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IntroductionA relay is an electronic control device, which has a control system (also called an input loop) and a controlled system (also called an output loop). It is often used in automatic control circuits. In fact, it is an automated switch using a smaller current to control a larger current. Therefore, it plays the role of automatic adjustment, safety protection, and converter in the circuit. Relay has the features of fast response speed, stable work, long service life and small size. In order to ensure that these performances can be better played, the test and maintenance of the relay (solid state relay) are particularly important. This paper will introduce main relay test parameters, how to test a relay, an example of an automotive relay test.Testing a RelayCatalogIntroductionⅠ Understanding Relays  1.1 Relay ParametersⅡ How to Test A Relay?  2.1 General Test Ideas  2.2 Types of Relay TestⅢ Relay for Life: Automotive Relay Test  3.1 Automotive Relay  3.2 Common Faults of Automotive Relays  3.3 Detection Method  3.4 Specific OperationⅣ One Question Related to Relay Test and Going Further  4.1 Question  4.2 AnswerⅤ Frequently Asked Questions about Relay TestⅠ Understanding Relays1.1 Relay ParametersMain relay parameters include rated working voltage, rated working current, coil resistance, contact load, etc.1) Rated working voltage refers to the voltage required by the coil when the relay is working normally. For DC relays it refers to DC voltage (Figure a), and for AC relays it refers to AC voltage (Figure b). Relays of the same type often have multiple assessed operating voltages for circuit requirements, and the specification number is added to the end of the component to distinguish.Figure 1. Relay Symbol2) The rated working current refers to the current required by the coil when the relay is working normally.Coil resistance refers to the DC resistance of the relay coil. When selecting a relay, you must ensure that it is rated working voltage and rated working current meet the requirements.Figure 2. Rated Working Current3) Contact load refers to the load capacity of the relay contact, also known as the contact capacity. For example, the contact load of the jzx-10m relay is:  DC 28v×2a or AC 115v×1a. When used, the voltage and current passing through the relay contact should not exceed the rated value, otherwise, the contact will be burned out and the relay will be damaged. A load of multiple sets of contacts of a relay is generally the same.Figure 3. Contact LoadRecommended Reading: Basic Knowledge of Relay Electronics Tutorial with Video    The Role of the Relay and Its Working Principle Ⅱ How to Test A Relay?Relays are widely used in power protection, automation, sport, remote control, measurement and communication devices, so it is very important to check and maintain the normal operation of relays. There are many types of relays. Therefore, the inspection of relays cannot only be judged by measuring the resistance value of the coil. It is necessary to adopt multiple detection methods according to different relay types.2.1 General Test Ideas1) measuring contact resistanceApply the specified working voltage to the relay coil, and use a multimeter to detect the on-off condition of the contact at the “R×1k” gear. When the power is not applied, the normally open contact does not work, and the normally closed contact conducts. When the power is turned on, you should be able to hear the pick-up sound of the relay. At this time, the normally open contact is conducting and the normally closed contact is opposite, and the switching contact should be switched accordingly. Otherwise, the relay is damaged. For multi-group contact relays, if some of the contacts are damaged, the remaining contacts can still be used.Figure 4. Relay Test 2) measuring coil resistanceThe resistance value of the relay coil can be measured with the multimeter at R×10Ω gear, so as to determine whether the coil is open. 3) measuring of pull-in voltage and currentUse an adjustable regulated power supply to input a set of voltage to the relay, and connect an ammeter in the power supply circuit to monitor. Increase the power supply voltage slowly, and when you hear the pull-in sound of the relay, write down the voltage and current. In order to be accurate, you can try several times to get the average value. 4) measuring the release voltage and currentSame test connection like the above. When the relay pulls in, then gradually reduce the supply voltage. When you hear the relay release sound again, write down the voltage and current at this time. Try several times to get the average release voltage and release current. Under normal circumstances, the release voltage of the relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it can't be used normally, which will affect the circuit stability resulting in abnormal operation. 2.2 Types of Relay TestElectromagnetic Relay TestFigure 5. Electromagnetic RelayThe multimeter is placed in the “R×100” or “R×1k” gear, and the two test leads (regardless of positive and negative) are connected to the two pins of the relay coil (shown in Figure 5). The indication of the multimeter should basically match the coil resistance of the relay. If the resistance value is obviously too small, it means that the coil is short-circuited locally; if the resistance value is 0, it means that there is a short circuit between the two coil pins; if the resistance value is infinite, it means that the coil is open or the pins are disconnected. Reed Relay TestReed relay is also one of the most commonly used relays. It consists of a reed switch and a coil, as shown in Figure 6. The reed switch is made by sealing two non-interconnected ferromagnetic metal strips in a glass tube, and the reed switch is placed in the coil. When the current passes through the coil, the magnetic field generated by the coil magnetizes the metal strips in the reed pipe, and the two metal strips attract due to opposite polarities to connect the controlled circuit. Several reed pipes can be placed in the coil, and they will act simultaneously under the action of the coil's magnetic field.Figure 6. Reed Relay Reed relay has a pair of coil pins and several pairs of reed switch pins, and there are corresponding marks on the shell for identification.Figure 7. Reed Relay Reed relays can also use a multimeter to detect their coils and contacts, and the detection method is the same as that of electromagnetic relays.Figure 8. Reed Relay Solid State Relay (SSR) TestThe input end can be tested with a multimeter. The multimeter is placed in the "R×10k" gear, the black test lead (the positive electrode of the battery in the meter) is connected to the positive electrode of the SSR input terminal, and the red test lead (that is, the negative electrode of the battery in the meter) is connected to the negative electrode of the input terminal of SSR. The hands should deflect more than halfway (Figure 9). Re-testing after swapping the two test leads, the hands should not move. If the needle deflects to the top or does not move regardless of the forward or reverse voltage access, the solid-state relay has been damaged.Figure 9. Solid State Relay  SSR You can also make a test circuit according to Figure 10. When the control voltage of the SSR input terminal is turned on, the light-emitting VD is on; when the control voltage of the SSR input terminal is cut off, the light-emitting diode VD is off.Figure 10. SSR Thermal Relay Test1) heating elements detectionThe heating element is composed of an electric heating wire or electric heating sheet, and its resistance is very small (close to 0Ω). The detection is shown in Figure 11. The normal resistance of the three groups of heating elements should be close to 0Ω. If the resistance is infinite (the digital multimeter displays the symbol "1" or "OL" for exceeding the range), the heating element is open.Figure 11. ① 200Ω gear is selected.② The red and black probes are respectively connected to the two ends of a heating element.③ The resistance is close to 0Ω, indicating that the resistor as a heating element is normal. 2) contact detectionThermal relays generally have a normally closed contact and a normally open contact. This detection includes working and non-working conditions. The first picture is the detection of the normally closed contact resistance when it is not in operation. Normally it should be close to 0Ω. Then the detection is taken in the opposite condition. Move the test rod, as shown in the second picture, simulates the over-current heating and bending of the heating element to make the contact action. The normally closed contact becomes an open circuit, and the resistance is infinite.Figure 12. ① 200Ω gear is selected.② The red and black probes are connected to both ends of the normally closed contact.③ The resistance is close to 0 Ω, indicating that the normally closed contact is closed.④ Move the test rod by hand.⑤ The out-of-range symbol "1" is displayed to indicate that the normally closed contact is open. Intermediate Relay TestThe electrical part of the intermediate relay is composed of coils and contacts, both of which use the resistance gear of a multimeter.1) The contact is detected when the control coil is not powered. Contacts include normally open contacts and normally closed contacts. When the control coil is power off, the normally open contacts are open and the resistance is infinite, at this time, the normally closed contacts are closed and the resistance is close to 0Ω. The above-mentioned detection of the normally open contact is shown in the figure below.Figure 13.① 200Ω gear is selected.② The red and black probes are connected to both ends of normally open contact.③ The out-of-range symbol "1" is displayed to indicate that the normally open contact is open. 2) Control coil detection of the intermediate relay is shown in Figure 14. Generally, the greater the rated current of the contact, the smaller the resistance of the control coil. This is because the greater the rated current of the contact, the larger the volume of the contact. Only a small control coil resistance (thicker line diameter) can flow through a larger current to produce a stronger magnetic field suction contact.Figure 14. ① 200Ω gear is selected for the gear switch.② Connect the red and black lead to the two pins of the control coil.③ The display of "6.60" indicates that the resistance of the control coil is 6.6kΩ.3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then use a multimeter to detect the resistance of the normally open and normally closed contacts. The normally open contact should be closed and the resistance should be close to 0Ω; the normally closed contact should be open and the resistance is infinite. Time Relay TestThe detection of time relay mainly includes contact normal state detection, coil detection and coil energization detection.1) Normal-state detection of contacts. It refers to the detection of the resistance of the contact when the control coil is not energized. The normally open contact is open and the resistance is infinite, while the normally closed contact is closed, and the resistance is close to 0Ω. Normal detection processes are shown in the figure below.Figure 15. ① 200Ω gear is selected for the gear switch.② The red and black lead is connected with two pins of a normally closed contact.③ The resistance is close to 0Ω, indicating that the normally closed contact is closed. 2) Detection of control coil. It is shown in Figure 16.Figure 16. ① 20kΩ gear is selected for the gear switch.② Connect the red and black lead to the two pins of the control coil.③ The display of "4.93" indicates that the resistance of the control coil is 4.93kΩ.3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then check whether the contact status has changed according to the characteristics of types of the time relay. For example, for a delay time relay, after a period of time delay, check whether the delay contact is closed (resistance is close to 0Ω) and whether the delay contact is disconnected (resistance is infinite).  Ⅲ Relay for Life: Automotive Relay Test3.1 Automotive RelayRelays are widely used in automotive circuits, such as starting system circuits, wiper circuits, and rear window heating circuits. When the vehicle starts, a larger starting current is required. If the ignition switch is used for direct control, the starting contacts will ignite and burn, which will affect the service life of the ignition switch and even cause serious consequences such as line ablation and fire. Using a relay to control a large current with a small current will not cause the above problems. When a certain voltage or current is applied to both ends of the electromagnetic relay coil, the magnetic flux generated by the coil passes through the magnetic circuit composed of the core, yoke, armature, and the working air gap of the magnetic circuit. Under the action of the magnetic field, the armature attracts the pole face of the iron core, making the normally closed contact opens and the normally open contact close. When the voltage or current at both ends of the coil is less than a certain value, the mechanical reaction force is greater than the electromagnetic attraction force, and the armature returns to the initial state: the normally closed contact is on and the normally open contact is off. One of the automobile relays functions is a switch; the other is load overload protection; the third is fault protection. 3.2 Common Faults of Automotive RelaysIncluding coil burnt, short circuit, insulation part aging, contact ablation, etc.1) Relay MalfunctionWhen the controlled circuit is required to be closed, the relay will not act, on the contrary, when the controlled circuit is not required to be closed, the relay will act. This kind of problem occurs mainly because the interference voltage in the circuit exceeds the allowable range of the drive circuit of the relay. When designing the circuit, pay attention to the factors that can cause interference (such as chip command errors, short circuits, grid fluctuations, etc.). 2) Relay BurnedThere are many reasons for burnout. For example, the actual switching current exceeds the rated switching current of the relay, and the actual inrush current exceeds the rated switching current of the relay. According to design experience, in order to avoid these problems, the rated current should be selected to be 2-3 times the actual switching current, and the impact current of the relay is 2-3 times the actual current. 