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IntroductionA p-n junction is an interface or a boundary between n-type and p-type semiconductor materials, inside a semiconductor. One of the crucial keys to solid state electronics is the nature of the P-N junction. For example, a PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. And the p-side or the positive side of the semiconductor has an excess of holes and the n-side or the negative side has an excess of electrons. Why pn junction exists? and How does it work? What is p-n junction diode?PN Junction IntroductionCatalogIntroductionIntroductionⅠ PN Junction Basic1.1 PN Semiconductor1.2 PN Junction ReviewⅡ PN Junction Characteristic2.1 Unidirectional Conductivity2.2 Reverse Breakdown2.3 Volt-Ampere Characteristic2.4 Capacitance CharacteristicⅢ Typical Example: Transistor PN JunctionⅠ PN Junction Basic1.1 PN SemiconductorN-type SemiconductorIn silicon crystal (or germanium crystal) doped with a small amount of impurity phosphorus element (or antimony element), since semiconductor atoms (such as silicon atoms) are replaced by impurity atoms, among the five outer electrons in the outer layer of phosphorus atoms four of them form covalent bonds with the surrounding atoms, and the extra electron is almost unbound and becomes a free electron more easily. Therefore, the N-type semiconductor has become a semiconductor with a higher concentration of electrons, and its conductivity is mainly due to the conduction of free electrons.P-type SemiconductorIn silicon crystal (or germanium crystal) doped with a small amount of impurity boron element (or indium element), since semiconductor atoms (such as silicon atoms) are replaced by impurity atoms, the three outer electrons in the outer layer of boron atoms and a semiconductor atom form a covalent bond, at this time, a "hole" is generated. This hole may attract bound electrons to "fill", making the boron atom a negatively charged ion. In this way, this type of semiconductor has a higher concentration of "holes" ("corresponding to" positive charges) and becomes a substance capable of conducting electricity.1.2 PN Junction ReviewP-N junction is formed by joining n-type and p-type semiconductor materials, which is a two terminal device that allows electric current in one direction and blocks electric current in another direction.Figure 1. How is PN Junction FormedOn a silicon wafer, different doping processes are used to form an N-type semiconductor on one side and a P-type semiconductor on the other side. We call the area near the interface of the two semiconductors a PN junction.After the P-type semiconductor and the N-type semiconductor are combined, since the free electrons in the N-type region are more and the holes are less, the concentration difference between electrons and holes appears at their junction. Due to the difference in the concentration of free electrons and holes, some electrons will diffuse from the N-type region to the P-type region, and some holes will diffuse from the P-type region to the N-type region. As a result of their diffusion, the P region loses holes, leaving negatively charged impurity ions, and the N region loses electrons, leaving positively charged impurity ions. The ions in the semiconductor cannot move arbitrarily in an open circuit, so they do not participate in conduction. These immovable charged particles form a space charge zone near the interface between the P and N zones. The thickness of the space charge zone is related to the concentration of dopants.After the space charge region is formed, due to the interaction between the positive and negative charges, an internal electric field is formed in the space charge region, the direction of which is from the positively charged N region to the negatively charged P region. Obviously, the direction of this electric field is opposite to the direction of carrier diffusion, which used to prevent diffusion.On the other hand, this electric field will cause the minority carrier holes in the N region to drift to the P region, and the minority carrier electrons in the P region to drift to the N region. The direction of the drift movement is just opposite to the diffusion movement. The holes drifting from the N region to the P region supplement the holes lost in the P region on the original interface, and the electrons drifting from the P region to the N region supplement the electrons lost in the N region on the original interface, which makes the electric charge is reduced and the internal electric field is weakened. Therefore, the result of drift motion is to narrow the space charge region and strengthen the diffusion motion.Finally, the diffusion of multiple carriers and the drift of minority carriers reach a dynamic balance. On both sides of the junction surface of the P-type semiconductor and the N-type semiconductor, a thin ion layer is left. The charge area formed by this thin ion layer is called a PN junction. The direction of the internal electric field of the PN junction points from the N to the P. It is also called the depletion layer, because lack of electrons.Figure 2. PN Junction Depletion RegionⅡ PN Junction Characteristic2.1 Unidirectional Conductivity(1) The PN junction is turned on when the forward voltage is applied.If the positive pole of the power supply is connected to the P area and the negative pole is connected to the N area, a part of the applied forward voltage is in the PN junction area, at this time, the PN junction is in a forward bias. The current flows from the P to the N, and the holes and electrons move to the interface, which narrows the space charge area. In addition, the current can pass smoothly. Its direction is opposite to the direction of the electric field in the PN junction, which weakens the internal electric field. As a result, the resistance of the internal electric field to the diffusion movement of the multitons weakens, and the diffusion current increases. The diffusion current is much larger than the drift current, and the influence of the drift current can be ignored, and the PN junction is in low resistance.(2) PN junction is cut off when reverse voltage is applied.If the positive pole of the power supply is connected to the N zone, the negative pole is connected to the P zone, and a part of the applied reverse voltage applies in the PN junction zone, and it is in reverse bias. Then the holes and electrons move away from the interface, which widens the space charge area, and the current cannot flow. The direction is the same as the direction of the electric field in the PN junction, which strengthens the internal electric field. The resistance of the internal electric field to the multiton diffusion movement is enhanced, and the diffusion current is greatly reduced. At this time, the drift current formed by the minority carriers in the PN junction region under the action of the internal electric field is greater than the diffusion current. The diffusion current can be ignored, and the PN junction exhibits high resistance.2.2 Reverse BreakdownWhen a reverse voltage is applied to the PN junction, the space charge region becomes wider and the electric field in the region strengthens. When the reverse voltage increases to a certain level, the reverse current will suddenly increase. If the external circuit cannot limit the current, the current will be so large that it will burn the PN junction. At this time, it is called the breakdown voltage. There are two basic breakdown ways, namely tunnel breakdown (also called Zener breakdown) and avalanche breakdown. The former has a breakdown voltage of less than 6V and has a negative temperature coefficient, and the latter has a breakdown voltage of greater than 6V and a positive temperature coefficient.2.3 Volt-Ampere CharacteristicThe volt-ampere characteristics of the PN junction are shown in the Figure 3, which visually shows the unidirectional conductivity.Figure 3. Volt-Ampere Curve of the PN JunctionThe volt-ampere characteristic shows by Where iD is the current passing through the PN junction, VD is the applied voltage at both ends of the PN junction, and VT is the voltage equivalent of temperature.2.4 Capacitance CharacteristicWhen a reverse voltage is applied to the PN junction, the positive and negative charges in the space charge region constitute a capacitive device. Its capacitance changes with the applied voltage, mainly including barrier capacitance (CB) and diffusion capacitance (CD). Both of them are non-linear capacitors. Ⅲ Typical Example: Transistor PN JunctionTransistors are one of the basic building blocks of modern electronics. In the diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material to form a simple pn-junction. While the transistor is a three terminal solid state device which is formed by connecting two diodes back to back. Hence it has got two PN junctions.Transistor NPN-Type and PNP-Type JunctionsTransistor Working StateThe transistor works like an electronic switch. It can turn a current ON and OFF. The basic idea behind a transistor is that it lets you control the flow of current through one channel by varying the intensity of a much smaller current that's flowing through a second channel.1) Cut-off State (C): The base current is zero.2) Amplified State (A): The transmitter junction is forward biased (that is, the voltage direction is P->N), and the collector junction is reverse biased.3) Saturation State (S): Both the emitter junction and the collector junction are forward biased.Working StateNPN-Type TransistorPNP-Type TransistorVb<Ve (C)Vc>Vb>Ve (A)Vb>Ve Vb>Vc (S)Vb>Ve (C)Vc<Vb<Ve (A)Vb<Ve Vb<Vc (S)In Figure (a), when there is no voltage input at b of the NPN transistor, no current flows between c and e, and the triode is in the cut-off state.In Figure (b), when a positive voltage is input to b of the NPN transistor, the negative electrons in the N region of e are attracted by the positrons of P region in b. Due to the effect of the power plant, and they rush (diffuse) to the base region, however, only part of the negative electrons collide with the positive electrons (recombination) to generate the base current, and the other part of the negative electrons gather near the collector junction. The negative electrons gathered in the collector junction pass through (drift) the collector junction due to the action of the