clamping) or downward (negative clamping).Bias Voltage: The bias voltage establishes the reference point (baseline) of the clamped waveform on the voltage axis.RC Time Constant: The product of capacitance (C) and load resistance (R) must be sufficiently large—typically RC ≥ 10T, where T is the period of the input waveform. This ensures the capacitor maintains its charge between cycles, preventing droop and maintaining clamping accuracy.Diode Selection: Choose diodes with low forward voltage drop (Schottky diodes for precision applications) and fast recovery time for high-frequency signals.Capacitor Selection: Use capacitors with low leakage current (film or ceramic types) to maintain charge stability.Ⅲ Practical Application: GPIO Protection Using Clamper CircuitsA practical application of clamping diodes is found in GPIO (General Purpose Input/Output) pin protection circuits. This example demonstrates the use of dual-diode clampers in the Qualcomm MSM8909 platform to prevent electrostatic discharge (ESD) damage and electrical overstress (EOS).Circuit AnalysisFigure 7. MSM8909 GPIO Internal Protection CircuitCircuit Configuration:Clamping diode D1: Cathode connected to VDD (positive supply rail), anode connected to GPIO pinClamping diode D2: Anode connected to GND (ground), cathode connected to GPIO pinProtection Mechanism:When input voltage > VDD: D1 conducts (forward-biased), D2 is off (reverse-biased). The GPIO pin voltage is clamped to approximately VDD + 0.6V, with excess current shunted to the VDD rail.When input voltage < GND: D1 is off (reverse-biased), D2 conducts (forward-biased). The GPIO pin voltage is clamped to approximately GND - 0.6V, with excess current shunted to ground.Normal operation (GND < Vin < VDD): Both diodes remain off, allowing normal signal operation without interference.This dual-diode configuration effectively limits the input voltage to the safe operating range of [GND - 0.6V, VDD + 0.6V], protecting the GPIO pin from ESD events and voltage transients.Diagnostic Procedure: Testing GPIO Protection DiodesTo determine whether a GPIO pin has been damaged by ESD or EOS, follow this multimeter-based diagnostic procedure:Equipment Required:Digital multimeter with diode test functionAnti-static wrist strap (recommended)Circuit schematic or pinout diagramTest Procedure:Power Down: Ensure the device is completely powered off and disconnected from all power sources.Test Diode D2 (Lower Clamp to GND):Set multimeter to diode test modeConnect RED probe to motherboard GNDConnect BLACK probe to the GPIO pin under testExpected Result: Forward voltage drop of 0.4-0.7V (typically 0.6V for silicon diodes)Failure Indication: Reading significantly outside this range indicates D2 damage:Very low reading (< 0.2V): Diode is shortedOpen circuit (OL or > 2V): Diode is openTest Diode D1 (Upper Clamp to VDD):Reverse probe connections:Connect RED probe to the GPIO pin under testConnect BLACK probe to VDD rail (or appropriate power pin)Expected Result: Forward voltage drop of 0.4-0.7VFailure Indication: Similar interpretation as D2 testReverse Bias Test (Optional):Reverse the probe connections for each testExpected Result: Open circuit (OL) or very high resistanceFailure Indication: Low resistance in reverse bias indicates diode breakdownImportant Notes:Always discharge any residual capacitance before testingSome modern ICs may have additional protection elements that affect readingsCompare readings with a known-good board when possibleDocument all measurements for troubleshooting recordsIf protection diodes are damaged, the internal GPIO circuitry may also be compromisedFrequently Asked Questions about Clamper Circuits1. What is a clamper circuit and what are its types?A clamper circuit is an electronic circuit that shifts the DC level of an AC signal to a desired voltage level without altering the shape of the waveform. Since the DC level is shifted, a clamper circuit is also called a level shifter. Clamper circuits utilize energy storage elements, primarily capacitors. A basic clamper circuit consists of a capacitor, a diode, a resistor, and optionally a DC bias voltage source. The main types are: positive clampers (shift waveform upward), negative clampers (shift waveform downward), and each can be either biased (with reference voltage) or unbiased (simple configuration).2. How do clamper circuits work?A clamper circuit operates by using a capacitor to store charge during one half-cycle of the input signal and a diode to control the charging and discharging process. During the half-cycle when the diode conducts, the capacitor charges to approximately the peak voltage of the input signal. During the opposite half-cycle, the diode blocks, and the capacitor voltage adds to (or subtracts from) the input voltage, effectively shifting the entire waveform