The Kynix Blog

IntroductionThe humidity sensor is a sensor that measures relative humidity, absolute humidity, or dew point. At present, such sensors are widely used, and are gradually developing in the direction of excellent environmental resistance, long life, and low price. This article will introduce the working principle, type, classification, application of humidity sensor, and an Arduino tutorial on real-time temperature and humidity monitor. In addition, some basic knowledge like the calculation of relative humidity and absolute humidity is also covered. Let’s waste no more time! The humidity sensor is a sensor that measures relative humidity, absolute humidity, or dew point. At present, such sensors are widely used, and are gradually developing in the direction of excellent environmental resistance, long life, and low price. This article will introduce the working principle, type, classification, application of humidity sensor, and an Arduino tutorial on real-time temperature and humidity monitor. In addition, some basic knowledge like the calculation of relative humidity and absolute humidity is also covered. If you are only interested about the Arduino guide, you can jump to that part from the category, and the video below is about the accuracy test of different temperature and humidity sensor for arduino, which can help you to choose the best one. Let’s waste no more time!Check this video to look for the best temperature and humidity sensor for ArduinoCatalogIntroductionCatalogI How do Humidity Sensors Work?1.1 Humidity1.2 Absolute Humidity and Relative Humidity1.3 How Humidity Sensors WorkII Classification and Common Types of Humidity Sensors2.1 Classification2.2 Comparison of Resistive and Capacitive Humidity Sensors2.3 Four Types of Humidity Sensors with More Applications2.4 Commonly Used Humidity Sensor ModelsIII Application of Humidity Sensors3.1 Typical Uses3.2 Application FieldsIV Arduino Entry Project: Real-time Temperature and Humidity Detector4.1 Hardware Preparation4.2 Software Preparation4.3 Circuit Connection4.4 Implementation CodeⅤ FAQI How do Humidity Sensors Work?1.1 HumidityBefore learning the humidity sensor, let's take a look at what humidity is. Humidity, a physical quantity indicating the degree of air dryness. At a certain temperature, the less water vapor contained in a certain volume of air, the drier the air; the more water vapor, the more humid the air. The degree of air humidity is called "humidity". In this sense, it is often expressed by physical quantities such as absolute humidity, relative humidity, comparative humidity, mixing ratio, saturation difference, and dew point; if it represents the percentage of the weight of water vapor in the wet steam to the total weight (volume) of the steam, it is called the humidity of the steam. The humidity that the human body feels comfortable with is: the relative humidity is lower than 70%. So the question comes again, what is the relative humidity?Figure1. What is Humidity?1.2 Absolute Humidity and Relative HumidityThe degree of dryness and humidity of the air, or the physical quantity that indicates how much water vapor it contains, is called humidity. The mass of water vapor contained in a unit volume of air is called absolute humidity. Because it is difficult to directly measure the density of water vapor, it is usually expressed by the pressure of water vapor. The absolute humidity of the air does not determine the speed of water vapor on the ground and the perception of humidity. People call the absolute humidity of air at a certain temperature and the percentage of saturated air pressure at the same temperature as relative humidity. To be more specific, absolute humidity refers to the mass of water vapor contained in a certain volume of air, and its unit is generally g/m3. The maximum absolute humidity is the highest humidity undersaturation. Absolute humidity is only meaningful together with temperature, because the amount of humidity that can be contained in the air varies with temperature, and the absolute humidity is also different at different temperatures, because the volume of air also changes with temperature. But the closer the absolute humidity is to the highest humidity, the smaller its change with temperature. The following is the formula for calculating absolute humidity:The symbols are:e-vapor pressure, the unit is Pascal (Pa)-The gas constant of water = 461.52J/(kg K) T-Temperature, the unit is Kelvin (K)m-the mass of water dissolved in the air, in kilograms (kg)V-The volume of air, in cubic meters (m). Relative humidity (RH)A hygrometer is recording relative humidity. The relative humidity is the ratio between absolute humidity and maximum humidity. Its value shows how high the saturation of water vapor is. Air with a relative humidity of 100% is saturated air. Air with a relative humidity of 50% contains water vapor that reaches half the saturation point of air at the same temperature. Water vapor in the air with a relative humidity of more than 100% generally condenses.  As the temperature increases, the air can contain more water, that is to say, the relative humidity will decrease when the temperature increases with the same amount of water vapor. Therefore, while providing relative humidity, temperature data must also be provided. The dew point can also be calculated from the relative humidity and temperature. The following is the formula for calculating relative humidity:The symbols are:ρw-absolute humidity, in grams/cubic meterρw,max-the highest humidity, the unit is g/m3e-water vapor pressure, the unit is PascalE-saturated vapor pressure, the unit is Pascals-specific humidity, the unit is g/kgS-the highest specific humidity, the unit is g/kg 1.3 How Humidity Sensors WorkGenerally, humidity sensors use the following four methods to detect humidity or condensation:(1) Measure the change in electrical impedance or capacitance caused by moisture absorption, separation, or condensation of moisture-sensitive materials.(2) Measure the difference in gas thermal conductivity due to changes in humidity.(3) Measure the change in the resonance frequency of the crystal vibrator due to changes in humidity or condensation.(4) Measure the attenuation and light absorption and reflection caused by alpha rays passing through water droplets due to changes in humidity.For example, the characteristic of a humidity-sensitive resistor is to cover a film made of moisture-sensitive material on the substrate. When water vapor in the air is adsorbed on the moisture-sensitive film, the resistivity and resistance value of the element will change. One characteristic can measure humidity.Figure2. How a polymeric membrane humidity sensor worksII Classification and Common Types of Humidity Sensors2.1 ClassificationAt present, there are many types of humidity sensors on the market, and their application ranges are also different. They are roughly divided into the temperature and humidity ranges used, which can be listed in the following table.IndustryScope of applicationOperating temperature and humidity rangeUsesTemperature(℃)Humidity(%PH)Home appliancesAir conditioning machine5~40 40~70 Air conditioning equipmentDryer800~40Clothes dryingElectronic range5~1002~100Food heating and conditioning controlVTR-5~6060~100Prevent condensationCarAutomatic anti-fog-20~8050~100Prevent condensationMedical treatmentTreatment device10~3080~100Respirator systemIncubator10~3050~80Air conditioning equipmentIndustryFiber10~3050~100SilkDryer50~1000~50Kiln industry, wood dryingPowder moisture5~1000~50Ceramic raw materialsDry food50~1000~50Food preservationElectronic Components Manufacturing5~400~50Magnetic head, LSI ICAgriculture, Forestry and LivestockHouse air conditioning5~400~100Air conditioning equipmentTea smoke anti-frost-10~6050~100Prevent condensationReptile feeding5~400~50Increase humidity, health managementTestConstant temperature and humidity tank-5~1000~100Precision measurementRF detector-50~400~100High-precision meteorological measurementHygrometer5~1000~100Control recording deviceOtherSoil moisture-20~500~100Plant cultivation, soil and sand collapseIn the above, we have introduced four methods for detecting humidity or condensation. These measurement methods must be selected according to the different test environments. The most commonly used measurement method on the market is the first one because its measurement and signal selection methods are quite simple and cheap. It can be divided into the following six categories if it is distinguished by the difference of its manufacturing materials:(1) Electrolytes such as LiCl.(2) Semiconductor materials such as Se and Ge.(3) MgCr2O4, ZnCr2O4, TiO2, SnO2 and other metal oxide fusion products.(4) Porous metal oxide film such as Al2O3.(5) A material made by dispersing conductive powder in a polymer material such as nylon.(6) Organic or inorganic polymer electrolyte membrane. Sensors made of moisture-sensitive materials can be roughly divided into 7 categories:(1) Electrolyte humidity sensor(2) Polymer humidity sensor(3) Ceramic humidity sensor(4) Crystal oscillator humidity sensor(5) Semiconductor humidity sensor(6) Thick-film humidity sensor(7) Condensation humidity sensorClassificationMoisture Sensitive MaterialDetection methodElectrolyteLiCl+Polyviny1 Alcho1 PolystyreneResistorSulfated filmResistorPotassium sulfate membraneResistorLiC1 saturated solutionResistorSemiconductorSe (Ge or Si) vapor deposition filmResistorSi+SiO2+PAPA (Polyamino Phenylacetylene)ResistorMetal oxides (ceramics)Fe3O4 Colloid coating filmResistorCr2O3 Ni2O3 Fe2O3ResistorGlass ceramic filmCapacitorFe2O3-K2O ceramicResistorZnO-Li2O-V2O5 ceramicResistorMg Cr2O4 type ceramicCapacitorPolymersAl2O3+ epoxy resinResistor or CapacitorMulti-emulsion resin filmCapacitor Organic materialCelluloid+CarbonResistorButyr CellaloseCapacitorResin carbonResistorPolyamid+ crystal oscillatorResonance frequency2.2 Comparison of Resistive and Capacitive Humidity Sensors(1) Resistive humidity sensorThe resistive humidity sensor is a sensor that uses the electrical characteristics of the humidity sensor (such as resistance value) to change with humidity to measure humidity. The humidity sensor is generally immersed in an insulating material with a hygroscopic substance, or through evaporation, It is made by coating and other processes to prepare a layer of metal, semiconductor, polymer film and powdered particles.  During the moisture absorption and dehumidification process of the moisture-sensitive element, the conduction state of the ion H+ decomposed by water molecules changes, so that the resistance value of the element changes with humidity.Figure3. Resistive humidity sensorAdvantage1) Compared with the capacitive type, the structure is simpler, and it is easier to achieve mass production and low price.2) There is no need to consider the capacity between the leads like capacitive sensors, so the sensor can be stretched at will, with greater design freedom.3) Since the characteristic is a logarithmic change (the degree of change is large), the humidity change is small for the resistance change. (According to this point, for example, the degree of influence of the deviation of the electrode on the characteristics is small, and the instability is also small. Even if there is a slight change, it is difficult to show when converted to humidity.) Disadvantages1) The temperature characteristic is larger than that of the capacitive type (0.5%rh/℃), and temperature compensation is usually required.2) Since the characteristic is a logarithmic change, if the logarithmic conversion is not processed, the linear characteristic will not be obtained.3) The low humidity range is difficult to detect due to high resistance. (About 20% rh is the limit) In addition, it is easily affected by interference. (2) Capacitive humidity sensorA capacitive humidity sensor is a commonly used instrument in humidity sensors. It uses polymer humidity and humidity-sensitive capacitors as the basic humidity-sensing component, and uses a single-chip microcomputer to analyze, process, display and remotely transmit the measurement results. The measurement accuracy is ±2.5 %. The capacitive humidity sensor is mainly composed of a glass substrate, a lower electrode, a humidity-sensitive material, and an upper electrode. The two lower electrodes are connected in series with the humidity-sensitive material and the two capacitors formed by the upper electrode. Humidity-sensitive material is a high molecular polymer whose dielectric constant changes with the relative humidity of the environment.  When the environmental humidity changes, the capacitance of the humidity sensor changes accordingly, that is, when the relative humidity increases, the humidity sensitive capacitance increases, and vice versa (the capacitance is usually between 48 and 56 pf). The sensor's conversion circuit converts the humidity-sensitive capacitance change into a voltage change, which corresponds to a change in relative humidity from 0 to 100% RH, and the output of the sensor changes linearly from 0 to 1v.Figure4. Capacitive soil moisture sensorAdvantage1) Generally speaking, low humidity starting from 0% rh can be detected.2) The capacitance value is relatively close to linear, and no logarithmic change is required.3) The temperature characteristic is smaller than that of the resistance type (about 0.05～0.1%rh/℃), and temperature compensation is not required for general use.4) In order to increase the capacitance value, the structure is made into a thin film, and there are more products with a faster response speed than the resistance type. Disadvantage1) If the sensor is extended with a lead wire, the capacitance value will change, so it is not suitable to extend the sensor alone. Also, if it is assembled into the device, it is difficult to change the position by the lead wire, so the design freedom is small.2) The amount of change is relatively small, but a small change in capacitance will cause a large error. Therefore, inexpensive sensors have a large deviation. (The sensors used for measurement also have very high accuracy, but these estimation formulas have been processed.)3) The fact that the amount of change is small can be said in terms of dependence. But a small change in capacitance will produce a large error. Therefore, a sensor with poor reliability will have a large humidity change.4) Although the amount of change is small, the deviation and temperature characteristics of other circuit parts will have a greater influence, so be careful when selecting circuit parts. 2.3 Four Types of Humidity Sensors with More Applications(1) Lithium chloride humidity sensor● Resistive lithium chloride hygrometerCertain metal salts (such as lithium chloride LiCI) have strong moisture absorption properties in the air, and their moisture absorption is a certain function of the relative humidity of the air, that is, the greater the relative humidity in the air, the more the moisture absorbed by the lithium chloride. At the same time, the electrical conductivity of lithium chloride, that is, the size of the resistivity changes with the amount of moisture absorption, the more water absorbed, the smaller the resistivity, and vice versa.  