The Kynix Blog

IntroductionTime relay refers to a kind of relay whose output circuit needs to make an obvious change (or contact action) after adding (or removing) the input action signal in a specified and accurate time. It is an electrical component used in a circuit with a lower voltage or a smaller current to switch on or off a circuit with a higher voltage and larger current.  With the development of electronic technology, electronic time relays have become mainstream products in time relays. Electronic intelligent digital display time relays using large-scale integrated circuit technology have many working modes, which can not only achieve long delay time but also have high time-delay accuracy, small size, convenient adjustment and long service life, making the control system simpler and more reliable. The time relay also has the function of automatic monitoring. Time relay and other equipment together can form a program space route to realize the automatic operation of the equipment.Time Relay Basics ExplainedCatalogIntroductionⅠ Time Relay Basics  1.1 What is a Time Delay Relay?  1.2 Time-delay Relay Working Principle  1.3 Timer Relay Structure  1.4 Timer Relay Parameters  1.5 Four-type Time Relay Contacts Ⅱ Understanding Time Delay in Relay CircuitⅢ Time Relay Classifications  3.1 According to Working Principle  3.2 According to the Delay ModesⅣ How to Wire Time Relay?Ⅴ Time Relay ApplicationsⅥ Time Relay SelectionⅦ Timer Relay Using Instructions  7.1 General Ideas  7.2 Two Points for Attention in Using Time RelaysⅧ Case Study: Time Relay Switch in Light CircuitⅨ Frequently Asked Questions about Time Delay Relay BasicsⅠ Time Relay Basics1.1 What is a Time Delay Relay?The time relay is a very important component in the electrical control system. In many control systems, use the time relay to achieve delay control. Time relay is an automatic control electrical appliance that uses the principle of electromagnetic or mechanical action to delay the closing or opening of contacts. Its characteristic is that there is a delay from the time the attracting coil gets the signal to the action of the contact. The time relay is generally used to control the motor starting process with time function. As above mentioned, the main function of the time delay is as an executive device in simple program control. When it receives the start signal, it starts timing. After the timing ends, its working contact opens or closes to promote the subsequent circuit work. Generally speaking, the delay performance of the time relay can be adjusted within the range of design, so as to facilitate the adjustment of its delay time. In addition, a time relay alone may not be able to do close. After closing for a period of time, it will open again. It is a cycle of time-delay closing and opening. However, configuring a certain number of time relays and intermediate relays can do it. 1.2 Time-delay Relay Working PrincipleTime relay is widely used in remote control, telecommunication, automatic control and other electronic equipment, and is one of the most important control components. When the coil is energized, the armature and the pallet are attracted by the core and move down instantaneously, making the action contact close or open. However, the piston rod and the lever cannot fall with the armature at the same time, because the upper end of the piston rod is connected to the rubber film in the air chamber.  When the piston rod starts to move downward under the action of the released spring, the rubber film is concave downward. The air in the air chamber becomes thinner, causing the piston rod to be damped and slowly descend. After a certain period of time, the piston rod descends to a certain position, and then the delay contact action is pushed through the lever to make the moving contacts open and close. The time from when the coil is energized to when the delay contact completes the action is the delay time of the relay. The length of the delay time can be changed by adjusting the size of the air inlet hole of the air chamber with a screw. After the suction coil is de-energized, the relay relies on the spring to recover. And the air is quickly expelled through the air outlet. 1.3 Time Relay StructureFigure 1. Air-damping Time Relay1 Coil5 Push plate9 Weak Spring13 Adjusting Screw2 Iron Core6 Piston rod10 Rubber Film14 Air Inlet3 Armature7 Lever11 Air Chamber Wall15 Micro Switch4 Reaction Spring8 Spring12 Piston16 Micro Switch 1.4 Time Relay ParametersTechnical parameters include rated voltage, contact working current, contact type and quantity, delay time, accuracy, ambient temperature, mechanical life and electrical life, etc. Now take the SJ23 series air-type time relay as an example, its technical parameters are as follows:1) Rated control capacity: AC300VA, DC60W (30W delay contact assembly).2) Rated voltage level: AC380V, 220V; DC220V, 110V.3) Rated voltage of the coil: AC110V, 220V and 380V.4) Maximum operating current of the contact: 0.79A at AC380V, 0.27A (momentary) and 0.14A (delay) at DC220V.5) Delay repeat error: ≤9%.6) Hot-state pull-in voltage: no more than 85% of the rated voltage of the relay. When the voltage drops from the rated value to 10% of the rated value in cold-state, it can be reliably released. And it can reliably release after reaching 110% of the rated voltage.7) The mechanical life is not less than 1 million times, and the electrical life is 1 million times (the DC life of the delay contacts assembly is 500,000 times). 1.5 Four-type Time Relay ContactsFigure 2. Time Relay SymbolsNOTC (normally-open, timed-closed): When the coil is not energized, the NOTC contact is normally open. It is closed by energizing the relay coil, but only within a specified time after the coil is continuously energized. The moving direction of the contact (close or open) is the same as that of a standard normally open contact. Since the delay occurs in the direction in which the coil is energized, this type of contact is normally open and on-delay. NOTO (normally-open, timed-open): Unlike the NOTC contact, the timed action occurs when the coil is de-energized. Since the delay occurs when the coil is de-energized, this type of contact is normally open and off-delay. NCTO (normally-closed, timed-open): When the coil is not powered on, the NCTO contact is normally closed. By energizing the relay coil, the contact is opened, but only within a specified time after the coil is continuously energized. The movement direction of the contact (closed or opened) is the same as the standard normally closed contact, but there is a delay in the opening direction. NCTC (normally-closed, timed-closed): The NCTC contact is similar to NCTO contact, because when the coil is normally closed when in de-energized and opened by energizing the coil. Ⅱ Understanding Time Delay in Relay CircuitSet delay time of a relay. Generally speaking, the delay performance of the time relay can be adjusted within the range of design, so as to facilitate the adjustment of its delay time in circuit. Time Delay Relay Circuit (Power-Off)If you are using an on delay relay, the delay will start immediately after the input signal is obtained. After the delay is completed, the executive part will output the signal to the control circuit. When the input signal disappears, the relay will immediately return to the pre-action status. It is opposite to an off delay relay. When the input signal is obtained, the execution part immediately has an output signal. After the input signal disappears, the relay needs a certain time to restore to the state before the action.Figure 3. Timer Relay StructureⅢ Time Relay Classifications3.1 According to Working PrincipleAccording to different working principles, time relays can be divided into air damping time relays, electric time relays, electromagnetic time relays, electronic time relays, etc. (1) Air damping time relayThe type is obtained by using the principle of damping when air passes through the small hole. Its structure is composed of three parts: electromagnetic system, delay mechanism and contact. The electromagnetic mechanism is a double-port direct-acting type, the contact system is a micro switch, and the delay mechanism adopts an airbag damper. (2) Electronic time relayUtilize the principle that the capacitor voltage in the RC circuit can't jump, and can only change gradually according to the exponential law, that is, the delay is obtained by electrical damping characteristics.Features: Wide delay range, high precision (generally about 5%), small size, shock resistance and easy adjustment. (3) Electric time relayUse the miniature synchronous motor to drive the reduction gear train to obtain the time delay.Features: The delay range is wide, up to 72 hours, and the delay accuracy can reach 1%. At the same time, the delay value is not affected by voltage fluctuations and environmental temperature.Its delay range and accuracy are unmatched by other time relays. Its disadvantages are complex structure, large size, short life, high price, and accuracy is affected by the power frequency. (4) Electromagnetic time relayUse the principle of slow attenuation of the magnetic flux after the electromagnetic coil is cut off to delay the release of the armature of the magnetic system to obtain the delay action of contacts. It is characterized by a large contact capacity, so the control capacity is large. However, the delay time range is small, and the accuracy is slightly worse. So it is mainly used in the control of DC circuits. 3.2 According to the Delay ModesBased on it, time relays can be divided into two types: on-delay type and off-delay type.(1) The on delay type time relay starts to delay immediately after receiving the input signal. After the delay is completed, its execution part outputs the signal to manipulate the control circuit. When the input signal disappears, the relay immediately returns to the state before the action.(2) The off delay type time relay is just the opposite. When the input signal is obtained, the execution part immediately has an output signal. After the input signal disappears, the relay needs a certain delay to restore to the state before the action. Ⅳ How to Wire Time Relay?The time relay is a very important component in the electrical control system. There are power-on delay types and power-off delay types. Based on the action type, there are electronic type and electric type, etc. Between them, the electronic type uses the principle of capacitor charging and discharging combined with electronic components to achieve delay action. There are many electric styles by using air bags and springs.Figure 4. Time Relay Wiring Schematics Time Relay Wiring:1) Control wiring: Consider it as a DC relay.2) Work control: Although the control voltage is connected, whether it plays a control role is determined by the timer on the panel.3) Function understanding: It is a switch,single-pole double-throw, with an active point, just like the active arm of a common knife switch.4) Load wiring: Connect the neutral wire of the power supply or the negative terminal.5) Working principle: When the timer is invalid, it is equivalent to the normal light in the switch-off state. When timing, the relay will act and the electrical appliances will be energized to work, which is equivalent to the normal light in the switch-on state.Take the power-on delay time relay as an example:Figure 5. On Delay Relay Contacts Wiring Ⅴ Time Relay ApplicationsIn Flash ControlTwo-time relays cooperate with each other to provide constant frequency on/off pulses of the contacts, sending intermittent power to the light.  In Furnace Safety Purge ControlBefore the combustion furnace can be safely ignited, the fan must run for a certain period of time to clean out any flammable or explosive steam in the furnace chamber. The time relay provides the required time parts for the furnace control work.In Electric Soft-start Delay ControlIt is not necessary to start a large electric engine by switching full power from a completely stopped state, and can reduce voltage softly start with less inrush current.In Conveyor Belt Sequence DelayWhen multiple conveyor belts are arranged to transport materials, the conveyor belts must be started in the reverse order (the last one is first, the first one is last) to prevent materials from accumulating on the moving conveyor which may be stop or move slowly. Ⅵ Time Relay SelectionThe selection of time relay is mainly due to delay mode and parameter coordination. The following aspects should be considered when selecting.(1) Delay mode selectionIt should be selected according to the requirements of the control circuit. The reset time after the action is longer than the inherent action time, so as to avoid misoperation or even no delay. This is especially important in the occasions of repeating delay circuits and frequent operations. (2) Type selectionFor occasions where the delay accuracy is not high, cheaper electromagnetic or air damping time relays are always used. On the contrary, for occasions where the delay accuracy is high, electronic time relays can be used. (3) Coil voltage selectionAccording to the voltage of the control circuit, the voltage at which the relay attracts the coil is selected. (4) Selection of power supply parametersIn the occasions where the power supply voltage fluctuates greatly, it is better to use air damping or electric time relays than the transistor type. And in the occasions where the power frequency fluctuates, electric time relays should not be used. In addition, when the temperature changes greatly, air damping type should not be used. When selecting a time relay, pay attention to the current type and voltage level of its coil (or power supply), and other factors, such as delay mode, contact form, delay accuracy and installation method according to the control requirements.Ⅶ Timer Relay Using Instructions7.1 General Ideas1) Keep the time relay clean, otherwise, the error will increase.2) Before use, check whether the power supply voltage and frequency are consistent with the voltage and frequency of the time relay.3) Choose the control time of the time relay according to user requirements. Regardless of the type of time relay, as long as the timing time is equal to the set time, its output contacts will act to achieve the purpose of the timing control circuit.4) For DC products, pay attention to wiring according to the circuit diagram and pay attention to the polarity of the power supply.5) After the time relay is out of working state, it should be reset immediately for the next use. If the repeated use interval is less than the preset time, the control circuit will be abnormal. What’s more, the power-on delay type is automatically reset after power off; and the power-off delay type is automatically reset after power on.6) Try to avoid using it in places with obvious vibration, direct sunlight, humidity and soil contact. 7.2 Two Points for Attention in Using Time RelaysThree Key Points1) Starting point of timingOn one hand, when selecting the timing point of the power-on delay time relay, you should choose to supply power to the time relay when the timing signal is sent by the control circuit that needs to perform timing. On the other hand, when selecting the timing point of the power-off delay type time relay, you should choose to cut off the power supply of the time relay when the control circuit that needs to send out the timing signal, so that the timing can be performed.2) Ending point of timingThe timing endpoint has two meanings: one refers to the point at which the set time is equal to the timing time; the other refers to the point at which the contract operates.3) Reset point of timingThe reset of the time relay is to clear the last timing content for the next use. If it is not reset, an abnormality will occur the next time it is used. Special attention should be paid to: the interval between two uses should be greater than the reset time, which is particularly important in electric time relays. The relationship between the starting point, ending point and reset point of timing1) After the time relay is used, there is a reset problem. Therefore, most of the control circuits are in the next level circuit by the time relay output. After the timing completion signal is accurately obtained, it is used to cut off the power supply of the time relay (power-on delay type), or power the time relay (power-off delay type).2) In the upper and lower control circuits of the time relay, there are components that cannot work at the same time. If the time relay cannot accurately operate the upper and lower control circuits at these points, it will cause the device to operate abnormally. Ⅷ Case Study: Time Relay Switch in Light CircuitControl requirements: Light 1 and light 2 are on at the same time, and light 2 is off in 30 seconds after light 1 is off. When light 1 is on, light 2 can be off at any time.According to the control requirements, explain through the following circuit diagram.Figure 6. Time Relay Switch in Light Circuit1) Press SB2, the contactor KM is energized and self-locked, and at the same time KT is also energized, and KT closes.2) After KT is turned on, the intermediate relay KA is also energized to work.3) At the same time, contact KM and contact KA are also closed at the same time, light 1 and light 2 are on.4) When the stop button SB1 is pressed, the contactor KM powers off, the contact KM opens, and the light 1 is off at the same time. Because of the existence of the power-off delay relay, KT is still on as well as the light 2. It goes out after the timing set by the time relay.5) When light 1 is on, and contact KA1 is turned on at any time, the time relay resets. KT disconnects and the light off.This is the typical application of an off delay relay. However, in the actual circuit, the control logic may be more complicated than this, so we must deeply understand the working principle and application of the time relay. Ⅸ Frequently Asked Questions about Time Delay Relay Basics1. What is time-delay relay?Time-delay, or time-release relays, allow necessary actions to happen at specific times in an electrical apparatus because they, in essence, act as a timer. 2. How does a time delay relay work?Time delay relays control the flow of electrical power and can be used to control power to many different types of electrical loads. Combining electromechanical output relay capability with control circuitry, these relays are pre-engineered to perform up to eleven time delay functions. 3. What is time delay relay circuit?Time Delay Relays. Time-Delay Relay. Relays are switches that are controlled by a circuit. Relays, in essence, send messages that tell something to start. When a car is started, the ignition only indirectly interacts with the battery of the car because a relay is sending the signal that tells the car to start. 4. How does a time delay relay work?Upon application of input voltage, the time delay relay is ready to accept trigger signals. Upon application of the trigger signal, the relay is energized and the preset time begins. ... Continuous cycling of the trigger signal at a rate faster than the preset time will cause the relay to remain energized. 5. How do you make a time delay relay?These relays provide a “Time Delay” between the energizing or de-energizing of the coil and movement of the armature. Such relays are called Time Delay Relays. A Time Delay Relay consists of a normal electromechanical relay along with a control circuit to control the relay operation and timing. 6. What is off delay relay?Abbreviated “NOTO”, these relays close immediately upon coil energization and open after the coil has been de-energized for the time duration period. Also called normally-open, off delay relays. 3: Normally-closed, timed-open. 7. How does an off delay timer relay work?Operation of Off Delay FunctionUpon application of input voltage, the time delay relay is ready to accept a trigger. When the trigger is applied, the output is energized. Upon removal of the trigger, the time delay (t) begins. At the end of the time delay (t), the output is de-energized. 8. What is the difference between off delay and on delay timer?As for Timer ON Delay, Timer starts by turning ON the timer trigger bit, and the timer output bit turns ON when the setup time has passed. As for Timer OFF Delay, the timer output bit turns OFF when the setup time has passed after the timer input bit had turned OFF. 9. How do you test a timer relay?Burden TestAdjust the timer with high time delay for example: 2 minutes.Energize the relay with 125V and measure the dc current.Note down the current before timer operates.After 2 minutes relay will pick up. Note down the current after operation.Calculate the relay power (W) = 125v x measured current. 10. What is the function of an time delay relay?Typical time delay functions include on-delay, repeat cycle (starting off), interval, off-delay, retriggerable one shot, repeat cycle (starting on), pulse generator, one shot, on / off delay, and memory latch.