3) Contact WeldingGenerally speaking, the temperature rise of the AC conversion relay coil is higher than that of the DC conversion relay. This is because of the eddy current loss and hysteresis loss in the magnetic circuit. In addition, when the AC conversion relay is operating at a voltage lower than the rated voltage, a bounce phenomenon may occur. This will cause burnout, welding of contacts and damage to the relay, or disconnection of the self-protection circuit. Therefore, measures must be taken to prevent fluctuations in the power supply voltage.In addition, regardless of the length of the fluctuation time, it will cause the failure of the relay. So ensure that there is a power supply with sufficient capacity. 4) Coil Temperature Rise is Too HighThe loss of magnetic materials such as copper wires and iron cores or the heat transfer of the contacts will cause the temperature rise. Therefore, the heat resistance of the insulating material and the distance between the relay and the heat-generating device should be paid special attention in the circuit design. 3.3 Detection MethodStatic detection: check the resistance of the coil and the resistance of the normally closed contact.Dynamic detection: energize the coil and detect the resistance of the normally open contact. 3.4 Specific OperationTurn on the ignition switch and hear whether there is a pull-in sound in the control relay or feel the relay with your hands for vibration. If so, it means that the relay is basically in routine. The failure of the circuit may be caused by other reasons. On the contrary, it means that the relay is faulty.Replace the relay to be tested with an identical working relay. Turn on the switch, and if the electrical equipment is working normally, it can be determined there is a problem with the relay to be tested.Use the multimeter Rx100Ω gear and combine the resistance of each pin of the circuit to analyze. If the conduction and disconnection are normal, it means that there is no problem with the relay, otherwise, it means the relay is faulty.Open the relay shell to check whether the contacts are ablated or oxidized. If there are bumps and rust on the contact, it means that the contact is ablated or oxidized and does not work properly.Check whether the coil is ablated or discolored. If the coil is ablated with jelly, the coil is black or has a gluey smell, which means the coil is short-circuited by ablation.Ⅳ One Question Related to Relay Test and Going Further4.1 QuestionWhat are the symptoms of a bad car relay?4.2 AnswerThe car suddenly stalls while operating. One of the most common symptoms of a failed ignition relay is a car that suddenly stalls while operating. Car not starting. Another symptom of a faulty ignition relay is a no-power condition.Dead battery. A dead battery is another symptom of a faulty ignition relay.Burned relay. Ⅴ Frequently Asked Questions about Relay Test1. How do you check if a relay is bad?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 2. How do you test a 12-volt relay? 3. How do you check an overload relay with a multimeter?CEP7 Overload Relay test proceduresMeasure the normal motor running current (i motor).Turn off the motor and let it cool for about 10 minutes.Calculate the following ratio: i (motor) / i (overload min FLA).Set the overload to its minimum FLA and turn on the motor.Wait for the overload to trip. 4. How do I test a solid-state relay?The SSR can be tested as described below if a load is connected. Connect a load and power supply, and check the voltage of the load terminals with the input ON and OFF. The output voltage will be close to the load power supply voltage with the SSR turned OFF. 5. Can a bad relay drain your battery?Battery drain or dead batteryA failed ECM power relay can also cause a battery drain or a dead battery. If the relay shorts, it can leave power on to the computer, even when the vehicle is turned off. This will place a parasitic drain on the battery, which will eventually cause it to go dead. 6. What happens when the main relay goes bad?The engine will not startIf the main relay is not supplying the engine computer with the power it needs, then the engine will not be able to crank and run the right way. Failing to get the main relay replaced will usually lead to the car being unusable. 7. How do you test a battery relay? 8. How do you test a protection relay?Protection relay self-test procedureThis will normally involve checking the relay watchdog circuit, exercising all digital inputs and outputs and checking that the relay analog inputs are within calibration by applying a test current or voltage. 9. How do you check if a relay is working?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 10. How do you test an electromagnetic relay?Grab a multimeter and set it to Ohms. Touch the leads across the electromagnet coil pins and measure resistance. Anywhere from 50-120 ohms is OK. Out of range or open means a bad electromagnet coil winding and time for a new relay.

I IntroductionSummary: A photoresistor, or light-dependent resistor (LDR), is a passive electronic component that decreases in resistance as light intensity increases. Driven by the expansion of IoT and smart home automation, the global photoresistor market is projected to reach $553.75 million by 2025. This guide covers LDR working principles, circuit diagrams, types, and step-by-step Arduino integration.Photoresistor or light-dependent resistor (abbreviated as LDR) or photoconductor is a special resistor made of semiconductor materials such as cadmium sulfide or cadmium selenide. Its working principle is based on the internal photoelectric effect. The stronger the light, the lower the resistance value. With the increase of the light intensity, the resistance value decreases rapidly, and the bright resistance value can be as small as 1KΩ or less. The photoresistor is very sensitive to light, and it shows a high resistance state when there is no light, and the dark resistance can generally reach 1.5MΩ.This article includes an overview of the basic information of the photoresistor and two Arduino tutorials for the photoresistor. The content is very comprehensive and detailed. You can choose the part you want to read or read the full text. We hope this article is helpful to you!II What is a Photoresistor?2.1 What is the Definition of a Photoresistor?A photoresistor (also known as a light-dependent resistor, LDR, or photo-conductive cell) is a passive electronic component that decreases its electrical resistance as the luminosity on its sensitive surface increases.2.2 What is the Symbol for a Photoresistor?The standard schematic symbols for a photoresistor are generally represented by the designators "RL", "RG", or "R", often accompanied by a resistor icon enclosed in a circle with incoming arrows indicating light. The following figure shows the schematic symbols of the photoresistor.Figure1. Photoresistor SymbolRecommended Reading: To learn more about Resistor Symbol.2.3 What is the Composition of a Photoresistor?(1) The structure of the photoresistorA photoresistor is primarily composed of a photosensitive semiconductor layer, a glass substrate or moisture-proof film, and comb-shaped ohmic electrodes.Figure2. The Structure of PhotoresistorMaterials for manufacturing photoresistorsThe materials used for manufacturing photoresistors are primarily semiconductors such as metal sulfides, selenides, and tellurides. Usually, coating, spraying, sintering and other methods are used to make a very thin photoresistor and comb-shaped ohmic electrode on the insulating substrate, and then the lead is taken out and encapsulated in a sealed housing with a light-transmitting mirror to prevent moisture from affecting its sensitivity.III How Does the Photoresistor Work?3.1 What is the Working Principle of Photoresistors?How does a photoresistor workThe working principle of a photoresistor is based entirely on the internal photoelectric effect, where incident light energy excites electrons into the conduction band, thereby lowering the component's electrical resistance.A voltage is applied to the metal electrodes at both ends of the photoresistor, and a current flows through it. When irradiated with light of a certain wavelength, the current will increase with the increase of light intensity, thereby achieving photoelectric conversion. After the incident light disappears, the electron-hole pairs generated by the photon excitation will recombine, and the resistance of the photoresistor will return to its original value.The photoresistor has no polarity and is purely a resistive device. It can be used with either DC voltage or AC voltage. The conductivity of a semiconductor depends on the number of carriers in the semiconductor conduction band. Why is the value of the photoresistor related to the wavelength of the incident light?Simply put, it is the effect of transitions between energy levels. Photons at different wavelengths have different energies, and an electron can only absorb one photon. After an electron absorbs a photon, whether it can be converted from non-conductive to conductive electrons depends on the photon’s Energy, and the number of electrons that can conduct electricity determines the resistance of the photoresistor. Therefore, the light wavelength also affects the resistance of the photoresistor.Recommended Reading: See more about light sensor, wavelength, spectrum and photometric physical quantity.3.2 What is the Internal Photoelectric Effect?The internal photoelectric effect is a phenomenon where the absorption of photons causes a change in the electrical conductivity of a semiconductor material. This differs from the external photoelectric effect, which involves the complete escape of electrons from the material's surface.The internal photoelectric effect can be divided into two main categories:Photoconductive effect: The change in electrical resistance due to light exposure.Photovoltaic effect: The generation of a photo-induced electromotive force (voltage) across a P-N junction. PhotoconductivityThe photoconductive effect is one of two internal photoelectric effects. The internal photoelectric effect refers to the phenomenon that the electrical conductivity of a semiconductor exposed to light changes or a photo-induced electromotive force is generated. Among them, the phenomenon that the conductivity of the semiconductor changes due to light is called the photoconductivity effect.Figure3. Energy Level of AtomPhotovoltaic EffectThe photovoltaic effect occurs when a P-type and N-type semiconductor are combined, creating a P-N junction. The process unfolds as follows:Electrons in the N-type semiconductor and holes in the P-type semiconductor diffuse, forming a depletion region near the interface.When light irradiates the P-N junction, photons with sufficient energy generate minority carriers (electron-hole pairs).Under the action of the internal electric field, excited electrons flow to the N-type region, and holes flow to the P-type region.This accumulation generates an additional electromotive force (voltage) at both ends of the P-N junction.If connected to an external circuit, current flows from the P region to the N region, producing usable electrical power.Figure4. Photovoltaic EffectIV How Do You Wire a Photoresistor Application Circuit?Schematic circuit for conventional applicationsFigure5. Schematic CircuitModule parameters:Working voltage: DC3.3-5VPhotoresistor Model: 5516Module pins: 3-pin or 4-pin (an additional analog output AO for 4-pin)Common circuit diagram （3）Photoresistor application circuit diagramThe following figure is a schematic diagram of the application of the photoresistor in the light control switch. The photoresistor is connected in series with the resistor R1. When there is no light, that is, the voltage across R1 does not reach the turn-on voltage of the Q1 transistor. Once exposed to light, the resistance of the photoresistor drops rapidly. The voltage across R1 rises and the transistor turns on, which causes the transistor Q2 in the subsequent stage to turn on, and finally, the switch K opens and the bulb works.Figure6. Common Photoresistor Application Circuit Diagram(4) Photoresistor dimming circuitThe following figure is a typical light-controlled dimming circuit. Its working principle is: when the surrounding light becomes weak, the resistance of the photoresistor RG increases, which increases the partial voltage added to the capacitor C, which in turn makes the thyristor's conduction angle is increased to achieve the purpose of increasing the voltage across the lamp. Conversely, if the surrounding light becomes brighter, the resistance of RG decreases, resulting in a smaller conduction angle of the thyristor, and the voltage across the lamp decreases at the same time, dimming the light, thereby controlling the illuminance of the lamp.Figure7. Photoresistor Dimming CircuitNote: The rectifier bridge in the above circuit must be a DC pulsating voltage, and it cannot be converted into a smooth DC voltage by capacitor filtering, otherwise the circuit will not work properly. The reason is that the DC pulsating voltage can not only provide the basic conditions for the zero-crossing shutdown of the thyristor, but also enable the charging of the capacitor C to start from zero every half cycle, and accurately complete the synchronous phase-shift triggering of the thyristor.V What Are the Types of Photoresistors?5.1 Classification by MaterialsBased on material composition, photoresistors are classified into intrinsic (pure semiconductor) and extrinsic (doped semiconductor) types. Polycrystalline and single crystal photoresistors can also be divided into cadmium sulfide (CdS), cadmium selenide (CdSe), lead sulfide (PbS), lead selenide (PbSe), indium antimonide (InSb) photoresistors, etc. 5.2 Classification by Spectral CharacteristicsSpectral TypeCommon MaterialsPrimary Applications (2026)Ultraviolet (UV)Cadmium sulfide, Cadmium selenideUV detection, environmental monitoringInfrared (IR)Lead sulfide, Lead telluride, Indium antimonideAstronomical detection, non-contact measurement, IR communicationVisible LightSelenium, Silicon, Germanium, Zinc sulfideIoT smart lighting, automatic street lights, exposure devices(1) Ultraviolet photoresistor: sensitive to ultraviolet rays, including cadmium sulfide, cadmium selenide photoresistors, etc., used to detect ultraviolet rays.