up or down. The RC time constant must be large enough (typically RC ≥ 10T) to maintain the capacitor charge between cycles, ensuring consistent clamping action.3. What is a diode clamper circuit?A diode clamper circuit is a specific implementation of a clamper that uses a diode as the switching element to control the charging of the capacitor. The circuit consists of a capacitor, a diode, and a resistor arranged to shift the waveform to a desired DC level. The diode's unidirectional current flow property ensures that the capacitor charges during one half-cycle and maintains its charge during the other half-cycle, creating the clamping effect. The diode's orientation determines whether the circuit functions as a positive or negative clamper.4. How many diodes are used in a clamper circuit?A basic clamper circuit requires a minimum of one diode, along with a capacitor and a resistor. However, protection circuits and bidirectional clampers may use two diodes connected in reverse parallel (anti-parallel) configuration to provide clamping in both positive and negative directions. Some advanced designs may incorporate additional diodes for improved performance, temperature compensation, or multiple voltage level clamping. An independent DC voltage source may also be added to create biased clamper circuits with adjustable reference levels.5. What is a clamping diode used for?Clamping diodes serve multiple purposes in electronic circuits: (1) Level Shifting: They shift AC signals to desired DC levels in signal processing applications. (2) Voltage Protection: They protect sensitive components from overvoltage conditions by limiting voltage excursions to safe levels (typically within ±0.6V of supply rails). (3) ESD Protection: In integrated circuits, clamping diodes protect GPIO pins and other I/O interfaces from electrostatic discharge damage. (4) Transient Suppression: They absorb voltage spikes and transients in power supply and signal lines. (5) Signal Restoration: In video and communication systems, they restore DC components that may be lost during AC coupling or transmission.6. What is the difference between a clipper and a clamper circuit?Clipper circuits cut off or limit portions of the input waveform that exceed certain voltage levels, fundamentally changing the waveform shape. Clamper circuits preserve the entire waveform shape but shift its DC level (vertical position on the voltage axis). Clippers are used for waveform shaping and overvoltage protection, while clampers are used for DC restoration and level shifting. Clippers typically use diodes with resistors, while clampers require capacitors in addition to diodes and resistors.7. Why is the RC time constant important in clamper circuits?The RC time constant (τ = R × C) determines how quickly the capacitor charges and discharges. For proper clamping action, the RC time constant must be much larger than the period of the input signal (typically RC ≥ 10T). This ensures that: (1) The capacitor charges quickly during the conducting half-cycle of the diode, (2) The capacitor maintains its charge during the non-conducting half-cycle with minimal voltage droop, and (3) The clamping level remains stable across multiple cycles. If the RC time constant is too small, the capacitor will discharge significantly between cycles, resulting in poor clamping performance and waveform distortion.ConclusionClamper circuits are essential components in modern electronics, providing DC level shifting and voltage protection across a wide range of applications. Understanding the operating principles of positive and negative clampers, both biased and unbiased configurations, enables engineers to design effective signal conditioning and protection circuits. The practical application in GPIO protection demonstrates the critical role of clamping diodes in safeguarding sensitive integrated circuits from ESD and overvoltage damage. Proper component selection, particularly regarding the RC time constant and diode characteristics, is crucial for optimal clamper circuit performance.Note: This article was originally published in 2020 and has been updated in 2025 to reflect current technology standards, correct technical inaccuracies, and include additional practical information about clamper circuit applications and diagnostics.