IntroductionPCB exists in every electronic device. A fully functional PCB is mainly used to create connections between components, such as resistors, capacitors, inductors, diodes, transistors, integrated chips, etc. It is the carrier of the entire logic circuit. Sound PCB design can save production costs, and achieve good circuit performance and heat dissipation effect. PCB designs vary in complexity according to product needs. This article mainly talks about wiring, one of the basics of PCB design.PCB Design: From Idea to Schematic to PCBCatalogIntroductionⅠ PCB Basics: Wiring RulesⅡ Three PCB Wiring MethodsⅢ PCB Design: Wire InspectionⅣ Complete PCB Design Projects Inspection4.1 General PCB Design Inspection Projects4.2 PCB Electrical Characteristics Checking Projects4.3 PCB Physical Characteristics Checking Projects4.4 PCB Mechanical Design Factors4.5 PCB Installation Requirements4.6 PCB Pull-out Requirements4.7 PCB Mechanical Considerations4.8 PCB Electrical Considerations4.9 Electronics Inspection Before Into A PCBⅤ ConclusionⅠ PCB Basics: Wiring Rules1. The area within 1mm from the edge of the PCB board and within 1mm around the mounting hole will not take wiring.2. The power line width should not be less than 18mil; the signal line width should not be less than 12mil; the cpu input and output lines should not be less than 10mil (or 8mil); the line spacing should not be less than 10mil.3. It is necessary noted that the power line and the ground line should be as radial as possible, and the signal line must not be looped.4. Ground circuit rulesThe loop area formed by the signal line should be as small as possible. The smaller the loop area, the less external radiation and the less interference from the outside. An example is shown in the figure below:5. Crosstalk controlHere crosstalk refers to the mutual interference caused by long parallel wiring between different networks on the PCB, which caused by the distributed capacitance and inductance between the parallel lines. The main measures to overcome it are:a. Increase the spacing of parallel wiring and follow the 3W rule. To ensure that the distance between the lines is large enough, when the distance between the line and the center of the line is not less than 3 times the line width (as shown in the figure below). If the line center distance is not less than 3 times the line width, 70% of the line electric fields will not interfere with each other, which is called 3W rule.b. Insert a grounded isolation wire between the parallel wires. Reduce the distance between the wiring layer and the ground plane.6. The direction control rules of routing:The routing directions of adjacent layers are orthogonal. Different signal lines in the same direction on adjacent layers should be avoided to reduce unnecessary interlayer crosstalk. When the signal rate is high, use a ground plane to isolate each wiring layer, in other words, isolate each signal line with ground line. The neighbouring wires used in the input and output end of the circuit shouldn’t be parallel to prevent the feedback, and it is best to add a ground wire between these wires.7. Open loop inspection rules for wiring:Generally, it is not allowed to have a floating wiring at one end, because of the "antenna effect" and unnecessary interference radiation and reception, which may bring unpredictable results.8. Impedance matching inspection rulesThe wiring width of the same network should be kept the same. Line width variations will bring uneven line characteristic impedance, and reflection will occur when the transmission speed is high. This situation should be avoided in the design. Under certain conditions, such as the lead wires of the connector and the similar structure of the lead wires of the BGA package, the change of the line width may not be avoided, so that the length of the middle inconsistent part should be minimized.9. Wiring closed loop inspection rules:Prevent signal lines from forming self-loops between different layers. Such problems are prone to occur in multilayer board design, and it will cause radiation interference. As shown below:10. The branch length control rule of wiring:Try to control the length of branches, and the general requirement is Tdelay≤Trise/20.11. Resonance rules of wiring:For high-frequency signal design, the wiring length must not be an integer multiple of its wavelength to avoid resonance.12. Line length control rules:In fact, it refers to the short-circuit rule. When designing, you should keep the wiring length as short as possible to reduce interference problems caused by unnecessary lines. Especially for some important signal lines, such as clock lines, be sure to place oscillators close to the device. In the case of driving multiple devices, the network topology should be decided according to the specific situation.13. Parallel input and output wires on the PCB board should be avoided as far as possible to avoid parallel. It is best to place a ground wire between the two wires to avoid circuit feedback coupling.14. Digital ground and analog ground should be separated. For low-frequency circuits, single-point parallel grounding should be used. High-frequency circuits should be grounded in series with multiple points. For digital circuits, the ground wire should be closed into a loop to improve anti-noise capability.15. The wiring and via distribution of the whole circuit board should be uniformity. When the outer signal of the circuit board has a large blank area, auxiliary lines should be added to make the lines distribution on the board basically balanced.16. The low-frequency circuit can be grounded at a single point in parallel, and the actual wiring can be connected in series and then grounded in parallel. The high-frequency circuit can be grounded in series with multiple points. The ground wire should be short and thick. For high-frequency components, a large area ground foil can be used. The ground wire should be as thick as possible. If the ground wire is a very thin, the ground potential will change with the current, which reduces the noise resistance.17. Multilayer boards should be as symmetrical as possible when designing the laminated structure, as well as the wiring density and copper layout of each layer to reduce warpage and reduce EMI during soldering.18. The signal line should not cross the power supply and ground. The signal reference plane should be as complete as possible.19. Impedance controlThe signal lines that need impedance control must be wired in strict accordance with the calculated data, in addition, it is necessary to tell manufacturers it. For signal lines that do not require it, the impedance should be calculated to prevent unnecessary interference.20. Grid copper should be used less in low frequency circuits. Although it can effectively reduce the problem of large area copper skin blistering. When using grid copper, you need to consider the electrical length of the grid line and the working frequency of the circuit board. If using grid copper, the power supply should also be coated with solid copper as much as possible.21. A group of buses with the same attribute should be wired side by side as much as possible, and the length should be as equal as possible. Ⅱ Three PCB Wiring MethodsThe wires should take the shortest route between components according to the specified wiring rules. Limit the coupling between parallel wires as much as possible. Good PCB design requires the minimum number of wiring layers, and also requires fair use of the widest wire and the largest pad size corresponding to packaging density. For example, rounded corners and smooth inner corners design may avoid some electrical and mechanical problems, therefore, sharp corners and sharp corners in the wire should be avoided. Here introduces three main PCB routing methods; right-angle wiring, differential wiring, and serpentine wiring to illustrate PCB layout:A. The influence of right-angle wiring on the signal is mainly reflected in three aspects:1. The corner can be equivalent to the capacitive load on the transmission line to slow down the rise time.2. Discontinuous impedance will cause signal reflection.3. The EMI generated by the right-angle tip reaches the RF field above 10GHz. Such a right-angle is likely to develop into the source of high-speed problems. B. To figure out what is differential wiring, you must first understand what is differential signal. In a word, the driving end sends two equal and inverted signals, and the receiving end judges the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces. Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:1. Have Strong anti-interference ability. Because the coupling between the two differential traces occurs, when there is noise interference from the outside, they are almost coupled to the two lines at the same time. However, the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely canceled.2. It can effectively suppress EMI. Due to the opposite polarity of the two signals, the electromagnetic fields radiated by them can cancel each other out. What’s more, the tighter the coupling, the less the electromagnetic energy leaked to the outside world.3. The timing positioning is accurate. Because the switch change of the differential signal is located at the intersection of the two signals. Unlike ordinary single-ended signals, which rely on the high and low threshold voltages to judge. Timing positioning is less affected by the process and temperature, and also more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signaling technology. C. Serpentine line is a type of wiring method often used in PCB layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when a signal is transmitted on a serpentine trace, the parallel line segments will be coupled in a differential mode. The smaller the S, the greater the Lp, the greater the coupling. It may cause the transmission delay to be reduced, also the signal quality is greatly reduced due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. The following are some suggestions when dealing with serpentine wring:1. Try to increase the distance (S) of parallel lines, at least more than 3H(H refers to the distance from the signal trace to the reference plane). As long as S is large enough, the mutual coupling effect can be almost completely avoided.2. Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.3. The signal transmission delay caused by the strip-line or embedded micro-strip line is less than that of the micro-strip. Theoretically, the strip-line will not affect the transmission rate due to differential mode crosstalk.4. For signal lines with high-speed and strict timing requirements, try not to take serpentine lines, especially in a small area.5. You can often use s-shaped routing at any angle, which can effectively reduce the mutual coupling.6. In high speed, the serpentine line has no ability so-called filtering or anti-interference, and can only reduce the signal quality, so it is better to use for timing matching.7. Sometimes you can consider the spiral routing method for winding. Simulation shows that its effect is better than normal serpentine routing.Ⅲ PCB Design: Wire Inspection1.Wire SpacingThe minimum spacing of wires must be determined to eliminate voltage breakdown or arcing between adjacent wires. The spacing is variable, it mainly depends on the following factors:1) Peak voltage between adjacent wires2) Atmospheric pressure (maximum working altitude)3) Coating layer4) Capacitive coupling parametersComponents with critical impedance or high-frequency components should be placed very close to reduce the critical stage delay. There is something need to pay attention to. Transformers and inductive components should be isolated to prevent coupling. Inductive signal wires should be laid orthogonally at right angles. Components that generate any electrical noise due to magnetic field movement should be isolated or rigidly installed to prevent excessive vibration.2. Whether the wire is short and straight without sacrificing function.3. Whether the restrictions on the wire width are complied with.4. There must be a minimum distance between wires, wires and mounting holes, wires and pads.5. Whether to avoid all the wires (including component leads) closer to parallel wiring.6. Whether sharp corners (≤90℃) are avoided in the wire pattern. Ⅳ Complete PCB Design Projects Inspection4.1 General PCB Design Inspection Projects1) Has the circuit been analyzed? Is the circuit divided into basic units to smooth the signal?2) Does the circuit allow short or isolated key leads?3) Where must be shielded, are they effectively shielded?4) Have you made full use of the basic grid graphics?5) Is the best size of the printed circuit board?6) Do you use the available wire width and spacing as much as possible?7) Has the preferred pad size and hole size been used?8) Are the base plate and the sketch consistent?9) Is less cross-wiring used? Do cross wires pass through components and accessories?10) Are the letters visible after assembly? Are their size and model correct?11) In order to prevent blistering, is there any window on the large area of copper foil?12) Are there tool positioning holes?4.2 PCB Electrical Characteristics Checking Projects1) Have you analyzed the influence of wire resistance, inductance, and capacitance, as well as the critical voltage drop on the ground?2) Does the wire spacing and shape meet the insulation requirements?3) Has the insulation resistance value been controlled and specified in key areas?4) Is the polarity fully recognized?5) According to geometric view, has the effect of wire spacing on leakage resistance and voltage been measured?6) Has the medium for changing the surface coating been identified?4.3 PCB Physical Characteristics Checking Projects1) Are all pads and their positions suitable for final assembly?2) Can the assembled PCB meet the shock and vibration conditions?3) What is the required spacing of standard components?4) Are the components that are not firmly installed or the heavier parts fixed?5) Is the heating element heat dissipation and cooling normally? Or is it isolated from the printed circuit board and other heat-sensitive elements?6) Are the voltage divider and other multi-lead components placed correctly?7) Is the arrangement and orientation of components easy to check?8) Has it eliminated all possible interference on the printed circuit board?9) Is the size of the positioning hole correct?10) Are the tolerances complete and reasonable?11) Have you controlled and signed the physical properties of all coatings?12) Is the ratio of via hole and lead diameter within an acceptable range?4.4 PCB Mechanical Design FactorsThe printed circuit board adopts mechanical methods to support the components, however, it cannot be used as an unique structural part of the entire device. On the edge of the printing plate, at least every 5 inches for a certain support. The factors