electric field. After reaching the collector area, it collides with the positrons gathered in c (N-type semiconductor terminal) to generate a collector current.It can be seen from this that the greater the base current, the greater the collector current. That is, when a small current is input to the collector, a large current can be obtained by the collector, and now the transistor is in an amplified state.It should be noted that when the base current reaches a certain level, the collector current no longer rises. At this time, the transistor loses its current amplification effect, and the voltage between the collector and the emitter is very small. The collector and emitter are equivalent to the on-state of the switch. At this moment, the transistor is in a saturated state.The working principle of the PNP transistor is the same as that of the NPN transistor, except that the direction of the bias voltage and the current are opposite, and the roles of electrons and holes are reversed. The PNP transistor uses Veb to control the positrons incident on the collector area from the emitter area through the base area, while the NPN transistor uses Vbe to control the negative electrons that enter the collector area from the emitter area through the base area.In addition, in a low-power design, the transistor control circuit will have a certain impact on the circuit. No matter it is NPN or PNP, there will be leakage current in the PN junction of the transistor. When the I/O controls the base voltage, in order to stabilize the base voltage, a pull-down resistor is generally added to the base of the NPN switch circuit. In the design of the PNP switch circuit, a pull-down resistor is added to the base. The pull-up and pull-down resistors are selected according to the control chip, transistor and circuit voltage. Frequently Asked Questions about PN Junctions Formed1. What is PN junction and how it is formed?P-n junctions are formed by joining n-type and p-type semiconductor materials, as shown below. ... However, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. 2. What is p-type and n-type?In silicon doping, there are two types of impurities: n-type and p-type. In n-type doping, arsenic or phosphorus is added in small quantities to the silicon. ... In p-type doping, boron or gallium is used as the dopant. These elements each have three electrons in their outer orbitals. 3. What is a PN junction diode?A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. ... By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. 4. What happens in a PN junction?A forward-biased PN junction conducts a current once the barrier voltage is overcome. The external applied potential forces majority carriers toward the junction where recombination takes place, allowing current flow. A reverse-biased PN junction conducts almost no current. 5. What is a PN junction used for?A p-n junction diode is a two terminal device that allows electric current in one direction and blocks electric current in another direction. In forward bias condition, the diode allows electric current whereas in reverse bias condition, the diode does not allow electric current.

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

CatalogⅠ IntroductionⅡ Function of Starter RelaysⅢ How does a Starter Relay Work with Others?Ⅳ Location of Starter RelaysⅤ Symptoms of a Bad Starter Relay5.1 the Vehicle doesn’t Start5.2 Starter Relay Remains on after Engine Started5.3 Starter Makes a Clicking Sound5.4 Vehicle Starts to IntermitⅥ How to Test a Starter Relay?Ⅶ How To Replace A Starter Relay?7.1 How to Remove a Starter Relay Correctly?7.2 How to Install Starter Relays?7.3 How to Wire Starter Relays?Ⅷ ConclusionⅨ Frequently Asked Questions about a Starter RelayⅠ IntroductionBefore your engine turns over and starts, a sequence of steps takes place when you get inside and start your vehicle. The starter relay is one of the most crucial and often overlooked – components of any vehicle's ignition system. A starter relay is a small electrical device located in the high-current motor's starting circuit. A relay is simply a remote switch that regulates the current in a high-current circuit. A starter relay in a car uses the modest ignition switch current to close the considerably more powerful starter circuit. The starter relay and the starter solenoid work together to run the starting mechanism in several automobile applications. In some cases, the ignition switch directly controls the starter solenoid circuit. These are often small vehicles with starting motors that do not require a lot of currents to operate. In this article, you will find all there is to learn about the starter relay: its function, its location in a vehicle and how it works. We also included information about the signs of a bad starter relay, how to test it, and how to replace or fix a bad one. keep reading. Video. How to Test Symptoms of a Bad Starter Relay Ⅱ Function of Starter RelaysBetween the vehicle's battery and the starter motor, a starter relay serves as an electrical circuit completer or circuit breaker. It assists in increasing the battery's current so that less current is required upon ignition. It's a switch between the starter solenoid and the starter motor, according to this description. When you turn the ignition key or push the start button on your car, it permits a large current to flow. The starter motor requires a considerable current, which the ignition switch cannot manage. It would burn out if it didn't have the relay. It is possible that if the starter relay fails, the vehicle will not start. As we'll see later, there are a variety of reasons why this component can fail. They also necessitate various treatments based on the nature and severity of the harm. Some can be repaired, and others require the installation of a new relay. Ⅲ How does a Starter Relay Work with Others?Figure 1. How does a starter relay work? When you turn on the ignition, your key activates the starter relay, which sends power to the starter solenoid, which then sends power to the starter motor. When you switch on the ignition key, a starter relay transmits small electric power to the starter solenoid, while the solenoid pulls a big current straight from the car batteries. This activates the solenoid, which sends electricity to the starter motor, which spins the flywheel. This procedure is followed by all modern starters. The starter relay is responsible for sending electricity to the solenoid, which engages the starter and turns the flywheel. When it comes to starting your car, the starter relay is crucial. Ⅳ Location of Starter RelaysFigure 2. Location of Starter Relays The position of the starter relay varies by vehicle type and model. The fuse box (also known as a power box), the fuse panel under the dash, or the right fender are all possible locations. It'll be under the hood, inside the large box with the black cover, in most autos. This is where a vehicle's fuses and relays are mounted, and it's also known as the fuse box. The box is normally mounted on the driver's side of the vehicle. Wires are coming in and out of the relay. However, many other relays in the car have a similar appearance. A starter relay that is mounted in the fuse box under the dashboard may be hard to find or even remove. It may not be difficult to locate starter relays that attach to the fender wall. These relays, which are usually of the cylinder type, can be identified by their mounting posts and leads. Refer to your repair handbook if you're not sure which one is the starter relay. Ⅴ Symptoms of a Bad Starter Relay The starter relay, like every other mechanical and electrical component in your vehicle, shows indicators of failure before finally stopping. Some of the signs of a damaged or worn-out starter relay are listed below. If you see these warning signs, schedule an appointment with a local ASE-certified mechanic to have your car thoroughly inspected, as these symptoms could suggest issues with other components. The starter solenoid and the motor should both stop working when we turn off the ignition switch. The main contacts in the starter relay have most likely welded together in the closed position if it doesn't operate in this sequence and the relay stays on even after the engine has started. If this happens, the starter relay will become trapped in the on position, causing damage to the starter, circuit, relay, and transmission flywheel if not treated instantly.Figure 3. Symptoms of a Bad Starter Relay 5.1 the Vehicle doesn’t StartA car that just won't start is one of the most telling indications of car trouble. While a multitude of underlying faults can prevent a car from starting, a defective starter relay is frequently at the root of the issue. To understand why you must first gain a better knowledge of the starter relay's function. When you turn the key in the ignition, your battery is jolted into action, releasing a burst of electrical energy. The starter motor is triggered by this energy and turns your engine over. However, before reaching the starter motor, the electrical impulse must first pass via the starter relay. The starter relay not only completes the electrical circuit but also increases the current from the battery. If your starter relay fails, the electrical signal from the battery to the starter motor will never reach it. As a result, no matter how many times you turn the key, your engine will not start. When you turn the key, you may hear a clicking noise if the circuit hasn't broken. In either situation, you should get professional assistance to evaluate the symptom and precisely diagnose the cause.5.2 Starter Relay Remains on after Engine StartedWhen you turn on your ignition, an electrical current is sent to the starter relay, which then passes it on to the starter solenoid. The starter motor rotates the flexplate to start the engine, and the solenoid supplies power to it. The starter solenoid and the motor should both stop working when we turn off the ignition switch. The main contacts in the starter relay have most likely welded together in the closed position if it doesn't operate in this sequence and the relay stays on even after the engine has started. If this happens, the starter relay will become trapped in the on position, causing damage to the starter, circuit, relay, and transmission flywheel if not treated instantly. It