IntroductionLC circuit, also called passive filtering circuit, is commonly used for harmonic compensation, which does not require additional power supply. It is generally composed of capacitors, inductors and resistors. This kind of filtering circuit is easy to design, but its pass-band magnification and cut-off frequency vary with the load, which is not suitable for occasions with high signal processing requirements. Passive filtering circuits are usually used in power circuits, like DC power rectification, or large current loads.LC (Inductor-Capacitor) CircuitsCatalogIntroductionⅠ LC Circuit TypesⅡ LC Circuit CharacteristicsⅢ LC Circuit Working FeaturesⅣ LC Circuit FunctionsⅤ LC Circuit CalculationⅠ LC Circuit TypesCommonly used filter circuits include passive filter and active filter. If the circuit are only composed of passive components (resistors, capacitors, inductors), it is called a passive filter circuit. The main forms of passive filtering are capacitive filtering, inductive filtering and compound filtering (including inverted L-type, LC-type , LC-π type and RC-π type, etc.). The filter circuit is not only composed of passive components, but also composed of active components (bipolar tubes, unipolar tubes, integrated operational amplifiers), which is called an active filter circuit, and its main form is active RC(resistor-capacitor) circuit.The LC circuit has advantages of simple structure, high reliability, and wide range of applications. Its main feature is that the resistance of the inductor is small, and the DC loss is small. What’s more, the inductance in LC circuit to alternating current is large, so that the filtering effect is good.LC circuits are divided into LC low-pass filters, LC band-pass filters, high-pass filters, LC all-pass filters, and LC band-stop filters according to their functions. According to tuning, they are further divided into single-tuned filters, double-tuned filters and three-tuned filters and so on. The LC filter design process mainly considers its resonant frequency, capacitor withstand voltage, and inductor withstand current. In addition, it is necessary to pay attention to that passive LC circuits are not easy to integrate.  Ⅱ LC Circuit CharacteristicsThe LC circuit is generally formed by a proper combination of inductors, capacitors and resistors, and parallel with the harmonic source. It not only for filtering, but also takes into account the role of compensation.Figure 1. LC Circuit TypesThe following are the characteristics of several LC circuits:1) The load impedance of the L-type filter is high, and the source impedance is low.2) The load impedance of the inverted L filter is low, and the source impedance is high.3) The load impedance of the T-type filter is low, and the source impedance is low.4) The load impedance of the Π filter is high, and the source impedance is high.In actual use, these features are generally followed. In practice, impedance is difficult to estimate, especially in high frequency bands. Due to parasitic parameters, the circuit impedance changes greatly, and the circuit impedance is also related to the working state of the circuit, so the debugging shall prevail in practice. Ⅲ LC Circuit Working FeaturesAccording to the different impedance of reactive components to AC and DC, the basic form of the LC circuit composed of capacitor C and inductance L is shown in the figure. Because the capacitor C is open to DC and has a small impedance to AC, C is connected in parallel at both ends of the load, while the inductor L has a small impedance to DC and a large impedance to AC, so L should be connected in series with the load.In electronic circuits, the inductor coil acts on the finite current of the alternating current. From the inductance formula XL=2πfL, it can be known that the larger the inductance L, the higher the frequency f, and the larger the inductive reactance. We already know that capacitors have the ability to "block DC and pass AC", while inductors have the function of "pass DC, block AC, pass low frequencies, and block high frequencies". If the direct current accompanied by many interference signals is passed through the LC filter circuit, most of the AC interference signals will be prevented by the inductance from being absorbed and turned into magnetic induction and thermal energy, and most of the rest will be bypassed to the ground by the capacitor, which can suppress the interference. So a relatively pure DC current at the output is getting.The inductance of the power supply on PCB is generally made of a very thick enameled wire wrapped around a round magnetic core coated with various colors. In addition, there are usually several tall filter aluminum electrolytic capacitors nearby, which is a classic LC filter circuit. In addition, PCB also uses a large number of snake-type lines and chip tantalum capacitors to form an LC circuit. Few people notice, when design LC circuit, the snake line folds back and forth on the circuit board, which can also be regarded as a small inductance.In short, the principle of the LC filter circuit is actually a combination of the basic characteristics of the L and C components. Because the capacitive reactance of the capacitor xc=2nfc will decrease as the signal frequency increases, and the inductance of the inductor xl=2f will