(2) Infrared photoresistors: mainly lead sulfide, lead telluride, and lead selenide. Photoresistors such as indium antimonide are widely used in missile guidance, astronomical detection, non-contact measurement, human disease detection, infrared spectroscopy, infrared communication and other national defense, scientific research, and industrial and agricultural production.(3) Visible light photoresistors: including selenium, cadmium sulfide, cadmium selenide, cadmium telluride, gallium arsenide, silicon, germanium, zinc sulfide photoresistors, etc. Mainly used in various photoelectric control systems, which account for a significant portion of the projected $553.75 million global photoresistor market in 2025. Applications include IoT smart lighting, automatic turning on and off of navigation lights, street lights and other lighting systems, automatic water supply and automatic water stop devices, automatic protection devices on machinery and "position detectors" Thickness detectors for thin parts, automatic exposure devices for cameras, photoelectric counters, smoke alarms, photoelectric tracking systems, etc.Figure8. Light Dependent ResistorVI The Main Parameters and Basic Characteristics of the Photoresistor6.1 What Are the Main Parameters of a Photoresistor?The main parameters of a photoresistor define its operational limits, sensitivity, and response time in various lighting conditions.1) Bright resistance (kΩ): refers to the resistance value of the photoresistor when exposed to light.2) Dark resistance (MΩ): refers to the resistance value of the photoresistor when there is no light exposure (dark environment).3) Maximum working voltage (V): refers to the highest voltage the photoresistor is allowed to withstand under the rated power.4) Bright current: refers to the current that the photoresistor passes when it is irradiated by light under the specified applied voltage.5) Dark current (mA): refers to the current that the photoresistor passes under the specified applied voltage when there is no light.6) Time constant (s): refers to the time required for the photoresistor to start from the light jump to stabilize 63% of the bright current.7) Resistance temperature coefficient: refers to the relative change of the resistance value of the photoresistor when the ambient temperature changes by 1°C.8) Sensitivity: refers to the relative change of the resistance value of the photoresistor with and without light irradiation.Figure9. LDR6.2 Basic Characteristics(1) Dark resistance and bright resistanceThe stable resistance value measured by the photoresistor under room temperature and total darkness is called dark resistance. The current flowing at this time is called dark current. For example, MG41-21 type photoresistor dark resistance is greater than or equal to 0.1M.The stable resistance value measured by the photoresistor at room temperature and under certain lighting conditions is called bright resistance. The current flowing at this time is called the bright current. The bright resistance of MG41-21 type photoresistor is less than or equal to 1k.    The difference between bright current and dark current is called photocurrent.    Obviously, the larger the dark resistance of the photoresistor, the better, and the smaller the bright resistance, the better, that is, the dark current should be small and the bright current should be large, so the sensitivity of the photoresistor is high.Figure10. Bright Current and Dark Current(2) Volt-ampere characteristicsUnder a certain illuminance, the relationship between the voltage applied across the photoresistor and the current flowing through the photoresistor is called the volt-ampere characteristic. The volt-ampere characteristic of the photoresistor is approximately a straight line, and there is no saturation phenomenon. Due to the limitation of power dissipation, the voltage across the photoresistor cannot exceed the maximum operating voltage during use. The dotted line in the figure is the allowable power consumption curve, from which the normal operating voltage of the photoresistor can be determined. (3) Photoelectric characteristics    The relationship between the photocurrent of the photoresistor and the illuminance is called the photoelectric characteristic. The photoelectric characteristics of the photoresistor are nonlinear. Therefore, it is not suitable as a detection element, which is one of the shortcomings of the photoresistor. In automatic control, it is often used as a switching photoelectric sensor.Figure11. Characteristics of the Photoelectric Effect(4) Spectral characteristicsFor incident light of different wavelengths, the relative sensitivity of the photoresistor is different. The spectral characteristics of various materials are shown in Figure 2.6.4. It can be seen from the figure that the peak value of cadmium sulfide is in the visible light region, and the peak value of lead sulfide is in the infrared region. Therefore, when selecting the photoresistor, the types of components and light sources should be considered in order to obtain satisfactory results. (5) Frequency characteristicsWhen the photoresistor is exposed to pulsed light, the photocurrent will reach a steady-state value after a period of time. When the light suddenly disappears, the photocurrent will not be zero immediately. This shows that the photoresistor has time-delay characteristics. Because different materials have different time delay characteristics of photoresistors, their frequency characteristics are also different. Figure 2.6.5 shows the relationship between the relative sensitivity Kr and the light intensity change frequency f. It can be seen that the use frequency of lead sulfide is much higher than that of thallium sulfide. However, most photoresistors have large time delays, so they cannot be used in situations where fast response is required. This is a defect of photoresistors. (6) Temperature characteristicsLike other semiconductor devices, the photoresistor is greatly affected by temperature. When the temperature increases, its dark resistance will decrease. Changes in temperature also have a great influence on spectral characteristics. Figure 2.6.6 is the spectral temperature characteristic curve of the lead sulfide photoresistor. It can be seen from the figure that its peak value moves to the short wavelength direction as the temperature rises. Therefore, in order to improve the sensitivity, or in order to receive far-infrared light, cooling measures are taken.Figure12. Temperature CharacteristicsSpectral Temperature Characteristics of Lead Sulfide Photoresistor A commonly used photoresistor is a cadmium sulfide photoresistor, which is made of semiconductor material. The resistance of the photoresistor changes with the intensity of the incident light (visible light). Under dark conditions, its resistance (dark resistance) can reach 1~10MΩ; under strong light conditions (100LX), its resistance (Bright resistance) Only a few hundred to thousands of ohms. The sensitivity of the photoresistor to light (the spectral characteristics) is very close to the human eye's response to visible light (0.4~0.76) μm. As long as the human eye can sense the light, it will cause its resistance to change. Therefore, when designing the light control circuit, the incandescent bulb (small electric bead) light or natural light is used as the control light source, which greatly simplifies the design.Figure13. Photoresistor Characteristic CurveThe corresponding resistance change of the photoresistor with the intensity of the incident light is not linear, so it cannot be used for the linear conversion of the photoelectricity. This is where the user should pay attention. Beginners can purchase a photoresistor (MG45 type), at night a 60~100W incandescent lamp, use a multimeter to directly measure the resistance of the photoresistor. When measuring, the photoresistor should be aimed at the light of the incandescent lamp, and then gradually distance from the lamp (from near to far), observe the change of the resistance value indicated by the multimeter, and the special characteristics of the photoresistor can be visually verified.Commonly used photoresistor models are sealed MG41, MG42, MG43 and unsealed MG45 (cheap price). Their rated power is below 200mW.VII How to Use a Photoresistor with Arduino?7.1 LED Control with Photoresistor and ArduinoLED Control with LDR (Photoresistor) and Arduino7.2 How to Measure Light Intensity Using a Photoresistor (Arduino)In the data collection of modern smart home systems, the measurement of light intensity is highly necessary. For example, indoor IoT lighting can be automatically adjusted according to the intensity of the light to provide users with the most comfortable environment. The tutorial here will use a photoresistor to cooperate with Arduino to complete the light data collection.(1) MaterialsArduino UNO development boardBreadboardPhotoresistor1K-10K resistance(2)Wiring method Figure14. Wiring MethodThe resistance of photosensitive resistors is very high in the condition of no light. The stronger the light, the smaller the resistance. By measuring the voltage variation on both sides of the photosensitive resistance, the variation of the photosensitive resistance can be known and the light intensity can be obtained. In the connection diagram, we find that a partial voltage resistor is connected in series for the photosensitive resistor.Figure15. CircuitIn the above figure, RL is a photoresistor, R1 is a series resistor, Vout=RLR1+RL∗Vin, in the dark, the resistance of RL will be very large, so Vout is also very large, close to 5V. Once the light is irradiated, the value of RL will decrease rapidly, so Vout will decrease accordingly. It can be seen from the above formula that R1 should not be too small, preferably around 1k~10k, otherwise the ratio will not change significantly. (3) CodeThe code part is very simple, just read the analog value of the interface connected to the photoresistor.1 light = analogRead(0);Open the serial monitor of Arduino, illuminate the photoresistor with the flashlight of the mobile phone, and observe the result:2 Serial.println("lignt :");3 Serial.println(light);7.3 Use Experiment of Arduino Photoresistor(1) MaterialsArduino UNO x1Photoresistor x1resistance 10K, 4.7K, 1K x several (or need one, but you can test the difference between different resistance values and data)(2)Wiring method Figure16. Wiring Method(3)Program#define AD5 A5 //Define analog port A5#define LED 13 //Define digital port 13 int Intensity = 0;//Illuminance value void setup() //Program initialization{   pinMode(LED, OUTPUT);//Set LED to output mode   Serial.begin(9600);//Set baud rate 9600} void loop() // Program body loop{   Intensity = analogRead(AD5); //Read the value of analog port AD5 and save it in the Intensity variable   Serial.print("Intensity = "); //Serial output "Intensity = "   Serial.println(Intensity); //The serial port outputs the value of the Intensity variable and wraps   delay(500); //Delay 500ms}(4) Power on, view serial dataTest Results:Figure17. Test ResultsThe above data is the change of the value with the flashlight and no light.(5) SummaryThe positive and negative poles are reversed and the values are reversed. The larger the resistance value, the larger the change range. Using 5V, the range is larger than 3.3V.Recommended Reading: Arduino&mBlock light sensorVIII How to Use Multimeter to Detect the Quality of Photoresistor?Measure the dark resistance: Use a black piece of paper to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinity. The larger the value, the better the performance of the photoresistor. If this value is very small or close to zero, it means that the photoresistor has been burnt through and damaged and can no longer be used.Measure the bright resistance value: Point a light source to the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If this value is large or even infinite, it indicates that the internal open circuit of the photoresistor is damaged and can no longer be used.Test intermittent light response: Align the light-transmitting window of the photoresistor with the incident light, and use a small piece of black paper to shake the upper part of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged.IX A Quiz about the PhotoresistorPhotoresistors, potentiometers, and thermistors are all ________.A. OutputsB. Digital inputsC. Analog inputsD. ThroughputsAnswer: CFrequently Asked QuestionsWhat is the difference between a photoresistor and a photodiode?A photoresistor is a passive component that changes resistance based on light intensity, making it slower but easier to use. A photodiode is an active semiconductor with a P-N junction that converts light into current, offering much faster response times for high-speed applications.Is a photoresistor an analog or digital component?A photoresistor is fundamentally an analog component. Its resistance changes continuously in response to varying light levels. However, when paired with a microcontroller like an Arduino and a voltage divider, its analog signal can be easily converted into digital data.What are the main types of photoresistors?Photoresistors are primarily categorized into intrinsic and extrinsic types. Intrinsic photoresistors use pure semiconductors like silicon, while extrinsic types use doped materials to detect longer wavelengths, such as infrared light, making them ideal for specialized sensors.How is a photoresistor used in smart home circuits?In modern IoT and smart home systems, photoresistors act as ambient light sensors. They automatically trigger actions like turning on outdoor security lights, adjusting indoor smart bulb brightness, or activating motorized blinds when sunlight reaches a specific threshold.