usually happens when the relay comes into contact with anything or is exposed to a lot of electricity. Because this problem has the potential to harm the entire starting system, you must diagnose and treat it right away. 5.3 Starter Makes a Clicking SoundThe relay usually functions on an all-or-nothing basis. Either it will send the entire electrical current, or it will not send anything at all. When the starter relay is destroyed, however, only a portion of the signal may be sent. The starter relay clicks, but the engine does not turn, indicating that the starter motor is not receiving enough electrical current from the relay. This can also be an indication of a low or dead battery. Only when it transmits enough electric current to the starter is the relay functioning. Lesser high power may harm the entire starter mechanism or cause the vehicle to not start, accompanied by an obnoxious clicking sound. Both could be caused by a corroded or aging relay with damaged contact points. Cleaning the contact points to guarantee correct flow or replacing an old relay are the only two options for repair. Scrape the rusted surface with sandpaper or a sand scraper to clean a corroded relay. You may replace the relay for greater output, or you could contact a professional mechanic.5.4 Vehicle Starts to IntermitWhen the starter relay is functioning, it transmits power to the starter each time it is engaged. However, debris, grime, and high heat can taint this component. The corrosion and residue in the circuit will limit the flow of electrical current. The starter relay may be forced to work intermittently as a result of these circumstances. A starter relay is a basic ignition system component with few moving elements, which is why it rarely fails. When it occurs, though, it is due to electrical conductivity issues. If the relay isn't faulty, there may be a damaged or corroded wire connection under the hood. Ⅵ How to Test a Starter Relay?Figure4.How to Test a Starter Relay? Materials:1. a fully charged battery2. a portable jumper cable3. baking soda, water and a wire brushTools:safety glasses and golves  Step 1Make sure the car is parked safely and that the transmission is in neutral or park. When working under the hood, you don't want the vehicle to move forward by accident. Step 2Get a fully charged battery and a portable jumper cord before conducting the test. Alternatively, you can check your car battery to make sure it's fully charged and not the source of your problem. Pay attention to how you connect the jumper cables to the battery terminals throughout the testing process. Step 3Examine the terminals on the battery and the starter. Make sure they're clear of rust, oil, dirt, and debris. Disconnect the minus battery cable and set it aside before cleaning the corroded terminals. Remove the positive battery cable from the battery and set it to the side. Ensure that the cables do not come into contact with the battery terminals by accident. Using baking soda, water, and a wire brush, clean rusted terminals. Clean the starter terminals if necessary. Disconnect the battery cables if possible. Step 4The cables from the starter solenoid to the starter relay should be followed. On the relay, there are four terminals. The two smaller wires are utilized to turn the relay "On" and come from the key switch circuit. The two larger wires run from the battery to the starter, carrying battery voltage. Remove the wires from the starter relay and mark the two smaller wires so they can be reconnected appropriately. Connect one end of a jumper wire to the chassis ground. The other end should be connected to terminal 86. Step 5Connect the positive battery post with a jumper wire. It's fine to leave the jumper wire attached for a short period now that the battery connection to the starter has been severed. Measure the resistance between terminals 30 and 87 with a digital voltmeter. It should have a resistance of less than one ohm. The relay is not working if the resistance is more than one ohm. Replacing the relay is necessary. Ⅶ How To Replace A Starter Relay?To the positive battery post, connect a jumper wire. It's okay to leave the jumper wire connected for a short time now that the battery connection to the starter has been disconnected. Between terminals 30 and 87, use a digital voltmeter to measure resistance. Less than one ohm of resistance is required. The starter relay will not work if the resistance is greater than one ohm. The relay must be changed. Materials: safety glasses, safety wireTools: pliers, wrenches, and wires 7.1 How to Remove a Starter Relay Correctly?You can open the hood and unhook the negative connection of the battery if it's a fuse box starter relay. Locate the fuse box next. It's usually the black-lidded box. Use the instructions if you can't find the starter relay. Determine the location of the starter relay using the information on the fuse box cover. Then, remove the starter relay. If it's a fender wall relay, follow the methods below to get rid of it. To begin, detach the battery terminals with a wrench. Second, disconnect the leads that connect to the relay's terminals. Remove the bolts that attach the leads to the posts on the relay with a wrench. Two large and two little posts will serve as connection points. Third, remove the relay from the fender by unscrewing the mounting screws. Figure6. Remove a Starter Relay 7.2 How to Install Starter Relays?The process of installing a fuse box starter relay is simple. There are no nuts or screws to tighten, and there is no need to worry about torque. Take your new relay with you. Push the relay in slowly and gently until it reaches the end of the seating, matching the pins with their slots in the fuse box. Replace the lid and reconnect the battery terminal that was detached when the old relay was removed. Follow these procedures to install a fender-mounted starter relay. Place the relay on the mounting surface and hold it there. Screw the relay to the fender wall by inserting and tightening the screws. Install the starter circuit and battery wires, being careful not to connect the wrong wire to the wrong post. Reconnect the battery cables that you have previously disconnected.Figure7. Install Starter Relays Test the starting system once the installation is complete. The vehicle should start without difficulty. Check the wires and connections if this is the case. Make that the connections are secure and the fender wall starter relay is properly wired. If you can't figure out the problem, you need to look at the other components of the starting system. Alternatively, get the car inspected by a mechanic. 7.3 How to Wire Starter Relays?Wires are installed on the connection posts of a fender-mounted starter relay. Typically, these leads are connected during the installation process. The relay must be wired appropriately to function safely. This diagram shows how to wire a four-connection starter relay.Figure8. How to Wire Starter Relays? Step1Disconnect the positive terminal of the battery. To avoid mishaps, secure the exposed end. You could do it using tape. Step2The thick starter solenoid cable can be found here. Connect it to one of the relay's large studs or posts. Tighten the mounting bolt to secure the connection. Because starter relays don't have polarity, it doesn't matter which big terminal you connect the wire to. Step3Obtain the wires for the ignition switch. They are usually thinner than starter cables since they only carry a limited quantity of current. One of the two wires should be connected to one of the small studs on the relay. Connect the remaining small post to the other wire. There will be only one small post on some relays. Connect the ignition wires to the mounting screw or bolt if this is the case. It can also be connected to any other part of the relay housing. This is because one of the relay's tiny terminals is normally grounded. Step4Connect the remaining thick wire to the single huge stud or post that remains. This is the cable that connects to the battery's positive terminal. Finally, turn on the ignition to test the relay wire. The engine should start and crank without difficulty. If it still doesn't work, double-check the wiring to make sure each cable is connected to the correct terminal and is securely fastened. Ⅷ ConclusionIt's crucial to check that starter relays are functional in vehicles that require them. It is a standard aspect of a vehicle's routine maintenance. It can save you from getting into trouble and being stranded in the middle of nowhere. Knowing how the relay works and how to recognize a failed one early on is the first step. We hope that this starter relay guide has provided you with enough knowledge to assist you in resolving starter relay problems that cause car starting problems. You can now recognize the signs of a malfunctioning starter relay, as well as how to test it. Ⅸ Frequently Asked Questions about a Starter Relay1.Can you fix a starter relay?If the starter relay fails, you won't be able to start the engine. Damage to the starter relay often happens from a bad power connection on the starter that causes it to short circuit. A nonworking relay cannot be dismantled for repair; you will have to install a new one in order to start the engine. 2.How easy is it to replace a starter relay?With the right tools and knowledge of what wire to connect to which terminal, the process to change a starter relay should be easy. The fuse box relay is even easier. It usually involves pushing in the new relay after pulling out the old one. 3.What is the starter relay replacement cost?Excluding the starter relay cost, expect to pay around $30. Adding the cost to buy the component, the total cost to install a new relay comes to about $50. 4.How long does a starter relay last?Typically, expect a starter relay to last more than 100 miles. These components are durable, having only a few moving parts and, therefore, minimal wear. The biggest threat to a starter relays lifespan is usually the contacts burning out. 5.Is a starter relay the same as a starter solenoid?Most often, a true starter relay is a small black cube plugged into an electrical fuse/relay box in the engine compartment, whereas a starter solenoid is (in most cases) attached directly to the starter on the engine (although it is sometimes located elsewhere in the engine compartment).  