that must be considered when selecting and designing printed circuit boards are as follows:1) The size and shape of the printed circuit board.2) The type of mechanical accessories and plug (seat) required.3) The environmental adaptability of circuits.4) According to some factors, such as heat and dust, install the printed circuit board vertically or horizontally.5) Some environmental factors that require special attention, such as heat dissipation, ventilation, shock, vibration, and humidity, dust, and radiation, etc.6) Physical support7) Install and fix.8) Disassemble4.5 PCB Installation RequirementsAccording to practical experience, the distance between the supporting points of a printed circuit board with a thickness of 0.031-0.062 inches should be at least 4 inches. For a printed circuit board with a thickness greater than 0.093 inches, the distance between the supporting points should be at least 5 inches. Taking this measure can improve the rigidity of the printed circuit board and avoid possible resonance. The following factors should be considered before deciding which mounting technology they use.1) PCB structure.2) Input and output terminals.3) Available equipment space.4) Convenience of loading and unloading.5) Type of attachments.6) Required heat dissipation.7) Required shieldability.8) The type of circuit and its relationship with other circuits.4.6 PCB Pull-out Requirements1) The influence of plugging tools on the installation distance between two printed circuit boards.2) When the plug-in tool used in the equipment, its size should be considered.3) A plug-in device is required, which is usually fixed to the printed circuit board assembly with rivets.4) As for the mounting frame of the printed circuit board, special design such as load bearing flange is required.5) The adaptability of the plug-in tool used and the size, shape and thickness of the printed circuit board.4.7 PCB Mechanical ConsiderationsThe characteristics of the board substrate that have an important influence on the printed circuit assembly are: water absorption, thermal expansion coefficient, heat resistance, flexural strength, impact strength, tensile strength, shear strength and hardness. All these characteristics affect the function and the production efficiency of the printed circuit board structure. For most applications, the dielectric substrate materials of the printed circuit board are as following:1) Phenolic impregnated paper2) Acrylic-polyester impregnated randomly arranged glass mat3) Epoxy impregnated paper4) Epoxy impregnated glass clothEach substrate can be flame retardant or combustible. The first 3 types mentioned above can be processed. The most common used material for printed circuit boards with metalized holes is epoxy-glass cloth. Its dimensional stability is suitable for high-density circuits and can minimize the occurrence of cracks in the metalized holes. One disadvantage of epoxy-glass cloth laminate is that it is difficult to punch in the usual thickness range of printed circuit boards. For this, all holes are usually drilled and copied and milled to form a print shape of the circuit board.4.8 PCB Electrical ConsiderationsIn DC or low-frequency AC applications, the most important electrical characteristics of insulating substrates are: insulation resistance, anti-isolation, printed wire resistance, and breakdown strength. In high frequency and microwave applications, include: dielectric constant, capacitance, and dissipation factors. In all applications, the current carrying capacity of printed wires is important.4.9 Electronics Inspection Before Into A PCB1) Check the rationality and correctness of the schematic diagram.2) Check the correctness of the component packaging of the schematic.3) The distance between strong and weak current lines, and the distance between isolation areas.4) Check the schematic diagram and PCB diagram to prevent the loss of the network table.5) Whether the package of the component matches the physical object.6) Whether the placement of the components is appropriate.7) Whether the components are easy to install and disassemble.8) Whether the temperature sensitive element is too close to the heating element.9) Whether the distance and direction of the mutual inductance components are appropriate.10) Whether the placement between the connectors is smooth.11) Easy to plug in and plug out12) Input and output13) Strong current and weak current14) digital and analog should be interlaced.15) Arrangement of elements on the upside and downside16) Check whether the directional component has been wrong flipped instead of rotated.17) Check whether the mounting holes of the component pins are suitable and whether it is easy to insert.18) Check whether the empty pin of each component is normal and whether it is a missing line.19) Check whether there are vias between the upper and lower wiring of the same net table. And the pads are connected through the holes, to prevent disconnection and ensure the integrity of the circuit.20) Silk screen printing should be clear, so that the operation of welding or maintenance can be easy.21) The arrangement of power and signal lines in the socket should ensure signal integrity and anti-interference.22) Pay attention to the proper ratio of pads and solder holes.23) Each plug should be placed on the edge of the PCB board as much as possible and easy to operate.24) Whether the size and distribution of the mounting holes on the PCB are appropriate to reduce the PCB bending stress.25) Pay attention to the height distribution of the components on the PCB to ensure easy assembly.Ⅴ ConclusionBased on the above mentioned rules, drawing the PCB schematics you need becomes easier. Decide what PCB you want to and install a PCB design software. PCB software is really helpful and powerful. Also a software can check your design to make sure the design does not contain errors such as traces that incorrectly touch, traces too skinny, or drill holes that are too small. For example, run the Electrical Rules Checker (ERC) to see if you’ve made any typical errors. There is less thing stopping you from making your first PCB, right? Frequently Asked Questions about PCB Design Diagram1. Which side of PCB is correct for soldering?The bottom side of the PCB is usually the side without components and the side that touches the solder wave during assembly. That is why sometimes it is also called SOLDER side. However more often, PCB are populated on both sides and the assembly process does not require wave soldering. 2. Which soldering method is suitable for soldering printed circuit board?Soldering Iron – Used to melt solder and connect component pins to board pads. A cheap soldering pencil may be sufficient, but a temperature-controlled solder station is best for high performance boards. Solder – An alloy of tin and lead with a low melting point. 3. What is PCB diagram?A PCB schematic is a simple two-dimensional circuit design showing the functionality and connectivity between different components. ... Once the blueprint has been completed, the PCB design comes next. The design is the layout, or physical representation of the PCB schematic and includes the copper track and hole layout. 4. Why we use PCB in soldering?PCB soldering is another term for the process of soldering electrical circuit boards. ... As the soldering iron melts this metal, it is then used a bit like glue to stick to pieces together. As the solder metal cools, it will re-harden into one large shape that connects the two parts. 5. How do you read a PCB board?Start with an easy analog circuit, such as a guitar distortion pedal, and work your way up to more complicated versions. Make a drawing of the top of the circuit board. Show the positions of the capacitors, integrated circuits, resistors, transistors and other components. Review it to make sure everything is included.

IntroductionRLC circuit is a circuit structure composed of resistance (R), inductance (L), and capacitance (C). The LC circuit is a simple example. RLC circuits are also called second-order circuits. The voltage or current in the circuit is the solution of a second-order differential equation, and its coefficients are determined by the circuit structure.If the circuit components are regarded as linear components, an RLC circuit can be regarded as an electronic harmonic oscillator.The natural frequency of this circuit is generally expressed as: (unit: Hz)RLC circuits are often used as band-pass filters or band-stop filters, and the Q factor can be obtained by the following formula:There are generally two types of RLC circuit composition: series and parallel.The animation above demonstrates the operation of the LC circuit (RLC circuit without resistors). The charge is transferred back and forth between the capacitor plate and the inductor. The energy oscillates back and forth between the electric field (E) of the capacitor and the magnetic field (B) of the inductor. The RLC circuit works similarly. The difference is that the oscillating current decays to zero over time due to the resistance in the circuit.CatalogIntroductionCatalogI RLC Series Circuit 1.1 What is Series RLC Circuit? 1.2 What is Transient Response of RLC Circuit? 1.3 Laplacian Domain 1.4 RLC Series Resonance Formula   1.5 Phasor Diagram of RLC Series CircuitII RLC Parallel CircuitIII The Difference Between Series Resonant Circuit and Parallel Resonant Circuit 3.1 Series Resonance 3.2 Parallel ResonanceIV Application of RLC Circuit Resonance 4.1 Application of Series Resonance Circuit 4.2 Application of Parallel Resonance CircuitV Frequently Asked Questions about RLC CircuitI RLC Series Circuit1.1 What is Series RLC Circuit?Figure1. RLC Series CircuitV-supply voltageI-circuit currentR-resistanceL-InductanceC-capacitanceIn this circuit, all three elements are connected in series with the voltage. The main differential equations can be obtained by substituting the constitutive equations of the three elements into Kirchhoff's voltage law (KVL). From Kirchhoff's voltage law:are the voltages across R, L, and C respectively, and V(t) is the voltage of the power supply that changes with time. Substituting the constitutive equation to get:In the case of a constant supply voltage, take the derivative of the above formula and divide by L to obtain the following second-order differential equation:This equation can be written in a more common form:α is called "attenuation", which is used to measure the attenuation rate of the transient response of this circuit when the external input is removed. ω0 is the angular resonance frequency. These two coefficients are given by:The damping coefficient ζ is another commonly used parameter, defined as the ratio of α to ω0:1.2 What is Transient Response of RLC Circuit?Figure2. Transient ResponseThe figure shows the underdamped and overdamped responses of the series RLC circuit. The critical damping is drawn with a thick red curve. These drawings are unified when L = 1, C = 1 and ω0=1. According to the value of different damping coefficient ζ, the solution of the differential equation has three different situations, namely: under-damping (ζ<1), over-damping (ζ>1), and critical damping (ζ=1).The characteristic equation of the differential equation is:The roots of this equation are:The general solution of this differential equation is the linear superposition of two exponential functions:The coefficients A1 and A2 are given by the boundary conditions of the specific problem.The following video introduces how to analyze RLC circuits by way of second order differential equations. Both parallel and series RLC configurations are discussed in it, looking primarily at Natural Response, but also touching on Step Response.RLC Circuit Response Explanation1.2.1 Over-damped responseThe over-damped response (ζ>1) is:Overdamping response is a transient current without oscillation attenuation.1.2.2 Underdamped responseThe underdamped response (ζ<1) is:Through the trigonometric identities, these two trigonometric functions can be expressed by a phased sine function:The underdamped response is an attenuated oscillation with a frequency of ωd. The rate of oscillation decay is α. The α in the index describes the envelope function of the oscillation. B1 and B2 (or B3 and phase difference φ in the second form) are arbitrary constants and are determined by boundary conditions. The frequency ωd is given by:This is the so-called damped resonance frequency or damped natural frequency. It is the frequency at which the circuit naturally vibrates when driven by no external source. The resonant frequency ω0 is the resonant frequency of the circuit when it is driven by an external source, and is often called the undamped resonant frequency in order to facilitate the distinction.1.2.3 Critical damping responseThe critical damping response (ζ=1) is:1.3 Laplacian DomainThe Laplace transform can be used to analyze the AC transient and steady-state behavior of the RLC series circuit. If the waveform generated by the above voltage source is V(s) after Laplace transform (where s is the complex frequency s=σ+iω), then Kirchhoff’s voltage law is applied in the Laplace domain:Among them, I(s) is the current after Laplace transform. Solve for I(s):After rearranging, the following formula can be obtained:1.3.1 Laplace admittanceSolve for Laplace admittance Y(s):The above formula can be simplified by using the parameters α and ωo defined in the above content, and we can get:1.3.2 Pole and zeroThe zero point of Y(s) is s such that Y(s)=0: s=0 and |s|⟶ ∞; the pole of Y(s) is s such that Y(s)⟶ ∞. Solve the quadratic equation. Get:The poles of Y(s) are the roots s1 and s2 of the characteristic equation of the differential equation mentioned above.1.3.3 Sine steady stateThe sine steady state can be represented by letting s=jω, where j is the imaginary unit. Substitute this into the amplitude of the above equation:The function of the current with ω as the variable isThere is a peak.In this special case, ω in this peak is equal to the undamped natural resonance frequency:1.4 RLC Series Resonance FormulaThe so-called series resonance formula refers to the study of the energy value of the voltage and current of the series circuit to reach the same phase, and the inductance of the inductance in the circuit and the capacitive reactance in the capacitor are equal in value. Therefore, in the study of the resistance characteristics of the circuit, In the case of a given terminal voltage, the maximum current is released, and the active power consumed will also be the maximum.Figure3. RLC series resonance formulaResonance definition: The energy of the L and C_ elements in the circuit are equal. When a reactance element in the circuit releases energy, the other reactance element must absorb the same energy, that is, energy pulsation occurs between the two reactance elements.  