increase as the signal frequency increases. If the capacitors and inductors are connected in series, parallel or mixed together, the impedance of their combination will also vary greatly depending on the frequency of the signal. This shows that different filter circuits will present a small or large reactance to a certain frequency signal, so that it can pass or block the frequency signal. Thereby filter circuit plays a role in selecting a certain frequency signal and filtering out a certain frequency signal.Figure 2. LC Resonating CircuitsⅣ LC Circuit Functions1) LC circuit can be used as a filter circuit used in some power supplies. If the DC power with many interference signals is passed through the LC filter circuit, most of the AC interference signals will be blocked by the inductance and become magnetic induction and thermal energy. Most of them are bypassed to ground by capacitors, which can suppress the effect of interference signals and obtain relatively pure DC current output.2) Have oscillation effect, or sometimes generate a clock signal for the single-chip microcomputer (some have an external clock signal). 3) It can also be used as a frequency selection circuit, or a transmitting oscillation circuit (but it must be a circuit that can send and receive signals), and it also serves for the generation of input signals from other circuits.4) Since the output voltage of the rectifier circuit is not all pure DC, the output of the rectifier circuit observed from an oscilloscope is very different from the DC, and the waveform contains a large pulsating component, which is called ripple. In order to obtain the ideal DC voltage, it is necessary to use an LC circuit composed of reactive components (such as capacitors and inductors) with energy storage function to filter out the pulsating elements of output voltage.Ⅴ LC Circuit Calculation1) The voltage of the RC tank capacitor is:Voltage=U*exp(-t/rc)U represents the initial value of the voltage, rc represents the resistance and capacitance, t is the elapsed time, and exp (-t/rc) represents the -t/rc power of e.Time constant τ=rcNamely the product of capacitance and resistance, voltage = U*exp(-t/τ) after introducing the time constantTherefore, the voltage change with zero input response is an exponential decay process, theoretically infinite time, but generally it is considered that the decay is over after 3 to 5 time constants.Therefore, the discharge time depends on the time constant τ=rc2) For the LC oscillating circuit, it is related to the product of the LC circuit.Specifically, for the general LRC loop,R>2*sqr (L/R)R=2*sqr(L/R)R>2*sqr (L/R)sqr (X) represents the root sign (X)Divided into three situations, roughly speaking, the discharge time depends on the value of R, L, C in the circuit, U is not equal to 0 and I = 0, the capacitor discharges through L, RTwo eigenvalues can be obtained by solving second-order partial differential equations such as:p1=-(R/2L)+spr[(R/2L)*(R/2L)-1/LC]p1=-(R/2L)-spr[(R/2L)*(R/2L)-1/LC]Capacitor voltage=[U/(p2-p1)]*[p2exp(p1*t)-p1exp(p2*t)]Based on this, the relationship between capacitor discharge time and LRC can be analyzed. Frequently Asked Questions about LC Circuit1. What is meant by LC circuit?An LC circuit is a circuit that uses the elements inductor (L) and capacitor ( C). This circuit is also referred as resonating circuit, tank circuit, or tuned circuit. 2. What does an LC circuit do?LC circuits are used either for generating signals at a particular frequency, or picking out a signal at a particular frequency from a more complex signal; this function is called a bandpass filter. 3. Where are LC circuits used?The LC circuit is used to select or generate a specific frequency signal. The application of LC circuits is reflected in many electronic devices, especially radio devices, such as transmitters, radio receivers and television receivers, amplifiers, oscillators, filters, tuners and frequency mixers. 4. What are the properties of LC circuit?An LC circuit is a closed loop with just two elements: a capacitor and an inductor. It has a resonance property like mechanical systems such as a pendulum or a mass on a spring: there is a special frequency that it likes to oscillate at, and therefore responds strongly to. 5. What is LC in LC oscillation?LC oscillations- The electric current and the charge on the capacitor in the circuit undergo electrical LC oscillations when a charged capacitor is connected to an inductor. The electrical energy stored in the capacitor is its initial charge which is named as q_m.