{ "@context": "https://schema.org", "@graph":[ { "@type": "Article", "headline": "Photoresistor Basics: Working Principle, Types, and Arduino Tutorial", "datePublished": "2020-06-19T00:00:00Z", "dateModified": "2026-03-14T15:57:00+08:00", "author": { "@type": "Organization", "name": "ApogeeWeb" }, "publisher": { "@type": "Organization", "name": "ApogeeWeb" } }, { "@type": "FAQPage", "mainEntity":[ { "@type": "Question", "name": "What is the difference between a photoresistor and a photodiode?", "acceptedAnswer": { "@type": "Answer", "text": "A photoresistor is a passive component that changes resistance based on light intensity, making it slower but easier to use. A photodiode is an active semiconductor with a P-N junction that converts light into current, offering much faster response times for high-speed applications." } }, { "@type": "Question", "name": "Is a photoresistor an analog or digital component?", "acceptedAnswer": { "@type": "Answer", "text": "A photoresistor is fundamentally an analog component. Its resistance changes continuously in response to varying light levels. However, when paired with a microcontroller like an Arduino and a voltage divider, its analog signal can be easily converted into digital data." } }, { "@type": "Question", "name": "What are the main types of photoresistors?", "acceptedAnswer": { "@type": "Answer", "text": "Photoresistors are primarily categorized into intrinsic and extrinsic types. Intrinsic photoresistors use pure semiconductors like silicon, while extrinsic types use doped materials to detect longer wavelengths, such as infrared light, making them ideal for specialized sensors." } }, { "@type": "Question", "name": "How is a photoresistor used in smart home circuits?", "acceptedAnswer": { "@type": "Answer", "text": "In modern IoT and smart home systems, photoresistors act as ambient light sensors. They automatically trigger actions like turning on outdoor security lights, adjusting indoor smart bulb brightness, or activating motorized blinds when sunlight reaches a specific threshold." } } ] }, { "@type": "HowTo", "name": "How to Use Multimeter to Detect the Quality of Photoresistor", "step":[ { "@type": "HowToStep", "name": "Measure the dark resistance", "text": "Use a black piece of paper to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter remains basically unchanged, and the resistance value is close to infinity. The larger the value, the better the performance of the photoresistor. If this value is very small or close to zero, it means that the photoresistor has been burnt through and damaged and can no longer be used." }, { "@type": "HowToStep", "name": "Measure the bright resistance value", "text": "Point a light source to the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If this value is large or even infinite, it indicates that the internal open circuit of the photoresistor is damaged and can no longer be used." }, { "@type": "HowToStep", "name": "Test intermittent light response", "text": "Align the light-transmitting window of the photoresistor with the incident light, and use a small piece of black paper to shake the upper part of the light-shielding window of the photoresistor to make it receive light intermittently. At this time, the pointer of the multimeter should swing left and right with the black paper. If the pointer of the multimeter always stops at a certain position and does not swing with the shaking of the paper, it means that the photosensitive material of the photoresistor has been damaged." } ] } ]}

Executive Summary: Potentiometers in 2026A potentiometer (or "pot") is a three-terminal variable resistor used to control voltage, current, and signal processing in electronic circuits. While traditional mechanical pots remain vital for audio and industrial controls, 2026 trends favor high-precision Digital Potentiometers (DigiPots) and MEMS-based sensors for IoT and robotics applications. This guide covers the latest classification standards, modern applications, and essential maintenance tips for optimal circuit performance.Ⅰ. What is a Potentiometer? (2026 Overview)A potentiometer is a critical three-terminal passive electronic component that functions as an adjustable voltage divider. Ideally suited for applications requiring user-generated input or position sensing, it consists of a resistive element (track) and a sliding wiper. If only two terminals are configured (one end and the wiper), it functions as a variable resistor or rheostat. While the core mechanical construction of potentiometers remains consistent, modern 2026 manufacturing has introduced durable conductive plastics and cermet materials to extend lifecycle and precision.Figure 1: Potentiometer Basics and Modern ClassificationsⅡ. How are Potentiometers Classified?Potentiometers are primarily classified by their movement mechanism (Linear vs. Rotary) and their control method (Analog Mechanical vs. Digital). Understanding these distinctions is crucial for selecting components for 2026-era PCB designs.Table 1. Industry Standard Potentiometer Classifications (2026)Potentiometer TypeClassification Criteria & Modern StandardsBy Resistor MaterialElectrical performance relies on the track material. Common types include wire wound (high power), synthetic carbon film (consumer audio), metal glass glaze, and conductive plastic (high durability >1M cycles). Specialized types use metal foil or metal oxide film for precision aerospace applications.By Working Environment Selected based on IP ratings and stress factors: High-precision, high-resolution, high-power, or high-temperature variants. Modern sealed potentiometers are essential for automotive and outdoor IoT sensors.By Adjustable ResistanceIncludes fully adjustable (user interface), semi-adjustable (trimpots for calibration), and fine-adjustment types. Non-contact options (Hall Effect, Photo-electric) are gaining market share in 2026 to eliminate mechanical wear and "wiper noise."By Construction MechanicsRotary: Standard rotation is 270°–300°. Multi-turn: Uses a worm gear for 10-20 turns, vital for precision calibration.Linear Slider: Standard in audio mixing consoles for visual feedback of levels (faders). By Gang Number Single Gang: Controls one circuit.Dual/Ganged: Two pots on a single shaft, standard for stereo audio volume control to maintain channel balance.By Resistance Taper (Law)Linear (Type B): Resistance changes proportionally to angle. Used for voltage division.Logarithmic (Type A/Audio): Resistance changes logarithmically to match human hearing. Essential for volume controls.By Actuation MethodManual: Hand-turned knobs or sliders.Motorized: Equipped with a servo motor for remote control and "flying fader" automation in studio consoles. Ⅲ. Detailed Guide: Common Types & ApplicationsSelecting the right potentiometer requires understanding the trade-offs between mechanical endurance, noise levels, and precision.3.1 Breakdown of Key Potentiometer VarietiesRheostat (Variable Resistor)A rheostat functions strictly as a two-terminal variable resistor used to control current. While historically significant, in 2026, the term "rheostat" is considered legacy. Modern efficiency standards prefer PWM (Pulse Width Modulation) for current control over resistive rheostats, which dissipate excess energy as heat. However, they remain useful in simple educational circuits or rugged high-power calibration scenarios. Rotary PotentiometersThe rotary potentiometer is the most ubiquitous interface component. It features a resistive track (Carbon or Cermet) and a wiper connected to a rotating shaft. Key Specs: Typical tolerance is 10%-20%. Rotation is usually 270°. 2026 Applications: Smart home dial interfaces, car audio amplifiers, light dimmers (triac control), and older telecommunication equipment. Logarithmic (Audio) PotentiometersAlso known as "Audio Taper" pots. The resistance changes logarithmically to mirror the Weber-Fechner law of human hearing. At the start of rotation, resistance changes slowly; as the angle increases, the resistance shifts rapidly. This ensures that volume adjustments sound natural and linear to the human ear. Linear Potentiometers (Faders)In linear potentiometers (slide pots), the resistance varies directly with the physical distance the slider moves. They are indispensable in audio mixing consoles (EQ, Faders) because they provide immediate visual feedback of the setting. They are also used for precise voltage division in test equipment and battery internal resistance measurements. Digital Potentiometers (DigiPots)The 2026 Standard: Digital potentiometers are CMOS integrated circuits that replace mechanical wipers with digital switching networks. Controlled via protocols like I2C or SPI, they offer vibration immunity, zero mechanical wear, and programmable precision. Use Cases: Programmable gain amplifiers (PGAs), sensor calibration, LCD contrast control, and smart IoT devices where physical knobs are unnecessary. Membrane (Soft) PotentiometersConstructed by printing carbon/graphite onto a flexible substrate. When pressed, the top layer contacts the bottom resistive track. These are ultra-low profile and widely used in modern wearables, medical devices, and robotics tactile sensors. While they have lower accuracy than wirewound types, their form factor is unmatched for compact designs. Wirewound PotentiometersKnown for high power handling and thermal stability. A resistance wire is wrapped around an insulating core. Pros: High precision, high temperature tolerance. Cons: "Zipper noise" (resolution steps) and inductance issues at high frequencies. 3.2 Critical Application: Potentiometric Position SensorsPotentiometers remain one of the most cost-effective methods for measuring displacement in industrial machinery.3.2.1 Resistive Position SensorsThese function as passive transducers. By applying a stable reference voltage across the fixed terminals, the voltage at the wiper becomes directly proportional to the shaft's angle or linear position. They are widely used in joystick controls, valve positioning, and robotics joint feedback.Figure 2. Simple Position Sensor Circuit 3.2.2 Capacitive Position Sensors (Non-Contact Alternative)For environments where mechanical wear is a concern, capacitive sensors are the superior 2026 choice. They measure position by detecting changes in capacitance between a sensor plate and the target object. These are dominant in high-precision semiconductor manufacturing, touchscreens, and fluid level sensing where physical contact is impossible.Ⅳ. Best Practices: How to Use Potentiometers SafelyTo ensure longevity and signal integrity in your circuits, adhere to these maintenance and installation guidelines:Chemical Safety: Avoid exposing polycarbonate synthetic resin pots to ammonia, ketones, or high-acid chemicals, which degrade the housing.Flux Management: Do not use water-compatible fluxes. Poor quality soldering can lead to oxidation, causing "scratchy" audio or open circuits.Soldering Temp: Prevent heat damage. Pin terminals: 235°C ±5°C. Wire terminals: 350°C ±10°C. Keep solder >1.5mm from the body.Internal Contamination: Ensure no flux enters the internal mechanism during soldering to prevent contact noise.Voltage vs. Current: Potentiometers are designed for Voltage Division. Avoid using them as Rheostats (current limiters) for high loads, as the wiper contact resistance cannot handle high current.Moisture Control: Prevent condensation. In humid environments (like marine audio), use sealed or IP67-rated potentiometers.Mechanical Installation: Do not overtighten mounting nuts. For slide pots, ensure screw length doesn't penetrate the casing and jam the fader.Knob Assembly: Support the back of the shaft when pushing on knobs to prevent internal wiper collapse.Temperature Effects: Lubricant viscosity changes with temperature. For sub-zero applications, specify low-temperature grease to maintain smooth rotation.Signal Interference: Keep shafts/sliders short to minimize acting as an antenna for EMI (Electromagnetic Interference).Thermal Derating: Carbon film power ratings drop above 70°C. Ensure adequate ventilation. Ⅴ. Advanced FAQs and Industry Trends5.1 What are the primary potentiometer types?The industry divides them into Linear (Slide) and Rotary types. A third category, Digital Potentiometers (DigiPots), is now standard for automated systems, while Membrane "Soft Pots" serve ultra-thin form factors.5.2 Quick-Fire FAQsFrequently Asked Questions (2026 Updated)1. What is the main function of a potentiometer?It acts as a variable voltage divider to measure EMF, control volume/gain, or sense position. In 2026, it is also a key component in Human-Machine Interfaces (HMI). 2. What are the top 3 modern applications?User Interface: Volume and tone control in audio gear.Robotics: Position feedback servos (servo motors).Industrial: Linear displacement sensors in injection molding machines. 3. How does a potentiometer differ from a rotary encoder?Potentiometers are absolute position sensors (they know where they are on power-up) but have limited rotation. Encoders are digital, have infinite rotation, but typically require homing (calibration) on startup. 4. Why do audio pots use a logarithmic taper?Because human hearing is non-linear. A logarithmic taper matches the ear's sensitivity, creating a smooth perceived increase in volume.{ "@context": "https://schema.org", "@type": "TechArticle", "headline": "Potentiometer Ultimate Guide 2026: Types, Applications, and Selection", "datePublished": "2020-06-08", "dateModified": "2026-01-09", "description": "A comprehensive engineering guide to Potentiometers in 2026. Covers Linear vs Rotary, Digital Potentiometers, wiring diagrams, and maintenance tips for modern electronics.", "articleBody": "A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider...", "mainEntity": { "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "What are the main types of potentiometers?", "acceptedAnswer": { "@type": "Answer", "text": "The three main categories are Rotary Potentiometers (knobs), Linear Potentiometers (sliders/faders), and Digital Potentiometers (DigiPots) which use ICs for control." } }, { "@type": "Question", "name": "What is the difference between a potentiometer and a rheostat?", "acceptedAnswer": { "@type": "Answer", "text": "A potentiometer uses three terminals to divide voltage. A rheostat uses only two terminals (one fixed, one wiper) to restrict current. Modern circuits prefer potentiometers or PWM over rheostats for efficiency." } }, { "@type": "Question", "name": "Where are potentiometers used in 2026?", "acceptedAnswer": { "@type": "Answer", "text": "They are used in audio volume controls, robotic arm position sensing, industrial joystick controls, and as digital calibration trimmers in IoT devices." } } ] }}

