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IntroductionThe transistor is one of the basic semiconductor components, which has the function of current amplification in electronic circuit. It is made of two PN junctions very close to each other on a semiconductor substrate. Two PN junctions divide the entire semiconductor into three parts: The middle part is the base area, and the two sides are the emitter and the collector. What is NPN Transistor? For BeginnerCatalogIntroductionⅠ NPN Transistor Arrangement and SymbolⅡ How Do NPN Transistors Work?Ⅲ NPN Transistor Uses: A Controllable ValveⅠ NPN Transistor Arrangement and SymbolBefore explaining the principle, let's first understand the basic structure and symbols of the NPN transistor. To identify the NPN transistor pins, it will be Collector (c), Base (b) and Emitter (e).Figure 1. NPN Transistor Structure and SymbolNPN transistor is composed of two N-type semiconductors and one P-type semiconductor. Generally, an NPN transistor has a piece of P-type silicon (the base) sandwiched between two pieces of N-type (the collector and emitter). The arrangement is shown in the Figure 1. Ⅱ How Do NPN Transistors Work?Here is the main description to illustrate the basic principle and function of NPN transistors.1) Current AmplificationThe following analysis is only for NPN silicon transistors. As shown in the figure above, we call the current flowing from the base B to the emitter E the base current Ib; the current flowing from the collector C to the emitter E is called the collector current Ic. The directions of these two currents are both flowing out of the emitter, so an arrow is used on the emitter E to indicate the current direction.The amplification function of the transistor is: the collector current is controlled by the base current (assuming that the power supply can provide a large enough current to the collector), and a small change in the base current will cause a large change in the collector current: the change in the collector current is β times the change in the base current, that is, the current change is amplified by β times, so we call β the magnification of the transistor (β is generally much larger than 1). If we add a changing small signal between the base and the emitter, it will cause a change in the base current Ib. After the change in Ib is amplified, it leads to a big change in Ic. If the collector current Ic flows through a resistor R, it can be calculated according to the Ohm's Law formula U=R*I, and the voltage on this resistor will change greatly. According to the voltage on this resistor, so we can get the amplified voltage signal. In short, the change satisfies a certain proportional relationship.2) Bias CircuitWhen the transistor is used in the actual amplifier circuit, it is also necessary to add a suitable bias circuit. There are several reasons for this. First of all, due to the non-linearity of the transistor's BE junction (equivalent to a diode), the base current must be generated after the input voltage reaches a certain level (for silicon tubes, 0.7V is often used). When the voltage between the base and the emitter is less than 0.7V, the base current can be considered as zero. However, in practice, the signal to be amplified is often much smaller than 0.7V. If no bias is applied, such a small signal is not enough to cause a change in the base current (because when it is less than 0.7V, the base current is all 0).Add a suitable current to the base of the transistor (called the bias current, and the resistor in the figure used to provide this current, is called the base bias resistor). When a small signal follows this bias current are superimposed together, a small signal will cause a change in the base current, and the change in the base current will be amplified and output on the collector. Another reason is meeting the requirement of the output signal range. If there is no bias, then only those increased signals will be amplified, but the decreased signals will be invalid (because the collector current is 0 when there is no bias, and it cannot be reduced). With bias, let the collector have a certain current in advance. When the input base current becomes smaller, the collector current can be reduced; when the input base current increases, the collector current increases. Both the reduced signal and the increased signal can be amplified.3) NPN Transistor SwitchLet's talk about the saturation mode of the transistor. As shown in the figure above, because of the limitation of resistance Rc (Rc is a fixed value, then the maximum current is U/Rc, where U is the power supply voltage), the collector current cannot increase indefinitely. When the base current increases and the collector current cannot continue to increase, the transistor enters a saturated state. The general criterion for judging whether the transistor is saturated is: Ib*β>Ic.In a saturation state, the voltage between the collector and the emitter of the transistor will be very small, which can be understood as a switch. In this way, when the base current is 0, the collector current is 0 (this is called the triode cut-off), which is equivalent to the switch off; when the base current is large, it is equivalent to the switch on. In cut-off and saturation state, a transistor is equal to a switch.4) Operational StateIf we replace the resistor Rc with a bulb in the above figure, then when the base current is 0, the collector current is 0, so the bulb is off. If the base current is relatively large (greater than the current flowing through the bulb divided by the magnification β), the transistor will saturate, and the bulb will light up. Since the control