Therefore, the relative humidity of the air can be determined according to the change in resistivity of lithium chloride. Lithium chloride resistance hygrometer is a meter made of the characteristics of resistivity change after lithium chloride absorbs moisture. The first lithium chloride electric humidity sensor based on the principle of resistance-humidity characteristics was developed by F.W.Dunmore of the American Bureau of Standards. This kind of element has high precision, simple structure, low price, suitable for a series of advantages such as measurement and control of normal temperature and humidity. ● Dew point lithium chloride hygrometerThe dew-point lithium chloride hygrometer was first developed by Forboro Company in the United States. This type of hygrometer is similar to the above-mentioned resistive lithium chloride hygrometer, but its working principle is completely different. In short, it uses the saturated vapor pressure of a saturated aqueous solution of lithium chloride to work with temperature. (2) Carbon humidity sensorThe carbon humidity sensor was first proposed by EKCarver and CWBreasefield in the United States in 1942. Compared with commonly used sounding elements such as hair, casing and lithium chloride, the carbon humidity sensor has a fast response speed, good repeatability, The advantages such as no erosion effect and narrow hysteresis ring are eye-catching. The uncertainty of measurement using carbon humidity sensor does not exceed ±5%RH, the time constant is 2～3s at positive temperature, the hysteresis is generally about 7%, and the specific resistance stability is also better. (3) Alumina hygrometerThe outstanding advantage of alumina sensor is that the volume can be very small (for example, the humidity sensor used in the radiosonde is only 90μm thick and 12mg weight), high sensitivity (the lower limit of measurement reaches -110℃ dew point), and the response speed is fast (generally 0.3 s to 3s), the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing program, and so on. In addition, it is also suitable for measuring moisture in liquids. (4) Ceramic humidity sensorCeramic humidity sensor is also called metal oxide humidity sensor, because its humidity sensing material is made of metal oxide powder through pressure molding and sintering into ceramics. Due to the degree of sintering, many porous objects can be obtained, and water vapor will be adsorbed on the porous surface to form an adsorption layer, and the H+ ions in the adsorption layer will form current carriers due to the adhesion of water vapor. When the humidity is high, the current attached to the layer of water vapor in the adsorption easily flows. The ceramic humidity sensor utilizes this property to convert the humidity change into the output of the impedance value change.Figure5. Heating purification type ceramic humidity sensor2.4 Commonly Used Humidity Sensor ModelsAt present, the main manufacturers and typical products producing integrated humidity sensors are Honeywell (HIH-3602, HIH-3605, HIH-3610), Humirel (HM1500, HM1520, HF3223, HTF3223), Sensiron (SHT11, SHT15) type). These products can be divided into the following four types: Linear voltage output integrated humidity sensorTypical products are HIH3605/3610, HM1500/1520. Its main feature is the use of constant voltage power supply, built-in amplifier circuit, can output a volt-level voltage signal proportional to the relative humidity, fast response, good repeatability, and strong anti-pollution ability. Linear frequency output integrated humidity sensorThe typical product is HF3223 type. It adopts a modular structure and is a frequency output integrated humidity sensor. The output frequency is 8750Hz (type value) at 55%RH. When the relative humidity changes from 10% to 95%, the output frequency is reduced from 9560Hz to 8030Hz . This kind of sensor has the advantages of good linearity, strong anti-interference ability, easy to be equipped with digital circuits or single-chip computers, and low price. Frequency/temperature output integrated humidity sensorThe typical product is HTF3223. In addition to the functions of HF3223, it also adds a temperature signal output terminal and uses a negative temperature coefficient (NTC) thermistor as a temperature sensor. When the ambient temperature changes, the resistance value changes accordingly and is drawn from the NTC terminal, and the temperature value can be measured with a secondary meter. Single-chip intelligent humidity/temperature sensorIn 2002, Sensiron took the lead in the world to successfully develop the SHT11 and SHT15 intelligent humidity/temperature sensors. The overall dimensions are only 7.6 (mm) × 5 (mm) × 2.5 (mm), and the size is similar to that of a match head. Before leaving the factory, each sensor has been precision-standardized in the temperature room, and the standard coefficients are compiled into corresponding programs and stored in the calibration memory.  The relative humidity can be automatically calibrated during the measurement process. They can not only accurately measure relative temperature, but also temperature and dew point. The measurement range of relative temperature is 0-100%, the resolution is up to 0.03%RH, and the highest accuracy is ±2%RH. The measuring temperature range is -40℃～+123.8℃, and the resolution is 0.01℃. The accuracy of measuring dew point is <±1℃. When measuring humidity and temperature, the digits of the A/D converter can reach 12 and 14 bits respectively.  Using the method of reducing the resolution can increase the measurement rate and reduce the power consumption of the chip. The products of SHT11/15 have good interchangeability, fast response speed, strong anti-interference ability, do not need external components, adapt to various single-chip microcomputers, and can be widely used in medical equipment and temperature/humidity adjustment systems.Figure6. HTS221 Capacitive Digital Humidity SensorIII Application of Humidity Sensors3.1 Typical UsesWork in any industry is inseparable from the air, and the humidity of the air is directly related to work, life, and production, making the monitoring and control of humidity more and more important. The main applications of humidity sensors are as follows: (1) Climate monitoringWeather measurement and forecasting are of great significance to industrial and agricultural production, military and people’s lives, and scientific experiments. Therefore, humidity sensors are essential humidity measuring equipment. For example, resin swelling humidity sensors have been used in meteorological balloon humidity measuring instruments. on. (2) Greenhouse breedingModern agriculture, forestry, and animal husbandry industries have a considerable number of greenhouses. The humidity control of the greenhouse is as important as temperature control. Controlling the humidity in a suitable range for the growth of crops, trees, livestock and poultry is one of the conditions for reducing pests and diseases and increasing yield. (3) Industrial productionIn the textile, electronics, precision machinery, ceramic industry and other sectors, air humidity directly affects the quality and output of products, and must be effectively monitored and regulated. (4) Storage of goodsVarious items have certain adaptability to the environment. If the humidity is too high or too low, the product will lose its original performance. For example, in high-humidity areas, electronic products are seriously damaged in the warehouse, non-metal parts will become moldy, and metal parts will corrode and rust. (5) Use protection of precision instrumentsMany precision instruments and equipment have higher requirements for the working environment. The environmental humidity must be controlled within a certain range to ensure their normal operation and improve work efficiency and reliability. For example, the working humidity of the telephone program-controlled switchboard is better at 55% ±10%. Too high temperature will affect insulation performance, and too low temperature will easily generate static electricity and affect normal operation.3.2 Application Fields(1) Humidity measurement system● When the temperature is below 70°C (usually above -40°C), if the environment is clean, use a polymer sensor, and use a ceramic sensor (heating cleaning regeneration type) for serious pollution. Because of its heating and cleaning process, it cannot be measured continuously and consumes a lot of energy (1-10W). However, it has a long life and can choose a sensor with a longer heating and cleaning cycle during use, such as a chloroapatite ceramic sensor, which is washed once every 2 to 3 months. In addition, the internal heating type consumes less energy than the external heating type. ● Measure the humidity in the range of 70～100℃, use ceramic sensors with heating and cleaning, and perform linearity and temperature compensation to improve accuracy. In order to achieve higher accuracy, a microcomputer is required. Frequent heating and cleaning are required at high temperature and humidity. For example, when the RH is above 80%, it needs 30S cleaning once. It is best equipped with an automatic heating cleaning device. ● Measure the humidity in the range of 100～150℃. In the world, ceramic humidity sensors are mostly used to make high temperature humidity meters. (2)Automatic control of industrial processesIn order to improve product quality and energy-saving, ceramic humidity sensors are usually used for control in product drying systems, reactor humidity control, boiler water vapor leakage detection, integrated circuits, or air conditioning in magnetic head processing plants; the humidity control of various air conditioning systems, medical systems can be carried out with polymer or ceramic humidity sensors. (3) Steam leak detection systemIn thermal power stations, nuclear power plants, steam locomotives, boilers and other high-temperature and high-pressure equipment, in order to prevent gas leakage and prevent personal accidents, humidity sensors can be used for leak detection. (4) Other systemsIn-home appliances, the humidity sensor can be used for humidity measurement of humidifiers, dehumidifiers, air conditioners, wine cabinets, clothes dryers, etc.IV Arduino Entry Project: Real-time Temperature and Humidity Detector4.1 Hardware PreparationArduino UNO oneA temperature and humidity sensorOne PCF8574T adapter board1602LCD oneA piece of breadboardSeveral connecting lines4.2 Software PreparationArduino IDE4.3 Circuit ConnectionThis project directly uses the PCF8574T adapter board to drive the 1602 LCD display, which will save a lot of Arduino IO ports and save a lot of wiring troubles. PCF8574T adapter board contains four interfaces: VCC, GND, SDA and SCL. Make these connections respectively: VCC - 5V, GND - GND, SDA - A4, SCL - A5. The temperature and humidity sensor contains 3 pins, viewed from the side with the mesh, from left to right are DATA, VCC, and GND. Make connections like this: DATA - A0, VCC - 3.3V, GND - GND.Figure7. Circuit Connection4.4 Implementation CodeFigure8. Experiment Result/** Use temperature and humidity sensor to detect information and display it on the LCD*/#include "Wire.h" // Import libraries needed to drive LCD#include "LiquidCrystal_I2C.h"#include "dht.h" // Import dht library for temperature and humidity sensor#define dht_pin A0 // Connect the data port of the temperature and humidity sensor to A0dht DHT;// Set up LCDLiquidCrystal_I2C lcd(0x27,16,2); // 0x27 is the address of the I2C busvoid setup() {// Delay waiting for system initializationdelay(1000);// Initialize LCDlcd.init();// Turn on the screen backlightlcd.backlight();// LCD screen displays Humidity(%):lcd.print("Humi(%): ");// LCD screen displays Temp(C):lcd.setCursor(0, 1);lcd.print("Temp(C): ");}void loop() {// Read the data of the temperature and humidity sensorDHT.read11(dht_pin);// LCD displays the collected temperature and humidity datalcd.setCursor(8,0);lcd.print(DHT.humidity,1);lcd.setCursor(8,1);lcd.print(DHT.temperature,1);delay(1000);} After the code is compiled without any problem, click the button to upload it to the Arduino UNO board. After the programming is no problem, you can observe the result on the LCD. If there is no change in the humidity, you can try to breathe a sigh of relief at the sensor and you can observe the change in value.Ⅴ FAQ1. What are humidity sensors?A humidity sensor (or hygrometer) senses, measures and reports both moisture and air temperature. The ratio of moisture in the air to the highest amount of moisture at a particular air temperature is called relative humidity. Relative humidity becomes an important factor when looking for comfort. 2. How do humidity sensors work?Humidity sensors work by detecting changes that alter electrical currents or temperature in the air. ... A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide's electrical capacity changes with the atmosphere's relative humidity. 3. How many types of humidity sensors are there?There are three primary types of humidity sensors employed which are defined around what approach is used to sense humidity and deliver an electrical signal that can be used to establish the value. These types of humidity sensors include Capacitive humidity sensors. Resistive humidity sensors. 4. Is a humidity sensor analog or digital?Humidity sensors measure and report moisture levels in two distinct ways - analog or digital (aka discrete). Digital sensors are able to monitor conditions for operation within a specified range. ... Analog sensors are more advanced and provide continuous visibility to current conditions through accurate measurements. 5. Why is a humidity sensor used?Humidity sensors are electronic devices that measure and report the moisture and air temperature of the surrounding environment where they are deployed e.g., in air, soil, or confined spaces. Humidity measurements indicate the concentration of water vapor present in the air. 6. What is an absolute humidity sensor?The ABS-300 is a thermal conductivity absolute humidity sensor. This sensor measures absolute humidity by quantifying the difference in thermal conductivity of dry air and air containing water vapor. ... If temperature and pressure are known the absolute humidity easily converts to relative humidity. 7. How accurate is a humidity sensor?Digital relative humidity sensors are typically accurate to plus/minus 3% relative humidity throughout the entire 0-100% RH range, but closer to plus/minus 2% at 50% RH. ... The simplest way to calibrate a relative humidity sensor is with table salt and water in an airtight container. 8. What is a humidity transducer?Humidity transducers are normally used in laboratories connected to a controller to keep a constant humidity there. ... Humidity transducers can transform a physical quantity of air humidity into a standard signal which is transferred to a controller. 9. How does the humidity sensor sense the moisture in the air?A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide's electrical capacity changes with the atmosphere's relative humidity. These types of sensors are used for weather, commercial and industrial applications. Resistive humidity sensors utilize ions in salts to measure the electrical impedance of atoms. As humidity changes, so do the resistance of the electrodes on either side of the salt medium. State-of-the-art resistive humidity sensors use ceramics to overcome areas where condensation occurs. Thermal conductivity sensors measure changes in heat to detect humidity. Two thermal sensors conduct electricity based upon the humidity of the surrounding air. One sensor is encased in dry nitrogen as a comparison to the other sensor which measures the ambient air. The difference between the two measures the humidity. 10. What is the difference between a Temperature sensor and a Humidity sensor?Temperature Sensor: Temperature is the most common environmental parameter. Temperature plays an important role in our homes and industries. Over the past few years, we are able to monitor and control environmental parameters with the help of temperature sensing devices. A temperature sensor is an electronic device that is used to detect and measure accurate temperature levels in different environmental conditions. There are many affordable temperature sensors are available in the market to measure the accurate temperature level. Humidity Sensor: Humidity is another most measurable environmental parameter. The high humidity levels in our homes and warehouses increase the chances of damaged products and things. In the past, we were not able to detect the accurate humidity level due to a lack of sensing devices. The humidity sensor is an electronic device uses to measure the humidity level and make changes in the humidity level through our mobile phone from anywhere. The humidity sensor detects the humidity level in the water, air and in soil. We can easily access humidity sensors in our homes and business. 