When series resonance occurs:Inductive reactance XL = capacitive reactance XCSource voltage U = resistance voltage URInductor voltage UL = Capacitor voltage UCInductor's reactive power QL = Capacitor's reactive power QCTotal circuit impedance Z=resistance value RApparent power S = resistance power PExplanation: When the circuit resonates, it must have two components: inductor L and capacitor C, and the frequency corresponding to resonance is called "resonant frequency" or resonant frequency, generally we use fr to indicate.1.5 Phasor Diagram of RLC Series Circuit(1) Phasor diagram of voltage and currentU&=U&R+U&L+U&CFigure4. Phasor diagram of voltage and currentFigure5. Phasor diagram of voltage and current(2) Voltage triangleThe relationship between the voltage triangle and the impedance triangle: divide the effective value of the voltage triangle by I to get the impedance triangle.Figure6. Voltage triangle● The relationship between the total voltage and the effective value of each part of the voltage:● The effective value relationship between total voltage and total current: U=I|Z|● The phase difference relationship between total voltage and total current:II RLC Parallel CircuitFigure7. RLC Parallel CircuitV-supply voltageI-circuit currentR-resistanceL-InductanceC-capacitanceThe characteristics of the RLC parallel circuit can be handled by the duality (electrical circuits) of the circuit. The RLC parallel circuit is treated as the dual impedance of the RLC series circuit, so it can be analyzed in a similar way to the RLC series circuit.The attenuation α of the RLC parallel circuit can be obtained by the following formula:If the factor of 1/2 is not considered, the damping coefficient of the RLC parallel circuit is exactly the reciprocal of the damping coefficient of the RLC series circuit.Frequency domainAdd the admittance of each element in parallel to obtain the admittance of this circuit:After capacitors, resistors, and inductors are connected in parallel, the impedance at the resonance frequency is the maximum, which is the opposite of the case where capacitors, resistors, and inductors are connected in series. The RLC parallel circuit is an antiresonator.In the figure below, it can be seen that if a constant voltage is used for driving, the frequency response of the current has a minimum value at the resonance frequency ω0=1/√LC. If it is driven by a constant current, the frequency response of the voltage has a maximum value at the resonance frequency, which is similar to the frequency response graph of the current in an RLC series circuit.Figure8. Sinusoidal steady state analysisNormalize with R = 1 ohm, C = 1 Farad, L = 1 Henry, and V = 1.0 VoltIII The Difference Between Series Resonant Circuit and Parallel Resonant CircuitIn an AC circuit containing resistance, inductance and capacitance, the voltage at both ends of the circuit and its current are generally out of phase. If the circuit parameters or the power supply frequency are adjusted to make the current and the power supply voltage in phase, the circuit is resistive, which is called resonance for the working state of the circuit at this time.Resonance is a specific phenomenon of sinusoidal AC circuits. It is widely used in electronics and communication engineering. However, in power systems, resonance may damage the normal operation of the system.Resonance is generally divided into series resonance and parallel resonance. As the name implies, series resonance is the resonance that occurs in a series circuit. Parallel resonance is the resonance that occurs in a parallel circuit.3.1 Series Resonance3.1.1 IntroductionIn a series circuit composed of resistance, inductance and capacitance, when the capacitive reactance XC and the inductive reactance XL are equal, that is, XC=XL, the voltage U and the current I in the circuit have the same phase, and the circuit presents pure resistivity. This phenomenon is called series resonance. When the circuit is in series resonance, the total impedance in the circuit is the smallest, and the current will reach the maximum. 3.1.2 Conditions for the occurrence of series resonanceIn order to resonate in a series circuit, certain conditions must be met.When UL=UC, that is, XL=XC,. Voltage and current are in phase, and series resonance occurs in the circuit. From ωL=1/ωC, ω0=1/√LC can be obtained, and the resonance frequency is f=f0=1/2π√LC. 3.1.3 Characteristics of series resonance circuit● Minimum total impedance● When the power supply voltage is constant, the current is the largest● The circuit is resistive, and the voltage on the capacitor or inductor may be higher than the power supply voltage 3.1.4 Energy changes in the circuit at resonanceThe circuit absorbs Q=0 from the power supply, and the circuit energy exchanges between the electric field and the magnetic field inside the circuit during resonance. The power supply only provides energy to R.High voltage may damage the device. Series resonance should be avoided in the power system. And series resonance is widely used in radio engineering.3.2 Parallel Resonance3.2.1 IntroductionIn a circuit where an inductance and a capacitor are connected in parallel, when the size of the capacitor just makes the voltage and current in the circuit have the same phase, that is, when the power supply is consumed by resistance and becomes a resistance circuit, it is called parallel resonance.Parallel resonance is a complete compensation. The power supply does not need to provide reactive power, only the active power required by the resistance. At resonance, the total current of the circuit is the smallest, and the current of the branch is often greater than the total current of the circuit. Therefore, parallel resonance is also called current resonance.When parallel resonance occurs, a large current flows in the inductance and capacitance components, which will cause the fuse of the circuit to blow or burn the electrical equipment; however, it is often used in radio engineering to select signals and eliminate interference. 3.2.2 Parallel resonance conditionsIn the following two types of circuitsFigure9. Two types of circuitsThe resonant frequency formula of (a) has been discussed above, and (b) is determined by,We can get.Under normal circumstances, the coil resistance R is much smaller than XL, therefore, ignoring R we can getthat is f=f0=1/2π√LC. 3.2.3 Features of parallel resonant circuit● When the voltage is constant, the current is the smallest at resonance● Maximum total impedance● The circuit is resistive, and the branch current may be greater than the total currentIV Application of RLC Circuit Resonance4.1 Application of Series Resonance CircuitThe use of series resonance to generate power frequency high voltage, which is used in high voltage technology to do withstand voltage test for power equipment such as transformers, can effectively find dangerous concentrated defects in the equipment, and is the most effective and direct way to test the insulation strength of electrical equipment Methods. Used in radio engineering, series resonance is often used to obtain a higher voltage.In the radio, the series resonance circuit is often used to select the radio signal. This process is called tuning. The following figure shows a typical circuit.Figure10. A typical circuit for tuningWhen the electric waves of various signals of different frequencies generate electric signals of different frequencies on the antenna, they are induced to the coil 2L through the coil 1L. If the oscillation circuit resonates to a certain signal frequency, the current of the signal in the loop is the largest, and a voltage CU higher than the signal voltage Q times is generated across the capacitor. For other signals of various frequencies, because no resonance occurs, the current in the loop is very small, which is suppressed by the circuit. Therefore, the capacitor C can be changed to change the resonant frequency of the loop to select the desired radio signal.4.2 Application of Parallel Resonance CircuitThe application of LC parallel resonant circuit in communication electronic circuit is determined by its characteristics. Specifically, it mainly includes three categories. One is working in resonance, as a frequency-selective network application. At this time, it appears as a large resistance and outputs a larger voltage under the excitation of current; the second is working in detuning The state, present as inductive or capacitive at this time, together with other inductances and capacitors in the circuit, satisfies the oscillation conditions of the three-point oscillation circuit to form a sine wave oscillator; the third is to work in a detuned state, that is, to work on the amplitude-frequency characteristic curve Or one side of the phase-frequency characteristic curve to realize amplitude-frequency conversion, frequency-amplitude conversion, frequency-phase conversion, and phase-frequency conversion to form an angle modulation and demodulation circuit. (1) LC parallel resonant circuit used as frequency selective matching networkFrequency selection is to select useful frequency components from the input signal and suppress useless frequency components or noise. In communication electronic circuits, the LC parallel resonant circuit is the most commonly used as a frequency selection network. It is widely used in high-frequency small-signal amplifiers, Class C high-frequency power amplifiers, mixers and other circuits. The common feature of these circuits is that the LC resonant circuit is not only a frequency-selective network. Through the connection of the transformer, it also plays the role of impedance transformation, reducing the impact of the amplifier tube or the load on the resonant circuit, and obtaining better selectivity. . (2) The LC parallel resonant circuit of the overtone crystal oscillator as a capacitorUnder the action of the applied alternating voltage, in the mechanical vibration generated by the quartz crystal, in addition to the fundamental frequency mechanical vibration, there are many odd frequency overtones. When a crystal oscillator with a very high operating frequency is required, overtone crystal oscillators are often used. The figure below shows the overtone crystal oscillator.Figure11. Circuit composition and reactance curve of L1C1 circuitIn the above figure, the quartz crystal and the CL branch are inductive. The quartz crystal, C2, and L1C1 loop together form a three-point oscillator. According to the composition principle of the three-point oscillator (shooting the same), the L1C1 resonant circuit should be capacitive. Assuming that the quartz crystal in the figure is working at the 5th overtone frequency, the nominal frequency is 5 MHz. In order to suppress the parasitic oscillation of the fundamental frequency and 3rd overtone, the L1C1 loop should be tuned between the 3rd and 5th overtone frequency, that is, 3~ Between 5 MHz.  From the reactance characteristic curve of the L1C1 resonant circuit shown in Figure (b), it can be seen that for the 5th overtone frequency of 5 MHz, the L1C1 circuit is capacitive, and the circuit meets the three-point oscillation condition and can oscillate. For the fundamental and third harmonics that are less than the resonance frequency of the L1C1 loop, the loop has an inductive characteristic, which does not conform to the principle of different components and cannot produce oscillation. For overtones of 7 times and above, although the L1C1 circuit is also capacitive, the equivalent capacitance at this time is too large, the amplitude starting conditions cannot be met, and the oscillation cannot be generated. (3) LC parallel resonant circuit that realizes the functions of amplitude-frequency conversion and frequency-phase conversionThe phase-frequency characteristic of the impedance of the LC parallel resonant circuit is a monotonous curve with a negative slope. The linear part of the curve can be used to perform a linear conversion between frequency and phase. This is mainly used in the phase frequency discrimination circuit; the same, the LC parallel resonant circuit The linear part of the impedance's amplitude-frequency characteristic curve can also perform the linear conversion between frequency and amplitude, so it has also been applied in the slope frequency discrimination circuit.V Frequently Asked Questions about RLC Circuit1. Is LCR and RLC circuit the same?Yes. An RLC circuit (also known as a resonant circuit, tuned circuit, or LCR circuit) is an electrical circuit consisting of a resistor (R), an inductor (L), and a capacitor (C), connected in series or in parallel. This configuration forms a harmonic oscillator. 2. What is the resonant frequency of the RLC circuit?What is Resonance in the RLC circuit? Resonance is the phenomenon in the electrical circuit, where the output of the circuit is maximum at one particular frequency. And that frequency is known as the resonant frequency. At the resonant frequency, The capacitive reactance and inductive reactance are equal. 3. Is the RLC circuit linear?In an RLC circuit, the most fundamental elements of a resistor, inductor and capacitor are connected across a voltage supply. All of these elements are linear and passive in nature. 4. What is the bandwidth of the RLC circuit?The bandwidth of any system is the range of frequencies for which the current or output voltage is equal to 70.7% of its value at the resonant frequency, and it is denoted by BW. 5. What is the second-order circuit?A second-order circuit is characterized by a second-order differential equation. It consists of resistors and the equivalent of two energy storage elements. 6. What is the first-order circuit?A first-order circuit can only contain one. energy storage element (a capacitor or an. inductor). The circuit will also contain. 7. What is the half-power frequency?The frequencies for which current in a series RLC (or a series tuned) circuit is equal to 1/√2 (i.e. 70.71%) of the maximum current (current at resonance)are known as Half Power Frequencies. 8. What is the natural response of the RC circuit?The natural response tells us what the circuit does as its internal stored energy (the initial voltage on the capacitor) is allowed to dissipate. It does this by ignoring the forcing input (the voltage step caused by the switch closing). The "destination" of the natural response is always zero voltage and zero current. 9. What is the difference between first-order and second-order filters?The main difference between a 1st and 2nd order low pass filter is that the stopband roll-off will be twice the 1st order filters at 40dB/decade (12dB/octave) as the operating frequency increases above the cut-off frequency ƒc, point as shown. 10. What is the use of a resonant circuit?One use for resonance is to establish a condition of stable frequency in circuits designed to produce AC signals. Usually, a parallel (tank) circuit is used for this purpose, with the capacitor and inductor directly connected together, exchanging energy between each other. 