I IntroductionSolid-state relays(SSRs) have unparalleled advantages over other relays, because it can make and break the circuit without contact or spark. In addition, with the progress of technology, the maturity of manufacturing and the decline in price, solid-state relay has become widely used day by day. At the same time, its position in the global market is becoming more and more important.This article will introduce what is a solid-state relays, its structure and working principle, solid-state relay wiring, advantages and disadvantages, and the difference between soild-state relay and conventional relay.Figure 1. Solid-State RelayCatalogI IntroductionII What is a Solid-State Relay(SSR)?III Structure and Working Principle of SSRs  3.1. Structure  3.2. Working PrincipleIV SSR Wiring, Advantages and Disadvantages of SSR  4.1. SSR Wiring  4.2. Advantages of SSRs  4.3. Disadvantages of SSRsV Difference between SSRs and Conventional Relays  5.1. A Brief Introduction to Conventional Relays  5.2. Differences between SSRs and Conventional Relays  5.3. Reasons for Choosing SSRsVI Questions Related to SSR WorkII What is a Solid-State Relay(SSR)?Solid-state relay (hereinafter abbreviated as "SSR") is a new type of contactless switching device composed entirely of solid-state electronic components, which makes use of the switching characteristics of electronic components (such as switching transistor, bi-directional thyristor and other semiconductor devices) to achieve the purpose of turning on and off the circuit without physical contact and spark, so it is also called "contactless switch". SSR is a four-terminal active device, in which two terminals are input terminals and the others are output terminals. It not only has the function of amplifying and driving, but also has the function of isolation, so it is very suitable for driving high-power switching actuators. Compared with electromagnetic relays, SSRs are more reliable, have a longer life, faster speed, and has less interference with the outside world, so this is the reason why it has been largely used.What is a Solid-state Relay?III Structure and Working Principle of SSRs3.1. StructureA solid-state relay consists of three parts: input circuit, isolation (coupling) and output circuit.3.1.1 Input circuitAccording to the type of input circuit, the input circuit can be divided into the DC input circuit, AC input circuit and AC/DC input circuit. Some input circuits also support TTL/CMOS and have the function of positive and negative logic control and inversion, which makes it easier to connect with TTL/CMOS circuits. For control signals with a fixed control voltage, a resistive input circuit is used. The control current is guaranteed to be greater than 5mA, for the large range of changes in the control signal (such as 3~32V) is the use of a constant-current circuit, to ensure that the current in the whole range of voltage changes in more than 5mA reliable operation. 3.1.2 Isolation (coupling)For solid-state relays, there are two ways to isolate and couple the input and output circuits: photoelectric coupling and transformer coupling. Photoelectric coupling usually uses photodiode-phototransistor, photodiode-bi-directional light-controlled thyristor, photovoltaic cell to realize the isolation control between the control side and the load side; High-frequency transformer coupling is the use of input control signals generated by a self-excited high-frequency signal coupled to the secondary, after detection and rectification, logic circuit processing to form a driving signal. 3.1.3 Output circuitThe power switch of SSR is directly connected to the power supply and the load terminal to realize the on-off switching of the load power supply. The main use of high-power crystal transistor (switch-Transistor), unidirectional thyristor (Thyristor or SCR), bidirectional thyristor (Triac), power field-effect transistor (MOSFET), insulated gate bipolar transistor (IGBT). The output circuit of the solid-state relay can also be divided into DC output circuit, AC output circuit and AC / DC output circuit. According to the type of load, it can be divided into DC solid-state relay and AC solid-state relay. For DC output, bipolar devices or power field-effect transistors can be used. For AC output, two thyristors or one triac are usually used. AC solid-state relay can be separated into single-phase AC solid-state relay and three-phase AC solid-state relay. Ac solid-state relays can be divided into random AC solid-state relays and zero-crossing AC solid-state relays according to the timing of turn-on and turn-off. 3.2. Working PrincipleSSR can be divided into two types of AC type and DC type according to the occasion of use. They are used as load switches on AC or DC power sources, and cannot be mixed. The following uses the AC SSR as an example to illustrate its working principle. Figure 1 is a block diagram of its working principle. The components ①～④ in Figure 1 form the main body of the AC SSR. From an overall perspective, the SSR has only two input terminals (A and B) and two output terminals (C and D).Figure 2. Working Principle of SSRWhen working, as long as a certain control signal is added to A and B, you can control the "on" and "off" between C and D, so as to realize the function of "switch". The function of the coupling circuit is to provide a channel between the input and output terminals for the control signals input at the A and B terminals, but electrically disconnect the (electrical) connection between the input terminal and the output terminal in the SSR, to prevent the output from affecting the input. The element used in the coupling circuit is the "optical coupler", which has high sensitivity, high return speed, and high tolerance between the input and output terminals. Because the input terminal is a light-emitting diode, it makes it easy for the input end of the SSR to match the input signal level. When in use, it can be directly connected with the computer output interface, that is, it is controlled by "1" and "0". The function of generating power lines is to generate signals that meet the requirements and to turn on the work of circuit 4, but because the off-line lines do not add special control circuits, they produce dry radiation and use pollution generators such as high-order waves or spikes, so a "zero-crossing control circuit" is specially built for this purpose. The "zero-crossing" means that when the control signal is added and the AC is over zero, the SSR is in the state; and after opening the control signal, the SSR has to wait for the junction point (zero potential) between the positive half-cycle and the half-cycle of the AC cycle before the SSR becomes stable. This design can prevent the interference of high-frequency waves and the pollution of electricity.The absorption circuit is designed to prevent the spike and surge (surge) from the power source from turning on the switching devices to the switching and operation of the controllable silicon tube (or even the operation). It is generally used to use an "R-C" series absorption circuit or non-destructive resistor (thermistor resistor).  IV SSR Wiring, Advantages and Disadvantages of SSR4.1. SSR WiringWhen the output of the relay is electrified to the coil and the output voltage is wired according to the load voltage, the contacts will close and the lamp will light up after energizing, as shown in the figure below.Figure 3. SSR WiringLooking at the physical wiring diagram for the solid-state relay below, you can see that the equipment has parameters 1, 2, 3, 4, and 60A. 60A of which represents the indicator light in action (there are two states, on and off). The INPUT word in the middle of 3 and 4 indicates the incoming terminals, and the OUTPUT word in the center of 1 and 2 indicates the outgoing terminals. Therefore, be careful not to mistakenly connect when wiring. 3 and 4 are used as trigger signals to control the on and off actions of 1 and 2. The SSR in this physical wiring diagram cannot be regulated, and some can adjust the DC voltage to adjust the voltage of the 3 and 4 outlets.Figure 4. SSR Physical Wiring DiagramThe physical wiring diagram of the solid-state relay used for electromechanical equipment is as follows, but it is generally widely used in the chemical industry, coal mine, and other fields, and requires explosion-proof and corrosion resistance.Figure 5. SSR Wiring DiagramSSRs are non-contact switching devices with relay characteristics that use semiconductor devices as switching devices instead of conventional electrical contacts. The single-phase SSR is a 4-terminal active device that includes two input terminals and two output terminals. Opto-isolated, after connecting the input terminal to a specific current value with a DC or pulse signal, you can change the output terminal from the off state to the on state.Figure 6. SSR Physical Wiring Diagram4.2. Advantages of SSRsLong Life Expectancy and High ReliabilityThe solid-state relay has no mechanical parts, solid-state device completes the contact function. It has no moving parts and can operate in high shock and vibration environments. The components of solid-state relays, due to their unique characteristics, determine the longevity and high reliability of solid-state relays. High Sensitivity, Low Control Power, Good Electromagnetic Compatibilitysolid-state relay has a wide input voltage range, low driving power, compatible with most logic integrated circuits without additional buffers or drivers. Fast TransitionsSolid-state relays use solid-state devices, which allow switching speed from milliseconds to microseconds. Low electromagnetic Interferencesolid-state relays have no input "coil" and no ignition arc and rebound, which reduces electromagnetic interference. Most AC output solid-state relays are zero voltage switches that turn on at zero voltage and turn off at zero current, reducing sudden interruptions in the current waveform and thereby reducing switching transient effects.4.3. Disadvantages of SSRsAfter conduction, The tube voltage drop is large, the forward voltage drop of SCR or bi-directional silicon can reach 1-2V, and the saturation voltage drop of the high power transistor is also between 1-2V. The on-resistance is higher than the contact resistance of mechanical contacts.Even after the semiconductor device is turned off, there can still be several microamperes to several milliamperes of leakage current, so ideal electrical insulation cannot be achieved.Because of the large tube voltage drop, large power consumption and calorific value after conduction, the volume of a high-power solid-state relay is much larger than that of the electromagnetic relay with the same capacity, and the cost is also high.The temperature characteristics of electronic components and the interference resistance of electronic circuits are poor, and the radiation resistance is also poor. If no effective measures are taken, the operating reliability will be low.Solid-state relays are more sensitive to overload and must be protected from overload by fast fuses or RC damping circuits. A load of a solid-state relay is clearly related to ambient temperature: as the temperature increases, the load capacity will decrease rapidly.The main drawbacks are the presence of voltage drops in the on-state(need corresponding heat dissipation measures), leakage currents in the off-state, AC and DC not universally usable, a small number of contact groups. In addition, indicators such as overcurrent, overvoltage, voltage rise rate, and current rise rate are poor.Figure 7. SSRV Difference between SSRs and Conventional Relays5.1. A Brief Introduction to Conventional RelaysIt generally consists of a relay coil and dynamic and static contacts. The movable contact acts through the electromagnetic attraction of the relay coil, thus realizing the connection and disconnection of the circuit. That means, there is a mechanical movement. When the current reaches a certain level, the contacts will spark. Its low price and simple structure can be attractive, but sparks and mechanical movements during the operation have a certain impact on its life span. The advantages of traditional relays are simple to drive, good insulation, and good resistance to short-term overload.The disadvantages of conventional relays are large size (cumbersome),  slow response (max ms level), and high power consumption to drive them.5.2. Differences between SSRs and Conventional RelaysAll-solid-state relays use electronic components, so they have many advantages compared with traditional relays, but they also have some limitations to some extent. The following table shows the advantages and disadvantages of solid-state relays and traditional relays.Conventional RelaysAdvantagesDisadvantages*Low residual output voltage*No heat sink required*Cheap*Can provide multiple sets of contacts and normally open normally closed contacts*No leakage current*AC and DC compatible*Compact size*Maximum switching frequency is limited (5-10Hz)*Noise*Electromagnetic interference*Limited contact life*The switching action cannot be fully synchronized*Contact bounce*Poor operating performance of high current, resulting in arc.*Interface is required to connect with digital circuit*High control power, usually higher than 200mWSSRsAdvantagesDisadvantages*Low control power, usually 10-50mW*Synchronous switch*low electromagnetic interference in synchronous switch mode *longer life, 50-100 times than that of traditional relays.