  Light emitting diode (LED) is a light source that meets the requirements of green lighting. LEDs are safe, efficient, environmentally friendly, long-lived, responsive, small in size and robust in construction with many features that are unmatched by ordinary light-emitting devices. Moreover, it is one of the first semiconductor devices and is widely used. Currently, LEDs are widely used as indicators for various electronic products and as light sources for fiber optic communication.     How dose a diode work?     Catalog I What is a diode? II How dose a diode work? III What is diode characteristics? IV What are diode parameters? V What are the types of diodes? FAQ   I What is a diode?   Diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.) . It has a unidirectional conductivity, that is, the diode anode and cathode to add a forward voltage, the diode conducts. When the reverse voltage is added to the anode and cathode, the diode cuts off. Therefore, the on and off of the diode is equivalent to the on and off of the switch.   In almost all electronic circuits, semiconductor diodes are used. The use of semiconductor diodes in the circuit can play a role in protecting the circuit, extending the life of the circuit. The development of semiconductor diodes has made integrated circuits more optimized and has played an active role in various fields. Diodes have many roles in integrated circuits and maintain the proper functioning of the integrated circuits.     Diodes were one of the first semiconductor devices to be created, and their applications are very widespread. Especially in a variety of electronic circuits, the use of diodes and resistors, capacitors, inductors and other components to make a reasonable connection to form a circuit of different functions, you can achieve a variety of functions such as rectification of alternating current, detection of modulated signals, limiting and clamping, and voltage regulation of the supply voltage. Whether in common radio circuits or in other household appliances or industrial control circuits, diodes can be found.   A diode is made of a PN junction with corresponding electrode leads and a tube housing package. The diode has two electrodes, the electrode leading from the P area is the positive electrode, also known as the anode; the electrode leading from the N area is the negative electrode, also known as the cathode.       Diode structure         There are many kinds of diodes:  - According to the semiconductor materials used, they can be divided into germanium diodes and silicon diodes.   - According to their different uses, they can be divided into detector diodes, rectifier diodes, zener diodes, switching diodes, etc.   - According to the structure of the tube core, they can be divided into point-contact diodes, surface-contact diodes and planar diodes.     --  The point contact diodes are pressed on the smooth surface of the semiconductor wafer with a thin metal wire. With pulse current, one end of the contact wire is sintered firmly with the wafer to form a "PN junction". Due to point contact, only small currents (no more than a few tens of mA) are allowed, which is suitable for high frequency low current circuits, such as radio detection, etc. The area of "PN junction" of surface contact diode is large, which allows large currents passing through and is mainly used in "rectifying" circuits that convert AC to DC.    --  Planar diode is a kind of special silicon diode. It not only can pass through large current, but also has stable and reliable performance. It is widely used in switching, pulse and high frequency circuits.   II How does a diode work? The crystal diode is a p-n junction formed by p-type semiconductor and n-type semiconductor. It forms a space charge layer on both sides of the interface and has a self-built electric field. When there is no applied voltage, the diffusion current caused by the carrier concentration difference on both sides of the p-n junction is equal to the drift current caused by the self-built electric field, so it is in an electric equilibrium state. When the external positive voltage is biased, the mutual suppression of the external electric field and the self-built electric field results in the increase of the carrier diffusion current, which is shown in the conduction region below. When the external reverse voltage is biased, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current I0 independent of the reverse bias voltage value within a certain reverse voltage range, which is shown in the cut-off region below. When the applied reverse voltage is high enough to a certain extent, the electric field intensity in the space charge layer of p-n junction reaches the critical value to produce the multiplying process of the carriers, resulting in a large number of electron hole pairs and a numerical reverse breakdown current is generated, known as the diode breakdown, which is shown in the breakdown region below. III What is diode characteristics? The most important characteristic of the diode is the unidirectional conductivity. In the circuit, the current can only flow from the positive electrode of the diode, and flows out from the negative electrode . The forward and reverse characteristics of the diode are illustrated by simple experiments.   3.1 Forward characteristics In electronic circuits, if the diode is connected to the high potential terminal and the negative electrode to the low potential terminal, the diode will be switched on. This connection is called forward bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode still cannot be switched on, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (about 0.6 V of the silicon tube) can the diode be truly switched on. The voltage at both ends of the diode after conduction is called the forward voltage drop of the diode.   3.2 Reverse characteristic In the electronic circuit, the positive end of the diode is connected to the low potential end, the negative electrode is connected to the high potential terminal, and the diode is in the cutoff state. This mode of connection is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called leakage current. When the reverse voltage of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose the single direction conduction characteristic. This state is called the breakdown of diode.   IV What are diode parameters? The technical specifications used to test the performance of diodes are called diode parameters. Here are some of the main parameters in diode testing:   4.1 Rated forward working current (IF) Refers to the maximum forward current that is allowed to pass through the diode when it is in continuous operation over a long period of time. When a larger current passes through the diode, the dice is heated and the temperature rises, and when the temperature exceeds the allowable limit, the dice is overheated and damaged. Therefore, it should not exceed the diode rated forward operating current value when the diode is in use. Eg. The rated forward working current of DFM is 1A   4.2 Forward Voltage(VF) Refers to the voltage at both ends of the diode when the rated forward working current IF is passed through the diode. Eg. The voltage at both ends of the diode is about 0.9V when the forward working current of DFM is 1A.   4.3 Maximum reverse operating voltage (VR) When the reverse voltage at both ends of the diode is raised to a certain value, the diode will be broken down and the unidirectional conductivity will be lost. In order to ensure the safety of operation, the maximum reverse operating voltage is specified. Eg. The maximum reverse operating voltage of DF10M is 1100V and the breakdown voltage is about 1400V   4.4 Reverse current IR Refers to the reverse current that flows through the diode when the maximum reverse operating voltage VR is applied to both ends of the diode. The smaller the reverse current, the better the unidirectional conductivity of the diode will be. Eg. When the reverse voltage of DF10M is 1100V, the VR is about 0.2uA.   4.5 Reverse critical current (IZ) Refers to the reverse current of the diode increases sharply to close to the breakdown phenomenon.   Eg. Set the IZ of DF10M to 0.1 Ma (Ma) 4.6 Reverse critical voltage (VZ) Refers to the reverse voltage of the diode when the reverse current is IZ. If the reverse voltage is greater than this value, the reverse current increases dramatically and the unidirectional conductivity of the diode is destroyed, thus causing reverse breakdown. Eg. The VZ is about 1300V when IZ of DF10M is 0.1mA.   4.7 Reverse recovery time (Trr) When diodes are in low frequency applications, it generally do not need to consider its conduction to the cut-off, or cut-off to the transition time. But if the diode works in a high-speed switching circuit environment, when diode suddenly turns to reverse bias from the forward biased conduction state, it will take a certain time to become a cut-off state, which is called reverse recovery time. But if the diode works in a high-speed switching circuit environment, when diode suddenly turns to reverse bias from the forward biased conduction state, it will take a certain time to become a cut-off state, which is called reverse recovery time. Eg. The maximum Trr of EDF1DM is 50nS.   V What are the types of diodes?   5.1 Light emitting diode Light emitting diode, also called LED, is a semiconductor diode that converts electrical energy into luminous energy. Like ordinary diodes, LEDs are made up of a PN junction and have unidirectional conductivity. When a forward voltage is applied to a light-emitting diode, Holes injected from P region to N region and electrons injected from N region to P region are recombined with N region electrons and P region holes in several microns near PN junction to produce spontaneous emission fluorescence.    The energy states of electrons and holes in different semiconductor materials are different. When electrons and holes are combined, the energy released is different. The more energy is released, the shorter the wavelength of light is. The commonly used diodes are red, green or yellow light.The reverse breakdown voltage of a light-emitting diode is greater than 5 volts. Its forward volt-ampere characteristic curve is so steep that it must be used in series to control the current passing through the diode. The current limiting resistance R can be calculated by the following formula: R=（E－UF）/IF . In this formula, E is the power supply voltage, UF is the forward voltage of LED, IF is the running current of LED.   5.2 Zener diode Zener diode, is also called voltage stabilizing diode. By using the reverse breakdown state of pn junction, the current can be changed in a wide range and the voltage is basically unchanged, thus form a diode which has voltage stabilizing function. This diode is a semiconductor device with high resistance until it reaches the critical reverse breakdown voltage.  The following picture is a typical Zener diode application circuit diagram: At this critical breakdown point, the reverse resistance is reduced to a very small value, where the current increases and the voltage remains constant in this low resistance region, and the Zener diode is divided according to the breakdown voltage, because of this characteristic, The regulator is mainly used as a voltage regulator or voltage reference element. Zener diodes can be connected in series for use at higher voltages, and higher stable voltages can be obtained by serializing them.   5.3 Switching diode  Working principle: The semiconductor diode is equivalent to switch-on when it is turned on (the circuit is turned on), and is equivalent to switch-off when it is turn-off (the circuit is cut off), so the diode can be used as a switch. The common used model is 1N4148. Due to the unidirectional conductivity of semiconductor diodes, the PN junction is on at positive bias, and the resistance is very small at the on-state, which ranges from tens to hundreds of ohs. At reverse bias, it is in a cut-off state, and its resistance is very large. Generally, silicon diodes are above 10 μ Ω and germanium diodes have tens to hundreds of kilos. By using this property, the diode will play the role of controlling the current on or off in the circuit and become an ideal electronic switch.   At high frequency, the barrier capacitance of the diode exhibits extremely low impedance and is parallel to the diode. When the capacitance of the barrier itself reaches a certain level, the switching performance of the diode will be seriously affected. In extreme conditions, the diode will be short-circuited, and the high-frequency current will no longer pass through the diode, but will pass directly through the barrier capacitance, and the diode will fail to work. The barrier capacitance of the switching diode is generally small, which is equivalent to blocking the barrier capacitance path and achieving the effect of maintaining good unidirectional conductivity at high frequency.  