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. These tools safely dissipate the stored electrical energy as heat without creating dangerous sparks or damaging the capacitor terminals." } }, { "@type": "Question", "name": "How long does it take for a capacitor to discharge naturally?", "acceptedAnswer": { "@type": "Answer", "text": "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." } }, { "@type": "Question", "name": "Why shouldn't I use a screwdriver to discharge a capacitor?", "acceptedAnswer": { "@type": "Answer", "text": "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." } }, { "@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." } ] } ]}

IntroductionWhat is the difference between a three-phase AC motor and a single-phase AC motor? If you pay attention to it, you will find that single-phase AC motors have more equipment than three-phase AC motors, which is the start capacitor (starting capacitor). The most common one is in various household appliances. Almost all household appliances with motors are equipped with start capacitors. This article will start with the explanation of the principle of the motor start capacitor, and then describe in detail the failure phenomena, causes and test methods of the start capacitors in the two common home appliances, air conditioners and fans. In addition, the article will also explain some basic questions about start/run capacitors. If you want to learn the related knowledge of motor start capacitors, this article must be worthy of your reading.How to test a RUN or START CAPACITOR the CORRECT wayCatalogIntroductionCatalogI The Principle and Causes of Damage of Start Capacitor 1.1 How Does the Motor Work? 1.2 How Does the Start Capacitor Work? 1.3 Reasons for the Burning of the Start CapacitorII Troubleshooting of Start Capacitor in Air Conditioner 2.1 Functional Characteristics of Start Capacitor 2.2 How to Quickly Figure out Whether the Start Capacitor is Damaged 2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage? 2.4 Symptoms and Test MethodsIII How Test the Motor Start Capacitor of a Fan?IV Relevant Knowledge in the Step of Start Capacitor Test 4.1 How to Choose Start Capacitor? 4.2 Precautions for Replacing the Start CapacitorV How to Test the Motor Start Capacitor without a MultimeterVI Frequently Asked Questions about Start Capacitors 6.1 Which Motors are Served by the Start Capacitor? 6.2 Why Does a Three-phase Motor do not Need a Start Capacitor? 6.3 The Relationship Between Start Capacitors and Motor 6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?VII QuizⅧ FAQI The Principle and Causes of Damage of Start Capacitor1.1 How Does the Motor Work?The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and a capacitor is needed to separate the phases. The purpose is to make the current in the two windings produce a phase difference of nearly 90 ゜ to generate a rotating magnetic field. The capacitive induction motor has two windings, namely the starting winding and the running winding. The two windings are 90 degrees apart in space. A large-capacity capacitor is connected in series to the starting winding. When the running winding and the starting winding pass single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current in the running winding due to the action of the capacitor, thus reaching the maximum value first. Two identical pulsed magnetic fields are formed in time and space so that a rotating magnetic field is generated in the air gap between the stator and the rotor. Under the action of the rotating magnetic field, an induced current is generated in the motor rotor, and the current interacts with the rotating magnetic field. The electromagnetic field torque causes the motor to rotate.Figure1. Electric-Motor1.2 How Does the Start Capacitor Work?A single-phase power supply is different from a three-phase power supply in that a three-phase rotating magnetic field is generated in the motor. The principle of capacitor starting of single-phase motor is: using the principle that the current of the capacitor in the circuit is advanced by 90 degrees so that a magnetic field of 90 degrees ahead of the main winding is generated in the starting winding so that there will be an alternating 90-degree angle in the motor. The magnetic field, to put it plainly, uses the phase-shifting principle of the capacitor to transform a single-phase power supply into a two-phase power supply of 90 degrees to each other, and a rotating magnetic field of 90 degrees to each other is generated in the motor. Maybe it is easier to understand to explain from this aspect. The start capacitor is to give the motor a thrust when the motor is started so that the motor can turn from moving to rotating. Without it, when a single-phase AC motor starts, it will shake at the origin instead of rotating; the start capacitor is a two-phase AC motor, so the magnetic field cannot exert force on the rotor without it, and of course, it is impossible to rotate.Figure2. Capacitor Start Run Induction Motor1.3 Reasons for the Burning of the Start CapacitorGenerally, the start capacitor is not easy to burn, because its working time is very short, and it is thrown off by the centrifugal switch at the moment of starting, with no current flowing through the start capacitor. However, not easy to burn does not mean that it will never burn. If the start capacitor burns out, the possible reasons are as follows:① Capacitors have low voltage resistance or poor quality, so it is best to use capacitors with a voltage resistance of 500V. ② The centrifugal switch will often produce an arc when it is turned off. It is possible to burn the switch to the motor. After the switch is started, the switch cannot be turned off. There is always current through the capacitor. It is easy to burn the secondary winding of the motor and the start capacitor within a certain period of time. ③ The capacity of the selected capacitor is too small, and the starting current exceeds the allowable value of the capacitor. ④ The motor is bored or the bearing is damaged. It is difficult for the motor to start the centrifugal switch within a certain period of time and it is difficult to reach the disconnected speed, and the start capacitor is easy to burn.Figure3. Deteriorated CapacitorII Troubleshooting of Start Capacitor in Air Conditioner2.1 Functional Characteristics of Start CapacitorThe start capacitor is an important part of the auxiliary compressor to start. The capacitor is a large-capacity capacitor (1~6uF), which is used to provide starting current for the auxiliary winding of the motor to assist the compressor to start. The start capacitor is generally fixed on the bracket or support plate above the compressor, and the pin is connected to the starting end of the compressor.2.2 How to Quickly Figure out Whether the Start Capacitor is DamagedFirst of all, it depends on what kind of capacitors are used in electrical parts.● If the compressor start/run capacitor is damaged, the compressor cannot start or run intermittently.● If the capacitor of the fan motor is damaged, some malfunctions such as excessive exhaust temperature, excessive exhaust pressure, compressor overload, and small air output will occur during the operation of the air conditioner.2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage?The capacitor of the air conditioner compressor is installed in the outdoor unit. Due to the high temperature of the outdoor unit (ambient temperature + temperature emitted by the condenser), capacitors that have been used for a long time will easily dry up and fail the electrolyte.When replacing capacitors, be sure to use high-quality capacitors. The capacity must be the same, and the withstand voltage must not be lower than the original standard.Figure4. Air Conditioner Outdoor Unit2.4 Symptoms and Test Methods① Smoke.② Cannot operate normally.③ The display cannot be displayed normally.④ The power supply cannot be charged and discharged normally.⑤ Can not heat normally.⑥ The power factor compensates the capacitor damage, resulting in a waste of electricity. Different symptoms of damage to compressor start capacitor and external motor start capacitor:There are two capacitors in the outdoor electromechanical packaging, the larger one is the compressor capacitor, and the smaller one is the external motor capacitor. Different capacitors have different failure phenomena. ① Compressor capacitor damagethe performance phenomenon is that normal compressor startup will be accompanied by loud noise and jitter. If the capacitor is damaged, you will feel a slight jitter in the compressor by pressing the casing above the compressor, and the sound is like the sound of current passing. The compressor will stop running after a period of time. ② The damage of the external motor capacitorAfter the compressor is working normally, the external motor stops working for a period of time. If there is a fault code, there will be high-pressure protection and compressor exhaust pipe temperature protection.Figure5. Basic Electrical Controls of Air-Conditioning UnitsTest methods of air conditioner start capacitor● Method 1:The start capacitor of the air-conditioning compressor is a large-capacity electrolytic capacitor. When testing, using the capacitance setting of the digital multimeter to determine whether there is any abnormality.Under normal circumstances, the capacitance of the capacitor used to detect the capacitance of a multimeter should be the same or very close to the nominal capacitance, otherwise, the start capacitor is mostly deteriorating, such as dry electrolyte, leakage, etc., which should be replaced. ● Method 2:In addition to using a multimeter to test its capacitance, the ohmic setting of a pointer multimeter can also be used to test the charge and discharge performance of the start capacitor.Steps:① Connect the red and black test leads to the two poles of the compressor start capacitor.