IntroductionDo you know buffer amplifier or isolation amplifier? The operational amplifier is an extremely efficient and versatile device. As we all known, the op amp is a component that amplifies the weak signal, which can be made into different forms according to the circuit requirements, and the voltage follower is one of them. Most voltage follower circuit will use an Op-amp. A follower is specifically an op amp wired to have a gain of +1. IE, the output is the same polarity and voltage as the input. That is, the output signal is exactly the same as the input signal. Here op amp voltage follower is used to isolate the signal and enhance load capacity. Op-amp as Voltage FollowerIntroductionⅠ Voltage Follower OP AmplifierⅡ Voltage Follower Characteristics2.1 Op Amp Impedance Matching2.2 Buffer Amplifier & Isolation AmplifierⅢ Op Amp Follower Circuit Analysis3.1 Op Amp Voltage and Load3.2 Op Amp Voltage Follower Stability3.3 Op Amp Phase Difference Problem3.4 Adding Feedback ResistanceⅣ Op Amp Voltage Follower ApplicationⅠ Voltage Follower OP AmplifierThe op amp follower sacrifices the voltage amplification factor in exchange for the performance of increasing the input impedance and reducing the output impedance. Because the gain of the op amp is extremely high, the input impedance of the op amp follower tends to be infinite, and the output impedance tends to zero. Within the rated output current range, the feedback voltage is equal to the output voltage, the output voltage is in phase with the input voltage, and the output voltage is slightly smaller than the input voltage. It should be noted that voltage follower is a special case of negative feedback amplifier (voltage series).Op amp voltage follower is actually a simple circuit structure which play a role in impedance matching. When a weaker signal is used to drive a relatively high current, voltage follower is often added in the middle, so that it can make weak signal stronger. It improves the load capacity to a considerable extent, while ensuring that the waveform and amplitude of the signal remain unchanged.For example, a single-chip microcomputer outputs a PWM signal to control LED lights. One LED does not require much current, so there is generally no big problem, however, when multiple LEDs need to light, current may definitely not large enough. If the current output is not enough, which may affect the signal output by the single-chip microcomputer, in this way, the voltage follower comes in handy.Ⅱ Voltage Follower Characteristics2.1 Op Amp Impedance MatchingWhen the op amp gain is approximately 1, that is, the magnification is approximately 1. The "follow" in the follower means that the voltage remains unchanged before and after, and the output waveform is almost not lost. It can be composed of transistors or operational amplifiers (best). Because the op amp input impedance is large and the output impedance is small, voltage follower can reduce the impact on the signal and improve the load capacity.2.2 Buffer Amplifier & Isolation AmplifierHere is a question, how to understand the buffering effect? Is the voltage of the former having a small impact on the back circuit? No, it is equivalent to a constant voltage source. Within the design requirement, no matter how the circuit connected to the subsequent stage changes, the output voltage is constant and does not change. In this way, the magnification or other performance of the previous stage can be kept unchanged. Otherwise, if the previous-stage input impedance is large, and the latter stage is small, the signal will definitely be distorted. For example, if a sinusoidal voltage waveform with a peak value of 10V, the sinusoidal peak value loaded to the latter stage may only be 8V. After adding a voltage follower, the waveform loaded on the input of the voltage follower will basically not change, and the input-output stage voltage ratio is very close to unity. So there will be no distortion.Since the output impedance of the voltage amplifier is generally relatively high, usually in the range of several kiloohms to tens of kiloohms. If the input impedance of the subsequent stage is relatively small, part of the signal will be lost in the output resistance of the previous stage. At this time, a voltage follower is needed to buffer from it. Another advantage of applying a voltage follower is that the op amp input impedance is increased, so that the capacity of the input capacitance can be greatly reduced, which provides a prerequisite guarantee for the application of high-quality capacitors.Another question, what about isolation? Because the op amp input impedance of the voltage follower is very large, it can be approximated as an open circuit. Of course, this open circuit is for the previous circuit. In this case, the previous circuit will not affect the subsequent circuit. However, "open circuit" means what, is it really open? No, the previous voltage is transmitted, but the corresponding current is not transmitted. This is the isolation effect.For example, if the MCU outputs a PWM waveform, you