Classification: General switching diode, high speed switching diodes, ultra-high speed switching diodes, low-power switching diodes, high reverse voltage switching diodes, silicon voltage switching diodes and so on.   5.4 Variable capacitance diode( Varactor Diodes ) Variable capacitance diode, also known as varactor Diodes, are semiconductors that change the junction capacitance according to the voltage supplied. That is, as variable capacitors, they can be used in resonant circuits such as FM tuners and TV tuners and FM modulation circuits. Working principle: Varactor Diodes is a kind of special diode. When applied forward bias voltage, the depletion region of PN (positive and negative electrode) junction is narrowed and the capacitance becomes larger, which results in diffusive capacitance effect. However, the leakage current will be generated when the forward bias is added, so the reverse bias is supplied in application. In fact, we can think of it as a PN junction. If a reverse voltage V is added to the PN junction (the varactor diode is used in reverse direction), the electrons in the N-type semiconductor are directed to the positive electrode and holes in P-type semiconductor will be led to the negative electrode. Then forms a depletion layer that has neither electrons nor holes, and the width of the depletion layer is set to d, which changes with the reverse voltage V. In this way, when the reverse voltage V increases, the depletion layer d becomes wider and the diode capacitance C decreases (according to C=kS/d), and the reverse voltage decreases, the depletion layer width d becomes narrower and the diode capacity becomes larger. The change of reverse voltage V leads to the change of depletion layer, which changes the junction capacity of the variable capacitance diode. - Application: the varactor diode is a semiconductor device based on the principle of variable capacitance between PN junctions. It is used as a variable capacitor in high frequency tuning and communication circuits. As shown in the following figure, the reverse voltage of the diode is changed by changing the different R2. This will result in a change in the capacitance of the diode, thus changing the resonant frequency in which the varactor diode can pull out the full range of the required capacitance in the parallel resonant band-pass filter.   FAQ   1. What is diode and its symbol? Diode, an electrical component that allows the flow of current in only one direction. In circuit diagrams, a diode is represented by a triangle with a line across one vertex.   2. What is special about a diode? Some semiconductor junctions, composed of special chemical combinations, emit radiant energy within the spectrum of visible light as the electrons change energy levels. Simply put, these junctions glow when forward biased. A diode intentionally designed to glow like a lamp is called a light-emitting diode, or LED.   3. Are diodes AC or DC? It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. Diodes are also known as rectifiers because they change alternating current (ac) into pulsating direct current (dc). Diodes are rated according to their type, voltage, and current capacity.   4. Why do we use zener diode? Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits. The load voltage equals breakdown voltage VZ of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.   5. What is unit of diode? A diode is not a measurable quantity. Hence,it does not have a unit. Generally,for a diode,we measure characteristics like forward voltage drop,reverse voltage drop and reverse breakdown voltage which are usually measured in Volts.   6. Do diodes have resistance? Just like a resistor or any other load in a circuit, a diode offers resistance in a circuit. Unlike resistors, though, diodes are not linear devices. This means that the resistance of diodes does not vary directly and proportional to the amount of voltage and current applied to them.   7. Does diode reduce current? Ideally, diodes will block any and all current flowing the reverse direction, or just act like a short-circuit if current flow is forward. Unfortunately, actual diode behavior isn't quite ideal. Diodes do consume some amount of power when conducting forward current, and they won't block out all reverse current.   8. How are diodes classified? Diodes are classified according to their characteristics and are offered in a number of different types, including rectifiers, switching diodes, Schottky barrier diodes, Zener (constant voltage) diodes, and diodes designed for high-frequency applications.   9. What is the most common diode? The most commonly used signal diode is the 1N4148. This diode has a close brother called 1N914 that can be used in its place if you can't find a 1N4148. This diode has a forward-voltage drop of 0.7 and a peak inverse voltage of 100 V, and can carry a maximum of 200 mA of current.   10. What is the difference between a Zener diode and a Schottky diode? As their switching speed is very high, Schottky diodes recover very fast when the current reverses, resulting in only a very small reverse current overshoot. ... A special type of diode, called the Zener diode, blocks the current through it up to a certain voltage when reverse biased.   11. What is difference between Schottky diode and normal diode? In the normal rectifier grade PN junction diode, the junction is formed between P type semiconductor to N type semiconductor. Whereas in Schottky diode the junction is in between N type semiconductor to Metal plate. The schottky barrier diode has electrons as majority carriers on both sides of the junction.   12. Why it is called diode? A diode is called a diode because it has two distinct electrodes (i.e. terminals), called the anode and the cathode. A diode is electrically asymmetric because current can flow freely from the anode to the cathode, but not in the other direction. In this way, it functions as a one-way valve for current.   13. Is a diode the same as a resistor? Key Difference: A diode is a type of electrical device that allows the current to move through it in only one direction. ... A resistor is an electric component that is used to provide resistance to current in the circuit. They are mostly used to produce heat or light.   14. How much voltage can a diode take? Silicon diodes have a forward voltage of approximately 0.7 volts. Germanium diodes have a forward voltage of approximately 0.3 volts. The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.   15. Can a resistor replace a diode? Diodes only conduct in one direction whereas resistors conduct in both directions. Without analyzing the actual circuit the results would be unpredictable but, generally speaking, being that diodes & resistors are designed to do different things, substituting one for the other is something you wouldn't want to do.  

2026 Executive Summary: A thermal fuse (or thermal cutoff) is a non-resettable safety device that permanently breaks an electrical circuit when overheated to prevent fires. Essential for dryers, microwaves, and coffee makers, it must be replaced—never bypassed—once triggered. This guide covers testing with a multimeter, replacement steps for Maytag/Whirlpool dryers, and current industry safety standards.What is a Thermal Fuse? (Definition & Purpose)A thermal fuse, technically known as a thermal cutoff (TCO), is a single-use safety device that opens circuits to stop the flow of electricity in the event of dangerous overheating. Unlike circuit breakers that trip due to current surges and can be reset, a thermal fuse detects excessive ambient heat caused by component failure or poor airflow. Crucially for 2026 safety standards: Once a thermal fuse triggers, the internal element melts, and the component must be replaced immediately.Video Tutorial: Understanding the mechanics of thermal fuse replacement (Reference for electric fans).Unlike electrical fuses which react to current spikes, thermal fuses react purely to temperature thresholds (typically between 72°C and 240°C depending on the appliance). However, if an electrical short generates significant heat, it will also trigger the fuse. This article provides a comprehensive 2026 guide to thermal fuse identification, testing, and replacement.Ⅰ Key Facts: How Do Thermal Fuses Protect Appliances?Thermal fuses are the last line of defense against residential fires. They are not universal; they are rated by holding temperature (Th) and functioning temperature (Tf). In modern 2026 appliances—from smart dryers to industrial machinery—these sensors monitor heat levels to sever electrical flow instantly when a preset threshold is breached.Home Safety: They prevent clothes dryers from igniting lint buildup.Industrial Safety: They stop machinery from catastrophic overheating.Maintenance Required: They are part of a system that requires human maintenance, such as cleaning dryer vents to prevent the heat buildup that blows the fuse in the first place.Ⅱ Why Replace a Maytag Dryer Thermal Fuse?The thermal fuse serves as a fail-safe for the dryer's thermostat. If the cycling thermostat (designed to shut off heat at ~155°F) fails in the "closed" position, the heating element stays on. The thermal fuse is calibrated to blow at approximately 196°F (depending on the model) to cut power before the dryer catches fire. If your Maytag dryer spins but does not heat, or will not start at all, this fuse is the likely culprit.Ⅲ Step-by-Step: Replacing a Thermal Fuse in a Maytag DryerFollow these steps to safely replace a blown thermal fuse in a Maytag dryer.Power Down: Disconnect the dryer from the wall outlet (240V plugs). Pull the unit away from the wall. Remove the exhaust vent clamp. Locate and remove the ¼-inch hex screws securing the rear access panel.Locate the Component: Find the thermal fuse, typically located on the blower wheel housing (bottom right or left). It is usually a white plastic strip (approx. 2 inches) with two wires attached.Disconnect Wiring: Isolate the fuse by removing the two wire connectors. Tip: Use needle-nose pliers to pull the metal locking clips, not the wires themselves, to avoid damaging the harness.Remove the Old Fuse: Unscrew the single or double mounting screws (usually Phillips or 1/4" nut driver) holding the fuse to the blower housing. Remove the faulty part.Install the Replacement: Screw the new thermal fuse into place. Reattach the two wires to the terminals (polarity generally does not matter for these single-pole devices, but check your schematic).Check the Thermostat (Recommended): While the back is open, inspect the cycling thermostat (near the heater element). If the fuse blew, the thermostat might be sticky. Consider replacing both to prevent repeat failures.Reassemble: Reattach the back panel, secure all screws, reconnect the dryer vent (ensure it is clean), and plug the unit back in.Ⅳ Diagnosing a Blown Thermal Fuse: Symptoms & TestsA thermal fuse is a critical fire prevention component mandated in all dryers manufactured since the 1980s. When the exhaust temperature spikes dangerously high, the fuse trips, permanently disabling the heating element or the motor.4.1 Common Symptoms of FailureNo Start: On many electric models, a blown fuse cuts power to the start switch. The dryer is completely dead.No Heat (Gas/Electric): The drum spins, but the air remains cold. The fuse has cut the circuit to the gas valve or heating element coils.Incomplete Drying: Clothes take multiple cycles to dry because the heat cuts out prematurely (often a sign of a failing, but not yet blown, thermal cutout or clogged vent).4.2 Locating the FuseSafety First: Ensure the dryer is unplugged (220-volt shock hazard). The fuse is mounted on the exhaust duct or blower housing to monitor airflow temperature. Look for a thin white plastic strip or a small metal cylinder with two wires.4.3 The Bypass Test (WARNING: For Testing ONLY)Danger: Bypassing is a temporary diagnostic step, never a permanent fix. To verify failure, you can jump the two wires together. If the dryer starts/heats with the fuse bypassed, the fuse is blown. Do not run a load of laundry with a bypassed fuse; fire risk is high.4.4 The Multimeter Test (Preferred Method)The safest diagnostic method in 2026 is using a digital multimeter: 1. Set the meter to Continuity (sound wave icon) or the lowest Ohms (Ω) setting. 2. Touch probes to both fuse terminals (wires removed). 3. Good Fuse: Beep sound or 0-1 Ohms resistance (Closed Circuit). 4. Bad Fuse: No sound or "OL" / Infinity (Open Circuit).4.5 Root Cause AnalysisFuses rarely fail without cause. Before installing the new one, check for: 1. Clogged Vents: The #1 cause of failure. Clean the ductwork leading outside. 2. Crushed Hoses: Ensure the flex hose behind the dryer isn't kinked. 3. Blower Wheel: Check for lint buildup slowing the fan.Ⅴ General Guide: Testing and Replacing Dryer Thermal FusesStep 1: Consult DocumentationLocate the wiring diagram often hidden inside the dryer's control console or kickplate. This will identify the thermal fuse location and part number.Step 2: Isolate PowerUnplug the unit. For hardwired units, flip the double-pole 30-amp breaker to the OFF position.Step 3: Access the ComponentDepending on the brand (Samsung, LG, Whirlpool, GE): * Rear Access: Remove the back panel. * Front Access: Remove the top lid, control panel, and front face (common on lint-screen-on-top models). * Toe Panel: Some fuses are accessible via a lower inspection kickplate.Step 4: Disconnect WiringRemove wire spade connectors carefully. Inspect connectors for heat damage or charring; replace connectors if burned.Step 5: Verify ContinuityUsing your multimeter, test for continuity. Zero Ohms = Good. Infinite/OL = Bad.Step 6: Source the Correct PartCrucial: Fuses are temperature-specific. Never replace a 196°F fuse with a higher-rated one. Use the dryer's Model Number (found on the door frame) to find the OEM replacement.Step 7: InstallSecure the new fuse with its mounting screws. Ensure the sensing face sits flush against the duct for accurate readings.Step 8: Functional TestReassemble the cabinet. Run the dryer on "Air Fluff" (No Heat) first to check the motor, then "High Heat" to verify the heating element engages.