IntroductionA band pass filter is an electronic device or circuit that allows signals between two specific frequencies to pass. That is, allowing signals in a specific frequency band to pass while shielding other frequency bands. In other words, a band-pass filter attenuates frequency components in other ranges to an extremely low level, as opposed to the concept of a band-stop filter. For example, the RLC tank is an analog band-pass filter, it is a resistor - inductor - capacitor circuit (RLC circuit). These filtering circuit can also be made by connecting low-pass filters and high-pass filters.How to Design Band Pass Filter CircuitCatalogIntroductionⅠ Band Pass Filter Circuit CharacteristicsⅡ Band Pass Filter Parameters2.1 Center Frequency2.2 Cut-off Frequency2.3 Bandwidth2.4 Quality FactorⅢ Types of Band Pass Filter3.1 Active Band Pass Filter3.2 Passive Band Pass FilterⅣ Band Pass Filter Equation4.1 Cutoff Frequency of Band Pass Filters4.2 General Form of Second-order BPF Transfer Function4.3 Second-order Band Pass Filters4.4 High-Q second-order Band Pass Filters4.5 Dual-operational Amplifier BPF (High-Q)4.6 Second-order Band Pass Filters (Voltage-controlled Type)Ⅴ Band Pass Filter ApplicationsⅠ Band Pass Filter Circuit CharacteristicsAn ideal band pass filter should have a completely flat pass band, no amplification or attenuation. And all frequencies outside the pass band will be completely attenuated. In addition, the conversion outside the pass band is completed in an extremely small frequency range. But in fact, there is no ideal band-pass filter. Because the filter cannot completely attenuate all frequencies outside the desired frequency range, especially there is an attenuated but not isolated frequency range outside the desired pass band. This is usually called the filter roll-off phenomenon, and it is expressed in dB per decade of attenuation amplitude. Generally, the filter design should ensure that the roll-off range is as narrow as possible, so that the performance of the filter is closer to the design requirement. However, as the roll-off range gets smaller and smaller, the pass band becomes no longer flat and causes ripples.The high-pass filter has a low cut-off frequency, and the low-pass filter has a high cut-off frequency. When the high cut-off frequency is lower than the low cut-off frequency, combining the two circuits, and it is possible to design a band pass filter. The gain of the band pass filter is adjusted by the feedback resistor and the current limiting resistor.Figure 1. Band Pass Filter Circuit PartsA band pass filter with a high quality factor refers to a filter with a narrow pass band. In other words, a high-Q factor means that fewer unwanted frequency signals will pass. A low-Q factor means that the pass band is very wide, to allow a wider range of frequencies to pass through.  Ⅱ Band Pass Filter Parameters2.1 Center FrequencyIt usually defined as the midpoint between the two 3dB points of a band pass filter (or a band stop filter), generally expressed by the arithmetic average of the two 3dB points. It is a frequency when the impedance of the entire circuit is a real number.2.2 Cut-off FrequencyIt refers to the frequency point on the right of the low-pass filter and the frequency point on the left of the high pass filter in the pass band. That is, the boundary frequency. It is usually defined as a standard by 1dB or 3dB relative loss point. The band pass filter has two cutoff frequencies, the low cutoff frequency fp1 and the high cutoff frequency fp2. 2.3 BandwidthThe difference between two cut-off frequencies. The bandwidth is defined as B=fp2－fp1.2.4 Quality FactorThe reciprocal of the damping coefficient is called the quality factor, which is an important indicator of the frequency selection characteristics of band pass and band stop filters. In short, it is the ratio of the center frequency to the bandwidth. What’s more, it can be used to describe the shape of the transfer function graph. Ⅲ Types of Band Pass Filter3.1 Active Band Pass FilterFigure 2. Active Band Pass Filter CircuitThe active band pass filter is a cascade of high-pass and low-pass filters and amplifier components. The circuit diagram of the active band pass filter consists of three parts. The first part is the high-pass filter. Then, use the op amp for amplification. The last part of the circuit is the low-pass filter.3.2 Passive Band Pass FilterFigure 3. Passive Band Pass Filter CircuitPassive band pass filters are a combination of passive high-pass and low-pass filters. Passive filters use only passive components, such as resistors, capacitors, and inductors. Therefore, passive band pass filters are also used as passive components and do not use op amps for amplification. Ⅳ Band Pass Filter Equation4.1 Cutoff Frequency of Band Pass FiltersThe characteristic of the band pass filter is that the output signal amplitude in the pass band is independent from the frequency. When f<fp1 or f>fp2, the output signals attenuate quickly. The amplitude-frequency characteristics are shown in the figure:Figure 4. BPF Bandwidth(The broken line is the ideal BPF frequency characteristic, and the solid line is the actual BPF frequency characteristic)The resonance frequency is between fp1 and fp2, where the gain of the filter is the largest, and the bandwidth of the filter is the difference between fp2 and fp1.It can be seen from the frequency characteristics of BPF that it can be composed of LPF and HPF in series, as long as the fpL of LPF (ie, fp2 of BPF) is greater than fpH of HPF (ie, fp1 of BPF).4.2 General Form of Second-order BPF Transfer FunctionFrequency CharacteristicsWhere Aup is the pass-band magnification, center frequency , Q factor Normalized frequency characteristicsNormalized amplitude - frequency characteristicsFigure 5. Amplitude - frequency CharacteristicsFigure 6. Frequency CharacteristicsIt can be seen that the frequency characteristic of the band pass filter is completely determined by the center frequency ωo and the quality factor Q.When f>fo, as the frequency f increases, the amplitude increases. According to the definition of cutoff frequency, the denominator of amplitude-frequency characteristic , that is (since f>fo, take a positive value)1) Upper cutoff frequency（take a positive value）When f<fo, as the frequency f decreases, the output signal amplitude will decrease. According to the definition of cutoff frequency, the denominator of amplitude-frequency characteristic , that is (since f<fo, take a negative value)2) Lower cutoff frequency（take a negative), get the bandwidth When the center frequency fo and bandwidth B (or Q) are known, the upper and lower cutoff frequencies fp1 and fp2 can be calculated. On the contrary, when the upper and lower cutoff frequencies fp1 and fp2 are known, the center frequency fo and bandwidth B (or Q) can be calculated. (where ),  4.3 Second-order Band Pass FiltersA simple second-order band pass filter circuit is shown in the figure below, where R1 and C1 constitute a low-pass filter circuit, and C2 and R3 constitute a high-pass filter circuit.Figure 7. Second-order Band Pass Filter Circuit(1) Transfer FunctionIn order to reduce the amount of parameters matching, generally take C1=C2=CTake , , that is The transfer function can be obtained by using the node current method.(2) Frequency Characteristicswhere band-pass amplification (The negative sign means that the input and output are inverted. Because the filter circuit is an inverting filter.)Center frequency (C1=C2=C), Q factor When Aup, Q, and ωo are known, the resistance of each resistor is (R3 can be calculated with ωo/Q), (Aup<2Q2)When the pass band amplification factor Aup is small, Q should not be too large (that is, the simple second-order BPF has poor selectivity), otherwise R2 will become very small (R2 is generally greater than 1K), which will attenuate the input signal seriously. In order to make the system stable, Aup is generally between 1 and 10, and Q can be between 1 and 20.(3) Design StepsExample: It is known that Aup=5, center frequency fo=450Hz, bandwidth B=200Hz (). Try to calculate the parameters of the band-pass filter and verify.First, according to the center frequency fo, check the parameter table and determine C1, C2, and operational amplifier parameters according to the nominal value.fo=450Hz, take C1=C2=0.01uF(103 capacitor). Since the center frequency is not high, the requirement can be met by using LM358 operational amplifier.Second, calculate the resistance of each resistor. Among them, the range of R1 and R3 should be between 10K ~510K, and R2 should be between 1K ~100K, otherwise the capacitor C needs to be reselected.Substituting the relevant parameters into the above formula, the result is R1=15.9K, R2=15.5K, R3=159K.Third, use simulation software to verify on the computer, and try to take the nominal value of each relevant resistance. The simulation schematic diagram and simulation results are shown in the figure below. The result values obtained from the AC small signal analysis and transmission characteristic analysis basically meet the requirements.Figure 8. Filtering Circuit with LM358Figure 9. Simulation Schematic DiagramFigure 10. Voltage - Time Simulation (ui)Figure 11. Voltage - Time Simulation (ui, uo)The circuit requires a small number of components, and it can work with dual power supplies or with a single power supply (the non-inverting termination is connected to a 1/2Vcc bias potential). In fact, it is widely used in single power supply systems. Because the quality factor Q cannot be too high. Almost all band pass filter circuits with a larger bandwidth B (with a smaller Q value) adopt this circuit form. 4.4 High-Q second-order Band Pass FiltersThe high-Q second-order band pass filter circuit is shown in the following figure. This circuit can work with dual power supplies or single power supplies, which is convenient to use in a single power supply system. Since the Q value can be made larger, it is particularly suitable as a band pass filter.Figure 12. Bandpass Filter Circuit(1) Transfer FunctionIn order to reduce the amount of parameters matching, generally take C1=C2=CWhere , , that is , , getting The transfer function can be obtained by using the node current method.(2) Frequency Characteristics, where band-pass amplification Center frequency , Q factor In order to make the system stable, Aup and Q must be greater than 0, that is, 2RfR4-RFR3>0, which must be guaranteed . Adjusting can control Aup and Q. is more closer to 2, the greater the Aup and Q values. Adjust the capacitor C to select the center frequency ωo. When the values of Aup, Q and ωo are known, and the ratio between and is determined, the resistance of each resistor is , , (3) Design stepsExample: It is known that Aup=5, center frequency fo=1kHz, bandwidth B=50Hz (). Try to calculate the parameters of the band-pass filter and verify.First, according to the center frequency fo, check the parameter table and determine C1, C2, and operational amplifier parameters according to the nominal value. (Aup: 1 ~10)fo=1kHz, take C1=C2=0.01uF. Since the center frequency is not high, the requirement can be met by using LM358 operational amplifier.Second, according to the value of Aup and Q, initially determine the value of and .Since Aup and Q are large, is 1.8, is 0.5, and is 3.6.Third, calculate the resistance of each resistor. The range of R1 and R3 should be between 10K and 510K, and R2 should be between 1K and 100K. Otherwise, the ratio of and needs to be reselected.Substituting the relevant parameters into the above formula, the result is:R1=229K, R3=63.7K, R2=4.18K, R4=127.4K: RF takes 36K, Rf takes 10K.Fourth, use simulation software to verify on the computer, and try to take the nominal value of each relevant resistance. The simulation schematic diagram and simulation results are shown in the figure below. The result values obtained from the AC small signal analysis and transmission characteristic analysis basically meet the requirements.Figure 13. High-Q BPF CircuitFigure 14. Voltage - Frequency SimulationFigure 15. Voltage - Time Simulation4.5 Dual-operational Amplifier BPF (High-Q)The BPF circuit with high-Q value formed by dual operational amplifiers is shown in the figure. With fewer components, a very high-Q value can be obtained when the pass band amplification factor Aup is fixed equal to 2, so it is also a commonly used BPF circuit.Figure 16. Dual-amp BPF(1) Transfer functionAccording to the rule of futility, where, , that is , where , so (2) Frequency CharacteristicsCompared with the standard form of the second-order BPF transfer function, the following parameters can be obtained:pass-band magnification , , center frequency When R4=R5,R2=R3=R,C1=C2=C, Aup=2, , （）It can be seen that when Aup=2 (that is, when R4=R5), the value of Q can be very large.(3) Design stepsAccording to the center frequency fo, check the parameter table to determine C. When C is determined, the resistance R is calculated from the center frequency. Meanwhile, etermine R1 based on the Q value. 4.6 Second-order Band Pass Filters (Voltage-controlled Type)The second-order voltage-controlled BPF is shown in the figure. Among them, R1 and C1 constitute a low-pass filter, R2 and C2 constitute a high-pass filter. (The voltage positive feedback is introduced through the voltage R3 to form a voltage-controlled band-pass filter.)Figure 17. Second-order Bandpass Filter (voltage controlled)Rf/RF cannot be 3 to avoid self-excitation.(1) Transfer FunctionWhere ，that is , , so The transfer function can be obtained by using the node current method.In order to reduce the amount of parameters matching, generally take R1=R3=R,R2=2R,C1=C2=CWhere In order to make the system stable, the coefficient of the first term in the denominator must be larger than 0, that is, 3−Auf>0, in other words, Auf<3.(2) Amplitude - frequency Characteristicswhere band-pass amplification , center frequency , Q factor It can be seen that the closer Auf is to 3, the larger the Q value. The narrower the pass band B, and the better the selectivity.(3) Design StepsAccording to the center frequency, look up the table and initially determine C1=C2=Ccalculate resistance , that is , Calculate bandwidth based on upper and lower cutoff frequencies , Calculate the quality factor Calculate by Q and determine the resistances Rf and RF.As a special case, the center frequency fo=1KHz is known, so C1=C2=C=0.01uF，R2=2R=31.8K, getting Auf=2.95, that is . If Rf=10K, calculate RF=19.5K.For high pass and band pass filters, the output of the op amp is not required to be 0 at static state. And the single power supply operating mode can be selected. In the low-pass or band-stop filter circuit, it is a DC-to-AC DC amplifier circuit, which generally requires the circuit to work in a dual power supply state. Ⅴ Band Pass Filter ApplicationsThe filtering circuit has a wide range of uses.According to different frequency amplitude characteristics, filter circuits can be divided into low pass filter circuit (LPF), high-pass filter circuit (HPF), band pass filter circuit (BPF), band stop filter circuit (BEF) and all-pass filter circuit (APF) . The BPF is mainly used to highlight signals in useful frequency bands and weaken signals or interference and noise in other frequency bands to improve the signal-to-noise ratio. Therefore, band pass filters are often used in wireless receivers and transmitters to receive useful signals while preventing unwanted frequencies from passing through.In addition to the fields of electronics and signal processing, an specific application of band pass filters is in the field of atmospheric sciences. A very common example is to use it to filter the weather data in the last 3 to 10 days, only the cyclone as a disturbance remains in the domain. Frequently Asked Questions about Band Pass Filter1. What is a bandpass filter used for?A band pass filter is an electronic circuit or device which allows only signals between specific frequencies to pass through and attenuates/rejects frequencies outside the range. Band pass filters are largely used in wireless receivers and transmitters, but are also widely used in many areas of electronics. 2. What is the bandwidth of a bandpass filter?The bandwidth of a bandpass filter is usually defined as the 3 dB bandwidth. Similarly, the 1 dB bandwidth is the point at which the signal amplitude decreases by 1 dB from its maximum value (above and below the center frequency). 3. How does bandpass filter work?A bandpass filter works to screen out frequencies that are too low or too high, giving easy passage only to frequencies within a certain range. Band-pass filters can be made by stacking a low-pass filter on the end of a high-pass filter, or vice versa. Attenuate means to reduce or diminish in amplitude. 4. How is bandpass filter calculated?So all frequencies between the low cutoff frequecny and the high cutoff frequency are the passband of the bandpass filter. The gain of the circuit is determined by the formula, gain (AV)= -R2/R1. Thus, for example, to have a gain of 10, R2 must be 10 times the value of R1. 5. What is the use of band pass filter?Bandpass filters are widely used in wireless transmitters and receivers. The main function of such a filter in a transmitter is to limit the bandwidth of the output signal to the band allocated for the transmission. This prevents the transmitter from interfering with other stations.