current only needs to be a little larger than β of the bulb current, a small current can be used to control the on and off of a large current. If the base current increases slowly, the brightness of the bulb will also increase (which is a saturation process).The figure below is a basic transistor switch circuit. The base should connect a base resistor (R2), and the collector connects with a load resistor (R1).Operational ModeNPNCut-offUne<UonUc>UbActiveUbe>UonUc>UbSaturationUbe>UonUc<UbNPN transistor uses the B-E current (IB) to control the C-E current (IC). The E pole has the lowest potential, and usually the C pole has the highest potential during normal amplification, that is, VC>VB>VE.NPN base extremely high voltage, the collector and emitter are short-circuit and low-voltage, and the collector and emitter are open-circuit.NPN is suitable for two situations:If the input is a high level and the output needs a low level, NPN is better.If the input is a low level and the output needs a high level, NPN is better.2N2222 NPN Transistor PinoutⅢ NPN Transistor Uses: A Controllable ValveNPN is a component that uses b (base) current Ib to drive the current Ic flowing through CE, and its working principle is much like a controllable valve.Figure 2. A Controllable ValveThe blue water flow in the thin pipe on the left impacts the lever to open the valve of the large water pipe, allowing the larger red water flow to pass through the valve. The larger the blue water flow, the greater the red water flow in the big pipe. If the magnification is 100, then when the blue water flow is 1 kg/hour, then 100 kg/hour of water is allowed to flow through the large pipe. The principle of the transistor is the same. When Ib (base current) is 1mA, a current of 100mA is allowed to pass through Ice.Figure 3. NPN Transistor DiagramLet's analyze this circuit. If its magnification is 100, and ignore the base voltage. The base current is 10V÷10K=1mA, so the collector current should be 100mA. According to Ohm's law, the voltage on Rc is 0.1A×50Ω=5V. Then the remaining 5V is on the C and E poles of the transistor. Now if we let Rb be 1K, then the base current is 10V÷1K=10mA, according to the magnification of 100, is Ic 1000mA? If it is really 1A, then the voltage on Rc is 1A×50Ω=50V. The power supply voltage has been exceeded, and the transistors have become generators? This is not the case. See below:Figure 4. NPN Transistor Compared to A ValveContinue the metaphor. When the control current is 10mA, the valve on the main water pipe is opened to allow 1A current to flow, but can 1A be realized? No, because there is a resistor on it, it is equivalent to a fixed valve. It is stringed on top of the main water pipe. When the opening of the lower controllable valve is greater than the opening of the upper fixed resistor, the water flow will not increase any more, but will be equal to the water flow passing through the fixed valve opening above. Therefore, it is useless to open the lower transistor to a large opening. Therefore, we can calculate the maximum current of the fixed resistor 10V÷50Ω=0.2A, which is 200mA. That is to say, in the circuit, the base current increases and the collector current also increases. When the base current Ib increases to 2mA, the collector current increases to 200mA. When the base current increases again, the collector current will no longer increase, and it will not move at 200mA. At this time, the upper resistor also acts as a current limiter.  Let us understand the status of the IO in the microcontroller.Figure 5. AT89S51/52 The circuits with 24 IO ports of P1-P3 in the single-chip microcomputer are as shown in the figure above. Usually the purpose of using electronic circuits is to allow devices to obtain a certain current to make them work. For example, to make light-emitting diodes bright, a current of more than 1mA is generally required. However, the single-chip microcomputer is a smart chip. It can make logical analysis and judgments by detecting the voltage value of each IO port, and outputs high or low voltage as the result signal. Therefore, it can be seen that the IO ports of the single-chip microcomputer focus on voltage, not the current flowing through R and the transistor. Here what is the relationship between the voltage and current of the IO port in the single-chip microcomputer?  Continue the water pipe example.Suppose we let the valve of R open larger and let the control valve below be fully closed. At this time, as shown in Figure 6, it can be seen that the pressure at point P is the same as the water tank. When we fully open the following control valve, as shown in Figure 7, the water will flow through the pipeline with a large flow, and the pressure at point P is 0 at this time. This principle is very similar to electronic circuits. The logic quantity measured at the output point P is 1 (power supply voltage) or 0 (0 potential) by transistor turning off or on. However, there is a problem with this process, that is, when the output of point P is required to be 0, the transistor will be turned on very large, and the current flowing through it will be very large. There are 32 IO ports on the single-chip microcomputer, which consumes a lot of power. Look at Figure 8. If we close the upper valve R very small and close the lower control valve fully, then the pressure at point P will still the same as the water tank, which is the same as in Figure 6 above. When we open the control valve greatly, as shown in Figure 9, although the pressure at point P is also 0, the flow of water passing through at this time is greatly reduced. In this way, we can either output 1 or 0. So very little