Ⅵ Buying Guide: Where to Find Whirlpool Dryer Thermal Fuses in 2026If you own a Whirlpool (or sub-brands like Kenmore, Roper, Estate), sourcing a genuine thermal fuse is straightforward. Avoid counterfeit parts that may not blow at the correct temperature.6.1 Specialized Appliance Parts StoresLocal appliance repair depots carry high stock of "fast-moving parts" like thermal fuses. Bring your old fuse or your dryer's model number to ensure a perfect match. OEM (Original Equipment Manufacturer) parts are recommended for safety compliance.6.2 Local Hardware StoresBig-box hardware stores (e.g., Home Depot, Lowe's, Ace Hardware) often stock universal dryer repair kits. While convenient, ensure the temperature rating matches your failed fuse exactly.6.3 Authorized DealersPurchasing from an authorized Whirlpool dealer guarantees you are getting a genuine FSP (Factory Specification Part) that meets strict 2026 fire safety codes. 6.5 Online RetailersMajor platforms (Amazon, RepairClinic, PartsSelect) offer next-day delivery. Pro Tip: Search for the specific part number (e.g., "Whirlpool 3392519") rather than just "dryer fuse" to avoid compatibility issues. Check reviews to ensure the part doesn't fail prematurely.Ⅶ Myth Busting: Can You Reset a Thermal Fuse?The short answer is NO. A thermal fuse is a "one-shot" device. Unlike a thermal switch or a circuit breaker, a thermal fuse contains a pellet that melts physically when overheated. Once the internal link is severed, the circuit is permanently open.Note: Some motors have a thermal overload protector that clicks back on after cooling, and some high-limit thermostats on heater boxes auto-reset. However, the safety fuse on the exhaust duct is almost exclusively non-resettable to force the user to address the dangerous overheating issue (usually lint blockage) before using the machine again.Ⅷ How It Works: The Physics of Thermal CutoffsThermal fuses act as sacrificial links. They employ specific chemical compounds engineered to change state at precise temperatures.SW-1, 3, 5 Series: Spring-Loaded MechanismThese fuses use a compressed spring held back by a solid thermal pellet. Under normal operation, the current flows through a sliding contact. When the ambient temperature hits the "melting point," the pellet liquefies. This releases the spring tension, which physically pulls the sliding contact away from the lead, creating an air gap that stops the electricity instantly.SW-2 Series: Axial Fusible AlloyCommon in smaller electronics, these use a fusible metal alloy wire coated in a special flux inside a ceramic tube. Upon overheating, the alloy melts and surface tension (aided by the flux) causes the liquid metal to bead up and separate, breaking the connection irreversibly.Ⅸ Common Applications in 2026MICROTEMP® and similar thermal cutoffs (TCOs) provide secondary protection in millions of devices. They are the backup when primary controllers (thermostats/thermistors) fail.Ⅹ Engineering Considerations for Thermal Fuse Selection10.1 Optimal Sensor PlacementFor effective protection, the fuse must be located where the temperature rise is most rapid during a fault condition. Engineers use infrared thermography to map "hot spots" to ensure the fuse triggers before plastic components melt or combustion occurs.10.2 Holding vs. Functioning TemperatureSelection requires a buffer. If the operating temperature of a dryer is 150°F, using a 155°F fuse will cause "nuisance tripping" due to thermal inertia. Engineers typically select a rating 30-40°C above the maximum normal operating temperature to ensure longevity while maintaining safety.10.3 Managing Thermal GradientsIdeally, the entire fuse body should heat evenly. If one lead is attached to a massive heat sink and the other to a heat source, a thermal gradient forms. This can cause the internal pellet to deform slowly over time (aging), leading to premature failure even without a true overheating event.10.4 Maximum OvershootEven after a fuse blows, the temperature may continue to rise slightly (overshoot). The fuse housing materials (ceramic/plastic) must be rated to withstand this peak temperature without carbonizing, which could inadvertently re-conduct electricity across the blown fuse.Ⅺ Frequently Asked Questions (2026 Update)1. What is the main purpose of a thermal fuse?Its sole purpose is fire prevention. It acts as a "kill switch" for heat-producing appliances (dryers, coffee makers, microwaves) if the primary thermostat fails, preventing the device from reaching ignition temperatures.2. How is a thermal fuse different from a regular electrical fuse?A regular electrical fuse blows when there is too much current (Amps). A thermal fuse blows when there is too much heat (Temperature). While both open the circuit, they react to different stimuli.3. Can I use a thermal fuse in any appliance?They are specific to heat-generating devices. You will find them in hair dryers, clothes dryers, rice cookers, and electric motors (to detect overheating windings).4. Does a thermal fuse show physical signs of damage?Rarely. Unlike a glass electrical fuse where you can see a broken wire, a thermal fuse usually looks normal even when blown. You must use a multimeter continuity test to confirm if it has failed.5. Is replacing a thermal fuse difficult?It is considered an easy-to-moderate DIY repair. On most dryers, it requires removing the back panel (8-10 screws) and swapping the part, which takes about 15-30 minutes.6. How much does a thermal fuse cost in 2026?They remain affordable, typically costing between $8 and $15 for OEM parts. Avoid extremely cheap "generic" packs online as they may lack proper safety certifications (UL/CSA).ConclusionThermal fuses are the unsung heroes of household safety. While a blown fuse is inconvenient, it indicates the system successfully prevented a potential fire. Whether you are troubleshooting a Maytag, Whirlpool, or Samsung dryer, remember that the fuse is a symptom, not the disease. Always clean your dryer vents and check for airflow restrictions when replacing this component. By understanding the operation and proper testing of thermal cutoffs, you ensure your home appliances operate safely and efficiently in 2026.{ "@context": "https://schema.org", "@type": "Article", "headline": "Thermal Fuse Guide 2026: Testing, Replacement, and Safety Facts", "datePublished": "2022-03-25", "dateModified": "2026-01-09", "author": { "@type": "Organization", "name": "ApogeeWeb" }, "publisher": { "@type": "Organization", "name": "ApogeeWeb", "logo": { "@type": "ImageObject", "url": "https://www.apogeeweb.net/logo.png" } }, "description": "A comprehensive 2026 guide to thermal fuses (thermal cutoffs). Learn how to test, bypass (for diagnosis), and replace thermal fuses in Maytag and Whirlpool dryers.", "image": "https://www.apogeeweb.net/upload/image/20220325/2022032515162194.jpg", "mainEntity": [ { "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "What is the purpose of a thermal fuse?", "acceptedAnswer": { "@type": "Answer", "text": "A thermal fuse acts as a safety cutoff for heat-producing equipment. It disconnects the power if the appliance overheats, preventing fires." } }, { "@type": "Question", "name": "Can you reset a thermal fuse?", "acceptedAnswer": { "@type": "Answer", "text": "No, a thermal fuse is a one-time use device. Once it blows due to overheating, the internal element melts and the fuse must be replaced." } }, { "@type": "Question", "name": "How do you test a thermal fuse?", "acceptedAnswer": { "@type": "Answer", "text": "The most reliable way is using a multimeter set to continuity mode. If the meter beeps or reads near 0 ohms, the fuse is good. If it reads infinity (OL), the fuse is blown." } } ] }, { "@type": "HowTo", "name": "How to Replace a Thermal Fuse in a Maytag Dryer", "step": [ { "@type": "HowToStep", "text": "Disconnect the dryer from the wall outlet and remove the rear panel screws." }, { "@type": "HowToStep", "text": "Locate the thermal fuse on the blower housing (white plastic strip with two wires)." }, { "@type": "HowToStep", "text": "Remove the two wires from the fuse terminals using needle-nose pliers." }, { "@type": "HowToStep", "text": "Unscrew the mounting screws and remove the old fuse." }, { "@type": "HowToStep", "text": "Install the new thermal fuse and reconnect the wires." }, { "@type": "HowToStep", "text": "Reattach the back panel and test the dryer." } ] } ]}

IntroductionIn September of this year, Apple's new generation of watch Apple watch series 6 launched the blood oxygen measurement function. In the following month or two, smart watches with blood oxygen measurement function were released together, vivo WATCH, Huawei Watch GT2, Honor Watch GS Pro, Hua Mi Amazfit GTR 2 and GTS 2 flagship smart watches almost all include this feature. In October, Huami Technology released the Amazfit Pop, which is priced at only 299RMB, and is also equipped with blood oxygen measurement. The blood oxygen sensor has begun to penetrate into low-end and mid-range computers, which may set off a greater wave.So, what exactly is the blood oxygen measurement function on the smart watch? Is it reliable? After you finish reading this article, you will get the answer.CatalogIntroductionCatalogI Blood Oxygen & Blood Oxygen Saturation 1.1 What is Normal Blood Oxygen Level? 1.2 How Important is Blood Oxygen Saturation?II How Does a Smart Watch Measure Blood Oxygen?III What is the use of Measuring Blood Oxygen Saturation?IV Is it Reliable to Measure Blood Oxygen with a Smart Watch?V Comparison with Pulse Oximeter 5.1 What is a Pulse Oximeter? 5.2 How Does a Pulse Oximeter Work? 5.3 Main Components 5.4 Can a Smart Watch be Used as a Pulse Oximeter?VI Is it Necessary to Have an Oximeter at Home During Covid-19? 6.1 Can a Pulse Oximeter Diagnose Covid-19? 6.2 Is it Necessary to Buy a Pulse Oximeter?VII FAQI Blood Oxygen & Blood Oxygen Saturation1.1 What is Normal Blood Oxygen Level?Blood oxygen refers to the oxygen in the blood, and the human body's normal blood oxygen saturation is above 95%. The higher the oxygen content in the blood, the better the human metabolism. Of course, high blood oxygen content is not a good phenomenon. The blood oxygen in the human body has a certain degree of saturation. Too low will cause insufficient oxygen supply in the body, and too high will cause cell aging in the body.Figure1. Blood Oxygen1.2 How Important is Blood Oxygen Saturation?The cells in the human body rely on oxygen to survive. The oxygen that enters the human body through the respiratory system will be combined with hemoglobin and transported to various organs. Blood oxygen saturation is a measure of the percentage of hemoglobin combined with oxygen, so blood oxygen saturation is a key indicator to measure the health of the body. This parameter can be used to understand the oxygen content in the human blood. Under normal circumstances, the normal blood oxygen saturation is between 95% and 100%. If the blood oxygen saturation is less than 90%, it can be considered as hypoxemia. When blood oxygen saturation is too low, it means that there will be hypoxia, which will affect the central nervous system, liver, kidney and other important organs. Therefore, blood oxygen saturation is an important indicator, and the measured data can assist in judging the current physical health.Figure2. How Important is Blood Oxygen Saturation?II How Does a Smart Watch Measure Blood Oxygen?The blood oxygen measurement function of the smart watch actually judges health by measuring the human arterial blood oxygen saturation. The blood oxygen saturation specifically refers to the percentage of hemoglobin combined with oxygen in the blood, that is, the concentration of blood oxygen in the blood. Generally speaking, if the blood oxygen saturation is below 94%, it will be regarded as insufficient oxygen supply. Many clinical diseases will cause insufficient oxygen supply, which directly affects the normal metabolism of cells, and in severe cases, it can be life-threatening. Therefore, blood oxygen testing is very important for clinical medicine. The most primitive method of tracing blood oxygen measurement requires blood sampling first, and then electrochemical analysis by a blood gas analyzer to obtain the blood oxygen saturation. This method has complicated steps and cannot achieve continuous detection. However, with the development of clinical medicine, non-invasive blood oxygen measurement is now widely used. As long as a finger pressure photoelectric sensor is worn for the patient, continuous blood oxygen detection can be realized. The essence is to use red light with a wavelength of 660nm and near-infrared light with a wavelength of 940nm as the intake light source, measure