IntroductionLED lights cannot directly use the conventional mains grid voltage, because of the characteristics of LED lighting. In order to meet the special voltage and current requirements of LEDs, a specially designed voltage conversion device must be used to make LEDs work normally. This device is an LED driver. LED drivers are usually switching mode devices that convert the input voltage (Typically 120-220 VAC or 12 VDC) into a voltage at which the current drawn by the LED's is equal to its drive current. The drive current is regulated for optimum brightness, led service life, and battery life. A drive current lower than the maximum drive current of an LED can greatly prolong service life. As a key part of LED lighting, the quality of LED drivers directly affects the performance of LED lighting.Choose the Correct LED Drivers For LED LightsNo matter how good the quality of the LED driver is, failure and maintenance are inevitable. This article will analyze the 10 failures in LED lighting design and its application based on the relevant technology and practical experience of the LED driver.CatalogIntroductionⅠ LED Driver Failure Analysis1.1 Forward Voltage Drop (Vf) Range1.2 Power Margin and Derating Requirements1.3 LED Working Characteristics1.4 Test Session1.5 Different Load with Different Test Results1.6 LED Driver Circuit Problem1.7 Wrong Phase Wring1.8 Grid Fluctuation1.9 Frequent Line Trips1.10 Drive CoolingⅡ LED Driver Maintenance2.1 Multimeter to Detect LED Driver2.2 Identify LED Power SupplyⅢ LED Driver Circuit Modulation3.1 Pulse Width Modulation (PWM)3.2 Pulse Frequency Modulation (PFM)3.3 Sliding-Mode ModulationⅣ One Question Related to LED DriverⅠ LED Driver Failure AnalysisThe LED driver is measuring current passing through LEDs using sense resistor and then increase or decrease the voltage to maintain constant current continuously. LEDs are kind of diode so they need DC voltage to operate so most of the LED drivers are boost and can vary output supply in wide range (example, 16V to 38V). They also have dimming control by PWM signal from microcontroller OR by having a manual potentiometer to change sense resistor. According to them, LED driver failures are complex, but we can follow the steps below to analyse.1.1 Forward Voltage Drop (Vf) RangeLED lamp load end is generally composed of a number of LEDs connected in series, and its working voltage Vo=Vf×Ns, where Ns represents the number of LEDs. And the Vf of an LED varies with temperature. Generally, at a constant current, Vf becomes lower at high temperatures and becomes higher at low temperatures. The LED lamp load working voltage is VoL at high temperature, and VoH represents a value at low temperature. When selecting an LED driver, consider that the driver output voltage range is greater than VoL～VoH.If the maximum output voltage of the LED driver is lower than VoH, the maximum power of the lamp may not reach the actual power required at low temperature. If the minimum voltage of the selected LED driver is higher than VoL, the output of the driver may exceed the working range at high temperature. And LED driver will work unstable, making the lights flicker.Considering the overall cost and lamp efficiency, don’t blindly pursue the ultra-wide output voltage range of the LED driver. Because the driver voltage is only in a certain range, its efficiency is the highest. When the range is exceeded, the efficiency and power factor (PF) will deteriorate. In addition, if the design of the driver output voltage range is too wide, high costs and unoptimized efficiency will be made. 1.2 Power Margin and Derating RequirementsIn general, the nominal power of the LED driver refers to the data measured under the rated environment and rated voltage. Taking into account different applications, most LED driver suppliers will provide power derating curves in their product specifications (common load vs. ambient temperature derating curves and load vs. input voltage derating curves).As shown in Figure 1, the red curve represents the power derating curve when the input is 120Vac, and its load varies with the ambient temperature. When the ambient temperature is lower than 50℃, the LED driver is allowed to be 100% full load. When the ambient temperature is as high as 70℃, the LED driver can only be derated to 60% of the load. When the ambient temperature changes between 50℃~70℃, the driver load varies with the temperature linearly.Figure 1. Power Derating Curve (Load vs Ambient Temperature)The blue curve represents the power derating curve when the input is 230Vac or 277Vac, and its load varies with the ambient temperature. The principle is similar to the above mentioning.As shown in Figure 2, the blue curve represents the derating curve of the LED driver when the ambient temperature is 55°C, its output power varies with the input voltage. When the input voltage is 140Vac, the load of the driver is allowed to be 100%, and the input voltage will be adjusted downward. If the output power remains the same, the input current will rise, resulting in input terminal loss and lower efficiency. When the device temperature rises, exceeding the rated temperature, which may cause the device to fail.Figure 2. Power Derating Curve (Load vs Input Voltage)Therefore, when the input voltage is less than 140Vac, the output load of the LED driver is required to linearly decrease as the input voltage decreases. According to the above derating curve and corresponding requirements, when choosing a LED driver, actual applying needs are important, as well as the derating margin. 1.3 LED Working CharacteristicsWhen the required input power is a fixed value, such as a fixed error of 5%, the output current can only be adjusted to the specified power for each lamp. Due to different working ambient temperature and different lighting time, the power of each lamp will vary greatly.Although there are considerations for marketing and business factors. However, the volt-ampere characteristic of the LED lamp determines that the LED driver is a constant current source, and its output voltage varies with the series voltage Vo of the LED load. When the efficiency of the driver is basically unchanged, its input power changes with Vo. And meanwhile, the overall efficiency of the LED driver will increase after thermal equilibrium. Under the same output power, the input power will decrease compared to the boot time.Therefore, when formulating requirements, LED driver users should first understand the operating characteristics of LEDs. Avoid suggesting indicators that do not meet the principles of operating characteristics, and indicators that far exceed actual requirements, resulting in excess quality and cost waste. 1.4 Test SessionSample test problems, for example, multi-brand LED driver samples all failed during the test. The reason is that a self-dual voltage regulator is used to directly power the LED driver for testing. After power on, the voltage regulator is gradually adjusted from 0Vac to the rated operating voltage of the LED driver. This kind of test operation can easily make the LED driver start and work with the load when the input voltage is very small, but this situation will cause the input current to be far greater than the rated value. And the internal input terminal related components, such as fuses, rectifier bridges, thermistors, etc. will fail due to excessive current or overheating, damaging the LED driver.The correct test method is to adjust the voltage regulator to the rated operating voltage range of the LED driver, and then connect the driver to power-on test. Of course, technically improving the design can also avoid the failure caused by this kind of test misoperation. That is, a starting voltage limit circuit and an input undervoltage protection circuit are set at the input of the driver. When the input does not reach the start-up voltage set by the driver, the driver does not work. When the input voltage drops to the input undervoltage protection point, the driver enters the protection state. Although the driver has a self-protection function and will not fail, you must carefully understand whether the purchased LED driver product has this protection before testing (considering the actual application environment of the LED driver, most LED drivers currently do not have this set). 1.5 Different Load with Different Test ResultsOn the one hand, when the LED driver is tested with LED lights, the result is normal; on the other hand, when driver tested with an electronic load, the result may be abnormal. Usually this phenomenon has the following reasons:1) The output instantaneous voltage or power of the driver exceeds the working range of the electronic load instrument. (Especially in CV mode, the maximum test power should not exceed 70% of the maximum power of the load. Otherwise, the load may instantaneously have overpower protection when loading, causing the driver to fail to work.)2) The characteristics of the electronic load instrument used are not suitable for measuring constant-current device. And the load voltage gear jumping, result in the drive to fail to work.3) Because there is a large capacitor inside the input of the electronic load meter. The test is equivalent to connecting a large capacitor in parallel with the driver output, which may cause the driver's current sampling work to be unstable. As we all known, the LED driver is designed to meet the working characteristics of LED lamps. The most practical test method is to use the LED lamp as a load, and connects an ammeter and a voltmeter in series to test. 1.6 LED Driver Circuit ProblemThe following conditions often cause damage to the LED driver:Connect AC to the DC output terminal of the driver, causing the driver to fail.Connect AC to the DC/DC output or input of the driver, causing the driver to fail.Connect the output terminal of the constant current to the dimming light, causing the driver to fail.Connect the phase wire to the ground wire causing the driver has no output and the housing is charged. 1.7 Wrong Phase WringTake an international example: the rated working voltage between each phase line and the neutral line is 220V, and the voltage between the phase line and the phase line is 380V. If the driver is connected to two phase wires, after power on, the LED driver input voltage exceeds the rated range, which cause the product to fail.As shown in Figure 3, V1 represents the first phase voltage, V2 represents the second phase voltage, and R1 and R2 respectively represent the drivers normally installed on the line. When the neutral line (N) on the circuit is disconnected, the drivers R1 and R2 on the two branches are connected to the 380V voltage after being connected in series. Because of the difference in input internal resistance, when one of the drivers is charged to start, the internal resistance becomes smaller. Most of the voltage may be applied to another driver, causing the overvoltage damage. Therefore, it is recommended that switches or short-circuiters on the same distribution branch should be disconnected together, not just cut off the neutral line. What’s more, do not put the power distribution fuse on the neutral line to avoid bad effect of the neutral line on the circuit.Figure 3. Neutral Line Open Circuit Diagram 1.8 Grid FluctuationWhen wires of a transformer grid branch is too long and there is large power equipment on the branch, the grid voltage will fluctuate sharply when the large equipment starts and stops. It even causes the grid to be unstable. When the grid voltage exceeds 310V, the drive may be damaged (even if there is an LED lightning protection device, it is useless. Because the lightning protection device is to deal with pulse spikes of tens of uS level, and the fluctuation of the grid may reach tens of mS, or even hundreds of mS) . Therefore, special attention should be paid when there is large electric machinery on the street lighting branch power grid. It is best to monitor the fluctuation range of the power grid or to supply power to the grid transformer separately. 1.9 Frequent Line TripsToo many lights are connected on the same branch, which leads to overload on a certain phase and uneven power distribution among the phases, resulting in frequent line trips. 1.10 Drive CoolingAlthough the LED has high luminous efficiency, only a small part of the energy flowing through the LED is radiated in the form of visible light. And most of the remaining energy is consumed in the LED in the form of heat, so the LED generates more serious heat. When the driver is installed in a non-ventilated environment, the driver housing should be in contact with the lamp housing as much as possible. If possible, apply thermal glue or a thermal pad on the contact surface between the housing and the lamp housing to improve the heat dissipation of the LED driver and ensure the reliability of the driver.Ⅱ LED Driver Maintenance2.1 Multimeter to Detect LED DriverMeasuring the output voltage of the no-load LED driver with a multimeter, if the output voltage is not detected, does it mean that the driver is broken? Look at the following steps:1) The voltage of the non-isolated LED power supply in the no-load state is about 300V tested with a multimeter, and it is about 220V with a PFC.2) Isolating the LED power supply, the voltage in the no-load state, tested with a multimeter, is about 3-5V more than the total voltage of the rated LED series. However, although the output voltage can be tested under no load, it does not mean that it can be normal under load. At this time, it is necessary to connect the corresponding LED light board to see the performance of the LED lighting. If there is no flicker, the output voltage is also equal to the total voltage of LED lights in series connection. This situation can be considered normal, otherwise it fails. If there is no output voltage at no load, the power supply must be broken. 