*Fast response time*No mechanical moving parts*No mechanical strain*Compatible with digital circuits*Anti-vibration, anti-impact*Anti-corrosion and moisture-proof*No noise*There is residual output voltage 1-1.6V*The output can only be AC or DC, not compatible*Usually requires heat sink*Not suitable for small output signals*There is leakage current*Only single contactThese two tables show that in conventional switching applications, solid-state relays have no significant disadvantages over traditional relays. By comparison, we have to understand some of the limitations of solid-state relay applications, which will affect our final choice of the type of relay. Finally, we have to accept the idea that no relay can be used in all applications. The application of relays depends largely on the mechanical and electrical environment, so it is impossible to define a set of accurate selection parameters to guide users to make the best choice of relays. Therefore, the final choice of relays can only be made according to each specific application.Difference between Solid-state RELAY AND MAGNETIC RELAY5.3. Reasons for Choosing SSRs5.3.1. Life Expectancy of RelayWhen used correctly, the most important features of solid-state relays are long life expectancy and high reliability. In practical application, the contacts of solid-state relays can be used permanently, while the contacts of traditional relays will be affected by strain, corrosion, bonding and so on. Traditional relays will fail due to the damage of moving parts (springs, electromagnets). The life of solid-state relays is usually 50-100 times longer than that of traditional relays.5.3.2. Cheap PriceThe price is an important factor to consider in the selection of relays. Under the same technical requirements, the initial purchase cost of traditional relays is usually lower than that of solid-state relays. However, this does not take into account the service life of traditional relays and the costs incurred in the future due to monitoring, maintenance and replacement of traditional relays.5.3.3. Power ControlThe sensitivity of traditional relays to control signals is only one-twentieth of that of solid-state relays, that is, in the case of obtaining the same output power, the power required by traditional relays is usually 10-20 times that of solid-state relays. The power required for solid-state relay control is only 200-500mW, and the low power consumption can be directly compatible with digital circuit systems.5.3.4. Environmental ResistanceEnvironmental resistance is a very complex concept, but solid-state relays always have an advantage in this respect. A solid-state relay has good mechanical properties because it has no moving parts. The resin packaging shell of the solid-state relay makes it have good shock resistance, impact resistance and corrosion resistance. In addition, humidity has almost no effect on solid-state relays, only slightly reducing their insulation performance. However, traditional relays are very sensitive to humidity, and long-term high humidity will cause corrosion of traditional relays.5.3.5. Switching RateSwitching speed is also usually an important factor in choosing solid-state relays or traditional relays. Controlling the response rate is very important, even crucial, in some process control cargo machinery automation applications. In some applications where the special power factor is very low, the traditional relay can not be used. In addition, in some situations where the switch is stable and no jumping is allowed, traditional relays cannot be used.5.3.6. Electromagnetic RadiationSolid-state relays can switch the load when the circuit voltage crosses zero, thus limiting the transient phenomenon to a considerable extent and avoiding current surge and electromagnetic radiation. In some situations where the power factor is very low, the switch must be stable and the vibration is not allowed, so solid-state relays must be selected.VI Questions Related to SSR Work1. How do solid state relays work?A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. ... Packaged solid-state relays use power semiconductor devices such as thyristors and transistors, to switch currents up to around a hundred amperes. 2. What is the difference between a relay and a solid state relay?The main difference between solid state relays and general relays is that there is no movable contacts in solid state relay (SSR). In general, solid state relays are quite similar to the mechanical relays that have movable contacts. ... SSR provide high-speed, high-frequency switching operations. 3. Where are solid state relays used?The most common application of solid state relays is in the switching of an AC load, whether that is to control the AC power for ON/OFF switching, light dimming, motor speed control or other such applications where power control is needed, these AC loads can be easily controlled with a low current DC voltage. 4. How does solid state relay works in temperature controller?The SSRL Series solid state relays are used to control large resistance heaters in conjunction with temperature controllers. ... By applying a control signal, an SSR switches “on” the AC load current, just as the moving contacts do on a mechanical contactor. Three-phase loads can be controlled using 2 or 3 SSR's. 5. How do you make a solid-state relay circuit?DIY Solid State RelayStep 1: THINGS WE NEED.Step 2: OPTOCOUPLER.Step 3: Add positive pin of led to pin 1 of optocoupler.Step 4: Add 220 Ohm resistor to -ve pin of LED.Step 5: Add jumper to pin 2 of optocoupler which will go to +ve power supply.Step 6: Join source of triac to 4th pin of optocoupler. 6. What is a solid-state relay used for?AC output solid-state relays are used to control the flow of electrical energy in alternating current power systems. The control (equivalent to electro-mechanical relay coil) voltages can be either AC or DC. 7. How fast is a solid-state relay?The SSR output is activated immediately after applying the control voltage. Consequently, this relay can turn on anywhere along the AC sinusoidal voltage curve. Response times can typically be as low as 1 ms. The SSR is particularly suitable in applications where a fast response time is desired, such as solenoids or coils. 8. How do I know if my solid state relay is bad?Solid-state relays should be checked with an ohmmeter across the normally open (N.O.) terminals when control power is off. The relays should be open, switched to OL, and closed (0.2, the internal resistance of the ohmmeter) when control power is applied. 9. What causes solid-state relay failure?If an ambient temperature exceeds the rated value, the SSR output elements may be damaged. ... If the SSR is used with loose screws of its output terminals or imperfect solder, abnormal heat generation while current flowing causes the SSR to burn out. Perform the proper wiring and soldering. 10. Do solid-state relays leak voltage?Solid-State relays have leakage. If you want to repeatedly switch something on / off, use them. But when you want the SSR to be fully off, say after pressing an off switch, a mechanical relay should be across the load to take it off the SSR. If you aren't switching repeatedly, then use a mechanical relay.  

I Introduction What is a blown fuse? A fuse is an electrical safety device that operates to provide over-current protection of an electrical circuit. Its essential component is a galvanized wire with antimony tin alloy that blows (melts) when too much current flows through it, thereby interrupting the current. This process of self-protection is called a blown fuse. This passage introduces blown fuse in the household circuit and car, mainly including its working principle, causes of a blown fuse, and how to repair it.  CatalogI IntroductionII Blown Fuse in Household Circuit2.1 Blown fuse in Household Circuit2.2 Slow Blow Fuse VS. Fast Blow FuseIII Help3.1 Six Reasons Why Fuses Blow3.2 Causes of Circuit fault From the Perspective of Blown fuse3.3 How to Change Household Fuse?IV Blown Fuse In Car4.1 What Is A Blown Fuse In Car?4.2 Why does a Car Fuse Blow?4.3 What are the Symptoms of a Blown Car Fuse?4.4 How to Replace a Fuse in a CarV One Question Related to Resistor Classification5.1 Question5.2 AnswerVI FAQII Blown Fuse in Household Circuit2.1 Blown Fuse in Household CircuitWith the increase of fire and electricity consumption in modern households, the frequency of household fires is getting higher and higher, which has become a focus of public attention. And a considerable part of the fire is caused by the blown fuse. In the state of high voltage and large current, the fluctuation and surge of the grid voltage will cause the current in the power supply to increase instantaneously and cause the fuse to blow.Figure 1. Fuse Figure 2. Blown Fuse2.2 Slow Blow Fuse VS. Fast Blow FuseLiterally, the difference between a slow blow fuse and a fast blow fuse is reaction speed. One reaction is faster and one reaction is slower. From a technical perspective, slow blow fuses are more resistant to surges than fast-blow fuses. In other words, slow-blow fuses have a higher ability to withstand instantaneous pulse currents than fast-blow wires. They can resist the surge of the surge current brought by the switch and do not act, thus ensuring the normal operation of the device. Therefore, a slow blow fuse is also called a surge resistance fuse. To explain from a deeper level, a slow blow fuse has a relatively large melting heat value I2t, and the energy required when the fuse blows are relatively large. So for fuses of the same rated current, slow blow fuses are much more pulse-resistant than fast-blow fuses. Because the I2t of the slow blow fuse is larger than that of the fast blow fuse of the same specification when the total current of the circuit is too large and the fuse blows, the slow blow will be slower than the fast blow fuse. But it doesn’t mean slow blow fuse is worse than fast blow fuse. Because if there is a fault in the circuit, the over-current will not disappear automatically, and the energy of the continuous over-current will greatly exceed the I2t of the fuse. No matter what kind of fuse, it still will be blown. Therefore, the time difference between fast-blow fuses and slow blow fuses is not so important for the device requirements they protect. Only in the case of sensitive devices in the protected circuit that require special protection, the performance of the slow blow fuse will be affected. Therefore，two kinds of blow fuse will be used in different circuits. For example, a purely resistive circuit, or a circuit that needs to protect some more sensitive and expensive components, the fast-blow fuse is necessary. On the contrary, it is best to use slow blow fuses in the power input/output parts of some circuits. In addition to protecting IC circuits, slow blow fuses can be used in most cases where fast-blow fuses are used to improve pulse resistance; on the contrary, if fast-blow fuses are used in places where slow blow fuses are used, it will often cause the power to be turned off immediately. The fuse cannot work normally.III Help3.1 Six Reasons Why Fuses Blow(1) Overloaded CircuitToo many appliances are operating at the same time,  resulting in household electricity overload, so that the fuse is blown. This situation is especially likely to occur when using air conditioners, electric heaters or adding other larger power appliances. Solution: Cut back on all that power that’s tapping a single circuit. Find outlets on other circuits to plug into or unplug what you aren’t using. (2) Connection ProblemsIn some families, although the fuse selection is reasonable and the load is not too large, it still occurs blown fuse as soon as it uses larger power appliances such as air conditioners, electric heaters, and rice cookers. The reason may be that when the fuse is installed or replaced, the fuse and the screw are in poor contact, causing fire and heating, and the screws fixing the fuse are oxidized and burned out. (3) Short CircuitIf the fuse is replaced but still blows as soon as you closed the switch, there may be a short circuit. May be short circuit or load short-circuit. Electric kettles, rice cookers and other commonly used high-power electrical appliances and inferior electrical appliances are prone to short-circuit failure. Solution: Make sure the faulty device is unplugged and there is no damage to the outlet. First, test the circuit. Then check for any damage on or around the electrical panel. If you see any damage, call an electrician before doing anything else with it. If there is none, flip the breaker switch back to its operating position. If it trips again, though, call an electrician. (4) Ground FaultA particular type of short circuit, a "ground-fault," occurs if a hot wire comes in contact with a ground wire or a metal wall box or touches wood framing members. Ground faults can be especially dangerous when they occur in areas with high levels of moisture, such as kitchens or bathrooms, or outdoor locations. (5) PulseWhen the circuit is started or the power supply is unstable, an instantaneous large current causes the fuse to be disconnected. Also, the screw is not tightened when the fuse is installed, or the fuse is damaged, all will cause the fuse blown. (6) The Wrong Type of Fuse Was InstalledFuses come in many shapes, sizes, and configurations. Many look similar but actually have very different functions. A suitable fuse must be selected and used. The fusing current of the fuse is usually 1.5-2.0 times the rated current. For example, when the total power of all appliances in the household is more than 1100W, choose a 5 amp fuse, and use a number 20 fuse with a diameter of 0.98MM. When the current exceeds 7.5 amps to 10 amps, the fuse will automatically blow to achieve the purpose of protection. 3.2 Causes of Circuit fault From the Perspective of Blown FuseThe blown fuse is one of the common faults of the circuit. If not handled properly, it will cause a new fault. Therefore, we should learn to judge the cause of circuit failure from the fuse blown condition. (1)Situation: The fuse blows fast, often with a "snap" sound. Most fuses are turned into small round beads. The gap between the fuses is large, and sometimes the entire fuse is almost completely melted.Analysis: This situation indicates that the blown fuse is caused by a short circuit in the line. Therefore, the circuit can work normally only after finding the short-circuit place, eliminating the short-circuit fault, and then connecting the fuse. (2)Situation: The fuse is disconnected from the middle, the gap between the disconnection is small, and a very thin residual part is left at both ends.Analysis: This is caused by the fuse being too thin, or the passing current is too large. It should be installed with a slightly thicker fuse. (3)Situation: The connection between fuse and screw joints is blown out, and the disconnection gap is small.Analysis: This is because the fastening screw that fixes the fuse is not tightened, the fuse is not in good contact with the screw, the resistance is too large when the current passes, and the generated heat is also increased. Heat doesn't dissipate as fast as it produces, so the fuse blows. In this case, connect the fuse directly and tighten the fastening screws. (4)Situation: The fuse blowing position is uncertain and the gap is small.Analysis: This situation is generally caused by the longitudinal tension of the fuse or a gap in the radial position, and the local diameter of the fuse becomes thinner. Just replace it with a new one. 3.3 How to Change Household Fuse?