the light transmission intensity through the tissue bed, calculate the blood oxygen concentration and blood oxygen saturation, and display the results by the instrument. The principle of measuring blood oxygen with a smart watch is similar to acupressure measurement, but the difference is that the part illuminated by the light source of the watch is the wrist, which is not as "transparent" as the finger. Visible light and infrared light cannot penetrate, so it is more challenging. However, as a wearable device with a very high frequency of use, the stimulation brought by the development space of smart watches is far greater than the challenges they need to face.Figure3. How Does a Smart Watch Measure Blood Oxygen?III What is the use of Measuring Blood Oxygen Saturation?Because the blood oxygen saturation can be detected anytime and anywhere through smart wearable devices, the application scenarios of blood oxygen detection are very wide. (1) Assist in judging the state of sleep breathingIntermittent apnea may occur during sleep, which may cause insufficient oxygen supply. Through continuous blood oxygen detection, the blood oxygen saturation data during sleep can be recorded, and the data can be used to analyze whether there is hypoxia during sleep, so as to determine the sleep health status. In addition, like some people who often work overtime, they can also use the blood oxygen saturation detection function to judge the current state. If the blood oxygen saturation is low, you may need to take a quick rest, and that is friendly to your health.Figure4. Judging the State of Sleep Breathing by Blood Oxygen Saturation (2) Monitor the physical state during exerciseThe blood oxygen saturation detection function not only allows you to know your physical condition at any time in your daily life, but also can play a better health support role in some special scenes. For example, in outdoor extreme mountaineering and other sports scenes, you can know your physical condition at any time through blood oxygen saturation, determine whether you need to rest or adjust the exercise intensity, so as to better cope with various sports scenes.Figure5. Monitor the Physical State During Exercise (3) Monitor the health of parentsGenerally, the APP on the mobile phone can synchronize the health data of the smart wearable device, so even if you are away from home, as long as you equip your parents with a smart wearable device, you can learn about your parents’ blood oxygen data and some other health data through remotely synchronized data. , adding more details for your concern.IV Is it Reliable to Measure Blood Oxygen with a Smart Watch?At present, the blood oxygen measurement function on a smart watch has not been certified by NMPA or FDA, that is, it has reached the level of medical diagnosis. The blood oxygen monitoring module of the smart watch consists of three parts: an optical sensor, a front-end signal acquisition system, and an algorithm. The principle is based on the different absorption rates of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the blood to red light and infrared light, the red light and infrared light are irradiated to the skin through an optical sensor, and then the blood vessels under the skin are obtained. After the reflection of the red light and infrared light, the blood oxygen is calculated by the algorithm. The optical sensor is the core of the blood oxygen detection module, which is usually composed of several LED lights and diodes. Apple's official website shows that the blood oxygen sensor equipped with Apple Watch 6 consists of four groups of LED light clusters and four photodiodes, and is integrated in the crystal glass back. If a smart watch is required to measure both heart rate and blood oxygen, it can be achieved through the same optical sensor. Ams has launched an optical sensor that integrates blood oxygen measurement and heart rate measurement. Li Minghao, field application engineer manager at ams, said, “Heart rate measurement usually uses green light, and blood oxygen measurement uses red light and infrared light. Compared with optical sensors that can only measure heart rate, optical sensors that can simultaneously perform two functions The composition usually adds several red and infrared LEDs.Figure6. Samsung Galaxy Series Smart WatchAs the core of blood oxygen measurement, the accuracy of optical sensors is very important to the accuracy of measurement results. For example, deviations in the wavelength of the red light produced by the diode may cause inaccurate measurement results. "Smartwatch measuring blood oxygen is a signal obtained by measuring capillaries. The signal is very weak. Usually this signal is submerged on it, and the feedback signal may be only about 1% useful. Therefore, the design of optics and filtering is very important." Li Minghao said.Currently, companies in the industry are taking various measures to improve accuracy. The senior technical staff of Huami Technology said, "Any sensor has various non-ideal errors. In terms of control errors, Huami conducts strict testing and evaluation of all key parameters when selecting optoelectronic devices. Each product of Mi has undergone multiple measurements on the production line to ensure that the accuracy of the factory product parameters is within the design range. In addition, at the beginning of the design, select the relatively flat waveband of the human skin spectral absorption curve to limit the wavelength error to the blood oxygen The impact is within an acceptable range." Increasing the number of sensors is also one of the ways to improve accuracy. The vivo watch is equipped with a self-developed 5-core optical heart rate blood oxygen sensor to achieve blood oxygen measurement. "5-core refers to a multi-sensor system composed of 5 sensors, in which the circuit and optical design are the results of vivo's independent research. Compared with the single sensor or multi-sensor design in the industry, it has a larger signal receiving area and receiving capacity. Stronger, can achieve more accurate monitoring effects." said a senior technical staff of vivo. However, judging from the actual effect of the current smart watch for blood oxygen measurement, although there is an improvement in accuracy, there is still a certain gap between achieving medical-level blood oxygen monitoring. When the news that Apple Watch 6's blood oxygen measurement function was inaccurate, Apple came out to clarify that this function is only for health reference, not as a medical diagnosis standard. The accuracy requirements of consumer and medical products are different. Taking the heart rate as an example, there is no problem with a few beats on the wrist, but in medical treatment, there can only be a difference of 1-2 beats.V Comparison with Pulse Oximeter5.1 What is a Pulse Oximeter?Pulse oximeter has been a common medical device since the 1970s. It is most commonly used for people with respiratory diseases and sometimes for athletes and pilots who must monitor blood oxygen levels. They are mainly used for clinical testing and monitoring, but for certain groups of people, they may also be used at home.Figure7. Pulse Oximeter5.2 How Does a Pulse Oximeter Work?Based on the change in light absorption during arterial pulsation. Two light sources located in the visible red spectrum (660 nanometers) and infrared spectrum (940 nanometers) alternately illuminate the tested area (usually fingertips or earlobes). The amount of light absorbed during these pulses is related to the oxygen content in the blood. The microprocessor calculates the ratio of the two spectra absorbed, and compares the result with the saturation value table stored in the memory to obtain the blood oxygen saturation. A typical oximeter sensor has a pair of LEDs that face a photodiode through a translucent part of the patient's body (usually a fingertip or earlobe). One of the LEDs is red light with a wavelength of 660nm; the other is infrared light with a wavelength of 940nm. The percentage of blood oxygen is calculated by measuring the two wavelengths of light with different absorption rates after passing through the body. 5.3 Main ComponentsA microprocessor, memory (EPROM and RAM), two digital-to-analog converters that control LEDs, a device that filters and amplifies the signal received by the photodiode, and an analog-to-digital converter that digitizes the received signal to provide the microprocessor . The LED and photodiode are placed in a small probe that is in contact with the patient's fingertip or earlobe. Pulse oximeters generally also include small liquid crystal displays. 5.4 Can a Smart Watch Replace a Pulse Oximeter?The answer is no. Fitness bracelets (including some Garmin devices) and smart watches with the function of detecting blood oxygen levels cannot be used as medical equipment. Garmin said that blood oxygen saturation can help you understand your body's adaptation to high altitude (especially for mountain sports and adventure), and it can also be a reminder for symptoms of sleep apnea or overwork during exercise. But it also clearly states that these data cannot be used for medical purposes, nor can it diagnose, treat, cure or prevent any disease or condition. The location where the device is worn may affect accuracy. The fitness tracker is worn on the wrist instead of the fingertips, it is easier to move when worn on the wrist, and the blood concentration on the skin surface is lower than that on the fingertips, so the obtained blood oxygen measurement may have a large error. However, although fitness trackers and smart watches are not medical devices, if they do detect abnormalities and alert you, then you should pay attention.Figure8. Apple WatchVI Is it Necessary to Have an Oximeter at Home During Covid-19?One of the powerful aspects of COVID-19 is that some patients with very low blood oxygen levels do not feel it themselves and are not aware of the severity of their illness. This has led some doctors (especially in the United States) to recommend pulse oximeters at home. Sounds reasonable, doesn't it? However, you must know that although they are commonly used in hospitals, their value for healthy people at home is limited. Equipment problems or incorrect use may cause inaccurate readings, so it is not wise to rely solely on oximeters without comprehensive diagnostic support from medical staff. 6.1 Can a Pulse Oximeter Diagnose Covid-19?The pulse oximeter may indicate a problem with the blood oxygen level, which may be related to the coronavirus, but it is only part of a comprehensive diagnosis. Blood oxygen saturation can help clinical decision-making, but it cannot replace clinical evaluation, nor can it be diagnosed alone. Some doctors suggest that for patients with suspected symptoms of the new crown but not serious enough to be hospitalized, they can consider using a pulse oximeter at home for monitoring.Figure9. Covid Prevention6.2 Is it Necessary to Buy a Pulse Oximeter?If you don’t have any breathing problems and have never used it before, then you don’t need to buy a pulse oximeter. It is usually only used if recommended by a doctor. Professor Xu, a clinical assistant professor at the University of Hong Kong and honorary consultant of respiratory and intensive care medicine at the Royal Free Hospital in London, said that pulse oximeters are still useful as early home self-monitoring equipment. Those who are weak, prone to respiratory failure, suffer from chronic respiratory diseases or need oxygen therapy at home can prepare one. Some people want to buy one for a self-test to find out their "normal" oxygen level, just in case. In fact, if you are not a suspected patient and have no symptoms, there is no need to buy one. Without the help of medical personnel, the measured readings are not very useful, and if you have not used them before, misuse or misinterpretation of the numbers is easy to happen. Dr. Andy Whittamore, the clinical director of the British Asthma Association and the British Lung Foundation, suggested: "Any surveillance at home should be part of the diagnosis, but not a substitute for clinical advice. All people who are concerned about their symptoms should see a doctor. In addition, like many coronavirus-related products, such as masks, due to the promotion of pulse oximeters, demand has also begun to increase, so beware of unscrupulous sellers selling substandard products at high prices.  VII FAQ1. Can a smartwatch measure blood oxygen?Thankfully, tech companies like Apple, Samsung, Realme, and Garmin have been experimenting with Sp02 sensors on their wearables to allow users to monitor not just blood oxygen saturation but other health data, on the go. 2. How do smartwatches measure oxygen levels?Both medical and wearable oximeters use light to do so. Typically, a pulse oximeter features two LEDs with different light wavelengths — one red and one infrared. ... Most recent watches and fitness trackers usually have sensors that provide pulse ox sleep tracking too. 3. How is oxygen level in blood measured?A pulse oximeter measures how much light is absorbed by your blood. This tells us how much oxygen your blood contains. The pulse oximeter shines 2 lights through your fingertip or earlobe: one red light and one infrared light.  