2.2 Identify LED Power SupplyThe LED power supply is widely used in many applications. So how to distinguish the quality of LED power supply is particularly important. A few methods are briefly introduced below. LED Driver ICThe power of IC drive, the quality of IC directly affects the whole power supply. The lighting manufacturer should understand the IC design solution and calculate the cost of the driver, so as to purchase power products at a reasonable price. TransformersThe control chip can be regarded as the brain center of the power supply, while the transformers determine the power and temperature resistance. The transformer is responsible for the transfer of "AC to DC". However, the energy overload will damage the device. The core of the transformer is the magnetic core and the wire package. Electrolytic Capacitors and Ceramic CapacitorsThe quality and life requirements of input electrolytic capacitors are important. However, people tend to ignore the quality requirements of the output capacitor. In fact, the life of the output capacitor also has a great impact on the life of the power supply. The output end has a switching frequency of up to 60,000 times per second, which causes the parasitic resistance of the capacitor to heat up and produce substances similar to scale. Finally, the electrolyte heats up and bursts. Ceramic capacitors: The materials are divided into X7R, X5R and Y5V, and the actual capacitance value of Y5V can only reach 1/10 of the actual value. In addition, the nominal capacitance value only refers to when a capacitor works at 0V. Therefore, this tiny chip with poor options will also lead to a price difference in cost and greatly shorten the life of the power supply. Circuit Design and Welding ProcessJudgment of the pros and cons of the design: Aside from the professional point of view, it can be distinguished by some intuitive methods, such as the neat layout of the components, and soldering points. As for flying leads and manually adding components, it is a serious lack of techniques and efficiency. As we all know, the quality of mechanized production of wave soldering process is definitely better than manual welding. Because the machining process is more neat and uniform. Identification method: whether there is red glue on the back board.The flashing phenomenon of the lamp after a period of use is basically caused by the power supply or the weak welding of the lamp beads. However, it is extremely difficult to detect the virtual welding of products through aging, so AOI must be used to detect the quality of the power supply. Batch Inspection of Aging Racks and High Temperature Aging RoomsNo matter how good the power products are controlled by materials and production processes, they still need to be tested for aging. Because the incoming inspection of electronic components and transformers is difficult to control. Only through the aging of the entire batch of power supplies and the high temperature sampling inspection of the high temperature room. This is a wide-ranging screening to determine whether the materials have safety hazards. Ⅲ LED Driver Circuit ModulationThe LED driver circuit is divided into constant-voltage type and constant-current type according to the power supply to the LED. Constant-current switch type LED driver circuit samples the current flowing through the LED lamp, and gives the output control signal to control the on and off of the switching power tube, which aims to adjust the output current as the set value. The dimming control circuit mainly includes SCR dimming circuit, pulse width modulation (PWM), pulse frequency modulation (PFM), sliding mode control, PWM_PFM, PSM, etc. Let's take pulse width modulation (PWM), pulse frequency modulation (PFM), and sliding mode modulation to introduce in detail below. 3.1 Pulse Width Modulation (PWM)Pulse width modulation, shown in the figure below, refers to the stability of the output voltage by changing the on-time of the switching power tube in each cycle at a specific frequency. That is adjusting the duty cycle to obtain stable output voltage. When the output voltage changes due to the working environment, noise and other factors, the error amplifier samples the voltage change and sends the signal to the control circuit. The control circuit adjusts the duty cycle of the switching power tube signal to maintain the stability of the output voltage.Figure 4. PWM Modulation Based on BUCK StructureFigure 4 (a). Voltage ModeFigure 4 (b). Peak Current ModeFigure 4 (c). Average Current Mode3.1.1 Advantages of PWM(1) The PWM modulation method has high efficiency under heavy load, and has a good dynamic response to load changes.(2) The output ripple voltage is small and the linearity is high.(3) The frequency is stable, the duty cycle adjustment is not restricted, the control is simple, and both the current control mode and the voltage control mode are applicable.3.1.2 Disadvantages of PWM(1) The efficiency of PWM modulation method decreases at light load.(2) The transient response is slow during constant-voltage driving, and a more complicated compensation circuit is required.(3) Accurate current detection circuit is required for constant-current driving. 3.2 Pulse Frequency Modulation (PFM)The pulse frequency modulation is shown in the figure below. Under the condition of a certain on-time of the switching power tube, the output voltage can be controlled by adjusting the off time. When the output voltage changes, the error amplifier samples the feedback signal and sends the output signal compared with the reference signal to the control circuit. The control circuit analyzes the error signal and generates a square wave signal with constant pulse width and varying frequency to control the switch power tube to maintain the stability of the output voltage.Figure 5. Pulse Frequency Modulation Based on BUCK Structure3.2.1 Advantages of PFM(1) The PFM modulation has very high efficiency, better frequency characteristics and higher voltage regulation rate at light load.(2) The PFM modulation has a relatively high transmission signal-to-noise ratio and a good anti-interference ability.(3) The output voltage has a large adjustable range and low power consumption.3.2.2 Disadvantages of PFM(1) The efficiency of the PFM modulation will decrease under heavy load.(2) The frequency spectrum of the output ripple is scattered and irregular.(3) The load adjustment range is very small, resulting in high filtering costs. 3.3 Sliding-Mode ModulationSliding-mode modulation mode, the full name is sliding mode variable structure control,  is a discontinuous control. As shown in Figure 6, the sliding mode makes the system structure change purposefully according to its current state, which force the system to make small amplitude and high frequency up and down movements along the designed trajectory under response conditions. That is, sliding mode movement. Reduce the system's sensitivity to disturbances and load jumps.Figure 6. Sliding Mode Control Based on BUCK Structure3.3.1 Advantages and Disadvantage of Sliding ModeIt has the advantages of fast dynamic response, strong robustness and wide stability range, but it also has a problem that the operating frequency is not fixed. Ⅳ One Question Related to LED Driver4.1 QuestionHow long do LED drivers last?4.2 AnswerWhile the light function of an LED may last for years, drivers can give out much sooner. This is why we recommend name brand LED bulbs for the home, especially those with 25,000 hour rated lives. In general, high power white LEDs use much more current, and need of more complicated drivers. Frequently Asked Questions about LED Drivers Failure Analysis and Maintenance1. What is a LED driver IC?They are configured as either inductorless (charge pump) or switching regulator-based LED drivers that support driving white LEDs in series, parallel or combination. ... Topologies include boost regulator, buck regulator, buck/boost, SEPIC topology LED drivers, and more. 2. What is a LED driver used for?LED drivers are electrical devices that prevent damage to LEDs by regulating the forward voltage (VF) of the LED that changes with temperature, avoiding thermal runaway while delivering a constant current to the LED. LED drivers also aid efforts to meet new energy requirements (e.g., Energy Star). 3. How do I choose an LED driver?Use an LED driver with at least the same value as your LED(s). The driver must have a higher output power than your LEDs require for extra safety. If the output is equivalent to the LED power requirements, it is running at full power. Running at full power may cause the driver to have a shorter life span. 4. Why do LED drivers fail?LED FailureThe LEDs usually fail, because they have been connected to a constant LED driver in parallel. If the LEDs have failed you may want to also replace the LED driver. We usually recommend using a model with an adjustable output, and trimming down the output voltage slightly, to avoid over powering the LEDs. 5. How long do LED drivers last?Namely, the life of the driving circuit expires prior to when the LED stops emitting light or has its brightness dropped. The typical nominal lifetime of these elements is often times less than 25,000 hours, while the lifetime of LED itself could be as long as 50,000-100,000 hours. 6. Why do my LED drivers get hot?If the LED driver is trying to draw DC (not a balanced load circuit) then that can also cause the transformer to overheat.7. What is the difference between a transformer and an LED driver?LED drivers and electronic transformers for retrofit LED lighting are not interchangeable. They differ in output and load compatibility i.e. which LED lights they will work with. The fundamental difference between the two is that LED drivers output DC while electronic halogen transformers output 12VAC.

2026 Executive Summary: Discharging a capacitor safely is a critical maintenance step to prevent severe electrical shocks and equipment damage. In 2026, with the rapid expansion of high-voltage EV inverters and renewable energy grid systems, proper discharge protocols using dedicated resistors or discharge tools are more important than ever. This guide covers the working principles, safety procedures, and step-by-step methods for discharging both low and high-voltage capacitors safely.IntroductionMaintenance technicians and electronics hobbyists frequently experience electrical shocks from devices long after they have been unplugged. The primary component responsible for this dangerous phenomenon is the capacitor. From industrial power capacitors and modern EV inverter filter capacitors to the graphite coating of legacy CRT televisions, capacitors can store lethal amounts of electrical charge (often exceeding 300V) in fractions of a second. If maintenance personnel accidentally touch a charged terminal, the resulting electric shock can cause severe secondary injuries, such as falls or involuntary contact with other live circuits. Therefore, verifying zero energy state and manually discharging the capacitor is a mandatory safety protocol before beginning any repair work. This article details the industry-standard methods for safely discharging capacitors, explains the underlying physics of capacitance, and outlines the working principles of various capacitor types used in 2026. If you need to calculate the exact discharge rate of a capacitor under a known capacitance and charge it through a fixed value resistor, we recommend using Apogeeweb's Capacitor Safety Discharge Calculator.Figure 1. Apogeeweb's Capacitor Safety Discharge CalculatorTable of ContentsIntroductionTable of ContentsI. How Does a Capacitor Charge and Discharge?II. How to Safely Discharge a Capacitor?  2.1 Discharge Method After the Capacitor is Cut Off  2.2 Critical Safety Notes for Capacitor Discharge  2.3 The Physics of the Charging and Discharging ProcessIII. Three Methods to Discharge High Voltage Capacitors  3.1 What is a High Voltage Capacitor?  3.2 Step-by-Step High Voltage Discharge MethodsIV. How to Discharge Low Voltage Capacitors?  4.1 Short-circuiting with Wires  4.2 Using a Digital Multimeter  4.3 Safety PrecautionsV. How to Discharge a Filter Capacitor?  5.1 Discharge Techniques for Filter Capacitors  5.2 Calculating Charge and Discharge Time ConstantsVI. Fast Discharge Methods for Power Compensation and Electrolytic CapacitorsVII. How Does a Capacitor Bank Discharge Coil Work?  7.1 Principle of Capacitor Bank Discharge Coils  7.2 Influence of the Connection ModeVIII. How to Test Capacitors Using the Discharge PrincipleIX. Test Your Knowledge: Capacitor QuizX. Frequently Asked QuestionsI. How Does a Capacitor Charge and Discharge?When a capacitor charges, it stores electrical energy in an electrostatic field between two conductive plates, and when it discharges, it releases this stored energy back into the circuit to power a load. A capacitor is a passive electronic component composed of two conductive parallel plates separated by an insulating dielectric material.Figure 2. The Function of CapacitorsDuring the charging process, the power supply forces charged particles through the circuit, causing the potential difference between the two plates to gradually approach the voltage of the power source. Ultimately, opposite polarities of charge accumulate on the plates, bound by the dielectric, storing electrical energy as an electrostatic field. During the discharge process, the capacitor acts as a temporary power source. It moves the stored charged particles through a closed circuit, neutralizing the potential difference between the plates. The electrostatic field collapses, and the stored energy is converted into work consumed by the connected electrical load.II. How to Safely Discharge a Capacitor?To safely discharge a capacitor, you must create a controlled closed loop using a high-wattage resistor or a dedicated discharge tool to slowly neutralize the stored potential difference without creating dangerous sparks. Watch this demonstration video first:A practical capacitor lesson demonstrating the effects of uncontrolled discharge.2.1 Discharge Method After the Capacitor is Cut OffWhen a capacitor is disconnected from an energized circuit, it retains its stored voltage. If the circuit contains bleed resistors or other continuous loads, it will discharge slowly. Otherwise, it must be discharged manually by creating a short circuit through a resistive load (or a direct wire for very low voltages). When discharging, the external circuit and the capacitor form a closed loop. The excess electrons (negative charges) travel toward the positive electrode to achieve electrostatic balance. This current flow neutralizes the charges on both ends of the capacitor. Once neutralization is complete, the electric field disappears. However, because real-world circuits always contain some resistance, the charge decays exponentially. It trends infinitely toward zero but technically never reaches absolute zero, though it quickly drops to safe handling levels.Figure 3. Discharge Tools2.2 Critical Safety Notes for Capacitor DischargeAfter the capacitor is disconnected from the main bus, it must be discharged through a high-wattage discharge resistor or a special voltage transformer.Discharge must occur between the lead wires of the capacitor, and subsequently between the lead wires and the metal casing.The capacitor should be physically grounded only after the initial resistive discharge is complete.Before handling the capacitor, a test discharge must be performed by holding the discharge rod on the terminals for several seconds.Even if both sides of a capacitor bank are grounded, residual charges can remain. Each individual capacitor in a parallel group must be discharged separately.Exercise extreme caution with damaged capacitors. Internal disconnections can prevent standard grounding devices from fully discharging the unit.If the capacitor enclosure features an interlocking safety device, ensure the protective fence is only opened after the entire system is verified as grounded.2.3 The Physics of the Charging and Discharging ProcessSuppose a capacitor has upper and lower plates, with the upper plate connected