② The multimeter gear is set in ohm gear.③ Under normal circumstances, the pointer of the multimeter first swings to the right to a position, then slowly swings to the left, and finally stops at a fixed position.④If the pointer does not swing or the swing range is small, it indicates that the performance of the compressor start capacitor is poor.Suggested Reading: 5 Ways to Test Capacitors How to replace the start capacitor?If it is ensured that the air conditioner failure is caused by the damage of the compressor start capacitor itself, the damaged compressor start capacitor needs to be replaced. Replacing the start capacitor can generally be divided into three steps: removing the start capacitor, finding a replaceable start capacitor, and replacing the start capacitor. ①Remove the start capacitorThe compressor start capacitor is located on the circuit support board above the compressor. When disassembling, unplug the connecting wire and use a screwdriver to remove the fixing screw of the snap ring. ② Looking for replaceable capacitorsAfter removing the damaged compressor start capacitor, then select a suitable new start capacitor to replace it according to the specifications and volume of the damaged start capacitor.The specific content of how to select the start capacitor will be explained in detail below. ③Replace the start capacitorAfter selecting the compressor start capacitor, install the new compressor start capacitor in the outdoor unit, fix the metal fixing ring, reconnect the connecting cable, and then power on and test the machine to complete the replacement.Figure6. Motor Start CapacitorIII How Test the Motor Start Capacitor of a Fan?①Connect all parts on the circuit board of the air conditioner completely.②Turn on the power supply.③Use the remote control to adjust the temperature to make the fan motor rotate.④Connect the ground terminal of the oscilloscope probe to the ground terminal of the circuit board.⑤Use an oscilloscope probe to detect the white lead on the Hall element plug.⑥The oscilloscope shows signal waveform. When testing the start capacitor of the fan motor, a multimeter should be used to measure the resistance of the capacitor. Due to the large size of the capacitor, it is impossible to use the capacitor input jack for testing. At this time, you can use the multimeter pen to test and judge the quality of the capacitor by the change of the value displayed by the multimeter. If the resistance value displayed by the multimeter changes from small to large and then changes to infinity, it means that this is a good capacitor with charging and discharging functions. Check again after changing the test leads, the displayed value still changes from small to large, and changes to infinity. After measuring the capacitor, if the test leads are not replaced when the test is performed again, and the resistance value is displayed as infinite, it means that charging and discharging are not performed, but it does not mean that the capacitor is damaged. Therefore, when testing the capacitance, the test leads must be replaced no matter which multimeter is used.Figure7. Fan MotorRelated recommendation: How to Test Ceramic Disc Capacitor IV Relevant Knowledge in the Step of Start Capacitor Test4.1 How to Choose Start Capacitor?● How to calculate the starting and running capacitance of a single motorrun capacitance C=120000*I/2.4*f*U*cosφWhere: I is current; f is the frequency; U is the voltage; cosφ is the power factor, taking 0.5 to 0.7. The run voltage of the run capacitor is greater than or equal to (2～2.3) U.Start capacitor capacity = (1.5 ~ 2.5) run capacitor capacity.The run voltage of the start capacitor is greater than or equal to 1.42 U.(It is best when the voltage across the capacitor is 311V during operation) The working capacitor is 1-4UF per 100W, and the start capacitor is 4-10 times the working capacitor (the motor requires a larger starting torque). Empirical data, if your motor does not exceed 200W, the start capacitor will not exceed 100uF. If you run the capacitor, you can choose several values for the power-on test, and see which capacitor has the smallest current in the whole machine, then the capacity of the capacitor is the most Good value.) The capacity of the single-phase split-phase motor capacitor can be calculated by the empirical formula C=35000I/2PUfcos&Such as I=250W/220V=1.2AC=35000x1.2/2x1x50x220X0.8=24ufCan choose 350V30uf capacitance.Figure8. Starting Capacitor Table ● How to calculate the voltage across the run capacitor of a single-phase motor?① First of all, you must know the impedance value of the secondary winding. You can measure the resistance value by measuring the DC resistance with a multimeter. Then, the secondary winding is connected to a 12V AC voltage and the current value is measured. According to the winding impedance equal to the resistance and reactance in series, it can be calculated by phasor Out the winding inductance value. ② In normal operation, the capacitor is connected in series on the secondary winding, that is, the three equivalent parameters of winding resistance, winding reactance, and capacitance are connected in series and then connected to 220V voltage. It is easy to calculate the phasor according to the formula of the series circuit. Calculate the voltage on the capacitor. ③ When a single-phase motor is running, the voltage at both ends of the capacitor is generally above 300VAC, so the capacitor voltage is generally selected for a capacitor with a withstand voltage of 400V or more, and a capacitor with a voltage of more than 450V is better. ④ For the calculation of the capacitance withstand voltage, please refer to Article 2. First, measure the resistance R and reactance XL of the secondary winding, and then select the capacitance C according to the power of the motor to calculate the capacitive reactance Xc.The actual voltage across the capacitor during operation: Uc= Xc*220/(R+jXL-jXc); the withstand voltage value of the capacitor: Uce=1.3~1.5Uc.Figure9. Single Phase Motor Starting ● Detailed selection guide of start capacitor and run capacitorSingle-phase motor capacitor selection.Withstand voltage formula: U (capacitance) is greater than or equal to 1.5*USingle-phase run capacitor formula: C=1950×I/U×cosφ (using a capacitor, which is both a start capacitor and a run capacitor, is commonly used for small-capacity motors such as electric fans and washing machines)Start capacitor capacity formula: C=3500*I/U*cosφ (a capacitor is only used when starting, disconnected during normal operation, and switched with a transfer switch or a centrifugal switch.Dual-value capacitor run capacitor capacity formula: C=1200*I/U*cosφ (use 2 capacitors, one for operation and one for startup)Dual-value capacitor start capacitor capacity formula: C=(2～3)*C (run capacitor) C: Capacitor capacity: I: motor rated current, U: motor rated voltage, cosφ: power factor 0.7.Generally, there is no need to calculate. The run capacitor is 2~3μF per 100W, and the start capacitor is 2~3 times the run capacitor. The capacitor selection of the motor has strict requirements on the voltage, and it must be equal to or greater than 1.5 times the rated voltage of the motor. For a power supply with a rated voltage of 220V, the rated voltage of the capacitor cannot be lower than 400V. The capacitance value has a certain broadness, it doesn't matter if it is larger or smaller, especially the start capacitor, which can be selected at 2-6 times the working capacitor. ● How to choose the capacitor of single-phase asynchronous capacitor start the motorWe can calculate according to the following formulaPhase start capacitor capacity:C=350000*I/2p*f*U*cosφIn the formula: I---current;f-frequency;U---voltage;2p-the larger power factor is 2, and the smaller power factor is 4; cosφ---power factor (0.4～0.8).Split-phase start capacitor withstand voltage:The capacitor withstand voltage is greater than or equal to 1.42*U. Run capacitor capacity:C=120000*I/2p*f*U*cosφIn the formula: I---current;f-frequency;U---voltage;2p-take 2.4;cosφ---power factor (0.4～0.8). Run capacitor withstand voltage:The withstand voltage of the capacitor is greater than or equal to (2～2.3)*U.start capacitor capacity of double-value capacitor motor:C=(1.5～2.5)*operating capacitor capacity.Withstand voltage of start capacitor:The capacitor withstand voltage is greater than or equal to 1.42*U.4.2 Precautions for Replacing the Start CapacitorThe start capacitor is an important part of the electronic circuit. Once the start capacitor is broken, the motor cannot be started. The damaged start capacitor will only make a buzzing sound when it is energized for a short time, causing the current to surge, and long-time energization will cause severe overheating and even burn the motor, so it should be replaced immediately. And it is not difficult to judge that the start capacitor is broken. Most of the damaged start capacitors are bulging, and the surface will be burnt due to excessive current, and the rotor speed will be slow and weak. Of course, the most intuitive and accurate way is to use