CatalogⅠ What is a Ballast Resistor ?Ⅱ Types of Ballast Resistors2.1 Fixed Resistors2.2 Self-Variable ResistorsⅢ The Working Principle of  Ballast ResistorsⅣ Ballast Resistor – Uses and Applications4.1 Ballast Resistor in Fluorescent Lamps4.2 Ballast Resistor for Automotive Applications4.3 Ballast Resistor in a LED CircuitⅤ Symptons of Failture Ballast-ResistorⅥ Frequently Asked Questions about Ballast resistors Introduction  “Ballast” is generally considered as “something that provides stability in English dictionary.  ” Thus, when we refer to an electrical ballast, we are referring to an electrical device that is important in maintaining the stability of the electrical circuit. However, you might be wondering how it provides stability. Depending on the circuit in which they are used, some electric ballasts limit current while others limit voltage. By doing so, they reduce the risk of overvoltage or overcurrent in the circuit, thereby improving system stability. The complexity of electric ballast varies greatly. It can be as simple as a resistor, capacitor, inductor, or a combination of these, or as complex as the electronic ballast found in fluorescent lamps.  Ⅰ What is a Ballast Resistor ? A ballast resistor is an electronic component that is usually used to regulate the current in a circuit. Some devices, such as fluorescent lamps, can exhibit negative resistance characteristics, and an increase in current will result in a decrease in voltage. Negative resistance can cause damage to the power supply or equipment. Ballast resistors are usually connected in series with the negative load, taking advantage of the fact that all components in the series circuit receive the same current. Some ballasts are just series resistors, while others use capacitors and more complex components. Figure1: complex components Ⅱ Types of Ballast ResistorsA resistor is a current-limiting electronic component that can reduce circuit voltage and current. Ballast resistors are classified into three types: fixed, variable, and reactive.2.1 Fixed ResistorsThis genre of ballast resistors have a fixed resistance. A high resistance value is primarily taken considered for most applications. This genre of ballast resistor is commonly used in simple circuits with low-powered loads such as neon or LED lamps. This fixed resistor is also applied to control the ventilation fan speed. It employs a fixed ballast resistor with two center taps. The fan speed selector switch detours portions of the ballast. Therefore, the entire ballast resistor is suitable for full speed, while no section of the ballast resistor is suitable for the low speed.2.2 Self-Variable ResistorsThese ballast resistors have the property of resistance in response to changes in current, such as an increase in current increasing resistance and a decrease in current decreasing resistance. Incandescent lamps frequently use these ballast resistors. As the current through the lamp increases, the ballast resistor heats up, and the resistance rises with the temperature, as does the voltage drop across the resistor. When the current is down, the temperature of the ballast resistor decreases as well, as does its resistance and thus the voltage drop. The benefits of using this type of ballast resistor are that it provides more precise current control than a suitable fixed resistor. Another benefit is that the power lost in the resistive ballast is decreased because a smaller section of the overall power is lost in the ballast when compared to a fixed resistor. Ⅲ The Working Principle of  Ballast Resistors When 220V 50HZ AC power is applied to the switch closed circuit, the current flows through the ballast, and the lamp filament starter heats the filament（The starter was turned off in the beginning. The gas arc discharge in the jumping bubble in the starter caused the bimetallic sheet to heat and deform due to the application of an AC voltage greater than 190V, and the two electrodes were close together to form the filament heated by the passage.) Because there is no arc discharge when the two electrodes of the starter are close together, the bimetal cools and the two poles separate. As the ballast is inductive, when the circuit is suddenly interrupted, 600V is generated at both ends of the lamp for about 1ms. The pulse voltage is -1500V; the exact voltage value depends on the type of lamp. When the lamp discharges, the voltage at both ends drops immediately. At this point, the ballast limits the lamp current on the one hand while also supplying power on the other. There is a phase difference of 55° to 65° between the voltage and the working current of the lamp in order to keep the secondary starting voltage of the lamp stable.Figure2:  the working principle of  a ballast resistor Due to its simple structure, inductive ballast, as the first type of ballast to work with fluorescent lamps, has a relatively large market share. However, on account of its low power factor, poor low-voltage startup performance, heavy energy consumption, stroboscopic and many other shortcomings , its market is gradually being replaced by electronic ballasts. The energy consumption of inductive ballasts: 40W (lamp tube power) + 10W (inductive ballast self-heating consumption) is equal to the total power 50W consumption of the whole set of lamps and lanterns. An electronic ballast is a converter that converts low frequency alternating current power to high frequency alternating current power. Its basic operation is as follows: after passing through a radio frequency interference (RFI) filter, full-wave rectification, and a passive (or active) power factor corrector (PPFC or APFC), the industrial frequency power supply becomes a DC power supply. The output of high-frequency AC power of 20K-100KHZ is added to the LC series resonant circuit connected to the lamp via the DC/AC converter to heat the filament, but the lamp is "discharged" into the "on" state and then enters the light-emitting state. At this point, the high-frequency inductor limits the current increase, ensuring that the lamp tube receives the voltage and current required for normal operation. Various protection circuits, such as abnormal protection, surge voltage, and current protection, temperature protection, and so on, are frequently constructed to improve reliability. Ⅳ Ballast Resistor – Uses and Applications Ballast resistors are most commonly used to adjust the current to a negative resistance load. They can also be applied in other contexts. Fixed ballast resistors are common in low-power devices like light-emitting diodes (LEDs) and neon lights. LEDs are a positive resistive load that can benefit from ballast resistors. 4.1 Ballast Resistor in Fluorescent Lamps Fluorescent lighting, as we all know, is a popular and efficient lighting system. However, there is a disadvantage to using this type of lighting system. When directly connected to a voltage source, it heats up very quickly. This situation is due to the lamp's uncontrollable current draw as soon as its operation. A ballast resistor, connected in series with the lamp, is applied to the circuit to prevent overheating caused by excessive current draw. Therefore, the function of ballast resistor is to regulates the current and reduces the voltage. However,  for the lamp to light up, an arc has to be formed between its two electrodes. This necessitates a high starting voltage that is nearly equal to the supply voltage. The ballast resistor provides the required voltage during startup, and then immediately after an arc is established, it reduces the voltage while also regulating the current flow. Figure3: application in fluorescent lamp 4.2 Ballast Resistor for Automotive Applications Ballast resistors are generally included in the ignition kits of automotive machines such as automobile engines. Such devices are often called Ignition Ballast Resistors because of their application. The application of this device reduces the possibility of coil failure. It is connected between the ignition coil's primary voltage source and the coil stud. This connection helps to reduce the coil voltage and coil current, so the coil does not get as hot as it would without it, extending the coil's life. However, a high voltage equal to the primary voltage source is required to start the ignition engine. As a result, a jumper wire is frequently connected to the ballast resistor. This jumper wire provides the voltage required to start the engine.  Figure4: application in automotives 4.3 Ballast Resistor in a LED Circuit If the source voltage in an LED circuit is greater than the rated voltage of an LED lamp, the LED may be damaged. It is strongly advised to connect a ballast resistor in series with the lamp to avoid this case. By connecting the ballast resistor in this manner, the voltage across the LED is down to a tolerable level. The circuit diagram for the same is shown in the figure below.  Figure5: application in leds  The following formula is the value of resistance of the ballast :R = (E – Vf)/IFWhere: R= resistance of the ballast resistorE = voltage sourceVf = forward voltage of the LEDIF =Forward current of the LED. So, let's say you have an LED with a voltage rating of 4 volts, a forward current of 10 mA, and a voltage source of 6 volts. This means that the voltage across the LED should be 4 or less than 4 volts. As a result, the resistance of the ballast resistor should be R= (6-4)/0.010 = 200 or greater. Now that we've covered the applications briefly, let's look at the different types of ballast resistors on the market. Ⅴ Symptons of Failture Ballast-Resistor A ballast resistor is a device in your car that limits the amount of current flowing through an electric circuit. Because they did not have the benefit of circuit boards like most modern vehicles, ballast resistors are commonly found in older vehicles. Normal wear and tear can damage the ballast resistor over time, so there are a few things to look for if you suspect a bad or failing ballast resistor needs servicing. 1. Vehicle starts, then immediately goes out The most obvious symptom is that the vehicle starts but then dies as soon as you remove the key. If this occurs, Your Mechanic's experts will be able to measure the voltage coming from the ballast resistor and determine if it needs to be replaced. They will inform you of the condition of your ballast resistor once they have read the voltage. 2. Not starting at all The vehicle will not start if the ballast resistor is not functioning properly. Because it is an electrical system, it is best left to the professionals. The only way to get the vehicle running again is to replace the ballast resistor. 3.  Don’t jump the resistor Some people attempt to jump the resistor, which means that the ballast resistor is ignored and the extra current is routed to the points. The points are not designed to withstand the additional voltage, causing them to wear out and fail prematurely. This will result in a much more extensive repair than if the ballast resistor was replaced at the start. Furthermore, because you are tampering with electricity, it can be dangerous, especially if you don't know what you're doing. 4.  Let the vehicle beIf your ballast resistor is out of work, your vehicle will not start and you will need to have it towed to a mechanic's shop. Because the professionals at Your Mechanic make house calls, you will be able to save money on towing. Furthermore, because the vehicle will not start, it is not a dangerous situation as long as you leave it alone. Do not attempt to bypass the ballast resistor and do not continue to try to start the engine. Allow the professionals to repair it so you can get back on your way. Ⅵ Frequently Asked Questions about Ballast resistors 1. Is a ballast resistor necessary?　If the coil required a ballast resistor when it was used with points, then you must still use the resistor. If it didn't, then no resistor is necessary. The ballast resistor keeps the engine running by preventing the engine from receiving full amperage from the ignition once started.　 2. Why do ignition coils need ballast resistor?　In simple terms, the ballast resistor in a Mopar limits the amperage, or current flow, through the coil while the engine is running, thereby extending the life of the coil and breaker points of the distributor. 3. How do you know if a ballast resistor is bad?To test your ballast resistor you need an ohm meter or multimeter set to ohms. Remove the connectors from both sides of the resistor. The ohms should read between 1.8 and 5 ohms. You should be getting 9 volts to the positive side of the coil. 4. What causes a ballast resistor to fail?The resistance through the ballast resistor varies with current flow which varies with engine rpm. This naturally causes the resistor to expand and contract from heat. That's why they eventually fail. 5. Can a resistor wire go bad?　　The only way the wire will "go bad" is if it gets a break in it so it has infinite resistance (an open circuit). 6. Which circuit the ballast resistor is used?　　Ballasts can also be used simply to limit the current in an ordinary, positive-resistance circuit. Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system. 7. What is inside of a ballast?　A magnetic ballast (also called a choke) contains a coil of copper wire. The magnetic field produced by the wire traps most of the current so only the right amount gets through to the fluorescent light. That amount can fluctuate depending on the thickness and length of the copper wire. 8. What's the difference between a ballast and non ballast coil? Basically, a non-ballast coil is designed to produce full spark output with 12 volts on the input (+ terminal). A ballast coil is designed to produce the same spark output, but with only 6 to 9 volts on the input. 