• Guide(1) The fuse of Knife Switch① Find the main knife switch of the circuit and check the power supply to make sure that the fuse in the knife switch has blown.② Put on rubber shoes and rubber gloves. Bring the prepared pliers and spare fuse in advance.③ After power off, disassemble the upper and lower body shells of the knife switch and use the pliers to remove the old fuse that has melted. Be careful not to touch the electrical circuit.④ Replace with a new fuse and then check if there is a short circuit. If not, you can install the case and close it. (2) Household fuse① Open the fuse box with a screwdriver and then check to see if the fuse is blown.② Install a fuse of similar length to the previous fuse. After installation, check whether it is done. How to Replace Fuse Wire of a Blown Fuse• AttentionWhen replacing the fuse, please pay attention: remove the cover of the insulation box, and do not directly connect the fuse in the insulation box. When installing the fuse, the two ends of the fuse should be wound clockwise around the screw. It should not be over-tightened or loosened, so as not to damage the fuse or cause poor contact, otherwise, it will be easy to blow.IV Blown Fuse In Car4.1 What Is A Blown Fuse In Car?Just like many other appliances, the purpose of fuses in cars is to protect the electrical system within. The basic components of a fuse are one fuse element and two terminals. And the fuse itself works as a bridge between the vehicle and the particular system. If the system is overheated, the fuse element will heat up and melt, and, eventually, the circuit will stop working.4.2 Why does a Car Fuse Blow?The circuit of a general vehicle has gone through a very detailed experiment before leaving the factory, even at the beginning of the design. It should be fine, but why does the vehicle fuse still break? Normally, there are two main reasons for the breakdown of a car fuse, one is that the load of the electrical appliance is too large, and the other is the short circuit in the car circuit. The electrical appliance is overloaded. This is also the reason why most fuses blow.  After buying a car, some people randomly modify electrical equipment, changing headlights, car audio equipment, using inferior cigarette lighters, adapters, and high-power electrical appliances. The operating power of some electrical equipment may be higher than the factory-set value. Once used, it may cause the fuse of the lamp to blow and short circuit in the automobile circuit. After the car has been used for a long time, some of the wire rubber has deteriorated, which has exposed the metal wire and caused a short circuit. Some car owners will directly flush the engine compartment with water pipes, which may cause water in the fuse box and short circuit.4.3 What are the Symptoms of a Blown Car Fuse?After the car fuse is blown, the most intuitive performance is that the car's related functions are disabled and the car cannot drive normally. The battery has power but the vehicle cannot start. In this case, it is likely that the fuse responsible for starting the motor has burned out. When you find that the vehicle can not start, remember not to start continuously, which will cause the battery to discharge continuously and lead to complete power failure. While the vehicle is running, the tachometer display is normal, but the speedometer display is zero, and the ABS warning light is on, indicating that the fuse corresponding to the ABS is burned out.4.4 How to Replace a Fuse in a Car(1) Turn off the start switch and all electrical equipment(2) Disconnect the negative battery cable(3) Figure out the blown fuse.(4) Using needle-nose pliers or some small tweezers, remove the broken fuse.(5) Replace the blown fuse with another new fuse of the same amperage.If the replacement fuse is damaged, you should immediately go to the relevant car service station for inspection. How To Safely Change A Blown FuseV One Question Related to Blown Fuse5.1 QuestionHow does blown fuse work?A. By having pieces of metal wire that melt when current gets too highB. By having an electromagnet that will break a circuit if the current gets too highC. By having pieces of metal wire that melt when current gets too lowD. By having an electromagnet that will break a circuit if the current gets too low5.2 AnswerA  VI FAQ1. How do I know if a fuse has blown?Remove the fuse from its holder. In some cases, you may need a small screwdriver to unscrew the fuse holder cap. Look at the fuse wire. If there is a visible gap in the wire or a dark or metallic smear inside the glass then the fuse is blown and needs to be replaced. 2. What happens when a fuse blows?When a fuse blows, a metal filament inside the fuse has burned through, meaning that you'll need to replace the fuse with a new one. 3. How long do fuses last?Fuses never need to be replaced unless they are tripped/activated by a failing component or any other even with the circuits of the car. They are encapsulated in plastic and are in a vacuum inside the piece. As long as the current limit isn't reached, that wire will not burn out. 4. Can a blown fuse stop a car from starting?A blown fuse in the starter circuit could be the cause of a no-start problem. Broken or corroded wiring-Damaged or dirty wires to the battery or to the starter solenoid (or wires that are loose) can prevent sufficient power from reaching the starter. 5. Can a blown fuse cause a fire?Absolutely, an outlet that has blown a fuse can start a fire. If the problem was whatever was plugged into it, and that item is no longer there, there should be no problem, but if the problem was within the outlet itself it should be repaired immediately. The time to a fire could be from 2 minutes to 2 years. 6. Do fuses go bad with age?Yes indeed, fuses do expire from old age. Yes, fuses can get tired, but it's equally possible you have an intermittent problem with the switch or perhaps the starting capacitor (assuming it's not 3ph). First, replace the fuse and see if you get lucky, then diagnose further if needed. 7. What causes a fuse to burn?First, and most commonly, when too many lights or plug-in appliances draw power from the circuit, it can overload the capacity of the fuse and cause the metal ribbon inside the fuse to melt through. ... A mis-wired lamp, for example, can cause a short circuit and blown fuse if it is plugged into an outlet. 8. How do you fix a blown fuse in a plug?Lift it with your screwdriver to dislodge it. Replace the fuse with a fuse of the same amperage (check the appliance's instructions if unsure) Reassemble the plug, plug it into the wall and switch the device back on. If a blown fuse was the problem, your appliance should now be working again as normal. 9. How does a fuse in a plug work?A plug fuse is a safety device that is connected to an electrical circuit to prevent excessive current flow during fault conditions. Upon overload, the wire fuse element heats up and melts, or blows with a resounding bang, interrupting and cutting off the current flow. 10. Can a fuse look good and still be bad?Due to the way fuses are engineered, the likelihood that a fuse would become faulty without blowing is pretty slim, but there are rare instances in which a fuse might appear completely fine, even though no current runs through it. 

I IntroductionThe ballast has become an important additional device in the gas discharge light source circuit. Because most gas discharge lamps are made using arc discharge characteristics and have negative characteristics (also known as negative resistance characteristics) where the voltage decreases with increasing current, it is impossible to establish a stable operating point. In order to stabilize the discharge and limit the working current of the lamp, ballast must be installed in the gas discharge light source circuit. This article will introduce what is a ballast, how does the ballast work, its function, type, fault symptoms, and how to replace a broken ballast.What is a ballast?CatalogI IntroductionII What is a Ballast?III Types of Ballasts 3.1 Inductive Ballast(Magnetic Ballast) 3.2 Electrical Ballast 3.3 Resistance Ballast 3.4 Magnetic Leakage Ballast 3.5 Capacitive Ballast 3.6 LC BallastIV The Function of the BallastV How to Replace a Ballast? 5.1 How to Tell If the Lamp is Broken or the Ballast is Broken 5.2 Symptoms of Ballast Failure 5.3 Replacement ProcedureVI Several Common Terms for Ballasts 6.1 Ballast Loss 6.2 Ballast Factor 6.3 Ballast Efficacy Factor 6.4 Crest Factor 6.5 Power FactorVII One Quiz about the BallastVIII FAQII What is a Ballast?The ballast is a device that acts as a current limiter and generates an instantaneous high voltage on the fluorescent lamp. It is made by wrapping the enameled wire around an iron core made of silicon steel. Such a coil with an iron core, when instantaneously turned on / off and powered on, will generate a high voltage by self-induction, which is added to the electrodes (filament) at both ends of the fluorescent tube. This action is carried out alternately. When the starter (bubble jump) is closed, the filament of the lamp tube conducts heat through the current limit of the ballast; when the starter is open, the ballast will generate a high voltage on the filament at both ends of the lamp tube. The filament emits electrons to hit the fluorescent powder of the tube wall, and the starter repeatedly turns on and off a few times, thereby turning on the lamp. When the lamp emits light normally, the internal resistance becomes smaller, and the starter will always maintain an open-circuit state so that the current will work stably through the lamp and the ballast to make the lamp emit light normally.Figure1. BallastIII Types of Ballasts3.1 Inductive Ballast(Magnetic Ballast)3.1.1 DefinitionInductive ballast, also known as magnetic ballast, is an iron core inductance coil. The nature of the inductance is that when the current in the coil changes, the magnetic flux will change in the coil, which will generate an induced electromotive force. Thus hindering the current changes. 3.1.2 How Does the Inductive Ballast Work?When the switch is a closed circuit of 220V, 50Hz AC power, the electric current flows through the ballast to the starter, lamp filament, the filament heating (at the beginning of the starter is broken, due to a greater than 180V AC voltage, the starter has jumped the gas inside the bubble glow discharge, jump bimetallic strip is heated inside the bubble expansion deformation, two electrodes together, forming pathways to the filament heating).Figure2. The Circuit of Inductive Ballast When the two electrodes of the starter are close together because there is no arc discharge, the bimetallic sheet cools, and the two electrodes are disconnected. Due to the inductance of the inductive ballast, when the two electrodes are disconnected, the current in the circuit suddenly disappears. Therefore, the ballast generates a high pulse voltage, which is superimposed with the power supply voltage and is added to both ends of the lamp tube to ionize the inert gas in the lamp tube and cause arc discharge. (High pulse voltage-time is about 1ms 600V ~ 1500V, the exact voltage value depends on the type of lamp). During the normal lighting process, the self-inductance of the ballast serves to stabilize the current in the circuit. 3.2 Electrical Ballast3.2.1 DefinitionAn electrical ballast is a type of ballast, which refers to an electronic device that uses electronic technology to drive an electric light source to produce the required lighting. The electronic components convert the energy of the power grid to meet the voltage and current required by the matching lamps with AC or DC power.Figure3. Electrical Ballast 3.2.2 How Does the Electrical Ballast Work?The power frequency power supply becomes a DC power supply after passing through a radio frequency interference (RFI) filter, full-wave rectification, and passive (or active) power factor corrector (PPFC or APFC). Through the DC / AC converter, output high-frequency AC power of 20K-100KHZ, which is added to the LC series resonant circuit connected to the lamp to heat the filament, and at the same time, a resonant high voltage is generated on the capacitor and added to both ends of the lamp tube. However, the "discharge" of the lamp tube is turned into the "on" state and then enters the light-emitting state. At this time, the high-frequency inductance plays a role in limiting the increase of the current and ensuring that the lamp tube obtains the lamp voltage and lamp current required for normal operation.Figure4. How an Electrical Ballast Work 3.3 Resistance BallastIt regulates the lamp current by the voltage on the resistor proportional to the current. In the gas discharge light source circuit equipped with DC power supply, the application of resistance ballast is relatively simple in design and processing, but the power consumption is large and the efficiency is low. Resistance gas ballasts are also used in gas discharge light source circuits equipped with AC power sources. For example, in self-ballasted high-pressure mercury lamp circuits, tungsten wires are used as resistance ballasts. But generally speaking, the application of a resistor ballast in an AC circuit will affect the lamp current waveform, and make the luminous efficiency of the lamp decrease and the stability deteriorate, but it can improve the circuit power factor. 