the capacitance setting of a multimeter to measure the quality. Once we have confirmed that the start capacitor has failed, the things that should be noted when replacing the start capacitor:① After the start capacitor is discharged, there will still be part of the residual charge that cannot be discharged for a while, and an artificial discharge should be performed again. ② Since the failed start capacitor may have poor lead contact, internal disconnection or fuse blown, etc., part of the charge may not be discharged. Therefore, the maintenance personnel should wear insulating gloves before touching the failed start capacitor. Using the short-circuit wire to short the two poles of the faulty capacitor first, and then it can be removed and replaced by hand. ③ If multiple start capacitors are used in series, they should be discharged separately.Figure10. Replace the Start Capacitor④ When handling or replacing a malfunctioning start capacitor, disconnect the power supply of the start capacitor, disconnect the switch or unplug the plug, and discharge the start capacitor. ⑤ When discharging, first connect the grounding terminal of the grounding wire, and then use the grounding rod to discharge the start capacitor several times until there is no discharge spark or discharge sound, and then fix the grounding terminal. ⑥ It should also be noted that general users often ignore the instruction manual, and the precautions for use must be carefully understood and followed during installation. As we all know, the impedance of a capacitor is inversely proportional to frequency. As the frequency increases, the loss also increases. Measures should be taken to limit the harmonics and inrush current in the circuit. Capacitors always generate heat, so pay special attention to ventilation and cooling. After the reactive power compensation device is installed, during the trial operation, the system should be tested, and measures should be taken in time if over-voltage, over-current, oscillation, and harmonics are found, which is very necessary for the normal operation of the capacitor. V How to Test the Motor Start Capacitor without a MultimeterA DC voltmeter can be connected to the capacitor in parallel, and an insulating shaker can be used to charge the capacitor (note the + and-poles)(1) See if the voltage can rise to the rated voltage of the capacitor:① 0V, the capacitor is short-circuited.② Slowly rise to the rated voltage of the capacitor, it proves that the capacitor is good.③ Raised quickly to the rated voltage of the capacitor, and the insulation resistance is about the internal resistance of the DC voltmeter, then the capacitor is open.(2) When it is stable at the rated voltage value of the capacitor, look at the insulation resistance of the capacitor:①The insulation resistance is close to the internal resistance of the DC voltmeter, so the capacitance is good.②If the insulation resistance is less than the internal resistance of the DC voltmeter, it means that the leakage of the capacitor is large, and it is easy to generate heat and cannot be used.VI Frequently Asked Questions about Start Capacitors6.1 Which Motors are Served by the Start Capacitor?Although some electrical appliances seem to have similar principles, they are different in the selection of motors, such as electric fans and air conditioners. Most electric fans use single-phase motors. Single-phase motors have only one 220v live wire and one neutral wire, while air conditioners often uses the three-phase motor, which has three wires, 220v live wire, neutral wire, and 380v live wire. The most obvious difference between a single-phase motor and a three-phase motor is that the number of start capacitors is different. A single-phase motor is equipped with a start capacitor, while a three-phase motor has no start capacitor.6.2 Why Does a Three-phase Motor do not Need a Start Capacitor?Because the three-phase motor itself has three running windings and can generate a magnetic field by itself, the appearance of the magnetic field can effectively replace the start capacitor, so the three-phase motor is generally not equipped with a start capacitor. However, the start capacitor still plays an irreplaceable role in a single-phase motor, because there is only one running winding in a single-phase motor, which cannot form a rotating magnetic field, and the operation of electrical appliances can only rely on the start capacitor. In addition to the start capacitor in a single-phase motor, there is also a run capacitor. Although these two capacitors work together, the function of the start capacitor is much greater than that of the run capacitor, so once they start capacitor is damaged, the fan will make a lot of noise, The blade speed is reduced. If this happens to your electric fan, you might as well try to replace a start capacitor, the problem should be solved.6.3 The Relationship Between Start Capacitors and MotorAt present, in single-phase motors with low-power motors, the start capacitor is connected in series with the starting coil and then connected in parallel with the running coil to work at the same time. In order to speed up the start-up time of the high-power motor, a large capacitor to help start is added. After the motor is started, the additional large start capacitor is disconnected by the centrifugal switch. The smaller capacitor connected in series with the starting coil is responsible for the phase shift required during normal operation. Electric current, the power supply machine is operating normally. Is there a single-phase motor that is connected to the starting coil and connected in parallel with the running coil in the circuit from start to run and does not require other large capacitors to help start? Low-power motors are always used in the circuit. High-power motors have to add additional capacitors due to their large power and large starting distance.Figure11. Torque-speed characteristic6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?The start capacitor is used for phase separation, and the purpose is to make the current in the two windings produce a phase difference close to 90 ゜ to generate a rotating magnetic field, allowing the motor to run quickly in a static state. There is an automatic clutch switch in the motor. When the motor is started, the motor will continue to run due to inertia. When the speed reaches a certain speed, the start capacitor will be separated by centrifugal action and automatically connected to the run capacitor, and the motor will enter the normal working state. The function of the run capacitor is to keep the current in the two windings with a phase difference of 90° to generate a continuous rotating magnetic field. For motors with start capacitors, the rotational torque generated by the start capacitors is larger than that of the run capacitors, which is more suitable for starting with a load. Motors without start capacitors are not suitable for starting with a larger load.VII QuizThe starting capacitor of a single phase motor is(A) Electrolytic capacitor(B) Ceramic capacitor(C) Paper capacitor(D) None of the above.Answer: AⅧ FAQ1. What happens when a start capacitor goes bad?A motor connected to a run and start capacitor may still attempt to start if one or both of the capacitors has failed, and this will result in a motor that hums and will not remain running for long. ... In most cases of capacitor problems, such as damage or a loss of charge, the capacitor will need to be replaced. 2. What's the difference between a run capacitor and a start capacitor?The start capacitor creates a current to voltage lag in the separate start windings of the motor. The current builds up slowly, and the armature has an opportunity to begin rotating with the field of current. A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. 3. How do you test a start capacitor with an ohmmeter?To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. 4. How to test a motor start capacitor?Motor run capacitor failure symptoms include warm air flowing from the vents inside the home, the air conditioner taking more time than usual to kick on or it turns off before it is programmed to, or there is a constant low hum emitting from the machine that isn't typical. 5. How do I test a capacitor with a multimeter?To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. 6. How do you check if a capacitor is bad with a multimeter?If the capacitance value is within the measurement range, the multimeter will display the capacitor's value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty. 7. How do I test a capacitor without a multimeter?Just connect those two ends of the capacitor to a single-phase supply and switch it ON for a few seconds. Then take that two-terminal and short it, you will get a spark. 8. How can I check a capacitor?Put the Analog Multimeter in the Ohmmeter position and if there are multiple ranges, choose a higher range. Connect the leads of the capacitor to the multimeter probes and observe the readings on the multimeter. For a good capacitor, the resistance will be low in the beginning and will gradually increase. 9. Can I replace a start capacitor with a run capacitor?Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). A start capacitor can never be used as a run capacitor because it cannot handle current continuously. 10. Which is bigger, start or run capacitor?A lot of torque is necessary to start up an AC system, so a start capacitor will have greater capacitance than a run capacitor. 