Ⅰ IntroductionA high-pass filter (HPF), also called a low-cut filter or bass-cut filter, passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. That is, unnecessary low-frequency components or low-frequency signal interference are removed. High-pass filters use the same two topologies as the low-pass filters: Sallen-Key and MFB. The only difference is that the positions of the resistors and the capacitors have changed. In other words, High-pass filters are complementary to low-pass filters.Figure 1. High Pass RC Filter CircuitCatalogⅠ IntroductionⅡ High-pass Filter Basic2.1 Terminology2.2 High-pass Filter Circuit2.2 Cutoff FrequencyⅢ High-pass Filter Types3.1 Passive High-pass Filter and Active High-pass Filter3.2 First-order High-pass Filter and Second-order High-pass FilterⅣ High-pass Filter Transfer FunctionⅤ High-pass Filter OrderⅥ Difference between High-pass Filter and Low-pass FilterⅦ High-pass Filter Application7.1 Question7.2 AnswerⅡ High-pass Filter Basic2.1 TerminologyA high-pass filter is a combination device of capacitors, inductors, and resistors that allows signal components above a certain frequency to pass through and greatly suppress signal components below that frequency. Its characteristics can be described in time domain and frequency domain by impulse response and frequency response respectively. The latter is represented by a function whose frequency is an independent variable. The latter is a function representation of frequency as an independent variable, which is generally a complex variable function with complex variable jω as an independent variable, expressed as H (jω). The H(ω) and amplitude φ(ω) are functions of angular frequency ω, which are called the "amplitude-frequency response" and "phase-frequency response" of the system, respectively. They show the signal components of different frequencies in the excitation source through the amplitude and phase changes encountered in this system. It can be proved that the "frequency response" of the system is the of the "impulse response" of the system, based on the Fourier Transform. When a linear passive system can be represented by an N-order linear differential equation, the frequency response H (jω) is a rational fraction, and its numerator and denominator correspond to the right and left sides of the differential equation, respectively.RC High Pass Filter Explained 2.2 High-pass Filter CircuitBasic high-pass filters are constructed using resistors with capacitors or inductors. The high-pass filter composed of resistors and capacitors is called a high-pass RC filter, and the high-pass filter with resistors and inductors is called a high-pass RL filter.Figure 2. Simple Passive High Pass RC Filter Circuit2.2 Cutoff FrequencyGenerally, the cutoff frequency of the filter refers to the right frequency point of the low-pass filter passband and the left frequency point of the high-pass filter passband, that is, the frequency response point of the filter. Usually defined by 1dB or 3dB relative loss point. For a high-pass filter, its cutoff frequency is that frequency at which the output (load) voltage equals 70.7% of the input voltage (source). The high-pass filter is based on the insertion loss at a sufficiently high -passband frequency without parasitic stopband. Figure 3. High Pass Filter Cutoff Frequency Ⅲ High-pass Filter TypesThe following two classification methods are independent of each other. Active high-pass filters are more common, such as first-order active high-pass filters and second-order active high-pass filters.3.1 Passive High-pass Filter and Active High-pass FilterAccording to the different part devices, it can be divided into passive high-pass filter and active high-pass filter. Passive high-pass filterA kind of filters composed of passive components (resistor R, inductor L, and capacitor C), which is constructed using the principle that the reactance of capacitors and inductive components changes with frequency. The advantages of this type of filter are: the circuit is relatively simple, no DC power supply is required, and the reliability is high. The disadvantages are: the signal in the passband has energy loss, the load effect is more obvious, and electromagnetic induction is easily caused when using inductive components. When L is large, the volume and weight of the filter are relatively large, and it is not suitable in the low frequency domain. Active high-pass filterA filter consists of passive components (usually R and C) and active devices (such as integrated operational amplifiers). The advantage of this type of filter is that the signal in the passband has no energy loss, but also can be amplified, the load effect is not obvious, and the mutual influence is small when the multi-stage cascade is connected. It is easy to form a high-order filter using the simple method of cascade, and the filter is small, light, and does not require magnetic shielding (because no inductive components are used). The disadvantage is that the passband range is limited by the bandwidth of active devices (such as integrated op amps) and requires a DC power supply, and reliability is not as high as the passive filters, thus it is not suitable for high voltage, high frequency and high power.Figure 4. LM741 Active High-pass Filter Circuit 3.2 First-order High-pass Filter and Second-order High-pass FilterAccording to the mathematical characteristics, it is divided into a first-order high-pass filter and a second-order high-pass filter, third-order high-pass filterand so on.Figure 5. Circuit Diagrams of High-pass Filter First Order High Pass FilterThe first order high-pass filter requires a capacitor with a very high capacity, which causes very high fundamental losses, and therefore, it is rarely used.Figure 6. First-order High-pass FilterThe derivation process is as follows: CV = Q (C represents capacitance, V voltage, Q electric quantity)Loop current，whereLaplace transform: Differential form:,  Second-order High Pass FilterThe second-order high-pass filter has the best performance, but causes higher fundamental losses compared with the other type. The second-order filter means that the filter contains the second-order differential in the time domain expression, or the highest order of s of the transfer function denominator is 2, and the gain of the filter to DC component is 1.Second-order High Pass filter Circuit (voltage controlled)Figure 5. Second-order High Pass filter CircuitPassband gain:Transfer function:Frequency response:where, the expressing formula can be foundConclusion: When f is less than f0, the slope of the amplitude-frequency characteristic curve is + 40dB / dec; when Avp is greater than or equal to 3, the circuit self-excited. Multichannel feedback high-pass filterFigure 6. Multichannel Feedback High-pass FilterVoltage transfer function of this circuit:Relationship between circuit parameters and components:This circuit is designed with equal capacitance, that is, let C2 = C3 = C, C1 = | K | C, there is Ⅳ High-pass Filter Transfer FunctionHow to determine the transfer function of each high-pass filterThe high-order filter consists of a cascade of second-order filter sections and first-order filter sections, and each filter section has a specific transfer function. In addition, the cutoff frequency and quality factor Q should be determined for the second-order filter sections, and for the first order sections, only the cutoff frequency is to be determined. The general form of voltage transfer function of second-order high-pass filter:Wc is the cut-off frequency, and the amplitude-frequency characteristic around the cut-off frequency is closely related to Q. K is the passband gain, which is the amplification factor when the frequency tends to infinity in the high-pass filter. The general form of voltage transfer function of first-order high-pass filter:The meaning of Wc and K is the same as that of the second-order high-pass filter, but there has no Q value.When designing and calculating, to determine these parameters, it is necessary to check the normalized pole table of the filter to complete it through a certain conversion. (Whether a low-pass, high-pass, band-pass, or band-reject filter, its frequency characteristics can be obtained by frequency coordinate transformation of the low-pass filter, so the low-pass filter also called a original filter.)Figure 7. High-pass Filter Block Diagram A common voltage-controlled voltage source high-pass filter circuit is used as an example. Its circuit diagram and voltage transfer function are as follows:Figure 8. Voltage-controlled High-pass Filter CircuitComparing this formula with the general expression of the transfer function of the second-order high-pass filter, we can know the cutoff frequency in the voltage-controlled voltage source high-pass filter circuit: Ⅴ High-pass Filter OrderHow to determine the order of the high-pass filterThe order of Butterworth high pass filter isWhere n represents the order of filter, FC is the cut-off frequency of - 3dB, F2 is the frequency of a specified attenuation within the transition band, and A2 is the attenuation at F2. The order of Chebyshev high pass filter is:Where n is the filter order, fc is the equal ripple cutoff frequency, a1 is the gain fluctuation (dB) in the passband, f2 is the frequency of a specified attenuation in the transition band, and a2 is the attenuation at f2 Volume (dB).The fc in the above formula represents the equal-ripple cut-off frequency, that is, the frequency of attenuation in the transition zone at a1. If the -3dB cutoff frequency is used in the design, it must be converted to an equal ripple cutoff frequency to get the above formula. The following table shows the proportional relationship between the cut-off frequency such as the Chebyshev high-pass filter and the -3dB cut-off frequency.Table 1: Ratio FC / f-3db of equal ripple bandwidth to - 3dB bandwidth of Chebyshev high pass filtern23456a1=0.1dB1.94321.36901.21311.13471.0929a1=0.2dB1.67431.28351.15641.09921.0685a1=0.5dB1.38971.16751.09311.05931.0410a1=1dB1.21761.09491.05301.03381.0234 The high-order active high-pass filter is formed by cascading several second-order high-pass filters (first-order high-pass filters should be added for odd-order ones). Each filter is called a filter section, and each has its own independent and Q value. The filter formed after the cascade can constitute different types of filters, such as Butterworth filter and Chebyshev filter. Calculation of first-order high-pass filter section: Calculation of second-order high-pass filter section:  Ⅵ Difference between High-pass Filter and Low-pass FilterThe difference between a high-pass filter and a low-pass filter is that a high-pass filter allows high-frequency or AC component signals to pass and suppresses low-frequency or DC components. A low-pass filter is a filter that allows low-frequency or DC components in a signal to pass through and suppress high-frequency components or interference and noise. In general, the low-pass filter retains signals that are smaller than the cutoff frequency, while the high-pass filter retains signals that are larger than the cutoff frequency. Ⅶ High-pass Filter Application1) In the power system, a high-pass filter is used to filter out harmonics of a certain order and above during harmonic compensation.2) In audio systemThe most overlooked and most useful EQ tool is the high-pass filter, which can remove unwanted low-frequency noise from the audio source. The high-pass filter can be a part on the EQ equalizer, or it can be an independent plug-in or device. Usually our speakers, mixers or microphones are equipped with high-pass filters, because the low-cutting of the recorded human voice can make it easy to distinguish the noise, although this type of noise is very low and difficult to detect. For example, in most musical instruments, the high-pass filter is used to cut off the sound lower 100Hz. You will find that the sound work is miraculously clean, but this does not apply to low-frequency instruments such as kick drums and bass. Ⅷ Question Related to High-pass Filter and Going Further7.1 QuestionWhat is the bandwidth of high pass filter?7.2 AnswerThe bandwidth of the filter denotes the value of frequency from which signals are allowed to pass. For example, if the bandwidth of the high pass filter is given as 50 kHz it means that only frequencies from 50 kHz to infinity are allowed to pass. Frequently Asked Questions about High Pass Filter1. What is a high pass filter used for?A high-pass filter effectively cuts out the frequency response of a mic below a certain set point, allowing only the frequencies above this point to “pass” through as the audio signal. High-pass filters remove unwanted and excess low-end energy that otherwise degrades the audio signal. 2. How is the high pass filters formed?A high pass filter can be formed by placing a capacitor in series with an inverting gain stage. 3. What are the types of high pass filter?The various types of High Pass Filters include:High Pass R-C Filter CircuitHigh Pass R-L Filter CircuitInverted L type High-Pass-Filter CircuitT- Type High-Pass-Filterπ Type High-Pass-FilterHigh-Pass-Filter using Op-AmpButterworth High-Pass-Filter 4. When should I use high pass filter?A high-pass filter is a simple, but effective EQ curve that scoops out unwanted low frequencies from an audio source. Like most engineers, I use them at many points in my mixes to clean up woofy signals and tighten up arrangements. 5. What is the cutoff frequency of a high pass filter?The cutoff frequency for a high-pass filter is that frequency at which the output (load) voltage equals 70.7% of the input (source) voltage. Above the cutoff frequency, the output voltage is greater than 70.7% of the input, and vice versa.