water is consumed. The circuit in the single-chip microcomputer does exactly this. The resistance R on it is about 50K, and the maximum current is 5V÷50K=0.1mA. In other words, when P outputs 1, no current is consumed, and when P outputs 0, the current consumed is 0.1mA. Because of its large pull-up resistance R, for beginners, it is necessary to have certain methods to directly drive LEDs or other loads. Here to share the various situations when the IO port is connected to the load.Figure 10. AT89S51/52 & 74HC373Let's take a look at the situation of connecting TTL devices first. When P1.0 is connected to an input pin of 74HC373, and the input impedance of TTL is very high, about a few hundred K to M ohm level. We assume 500K resistor to P1.0 to ground. In this way, when the transistor is turned on, the P1.0 point is at a low level, and a current of 0.1mA flows through Rc and then through the transistor to the ground, and no current flows through Ri. When the transistor is cut off, the current flows through Rc and then flows to the ground through Ri. Due to the resistor voltage divider effect, there are partial voltages on Rc and Ri, and the voltage at point P1.0 is the divided voltage of Rc and Ri. Total current is 5V÷(50K＋500K)=0.009mA, then the voltage at point P1.0 is 0.009mA×500K=4.5V. TTL stipulates that output 2.4~5V is high level. So this connection is correct. Now let's take a look at the situation of using S51 to drive the LED.AT89S51 Correct ConnectionLet’s take a look at the situation in Figure 11. Obviously, only P1.0 is a high potential to light the luminous tube, so the transistor must be cut off. In this case, the current flows through Rc to the luminous tube and then to the ground. To make the luminous tube turn on, there must be a threshold voltage exceeding 2.1V at both ends of the luminous tube. Therefore, the current flowing through the luminous tube is (5V-2.1V)÷50K=0.058mA, which is too weak to conduct.Look at Figure 12. It can be seen from the figure that P1.0 must be at a low potential if the luminous tube turned on. The transistor of the P1.0 port must be turned on. At this time, the current flows all the way through Rc to the transistor and then to the ground. The other way consumes 2.1V on the luminous tube. Then current flows through with almost no resistance, but the maximum current of the triode of the IO port cannot exceed 15mA. If it exceeds, the triode will be burned out, so this connection method is incorrect. So how can these two connections be able to drive the light-emitting tube? See below: AT89S51 Incorrect ConnectionLooking at Figure 13, a resistor Ri is connected between P1.0 and Vcc. When the transistor is turned on, two currents will flow through its c, e pole, one is the 0.1mA current on the internal R, and the other is the current on Ri. In order to prevent the transistor from over-current and burn out, we must make sure the resistance value, Ri=5V÷15mA=0.333K, which is about 330 ohms. At this time, the current flowing through the transistor is about 15mA, and the light-emitting tube is not bright at this time. When the transistor is turned off, both currents will flow through the luminous tube. The current flowing through the internal resistance of S51 is (5V-2.1V)÷50K=0.06mA, which is so small that we can ignore it. The current flowing through Ri is (5V-2.1V)÷330Ω=0.0087A, which is 8.7mA. However, the current consumed when the luminous tube is off is greater than the current consumed when the luminous tube is on. If many IO ports are used to light up many LEDs, such a circuit is not economical.Look at Figure 14, after connecting a resistor in series with the luminous tube between Vcc and P1.0. When the transistor is turned on, the two currents will flow through the c, e after confluence. The current on the internal resistance is still 0.1mA. The current on the ce should be less 15mA. If exceeds 15mA, the resistance is determined as (5V-2.1V) ÷ 15mA = 0.193K, which is about 200 ohms. In this way, the current flowing through the luminous tube is about 15mA, and the luminous tube is on. When the transistor is cut off, it blocks the paths of these two currents, so no current is consumed. Low level P1.0 directly drives the light-emitting tube. It can be seen that this circuit consumes 15mA of current when the light-emitting tube is on, and does not consume current when it is off, so this circuit is effective. S51 direct drive digital tube generally also uses this principle. Frequently Asked Questions about NPN Transistor1. What is meant by NPN transistor?An NPN transistor is the most commonly used bipolar junction transistor, and is constructed by sandwiching a P-type semiconductor between two N-type semiconductors. An NPN transistor has three terminals– a collector, emitter and base. The NPN transistor behaves like two PN junctions diodes connected back to back. 2. How do NPN transistors work?The NPN transistor is designed to pass electrons from the emitter to the collector (so conventional current flows from collector to emitter). The emitter "emits" electrons into the base, which controls the number of electrons the emitter emits. ... The transistor is kind of like an electron valve. 3. What is a NPN transistor used for?NPN transistors are mainly used in switching applications. Used in amplifying circuit applications. Used in the Darlington pair circuits to amplify weak signals. NPN transistors are used in the applications where there is a need to sink a current. 4. Which is better PNP or NPN transistor?A NPN transistor has electrons as majority charge carriers whereas the PNP transistor has holes as majority charge carrier. ... mobility of electrons is more than hole,so as a result npn transistor are faster than pnp that's why they are preferred. 5. What does NPN mean?NPN stands for Negative, Positive, Negative. Also known as sinking.