want to use it to control the brightness of a small light bulb. However, the output capacity of the IO port of the general MCU is limited. You can directly use the PWM output from the IO port to drive one light bulb. More than one doesn't work. In this case, you can add a voltage follower, so that the voltage is still the original voltage, but the driving ability has improved. Of course, the output capacity is not increased out of thin air, but comes from the input power of the op amp. In electronics, the diode has current amplification capability, and its source of amplification capability also comes from the power supply.In Hi-Fi circuits, the controversy about negative feedback has been around for a long time. In fact, if there is no negative feedback, most amplifying circuits will not work well. However, due to the introduction of a large loop negative feedback circuit, the back EMF of the speaker will pass through the feedback circuit and be superimposed with the input signal. The sound quality is blurred and the clarity is reduced. Therefore, some of the final stages of the power amplifier adopt a circuit without large loop negative feedback, trying to eliminate the disadvantages by disconnecting the negative feedback loop. However, since the operating current of the final stage of the amplifier varies greatly, its distortion is difficult to control. Here, the function of the voltage follower is just for the application. Putting the circuit between the front stage and the power amplifier can cut off the interference effect of the back electromotive force of the speaker on the front stage, so that the clarity of the sound quality is greatly improved. Ⅲ Op Amp Follower Circuit Analysis3.1 Op Amp Voltage and LoadFigure 1. Op Amp Voltage Follower SchematicThe output and the inverting input terminal are connected in series with a 10k resistor to ensure excellent characteristics. An ac signal is input at the non-inverting input terminal. Of course, dc and ac are all okay, so you will get a very high voltage at the output terminal. AC voltage that is similar and has excellent load capacity, with buffering and isolation effects.3.2 Op Amp Voltage Follower StabilityThe problem of using a voltage follower to keep the operational amplifier stable, that is, how to reduce the oscillation in the amplifier circuit using negative feedback to maintain stability, there is still no final conclusion. The ideal operating state of the op amp is that the output voltage and the input voltage are in phase, that is, when the applied voltage at the negative input causes the output to increase, the op amp can reduce the increased voltage accordingly. However, there is always a difference in phase between the input and output in reality. When the phase difference between the output and the output is 180°, the negative input and the positive input are exactly the same, but the output that should have been reduced is enhanced. It becomes a state of positive and negative collapse. If it falls into this state in a specific frequency band and still maintains the original amplitude, then the output frequency and oscillation state will continue.Figure 2. Feedback Loop3.3 Op Amp Phase Difference ProblemThe main reason for the phase difference between the input and the output:1) Due to the inherent characteristics of op amps.2) Due to the characteristics of the other feedback loop in circuit.Figure 3. Gain-frequency, Phase-frequency CurveFig 3(a), Fig 3(b)and Fig 3(c) respectively represent the voltage gain-frequency characteristic and phase-frequency characteristic of the operational amplifier. As shown in the figure, the voltage gain and phase vary with frequency. The difference between the op amp gain and the gain after feedback (0dB when using a voltage follower) is the gain (feedback gain) of the feedback loop. If the feedback gain is less than 1 time (0dB), then, the phase changes by 180° and returns to the positive feedback state, the negative gain will gradually attenuate in the circuit and theoretically will not cause oscillation.On the contrary, when the phase changes by 180°, if the loop gain corresponding to the frequency is 1 time, the original amplitude will be maintained. If the loop gain corresponding to the frequency is greater than 1, the amplitude will gradually diverge. In most cases, in the process of amplitude divergence, the amplitude is limited due to the influence of nonlinear elements such as the maximum output voltage, and the oscillation state will be maintained.Therefore, the difference between the phase corresponding to the frequency when the loop gain is 0dB, 180° is an important factor for judging the stability of the negative feedback loop, and this parameter is called the phase margin. Unless otherwise specified, when a single amplifier is used