3.4 Magnetic Leakage BallastThe magnetic leakage ballast is a type of LC leading peak type circuit. The fundamental wave is mutated by auto-coupling boost and local magnetic saturation, and then it resonates with the working capacitor to obtain a higher open-circuit voltage and longer-lasting lamp operating current. It is a kind of high power factor lighting circuit, the line power factor reaches 0.90 ~ 0.97, which has its unique advantages for point HID light sources.Figure5. Magnetic Leakage Ballast 3.5 Capacitive BallastA capacitor is used as a ballast in a gas discharge light source circuit equipped with an AC power source. It cannot limit the instantaneous current of the lamp, only the total amount of charge that passes through the circuit in each half-cycle. In the low-frequency AC circuit, the lamp current waveform will be seriously distorted, forming a high pulse peak current (effective value is not large), which has a very harmful effect on the lamp electrode, resulting in a significant reduction in lamp life. Therefore, capacitors are rarely used as ballasts in low-frequency AC circuits. In higher frequency (20 ～ 100kHz) AC circuits, capacitors can be used as ballasts to obtain satisfactory results. The power consumption is small, the current waveform distortion is small, the volume is small, the weight is light, and there is no noise.Figure6. Circuit for Ballast-Ignitor-Capacitor-Lamp 3.6 LC BallastThere are the following two forms. (1) Ballast composed of inductor and capacitor connected in series. The designed capacitive reactance is usually about twice the inductive reactance, and the total impedance is capacitive. It belongs to the advanced ballast, that is, the phase of the lamp current leads to the phase of the power supply voltage. Compared with resistive ballast and inductive ballast, the power consumption is small, especially with good steady current characteristics, and the short-circuit characteristics when the lamp is started are also good. Combined with the use of hysteretic inductance ballast, it can greatly improve the power factor of the circuit. However, the repetitive ionization voltage provided every half cycle when the power supply voltage is commutated is low, and the repetitive ignition ability is poor.Figure7. Circuit of the LC Ballast for T5 28W Lamps (2) Ballast composed of magnetic leakage transformer and capacitor. Generally, it belongs to the advanced ballast. If the parameters are selected properly, the circuit power factor can be better improved. Its main performance is the same as the previous form of LC ballast. If a special leakage magnetic transformer structure is selected in the design to form an LC leading peak ballast, in addition to the above advantages, it can also greatly improve repeated ignition ability. This is an ideal ballast. The disadvantage is that the design and processing are more complicated. To better understand this part, you can see more details about LC circuit. IV The Function of the BallastLimit the Starting Current of the Lamp to A Suitable RangeStarting current refers to the current through the lamp within 30 seconds after the lamp is powered on or during the lamp preheating process. In general (especially in the state of lowest temperature), the starting current is much larger than the operating current of the lamp, so each lamp has a maximum starting current. If the starting current is too large, the service life of the lamp will be shortened; if the current is too small, the lamp cannot be preheated to the normal starting state or the process from glow discharge to arc discharge cannot be completed. The starting current provided by the ballast should not only start the lamp in a short time but also not affect the normal service life of the lamp. The Open Circuit Voltage Provided Is Sufficient for the Lamp to Start SmoothlyWhen the open-circuit peak voltage of the ballast is used as the starting voltage of the lamp, it must be sufficient to ionize the gas in the gas discharge lamp, that is, to generate a peak current that causes a glow-to-arc transition discharge between the electrodes, so that the lamp can start to work. High-pressure mercury lamps and metal halide lamps are more difficult to start at low temperatures, and the open-circuit peak voltage provided by the ballast must be sufficiently high.Figure8. How Electronic Ballast Functions Prevent the Lamp Power from Changing GreatlyAlthough the lamp has a certain range of voltage values during the design and delivery of the lamp, the voltage value of the lamp changes during actual use and throughout its life. This requires the matching ballast to adjust it within a certain range so that the lamp power does not change significantly. The ideal ballast should be such that the lamp power of the newly used lamp and the lamp near the end of its life are not too different. Working Current of Automatic Control LampStable impedance within a certain voltage range is the basic condition that the impedance ballast can control the working current of the lamp. The ballast uses the time change rate of the voltage proportional to the current to adjust the working current of the lamp. When the open-circuit voltage in a certain period causes the lamp operating current to increase, the inductive effect of the ballast will limit the rate of current increase; when the current starts to decrease, the inductive effect will prevent the rate of current decrease. V How to Replace a Ballast?5.1 How to Tell If the Lamp is Broken or the Ballast is Broken(1) The fluorescent lamp requires a ballast to meet the voltage required for the fluorescent lamp to start and work. If the lamp is broken, you can try another lamp to trouble the lamp cover. If it can be lit, the lamp is broken, if it is still not lit, the ballast is broken; (2) Using a universal meter to measure the disconnection of the lamp filament can also be judged: use a universal meter to measure the two ends of the fluorescent lamp separately. If the resistance is not zero, it means that the fluorescent lamp is broken, and the light can be shorted According to the method, if the inspection of the two components of the lamp tube and the light emitter is good, the ballast is broken.Fluorescent Light Troubleshooting and Repair: starter, bulb or ballast?5.2 Symptoms of Ballast Failure(1) The shell of the lamp or ballast appears black.(2) Use an electric pen to detect the ballast. There is no electricity at the incoming and outgoing ends.(3) Use an electric pen to detect that there is electricity at the incoming end, but no electricity at the outgoing end, which means that there is a disconnection inside the ballast.(4) If the shell of the ballast is charged with an electric pen, it means that the ballast has a leakage problem.(5) Use an electric pen to detect that there is electricity at the incoming and outgoing ends. The housing is not live, but the light is off. The light is still off when the trigger is replaced.(6) Use a multimeter with a resistance level of 200 to measure the resistance of the coil. The resistance is infinite.Figure9. Flurescent Ballast Tear Down5.3 Replacement Procedure(1) Open the lampshade: There are three clips around the lampshade that can be rotated. Use a screwdriver to turn the clip away to remove the lamp cover. Place the removed clip in a fixed place so that it can be reinstalled.(2) Observe the position of each component(3) Check whether the old lamp is broken. In general, a section with a longer ring lamp will appear gray. In this case, we should know that it is caused by the sublimation of a substance inside the lamp tube under a high-temperature environment.(4) Remove the lamp: After the lamp cover is opened, remove it, you can see there is a wire slot on the ring lamp. A wire extending from the lamp holder in the center is inserted into this slot. This wire is plug-in type, just pull it out! The ring tube inside is clamped and fixed by three bent metal pieces. This piece of metal is elastic and breaks apart with a screwdriver. You can remove the ring lamp. Pay attention to the wiring inside.(5) Replace the ballast: the ballast of the ring lamp, the two white wires are 220V power cords, there is no difference between positive and negative. The four small round holes on the box are the sockets for the lamp feet. Remove the chassis, disconnect the power cord, put the ballast down, and replace it with a new one.(6) Ring lamp installation: The steps are exactly the reverse when they are removed. Fix the new lamp, and fix the lamp with the three bent metal cards. Use a screwdriver to tighten. Then connect the wires. Stick it with tape and fix it. At the same time, clamp the wire slot on the ring lamp.(7) Install the lampshade: put back the three clips that were just removed. Fix the lampshade. Turn on the power. You can test whether the light is on. If it's on, it's a good replacement lamp. Precautions:(1) Be sure to cut off the main power switch before operation;(2) When removing the lamp cover, small parts such as cards and screws must be placed. In order to reinstall it finally.Figure10. Ballast KitsVI Several Common Terms for Ballasts6.1 Ballast LossThis value represents that the energy consumed by the electronic ballast itself is converted into heat energy instead of light energy. This value can be calculated by subtracting the power consumed by all lamp tubes from the total output power. Generally speaking, the traditional 40W dual-lamp ballast consumes about 22W, while the electronic ballast consumes about 7W. 6.2 Ballast FactorThis value can show the relative effect of the light output of the electronic ballast. The value is the percentage obtained by dividing the measured light output of the electronic ballast by the light output of the standard ballast light. Generally speaking, the higher the value, the better the light output effect. For electronic ballasts, it must not be less than 0.9, but there are also electronic ballasts designed to emphasize high output values and its light output ratio can be up to 1.18 to 1.28. 6.3 Ballast Efficacy FactorThis value can be obtained by dividing the light output ratio (Ballast Factor) by the ballast input power value (Input Power). In the US market, sellers usually use this value to measure and compare the pros and cons of the efficiency of various electronic ballasts. The higher the value, the better the efficiency of the electronic ballast.Figure11. Impedance Ballast6.4 Crest FactorIt is also called wave height rate. This value has a direct and critical impact on the life of the lamp tube. Most lamp tube manufacturers recommend that this value is preferably less than 1.7. Excessively high values can easily cause blackening of the lamp tube and shorten the service life of the lamp tube. The definition of the crest factor refers to the peak current divided by the average current when an electronic ballast is used to light a fluorescent tube. 6.5 Power FactorThis value can represent the efficiency value of the electronic ballast to convert the external input voltage and current into available power. The higher the power factor value, the better the company that supplies the power system (referred to as the power company). In order to encourage consumers to use electronic ballasts with high power factors, foreign power companies have adopted a subsidy policy, but consumers generally think that the higher the PF value, the more power they save. This is a wrong concept because the amount of power saved is not related to the PF value.Figure12. Inductive BallastVII One Quiz about the BallastQuestion: Which of the following ballasts are required to have thermal protection?A. The ballast of a fluorescent luminaire installed indoors, including a replacement ballast for this type of luminaireB. A simple reactance ballast in a fluorescent luminaire with straight tubular lampsC. A ballast in a fluorescent exit luminaireD. A ballast in a fluorescent luminaire used for egress lighting and energized only during a failure of the normal supplyE. All of the aboveAnswer: A  VIII FAQ1. What does a ballast do?In a fluorescent lighting system, the ballast regulates the current to the lamps and provides sufficient voltage to start the lamps. Without a ballast to limit its current, a fluorescent lamp connected directly to a high voltage power source would rapidly and uncontrollably increase its current draw. 2. Do you need a ballast with LED lights?LED technology does not require a ballast to regulate the amount of energy flowing to the lights. LEDs require less energy and can be sensitive to excess energy. A ballast bypass is a common procedure to remove the ballast from the existing fixture. 3. What is ballast and why is it important?Ballast is defined as any solid or liquid that is brought on board a ship to increase stability. Ballasting is essential if a ship is carrying a heavy load in one hold and a lighter load in another, or when the ship is empty or facing rough seas. 4. What happens when a ballast goes bad?If the ballast is bad, then the needle won't move. If you're using a digital multimeter, often the digital readout will possibly list a ‘1’ when it doesn't find a measurable resistance. 5. Is ballast and choke the same?A choke is an inductor designed to have a high reactance to a particular frequency when used in a signal-carrying circuit. An electrical ballast (sometimes called control gear) is a device intended to limit the amount of current flowing in an electric circuit. 6. How long should a ballast last?According to the Certified Ballast Manufacturers Association, the average magnetic ballast lasts about 75,000 hours or 12 to 15 years with normal use. The optimum economic life of a fluorescent lighting system with magnetic ballasts is usually about 15 years. 7. How do I know if my ballast is T8 or T12?If no markings are available, the size in diameter of the tube is the easiest way to determine the type you have installed. T8 tubes are 1-inch in diameter and T12 tubes are 1 1/2 -inch. 8. How often does a ballast need to be replaced?A typical ballast will generally last about 20 years, but cold environments and bad bulbs can decrease this lifespan significantly. You can get a new ballast at a hardware store or home center and install it in about 10 minutes. 9. Can you bypass a ballast on a fluorescent light?If the existing fluorescent tube fixture you want to replace has a non-shunted tombstone, you can proceed with the ballast bypass procedure. Nonetheless, if the existing fixture offers a shunted tombstone, you should replace it with a non-shunted variant. 10. What's in a light ballast?They're usually rectangular black boxes with wires coming out of one or both ends. Lighting ballasts for fluorescent light bulbs and HID lamps made before 1980 may contain polychlorinated biphenyls (PCBs). When the manufacture of PCBs was banned, existing equipment containing PCBs was allowed to remain in use.