Ⅰ AbstractIn portable electronic devices such as mobile phones, notebook computers, and small video cameras, lithium-ion batteries have developed rapidly due to their sound performance, such as high working voltage, large specific energy, long cycle life, low self-discharge rate, no memory effect and so on, which are compared with traditional NiCd batteries and NiMH batteries.Figure 1. Lithium-ion Movement in Li-ion BatteryCatalogⅠ AbstractⅡ Charging Characteristics of Lithium BatteriesⅢ Performance Description of Several Different Charging States3.1 On Standby3.2 Precharging3.3 Constant Current3.4 Constant VoltageⅣ Charging Process Analysis4.1 High Voltage Constant Current Mode4.2 Low Voltage High Current Mode4.3 High Voltage High Current ModeⅤ Li-ion Battery Charging Security5.1 Common Sense in the Daily Use of Batteries5.2 Charging RulesⅥ One Question Related to Lithium-ion Battery and Going Further6.1 Question6.2 AnswerThe charge and discharge of lithium-ion batteries do not transfer electrons through traditional methods, but energy changes occur through the entry and exit of lithium ions in the crystals of layered materials. Under normal charge and discharge conditions, the in and out of lithium ions cause changes in the interlayer spacing, but will not cause damage to the crystal structure, so lithium-ion batteries can be regarded as an ideal reversible battery. During charging and discharging, lithium ions come and go between the positive and negative electrodes of the battery, and they shake between the positive and negative electrodes like a rocking chair.Lithium-ion Battery Charging BasicCharging batteries is common for people's daily life, as we all know, Li-ion batteries play a very important role in our social life with their excellent performance, in order to get longest service life, proper charging of Li-ion batteries is essential. Li-ion battery charging mode is voltage limit and constant current, which is controlled by IC chip. The typical charging method is: detect the voltage of the battery to be charged firstly, if its voltage is lower than 3V, pre-charge is required necessarily, and the the charging current is 1 ≤ 10 of the set current. After the voltage rises to 3V, then transferring into the standard charging process. The standard charging process is: having constant current charging with set current. When the battery voltage rises to 4.20V, it is changed to constant voltage charging mode, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases till the current drops to 1/10 of the set charging current, the charging ends.The charging process of a Li-ion battery can be divided into three processes: trickle charging (low voltage precharging), constant current charge, and constant voltage charge. Ⅱ Charging Characteristics of Lithium BatteriesFigure 2. Typical Charge ProfileAs can be seen from the above figure, the charging current and voltage of the lithium battery are dynamically changed, which is determined by the chemical content of the lithium battery itself. Therefore, it is necessary to configure the performance of the charging IC according to the charging characteristics of the lithium battery itself to achieve a correct, safe and efficient use of the lithium battery. The "lithium-ion battery charging current" in the daily expression is for the charging current of fast charging. As a dynamic process, the optimal charging current of the lithium battery is actually divided into three stages. Ⅲ Performance Description of Several Different Charging StatesFigure 3. Li-ion Battery Process3.1 On StandbyThe standby state is handled in the following cases:1) The input voltage is lower than the minimum operating voltage of the circuit.2) After the battery voltage is approach to the limit.3) Using external switch to turn offmanagement IC to stop charge.Voltage and current characteristics in standby mode: The charging IC has no charging voltage output, and the IC input current is in the uA level, which can reduce power loss. 3.2 PrechargingAs shown in above figure. Optimal current during precharging: that is, when the initial/no-load voltage of the lithium battery is lower than the prechargeing threshold, it needs a pre-charging stage. For a single lithium-ion battery, this threshold is generally 3.0V, in the phase, the precharge current is about 10% of the current in the constant current charging phase. 3.3 Constant CurrentAs shown in the figure above, when the battery voltage is greater than the preset voltage threshold and less than the maximum voltage of 4.2V, the IC will charge the battery with the maximum charging current set by the external resistor. When the battery voltage is equal to the maximum charging voltage (near 4.2V), the charge stop.The best current for constant current charging: when stay in constant current stage, the voltage gradually rises, then enter the fast charging phase. Most of the constant current charging current is set between 0.5 and 1.0C, and the best set is 0.8C, because the battery can be full charged about two hours without consider other factors. The case is a good balance between charging time and charging safety.Several problems that should be paid attention to when batteries at constant current charging:1) In this state, the IC is in the state of maximum charging current, and the loss at this time is also the largest. The linear voltage drop loss calculation is L = (Vin-Vout) × Iout, it is necessary to pay attention to the maximum operating temperature of the IC.2) The increasing temperature due to the highest charging current, the IC will automatically reduce the maximum charge current, and this is why the charging current drops during overheating. 