What is a capacitor? Capacitor, a electronic component to hold charges, represented by the letter C. It composes of two metal electrodes between a layer of insulating dielectric. When a voltage is applied between the two metal electrodes, the charge is stored on the electrode, so the capacitor is an energy storage electrical part. Any of two conductors that are insulated and close to each other form a capacitor. In addition, the parallel plate capacitor consists of the electrode plate and the dielectric of the capacitor.  Capacitor is one of the widely used electronic components in electronic equipment. It is widely used in stopping DC and alternating AC, coupling, bypass, filtering, tuning loop, energy conversion, control and so on. Capacitor is different from capacitance. The capacitance is the basic physical quantity, the symbol C, the unit is F (Farah).   A video introducing basic knowledge of capacitors Catalog   I. Capacitor characteristics II. Functions of capacitor in electrical circuits III. How to use capacitors? IV. Capacitor types V. Capacitor volume VI. Charge and discharge of a capacitor VII. Matters needing attention when using capacitors VIII. Common fault of capacitor and treatment method FAQ   I. Capacitor characteristics  - It has the ability of charge and discharge, preventing DC current from passing through, allowing AC current to pass through.  - During the charge and discharge process, the charge on the bipolar plate accumulates, that is, the voltage is set up, therefore, the voltage on the capacitor will not change abruptly. Charging: two plates with the same amount of dissimilar charge, each plate with the absolute value of the charge is called capacitor volume. Discharging: positive and negative charges at both ends of capacitors are neutralized through conductors. During discharge, there is a transient current on the wire. Capacitor charge  - The capacitive reactance of capacitors is inversely proportional to frequency and capacity. When analyzing the capacitance, the frequency and capacity of the contacting signal must be analyzed. Formula of parallel plate capacitor The dielectric constant of vacuum εr=1, k is a constant of hydrostatic power, s is the positive area of two plates, and d is the distance between two plates. Explanation: the electric field in the parallel plate capacitor is uniform electric field. II. Functions of capacitor in electrical circuits In DC circuits, the effect of a capacitor is equivalent to a open circuit. Capacitors are one of the most commonly used electronic components to store charge. Capacitors are used in electronic circuits as low-pass, high-pass and band filters. A filter is a circuit that allows current and voltage of a specified frequency and waveform to pass through. A capacitor's reactance is inversely proportional to frequency. By controlling or changing the reactance, you can control the frequency allowed through the circuit. Capacitors also play a significant role in high-speed switching logic circuits. Such circuits' voltage level, which should be steady, can change with current fluctuation, thereby introducing noise or error signals. Decoupling capacitors are built into circuits to stabilize the current, minimizing noise signals. The effect of capacitor links with the structure of itself. The simplest capacitors are made up of polar plates at both ends and insulating dielectric (including air) at the middle. After electrification, the plate is charged, forming a voltage (potential difference), but the entire capacitor is non-conductive because of the intermediate insulation. However, the condition is that the critical voltage (breakdown voltage) of the capacitor is not exceeded. We know that any substance is relatively insulated, and when the voltage at both ends of the material increases to a certain extent, the material can conduct electricity. We call this voltage a breakdown voltage. When the capacitor is broken down, it is not an insulator. However, in AC circuits, the direction of the current changes with time, that is, this change has functional relation. The charging and discharging process of capacitors is time-dependent, and at this time, a varying electric field is formed between the plates, and this electric field is a function of the change with time. In fact, the current passes between capacitors in the form of an electric field. III. How to use capacitors? As a relatively common electronic component, capacitors have a wide range of uses. The following content gives you a brief introduction to the 9 most common scenarios where capacitors are used: Stopping DC, bypass (decoupling), coupling, filtering, temperature compensation, timing, tuning, rectifier, and energy storage. 1.Stopping DC: the function is to prevent the passage of DC and allow the AC to pass through. DC blocking capacitor 2. Bypass (decoupling): it provides a low impedance path for some parallel components in AC circuits.       Signal input and output   3. Coupling: as a connection between two circuits, AC signals are allowed to pass and transmitted to the next stage of the circuit. Coupling capacitor circuit model Capacitor as coupling component The purpose of using capacitor as coupling part is to transmit the front stage signal to the next stage, and to separate the influence of the DC of the former stage on the latter stage, so that the circuit is simple to debug and its performance is stable. The amplification of AC signal without capacitor will not changed, but the work points at all levels need to be redesigned. Because of the influence of the front and back stages, to debug at working points is very difficult and can hardly be realized at multistage. 4. Filtering: this is very important for the circuit, the capacitor behind the CPU is having this function basically. Impedance formula(filtering circuit) That is, the larger the frequency f, the smaller the impedance Z of the capacitance. At low frequency, the capacitance C can pass smoothly because of the large impedance Z, and at high frequency, the capacitance C is very small because of the impedance Z, which is equivalent to shorting the high frequency noise to the GND. 5. Temperature compensation: it improves the stability of the circuit by compensating for the influence of other components on the temperature adaptability.   Temperature compensation Analysis: because the capacity of the timing capacitor determines the oscillation frequency of the horizontal oscillator, the capacity of the timing capacitor must very stable and does not change with the humidity in the environment. Therefore, the capacitors with positive and negative temperature coefficients are used for temperature complementation. When the operating temperature increases, the capacity of Cl is increasing, while the capacity of C2 is decreasing, and the total capacity of two capacitors is the sum of the two capacitors after parallel connection. Because one capacity is increasing and the other is decreasing, the total capacity is basically stable. Similarly, when the temperature decreases, the capacity of one capacitor decreases while the other increases, and the total capacity is basically unchanged, which stabilizes the oscillation frequency and realizes the purpose of temperature compensation. 6. Timing: the use of capacitors in conjunction with resistors to determine the time constant of the circuit. Capacitor and resistor(timing) Inputting signal from low to high, after buffer 1 then input RC circuit. The characteristics of capacitor charging make the B point signal not change immediately with the input signal, but there is a gradual process of increasing. When it becomes larger to a certain extent, the buffer 2 flips over, resulting in a delay jump from low to high at the output end. 7. Tuning: having systematic tuning to circuits which related to frequency, such as cell phones, radios, and televisions. System tune Because the resonant frequency of the oscillation circuit is a functional relation of lc. It is fond that the ratio of maximum to minimum resonant frequency varies with the square root of capacitance ratio. Here the capacitance ratio refers to the ratio of the capacitance at the minimum reverse bias voltage to the capacitance at the maximum reverse bias voltage. Therefore, the tuning characteristic curve (bias voltage and resonant frequency) is basically a parabola. 8. Rectifier: switch on or off a semi-closed conductor component at a predetermined time. Rectification Filtering wave form 9. Energy storage: storage of electrical energy for release when necessary. For example, camera flashlights, heating devices, etc. (some capacitors now store energy at levels close to lithium batteries; a capacitor can store electricity as one-day power for a mobile phone.   IV. Capacitor types According to the analysis and statistics, capacitors are divided into the following 10 categories: 1. According to the structure: solid capacitor, variable capacitor and fine-tuned capacitor. 2. Classified by electrolytes: organic dielectric capacitor, inorganic dielectric capacitor, electrolytic capacitor, electrothermal capacitor and air-spaced capacitor. 3.According to the usage: high-frequency bypass  capacitor, low-frequency bypass  capacitor, filtering capacitor, tuning capacitor, high-frequency coupling capacitor, low-frequency coupling capacitor, small capacitor. 4. According to the different materials: ceramic capacitor, polyester capacitor, electrolytic capacitor, tantalum capacitor, advanced polypropylene capacitor etc. 5. High frequency bypass: ceramic capacitor, mica capacitor, glass film capacitor, polyester capacitor, glass-glazed capacitor. 6. Low frequency bypass: paper capacitor, ceramic capacitor, aluminum electrolytic capacitor, polyester capacitor. 7. Filter: aluminum electrolytic capacitor, paper capacitor, composite paper capacitor, liquid tantalum capacitor. 8. Tuning: ceramic capacitors, mica capacitors, glass film capacitors, polystyrene capacitors. 9. Low coupling: paper capacitor, ceramic capacitor, aluminum electrolytic capacitor, polyester capacitor, solid tantalum capacitor. 