to the positive electrode and the lower plate to the negative electrode. Upon connection to a DC power supply, a potential difference forms. The positive charges remain stationary in the atomic lattice, while the negative charges (electrons) are repelled from the negative terminal of the supply and accumulate on the bottom plate. This electron movement creates a growing potential difference across the plates. The voltage increases until it perfectly matches the power supply voltage, at which point the capacitor is fully charged and current ceases to flow. After disconnecting the power, the dielectric insulation prevents the charges from recombining. The potential difference persists until a conductive path is provided. When we discharge the capacitor by connecting the plates with a wire or resistor, electrons flow from the negatively charged plate back to the positive plate until electrostatic equilibrium is restored.Figure 4. Charge and DischargeIII. Three Methods to Discharge High Voltage Capacitors3.1 What is a High Voltage Capacitor?High-voltage capacitors are heavy-duty components designed to handle extreme electrical loads, commonly found in microwave ovens, medical imaging equipment, and modern EV charging infrastructure. They consist of outlet porcelain bushings, capacitive element groups, and a sealed steel shell. The internal elements use capacitor paper, film-paper composites, or pure synthetic films as the dielectric, with aluminum or platinum plates. To meet high withstand voltage requirements (often exceeding 10kV), internal capacitive elements are connected in series or parallel. Many modern high-voltage capacitors are equipped with internal bleed resistors designed to reduce residual voltage below 75V within 10 minutes of disconnection, though manual discharge remains a mandatory safety requirement.Figure 5. Microwave High Voltage Capacitor3.2 Step-by-Step High Voltage Discharge MethodsHigh-voltage capacitors must never be short-circuited directly with a screwdriver, as the massive instantaneous current can melt the contacts, vaporize the metal, and cause an explosion. Instead, the energy must be dissipated slowly using a resistive load. Here are three safe methods:Method 1: The High-Wattage Resistor MethodFirst, unplug the electrical power and verify the equipment is isolated from the mains.Obtain a 20,000-ohm, 2-watt (or higher, such as 5W) wire-wound resistor.Using insulated pliers, hold the resistor and touch its probes across the two terminals of the capacitor for several seconds.If the capacitor has three terminals, discharge between the outer terminal and the center terminal, then repeat for the remaining outer terminal.Method 2: The Alligator Clip MethodConnect one end of a high-wattage resistor to an insulated test lead and the other end to an insulated alligator clip. Wrap exposed connections in electrical tape.Clamp the alligator clip securely to the equipment's chassis ground wire.Use the test lead probe to touch the positive terminal of the capacitor. This routes the discharge safely to ground without generating sparks.Note: If discharging multiple capacitors consecutively, the resistor will generate significant heat. Use a 5W or 10W resistor for heavy-duty applications.Method 3: The Bulb or Soldering Iron MethodConstruct a discharge rig using a 100-200 watt incandescent bulb (or a 60-80W electric soldering iron) connected to two insulated probes.Touch the probes to the capacitor terminals. The bulb will flash brightly and dim as the capacitor discharges, providing a visual indicator of the voltage drop.Once the bulb is completely dark, verify the voltage is zero using a multimeter.IV. How to Discharge Low Voltage Capacitors?4.1 Short-circuiting with WiresFor low-voltage capacitors operating below 50V or with a capacity under 1μF, you can safely discharge them by directly short-circuiting the two poles with an insulated wire or screwdriver. While a screwdriver is common, it can leave carbonized burn marks on the terminals. Never use this direct short-circuit method for high-voltage or large-capacity capacitors, as the rapid energy release will create dangerous sparks and potentially damage the component.Figure 6. Shortcircuit4.2 Using a Digital MultimeterYou can safely discharge small capacitors using the resistance setting (Ohms) on a digital multimeter. Set the multimeter to a high resistance range (e.g., 100K or 200K ohms) and place the probes on the capacitor terminals. The internal battery of the multimeter will interact with the capacitor, and you will see the resistance reading climb or drop until it stabilizes, indicating the charge has been neutralized. Disconnect the probes immediately after the reading stabilizes to prevent reverse charging.Figure 7. Multimeter4.3 Safety PrecautionsBecause capacitors exhibit dielectric absorption (often called "battery action"), they can slowly rebuild a small residual charge even after being discharged. Always manually discharge a capacitor immediately before handling it, even if the equipment has been unplugged for days. When working with industrial capacitor banks, ensure the circuit breaker and isolation switches are open, and wear appropriately rated insulating gloves.V. How to Discharge a Filter Capacitor?5.1 Discharge Techniques for Filter CapacitorsA filter capacitor is an energy storage device installed at the output of a rectifier circuit to smooth out AC pulsations and provide a stable DC output. In a standard 220V AC to DC power supply, the voltage across the primary filter capacitor can reach approximately 310V DC. To discharge a 310V filter capacitor, the best tool is a 25W electric soldering iron or a dedicated power resistor. A 25W soldering iron has an internal resistance of about 2.2KΩ. According to Ohm's Law (I = U/R), the maximum initial discharge current is 310V / 2200Ω = 140mA. This low current safely drains a several-hundred microfarad capacitor in a few seconds without generating sparks. Avoid using a 25W incandescent bulb for 310V filter capacitors. The cold resistance of a 25W tungsten filament is only about 160Ω, resulting in an instantaneous current surge of nearly 2 amps, which can instantly burn out the filament.Figure 8. Filter Capacitors5.2 Calculating Charge and Discharge Time ConstantsThe charging and discharging time of a filter capacitor is dictated by the RC time constant (T = R × C), where R is the circuit resistance and C is the capacitance. The time required for the capacitor voltage to reach 63.2% of the supply voltage is one time constant. After 5 time constants (5T), the capacitor is considered 99.3% fully charged or discharged. Charging Calculation: If a rectifier diode has an internal forward resistance of 30Ω and the filter capacitor is 2200μF, the charging time constant is Tc = 30Ω × 0.0022F = 0.066 seconds (66ms). Discharging Calculation: If the connected load has a resistance of 300Ω, the discharge time constant is Td = 300Ω × 0.0022F = 0.66 seconds (660ms). It will take approximately 3.3 seconds (5 × 660ms) for the capacitor to fully discharge through the load once power is removed.Figure 9. (a) Charging Curve (b) Discharging CurveFigure 10. Bridge Rectifier CircuitRelated recommendation: Apogeeweb's time constant calculator.VI. Fast Discharge Methods for Power Compensation and Electrolytic Capacitors(1) Power Factor Compensation CapacitorsPower factor compensation capacitors, rated in kVAR, are large non-polar capacitors used to stabilize industrial power grids. Modern units are equipped with internal discharge resistors designed to drop the voltage below 50V within 3 minutes of disconnection. However, technicians must still manually discharge them using a high-power resistor probe before maintenance, as they operate at grid voltages (220V to 480V+).Figure 11. Reactive Power Compensation Supplied by Capacitors(2) Operating Skills for Electrolytic CapacitorsSmall Lead-Type: Can be directly short-circuited, though using a 100-ohm resistor is always safer for the component's lifespan.Snap-in/Horn Type: Use a 220V/60W bulb or a dedicated 1KΩ 5W resistor to discharge slowly.Large Bolt-Type: For high-voltage industrial electrolytics, use a dedicated discharge coil. Alternatively, a long (5-meter) coiled wire with alligator clips can be used; the length and coiling provide enough inductive and resistive load to dissipate the energy as heat safely.VII. How Does a Capacitor Bank Discharge Coil Work?A discharge coil is a mandatory safety device installed in parallel with high-voltage capacitor banks in substations. It rapidly drains the residual charge from the capacitors when they are disconnected from the grid, preventing dangerous overvoltage conditions during re-closing and ensuring the safety of maintenance personnel.7.1 Principle of Capacitor Bank Discharge CoilsThe discharge coil acts as a voltage transformer. Its primary winding is connected across the capacitor bank, bearing the full operating voltage. When the power is cut, the coil provides a low-resistance path for the DC charge to dissipate rapidly. For large capacity capacitor banks (≥1.7Mvar), a dedicated discharge coil must be used instead of a standard voltage transformer (PT) to prevent the PT from overheating and exploding under the massive discharge current.7.2 Influence of the Connection ModeDischarge coils are typically used in 66kV and below power systems. Their secondary windings are often wired in an open delta configuration to provide internal fault protection for the capacitor bank. If the discharge coil is used to directly monitor the capacitor's terminal voltage, jumper connection methods cannot be used, as they would distort the voltage readings required for accurate protective relaying.VIII. How to Test Capacitors Using the Discharge PrincipleYou can test the health of a capacitor (checking for breakdown, leakage, or failure) using an analog multimeter. This test relies on the multimeter's internal battery charging the capacitor, and the subsequent discharge behavior.Normal: The multimeter pointer swings rapidly to the right (low resistance) as the capacitor charges, then slowly returns to "∞" (infinite resistance) as it fully charges and blocks DC current.Leakage: The pointer swings to the right but fails to return all the way to "∞", stopping at a specific resistance value. This indicates the dielectric is leaking current.Open Circuit: The pointer does not move at all and stays at "∞". (Note: Very small capacitors under 5000pF may not show a visible swing on standard meters).Short Circuit: The pointer swings to "0" ohms and stays there, indicating the internal dielectric has completely failed.Figure 12. (a) Normal; (b) Breakdown; (c) FailureIX. Test Your Knowledge: Capacitor QuizWhich of the following determines the charging and discharging rate of a capacitor?a) Time constantb) Currentc) Powerd) VoltageAnswer: aExplanation: The time constant (T = R × C) in an RC circuit dictates the speed of charge and discharge. A smaller time constant results in a faster charging and discharging rate. Frequently Asked QuestionsWhat is the safest tool to discharge a capacitor?The safest tool to discharge a capacitor is a dedicated capacitor discharge pen or a high-wattage resistor (such as a 20,000-ohm, 5-watt resistor) connected to insulated alligator clips. These tools safely dissipate the stored electrical energy as heat without creating dangerous sparks or damaging the capacitor terminals.How long does it take for a capacitor to discharge naturally?A disconnected capacitor can take anywhere from a few minutes to several months to discharge naturally, depending on its internal leakage resistance and dielectric material. Because high-voltage capacitors can retain lethal charges for weeks, you must always manually discharge and verify them with a multimeter before handling.Why shouldn't I use a screwdriver to discharge a capacitor?Using a screwdriver to short-circuit a capacitor causes an instantaneous, uncontrolled energy release. This rapid discharge can generate dangerous sparks, melt the capacitor's terminals, damage the screwdriver, and potentially cause the capacitor to explode. Always use a proper resistive load to control the discharge rate safely.Can a capacitor hold a charge without power?Yes, a capacitor is specifically designed to store electrical energy and will hold its charge long after the main power supply is disconnected. This residual charge acts like a temporary battery, which is why capacitors pose a severe shock hazard in unplugged electronic devices and power supplies.{ "@context": "https://schema.org", "@graph":[ { "@type": "Article", "headline": "How to Safely Discharge a Capacitor: 2026 Guide", "datePublished": "2020-09-10", "dateModified": "2026-03-20", "author": { "@type": "Organization", "name": "Apogeeweb" }, "publisher": { "@type": "Organization", "name": "Apogeeweb" }, "description": "A comprehensive 2026 guide on how to safely discharge high and low voltage capacitors, including step-by-step methods, safety tools, and the physics of RC time constants." }, { "@type": "FAQPage", "mainEntity":[ { "@type": "Question", "name": "What is the safest tool to discharge a capacitor?", "acceptedAnswer": { "@type": "Answer", "text": "The safest tool to discharge a capacitor is a dedicated capacitor discharge pen or a high-wattage resistor (such as a 20,000-ohm, 5-watt resistor) connected to insulated alligator clips. 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This rapid discharge can generate dangerous sparks, melt the capacitor's terminals, damage the screwdriver, and potentially cause the capacitor to explode. Always use a proper resistive load to control the discharge rate safely." } }, { "@type": "Question", "name": "Can a capacitor hold a charge without power?", "acceptedAnswer": { "@type": "Answer", "text": "Yes, a capacitor is specifically designed to store electrical energy and will hold its charge long after the main power supply is disconnected. This residual charge acts like a temporary battery, which is why capacitors pose a severe shock hazard in unplugged electronic devices and power supplies." } } ] }, { "@type": "HowTo", "name": "How to Discharge a High Voltage Capacitor", "description": "Step-by-step instructions for safely discharging a high voltage capacitor using a high-wattage resistor.", "step":[ { "@type": "HowToStep", "name": "Isolate Power", "text": "First, unplug the electrical power and verify the equipment is isolated from the mains." }, { "@type": "HowToStep", "name": "Prepare the Resistor", "text": "Obtain a 20,000-ohm, 2-watt (or higher, such as 5W) wire-wound resistor." }, { "@type": "HowToStep", "name": "Discharge the Terminals", "text": "Using insulated pliers, hold the resistor and touch its probes across the two terminals of the capacitor for several seconds." }, { "@type": "HowToStep", "name": "Discharge Multi-Terminal Capacitors", "text": "If the capacitor has three terminals, discharge between the outer terminal and the center terminal, then repeat for the remaining outer terminal." } ] } ]}