3.4 Constant VoltageThe maximum charging voltage portion shown in the above figure, when it is detected that the battery voltage is equal to or close to the battery charging voltage, at this time, the charging mode will be stepped down with a constant charging voltage of 4.2V. When it is detected that the charging current is less than 1/10 of the maximum set current, charging will stop. Charging current during constant voltage charging: In the case of a single-cell lithium-ion battery, as the battery voltage rises to 4.2 V, the constant current charging ends and the constant voltage charging stage begins. In order to achieve the best performance, the voltage stabilizer tolerance should be better than +1%.At this stage, the voltage is keeping constant and the current is reduced, and this current reduction is a sequential decrement process. Most lithium battery protection selects 0.1C as the termination current, which means that the charging process enters the end state. Once charging is finished, the charging current drops to zero. The problem to be noted in this state is that the battery can be automatically turned off when the battery is charged to the highest setting voltage. At the same time, when the overvoltage protection of the IC is in the abnormal battery state, it can be automatically locked. Unlike nickel batteries, continuous trickle charging is not recommended. Because it will cause plate plating effect to the lithium metal, making batteries failure.The core of the best charging current of lithium battery is the current design of constant current charging. It should be emphasized that most portable lithium batteries should be designed to charge 0.5C~0.8C. For example 1400mAh capacity of iPhone battery(capacity mAh= current mA × time /h), choosing 0.7C, that is, Apple’s charging current is about 1A, so that most of the batteries between 0.5C~0.8C you can choose.When charging, the voltage of the battery should be detected first. If the voltage is lower than 3V, pre-charging should be performed first. When the charging current is 1/10 of the set current, 0.05C is selected generally. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is constant current charging with set current. Till the battery voltage rises to 4.20V, it is changed to constant voltage charging, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases, and when the current drops to 1/10 of the set charging current, the charging ends.Generally, the charging current of the lithium battery is set between 0.2C and 1C. The larger the current, the faster the charging, and the greater the heat of the battery. Moreover, when lies in excessive current charging, the capacity is not full, because the electro-chemical reaction inside the battery takes time. Ⅳ Charging Process Analysis Figure 4. Charging Characteristics of Lithium-ion Battery 4.1 High Voltage Constant Current ModeIn general, the charging process of the mobile phone is to first reduce the 220V charging voltage to the 5V charger voltage, and the 5V charger voltage reduce to the 4.2V battery voltage. During the entire charging process, if the voltage is increased, heat is generated, therefore, the charger will heat up and the phone will heat up. Moreover, the greater the power consumption, the greater the damage to the battery. 4.2 Low Voltage High Current ModeWhen the voltage is constant, the current can be increased by using a parallel circuit. Under this situation, the smaller the volume shared by each circuit after parallel shunting, each circuit has the smaller load damage, so as to the phones charging process. 4.3 High Voltage High Current ModeThis method increases the current and voltage at the same time, so that from the previous formula P=UI, we can know that this method is the best way to increase the power, but it will generate more heat when the voltage is increased. In this way, the more energy is consumed, but the voltage and current are not freely increased without limitation.The maximum charging current of a lithium battery is strictly determined by the structure of the battery. Therefore, the specifications of the lithium battery manufacturers are not consistent, some are set to 0.6C, and the highest current specification for portable lithium batteries is 1C.  Of course, the current design of pre-charging and constant voltage charging cannot be ignored. In the two processes, if the initial voltage is not lower than the pre-charging threshold of 3.0V, there is no pre-charging process. In general, there is a process to check batteries charging voltage that is beneficial to keep the long-term use of lithium batteriess.Ⅴ Li-ion Battery Charging Security5.1 Common Sense in the Daily Use of BatteriesMisunderstanding: “Battery activation”, charging for more than 12 hours in the first three times.For the “activation” problem of lithium batteries, many sayings are: charging time must be more than 12 hours, and repeat three times in order to activate the battery. This statement that “the first three charges have to be charged for more than 12 hours” is obviously a continuation of nickel batteries (such as nickel cadmium and nickel hydride), in other words, this kind of statement can be said to be misinformation of the other batteries. After a sample survey, it conformed that a considerable number of people have confused the charging methods of the two batteries. Lithium-ion battery activation does not require a special method, they will be activated naturally in