10. Small capacitors: metallized paper capacitor, ceramic capacitor, aluminum electrolytic capacitor, polystyrene capacitor, solid tantalum capacitor, glass-glazed capacitor, metallized polyester capacitor, polypropylene capacitor, mica capacitor. V. Capacitor volume Since capacitors are a container for storing charges, there is a problem of capacity. In order to measure the capacity of capacitors to store charges, the capacity is determined. A capacitor must store a charge under the action of an applied voltage. The amount of charge stored in different capacitors under voltage may also different. According to the international standard, when the capacitor is subjected to a 1V DC voltage, the value is the charge that can store in the the capacitor (that is, the electric quantity per unit voltage), which is expressed by the letter C. The basic unit of capacitance is the Farah (F). At 1V DC voltage, if the capacitor stores the charge is 1 Coulomb, the capacitance is set at 1 farah, and Farah is represented by the symbol F, 1 F=1 Q/ V. In practical application, the capacitance of capacitors is often much smaller than that of 1F, and is often used in smaller units, such as mF, μF, nF, pF, etc. The relationship between them is as follows:   1F=1000mF1mF=1000μF1μF=1000nF1nF=1000pF1F=1000000μF1μF=1000000pF VI. Charge and discharge of a capacitor When the capacitor is connected to the power supply, under the action of the electric field force, the free electron connected with the positive electrode of the capacitor moves through the power supply to the plate connected to the negative electrode of the power supply. The positive electrode is positively charged because of the loss of the negative charge; the negative electrode is negatively charged because of gaining negative charge. The positive and negative plates have the same charge size and the opposite sign, so the charge moves in a fixed direction to form a current. Due to the repulsive effect of the same charge, the initial current is maximum, and then decreases gradually. During the process of charge movement, the charge stored on the electrode plate of the capacitor increases continuously. When the voltage Uc between two poles of capacitor is equal to the power-supply voltage U, the charge stop moving. The current I=0, switch closed, through the wire connection, the capacitor plate charge neutralized. When K is closed, on the one hand, the positive charge of the capacitor C can be neutralized on the negative electrode; on the other hand, the negative charge of the negative electrode can also be moved to the positive electrode. The charge gradually decreases, the apparent current decreases and the voltage decreases to zero. VII. Matters needing attention when using capacitors Because the two poles of the capacitor have the residual charge, it is necessary to release the charge at first, otherwise the electric shock will occur easily. When dealing with the faulty capacitor, the circuit breaker and the upper and lower disconnector of the capacitor set should be opened first, and if the fuse protection is adopted, the fuse tube should be removed first. At this time, although the capacitor set has discharged  itself, there will still be part of the residual charge, therefore, it is necessary to carry out manual discharge. When discharging, the grounding end of the ground wire and the grounding grid should be fixed first, then the capacitor should be discharged with the grounding rod several times until there is no spark and discharge sound, and finally the ground wire is fixed again. Meanwhile, it should also be noted that if the capacitor has internal breakage, fuse failure or poor lead contact, there may be residual charges between the two poles, which will not be released during automatic discharge or manual discharge. Therefore, the operation or maintenance personnel should wear insulating gloves before contacting the faulty capacitor, and use a short line to connect the two poles of the fault capacitor to make it discharge. In addition, the capacitor with series connection should be discharged separately. VIII. Common fault of capacitor and treatment method (1) When the capacitor explodes, the power should cut off immediately and extinguish the fire with sand and dry-firefighter. (2) When the capacitor fuses, it shall report to the dispatch and open the circuit breaker of the capacitor after obtaining the consent. When the power supply is cut off to discharge it, external checks are carried out, such as whether there are flashover marks on the outside of the casing, whether the casing is deformed, the oil leakage and the short circuit of the earthing device, etc., and the insulation resistance between the poles and the ground is measured. Check whether the capacitor set connection is complete, firm, lacking of phase phenomenon. If not found fault phenomenon, it can be replaced after the investment. If the insurance still melts after power transmission, the faulty capacitor should be withdrawn and the rest should be power on. If the circuit breaker tripped at the same time as the fuse, at this time, don’t connect power supply. After the above inspection has been completed, the insurance must be replaced. (3) The circuit breaker of the capacitor tripped and the shunt safety was not broken, the capacitor should be discharged for three minutes before checking the power cable of the circuit breaker current inductor and the outside of the capacitor. If no anomaly is found, it may be due to external fault bus voltage fluctuations. After inspection, it may be put on trial; if not, a comprehensive test of the protection should be carried out. Through the above inspection, the test, if still can not find the reason, it is necessary to act according to the system, the capacitor is gradually tested. No trial test shall be made until the cause has been found.   FAQ   1. What is a capacitor used for? A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator).   2. What is capacitor and how it works? In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. ... Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric.   3. When should you use a capacitor? Power Supply Smoothing. This is the easiest and very widely used application of a capacitor. ... Timing. If you supply power to a capacitor through a resistor, it will take time to charge. ... Filtering. If you pass DC through a capacitor, it will charge and then block any further current from flowing.   4. What is capacitor and its types? The most common kinds of capacitors are: Ceramic capacitors have a ceramic dielectric. Film and paper capacitors are named for their dielectrics. Aluminum, tantalum and niobium electrolytic capacitors are named after the material used as the anode and the construction of the cathode (electrolyte).   5. Are capacitors AC or DC? When we connect a charged capacitor across a small load, it starts to supply the voltage (Stored energy) to that load until the capacitor fully discharges. Capacitor comes in different shapes and their value is measured in farad (F). Capacitors are used in both AC and DC systems (We will discuss it below).   6. What is the principle of capacitor? A capacitor is a device that is used to store charges in an electrical circuit. A capacitor works on the principle that the capacitance of a conductor increases appreciably when an earthed conductor is brought near it. Hence, a capacitor has two plates separated by a distance having equal and opposite charges.   7. Are capacitors dangerous? Capacitors may store hazardous energy even after the equipment has been de-energized, and may build up a dangerous residual charge without an external source. "Grounding" capacitors in series, for example, may transfer (rather than discharge) the stored energy.   8. What type of capacitor should I use? The general rule is always use a capacitor with a higher working voltage than the circuit it is used in. This is of particular importance in power supply circuits with high value electrolytic capacitors. The working voltage should always exceed the peak working voltage of the circuit by a minimum of 20%.   9. What is capacitor and its applications? Capacitor is a basic storage device to store electrical charges and release it as it is required by the circuit. Capacitors are widely used in electronic circuits to perform variety of tasks, such as smoothing, filtering, bypassing etc…. One type of capacitor may not be suitable for all applications.   10. Do capacitors change AC to DC? No, capacitor cannot convert AC to DC. Capacitor can add DC to AC so that zero reference of AC signal can be changed, in other words capacitor works as level shifter.   11. Can Capacitors store AC? Capacitors do not store AC voltage - it stores voltage. It's rated to handle 450 VAC; that means it can withstand an AC voltage being applied to it. In other words, the capacitor is non-polar (it has no positive or negative lead). Polar (or polarized) capacitors are best known as "Electrolytic" capacitors.   12. What is the difference between a capacitor and a battery? A battery is an electronic device that converts chemical energy into electrical energy to provide a static electrical charge for power. Whereas a capacitor is an electronic component that stores electrostatic energy in an electric field.   13. How much current can a capacitor handle? A 3.5V charger will charge the capacitor up to 3.5V only. You need a higher voltage DC source to charge the capacitor to higher potential. Remember, in your case, 100V is the maximum which capacitor can handle.   14. What happens when a capacitor fails? During a failure, half of the capacitor could fail open, which would result in overall capacitance being lost. Or half of the capacitor could fail short, which would result in the overall capacitance being halved.   15. Does type of capacitor matter? Yes, the type of capacitor can matter. Different types of capacitor have different properties. Some of the properties that vary between capacitor types: a. Polarised vs unpolarised b. Max voltage c. Equivalent Series Resistance (ESR) d. Lifetime (electrolytics are particularly bad in this case) e. Physical size (e.g. a 100,000 uF ceramic capacitor would be HUGE!) f. Tolerance of capacitance (again, electrolytics are bad here, often being +/- 20%