as a voltage follower, sufficient phase margin must be maintained (Fig 3b.).3.4 Adding Feedback ResistanceWhen the operational amplifier is used as a follower, when the internal resistance of the signal source is large, adding a feedback resistor with the same resistance as the internal resistance of the signal source can reduce the output offset voltage and improve the follow accuracy. The follower with feedback resistance has a certain current limiting protection effect on the circuit when the circuit is "blocked", which is its advantage.The voltage follower is originally a non-inverting operational amplifier. One of the common features of it is that a common-mode voltage is added to the non-inverting terminal and the inverting terminal.Once this common-mode voltage exceeds the allowable common-mode input voltage range, for example, if the inverting terminal signal is too large, it will cause the input stage transistor to saturate. The inverting terminal signal will be directly added to the second stage of the op amp, making the inverting input becomes non-inverting input, that is, negative feedback becomes positive feedback, and the output signal passes through the feedback loop to further saturate the input stage transistor. As a result of this, the amplifier is of course no longer in normal working condition. Even if the input signal is canceled, it will not immediately return to the normal state. This phenomenon is called blocking.When it occurs, if the feedback loop resistance is not large enough, the current in the feedback loop may burn the input stage transistors and even harm the second stage. In order to avoid blocking, in addition to choosing an op amp with a large common-mode input voltage range, a clamp circuit is often added to the input of the amplifier to ensure that the common-mode voltage at the input does not exceed the allowable range.Of course, in a small-signal inverting operational amplifier, especially in circuits with capacitive elements such as integrating operational amplifiers, blocking may also occur. The processing method is the same as that of the non-inverting amplifier. Ⅳ Op Amp Voltage Follower ApplicationIn many typical circuit designs, there will be an op amp follower before the AD converter. Whether this follower is necessary or not depends on the requirements of the circuit based on the understanding of the function of the follower. First analyze the role of the voltage follower here:The function of the voltage follower here is impedance transformation.Impact 1: The input impedance becomes very high, so that the impact on the input signal can be small.Impact 2: The output impedance becomes very low, and the impact of AD input impedance on the input signal can be very small.It can be seen that the follower is very meaningful. Secondly, analyze your own circuit and the signal under test to make a decision whether to use a follower. Here are some rules to confirm:1) If the output impedance of the signal is very small, then the Impact 1 can be ignored.2) If the input impedance of AD converter is very large, then two impacts can be ignored.3) If both impacts can be ignored, no voltage follower is necessary.4) If there is an impact, a voltage follower is needed. Frequently Asked Questions about Op Amp Voltage Follower1. Which amplifier is called as voltage follower Why?This means that the op amp does not provide any amplification to the signal. The reason it is called a voltage follower is because the output voltage directly follows the input voltage, meaning the output voltage is the same as the input voltage. 2. What is the use of voltage follower?A voltage follower can be used as a buffer because it draws very little current due to the high input impedance of the amplifier, thus eliminating loading effects while still maintaining the same voltage at the output. 3. What do you mean by voltage follower circuit?A voltage follower is also known as a unity gain amplifier, a voltage buffer, or an isolation amplifier. In a voltage follower circuit, the output voltage is equal to the input voltage; thus, it has a gain of one (unity) and does not amplify the incoming signal. 4. What is an op amp buffer?An op-amp voltage buffer mirrors a voltage from a high-impedance input to a low-impedance output. 8 min read. A voltage buffer, also known as a voltage follower, or a unity gain amplifier, is an amplifier with a gain of 1. It's one of the simplest possible op-amp circuits with closed-loop feedback. 5. What is an op amp buffer circuit used for?A buffer is a unity gain amplifier packaged in an integrated circuit. Its function is to provide sufficient drive capability to pass signals or data bits along to a succeeding stage. Voltage buffers increase available current for low impedance inputs while retaining the voltage level.