the normal use.The charge and discharge characteristics of lithium and nickel batteries are very different. All the professional technical data reviewed emphasize that overcharge and overdischarge can cause huge damage to lithium batteries, especially liquid Li-ion batteries. Therefore, charging is preferably performed in accordance with standard methods, especially for ultra-long charging of more than 12 hours. For example, the charging method described in the mobile phone manual is a standard charging method suitable for the mobile phone. It is not suitable to charge for a long time, also the battery is completely dischargedand thenThe lithium battery phone or charger will automatically stop charging when the battery is fully charged. There is no so-called “turbulent” charging over 10 hours for nickel battery chargers. If the lithium battery is fully charged, it will not be charged anymore continuously.Over-time charging and power off completely will cause over-charging and over-discharging, which will cause permanent damage to the positive and negative electrodes of lithium-ion batteries. At the molecular level, over-discharge will cause the anode carbon to release lithium ions excessively causing the layer structure collapses, and overcharging will hardly plug too much lithium ions into the negative carbon structure, and some of the lithium ions will no longer be released. Regular deep charge and discharge for battery calibrationLi-ion batteries generally have a management IC and a charge control IC. The management IC has a series of registers, which contain values such as capacity, temperature, ID, state of charge, and discharge times. These values will gradually change during use, so the main function of the “The batteries should be fully charged and discharged when used once a month or so” is to correct the improper values in these registers. 5.2 Charging RulesThe following rules should be noted when charging and discharging lithium ion batteries:Figure 5. Typical Li-ion Battery Discharging DiagramCharge currentItmust limited for li-ion batteries. Typically the maximum value is 0.8C, but lower values are more usually set to give some margin. Charge temperature  Itshould be monitored. The cell or battery must not be charged when the temperature is lower than 0°C or greater than 45°C. Short circuit protectionItis required to prevent damage or explosion as a result of short circuits. Over-voltage protectionItis required to prevent a voltage that is too high being applied across the battery terminals. Over-charge protectionItis required to stop the Li-ion charging process when voltage per cell rises above 4.30 volts. Reverse polarity protectionItis needed to make sure the battery is not charged in the wrong direction as this could lead to serious damage or even explosion. Over-discharge protectionItis required to prevent the battery voltage falling below about 2.3V dependent upon the manufacturer, when battery voltage less than 2.3V will make battery damage irreversibly. Over temperature protectionIt is necessary to prevent the battery operating in a high temperature, because heating will age batteries and reduce their service life. if the temperature rises too high. Temperatures above 100°C can cause irreparable damage.Ⅵ Questions Related to Lithium-ion Batteries1. How many years does a lithium ion battery last?three yearsThe typical estimated life of a Lithium-Ion battery is about two to three years or 300 to 500 charge cycles, whichever occurs first. One charge cycle is a period of use from fully charged, to fully discharged, and fully recharged again. 2. What is the difference between a lithium battery and a lithium ion battery?Lithium batteries feature primary cell construction. This means that they are single-use—or non-rechargeable. Ion batteries, on the other hand, feature secondary cell construction. This means that they can be recharged and used over and over again. 3. Why are lithium ion batteries bad for the environment?Recycling Lithium-IonUnwanted MP3 players and laptops often end up in landfills, where metals from the electrodes and ionic fluids from the electrolyte can leak into the environment. Because lithium cathodes degrade over time, they cannot be placed into new batteries. 4. Is there a better battery than lithium ion?Zinc-air batteries can be considered superior to lithium-ion, because they don't catch fire. The only problem is they rely on expensive components to work. 5. What is the best way to charge a lithium ion battery?Simple Guidelines for Charging Lithium-based BatteriesTurn off the device or disconnect the load on charge to allow the current to drop unhindered during saturation.Charge at a moderate temperature.Lithium-ion does not need to be fully charged; a partial charge is better.

IntroductionEarthing (also known as grounding) refers to the process of transferring the immediate discharge of the electrical energy directly to the earth by the help of the low resistance wire, for safety and functional purposes. Generally, the "ground" of electronic equipment has two meanings: one is to connect to the "earth". Taking the earth as the zero potential, connecting the metal shell of electronic equipment and the circuit reference point to the earth can protect the safety of equipment and personnel, such as protective earthing, lightning protection earthing, etc. In addition, the earthing in the weak current system does not necessarily mean the ground connected to the earth in the true sense. It has the effect of improving the stability of the system, shielding and protecting the electromagnetic compatibility of the system, and it can also be connected to the "earth" when necessary.What is Electrical Earthing?CatalogIntroductionⅠ Earthing Basic1.1 Electrical Earthing1.2 Earthing SymbolsⅡ What Are the Types of Earthing?Ⅲ Why Is Electrical Earthing Important?Ⅳ Earthing Q&A You Should KnowⅠ Earthing Basic1.1 Electrical EarthingAn earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground. Earthing is a therapeutic technique that involves doing activities that “ground” or electrically connect you to the earth. In modern earthing concepts, for line engineers, the meaning of this term is usually a reference point of line voltage; for system designers, it is often a cabinet or rack; for electrical engineers, it is safe earthing or connecting to the earth. A more general definition is a low impedance path for current to return to its source. Note that the requirements are "low impedance" and "path".1.2 Earthing SymbolsPE, PGND, FG: Protective ground or chassisBGND or DC-RETURN: Power supply (battery) returnGND: Work groundDGND: Digital groundAGND: Analog groundLGND: Lightning protection groundⅡ What Are the Types of Earthing?There are many types of earthing, including single-point earthing, multi-point earthing and mixed types of earthing. Among them, single-point earthing is divided into series earthing and parallel earthing. Generally speaking, single-point earthing is used for simple circuits, such as earthing distinctions between different functional modules, and low-frequency (f<1MHz) electronic circuits. When designing high frequency (>10MHz) circuits, multi-point earthing or multilayer boards (complete ground plane) should be used. The following are four specific earthing methods.1. Earth FloatingIn electronic design, a commonly used method is floating technology. In this method, the signal ground of the circuit board is not connected to the external public ground, thereby ensuring good isolation of the circuit. The circuit is well isolated from the external ground system and is not easily affected by the interference on the external ground system. However, static electricity is easy to accumulate on the circuit and cause electrostatic interference, which may generate dangerous voltage.Small-scale low-speed (<1mhz) equipment can use earth floating, a single-point connection to the ground by the metal shell.2. Single-point earthing in SeriesThis kind of earthing method is relatively simple, and there is no need to pay so much attention to the circuit board design. So it will be used more. However, this kind of circuit will have common impedance coupling, causing each circuit module to affect each other.3. Single point earthing in ParallelThis method of earthing, although getting rid of the common impedance coupling problem of series single-point earthing, but in actual use, it will introduce too much earthing wire annoying, as to which one needs to be comprehensively evaluated in the actual process. If the circuit board area allows, use the parallel mode, and if the connection between the various circuit modules is kept simple, then use the series mode. In general, there are power modules, analog circuit modules, digital circuit modules and protection circuit modules in the downloaded board. In this case, I use a parallel single-point earthing method.4. Multi-point EarthingMulti-point earthing is used more in daily circuit design, especially in multi-module circuit design. This earthing method can effectively reduce high-frequency interference problems, but it is also prone to cause earthing loops. This point must be fully considered in the design to improve the circuit stability. The working ground of small high-speed (>10MHz) equipment should be grounded at multiple points with its metal casing. The distance between earthing points should be less than 1/20 of the wavelength of the highest operating frequency, and the metal casing should be connected to the ground at a single point.In short, in the design of electronic circuits, the most important point is to reduce the loop area of the circuit, to improve the stability of electronic design and the EMC design of electronic systems. In the actual design, have comprehensive evaluation of the above various earthing technologies to achieve the purpose of improving system stability.Ⅲ Why Is Electrical Earthing Important?As for earthing function, the introduction of earthing technology was originally a protective measure to prevent electrical or electronic equipment from being struck by lightning. The purpose was to introduce the lightning current generated to the ground through the lightning rod, thereby protecting the building. At the same time, earthing is also an effective means to protect personal safety. When the phase line touches the equipment shell caused by some reason (such as poor insulation of wires, aging of wiring, etc.), the equipment shell will have dangerous voltages. The generated fault current will flow through the neutral line to the ground, thereby playing a protective role. With the development of electronic communication and other digital fields, it is no longer sufficient to consider only lightning protection and safety in the earthing system. For example, in a communication system, the interconnection of signals between a large number of devices requires each device to have a reference ground as the signal reference ground. And with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the earthing design, special attention must be paid to electromagnetic compatibility issues such as mutual interference between signals. Otherwise, improper earthing will seriously affect the reliability of system operation. Also, the concept of "earthing" has also been introduced in the high-speed signals return technology. Ⅳ Earthing Q&A You Should KnowThe following questions relay on electrical earthing science and grounding physics to explain how electrical charges from the earth can have huge effects on our life. And how do you discharge the electrical energy directly to the earth by earthing technology. Also these Q&A give you considerable attention for the earthing system design and installation.1. What is the difference between earth earthing and electrical earthing?The earth is an object with very low resistance and very large capacitance. It has the ability to absorb infinite charge, and meanwhile can maintain the potential unchanged. Therefore, it is used as the reference potential of a system electrically, that is electrical earthing. In addition, in electronic equipment, when transmitting current and signal conversion at various levels of circuits, a reference potential is required to prevent interference from external signals. This potential is called logical ground or floating ground.2. What is the difference between the ground potential and the logical ground potential?Since the earth can absorb infinite electric charge, the potential of the earth looks macroscopically zero. Due to the influence of the natural electric field and the artificial electric field in the earth, the potential of each point of the earth is different. In engineering, 20m away from the artificial electric field is regarded as zero potential (earthing potential). The electrical ground potential is related to the current injected into the ground by the electrical system. When a large current flows into the electrical ground, the electrical ground potential may reach a very high voltage, especially when the lightning current flows into the electrical ground. The instantaneous potential of the electrical ground can reach 100,000 volts. Therefore, a separate lightning protection earthing point cannot be located in a place where have pedestrians.3. What is the shell?Due to the damage of the insulation layer of the wire, the phase wire is in contact with the outer shell of the electrical equipment, which is called a bumping shell. If the insulation of the phase wires and the enclosure of the electrical equipment does not meet the specified requirements, the equipment cannot be put into use. The reason for the insulation drop may be moisture or damage to the insulation layer, which can be analyzed according to the environment in which the circuit equipment is used.4. What is the step voltage?When an electrical device has a short-circuit fault to the ground, the fault current flows from the fault ground to the ground electrode and returns to the power source. Therefore, an electric field is generated around the ground of the fault point and the ground electrode, which is away from the ground of the fault point or the ground of the ground electrode. The closer, the higher the potential, and the farther, the lower the potential. When the distance between the two feet of a person is about 0.8 meters, standing in this electric field, because the two feet are at different potential points, there will be a potential difference. This potential difference is called the step voltage.5. What is contact voltage?When the insulation of electrical equipment is damaged and a short-circuit occurs to the shell, people who touch the electrical equipment will have the risk of electric shock. To define the degree of danger, the potential of the equipment 0.8 meters away from the horizontal direction of the electrical equipment when it fails is measured. The potential difference between the two is called the contact voltage.6. What is the earthing resistance difference between the earthing electrode and the equipment?The ratio of the earthing voltage to the earthing current is called the earthing resistance of the earthing electrode. When measuring the earthing electrode resistance in a project, an ac voltage is artificially applied to the earthing electrode, and then the current flowing into the earthing electrode is measured. The ratio of the two is the earthing resistance. The earthing resistance of the equipment is the sum of earthing wires resistances.7. What are the classifications of earthing functions?Generally divided into two categories: protective earthing and functional earthing1) Protective earthing can be divided into the following 4 types:Protective earthing: earthing the exposed conductor part of the equipment is called protective earthing. Its purpose is to prevent electrical equipment insulation damage or leakage, which may cause electric shock when people touch it.Lightning earthing: Lead lightning into the earth to prevent electric shock or other property damage.Anti-static earthing: Introduce static charges into the ground to prevent the accumulation of static electricity from causing harm to the human body and equipment.Anti-corrosion earthing: Bury a metal body underground as a sacrificial anode or cathode to protect the metal body connected to it, such as a metal oil pipeline.2) Functional earthing can be divided into the following 4 types:Working earthing: In order to ensure the operation of the power system, earthing is done at an appropriate place in the power system, which is called working earthing. In an AC system, this point is generally a neutral point.Logic earthing: To obtain a stable reference voltage, the appropriate metal parts in the electronic equipment are used as the reference zero potential, and the electronic parts that need to obtain the zero potential are connected to this metal part. This method is called logic earthing.Shield earthing: Ground the metal shell or the metal net to protect the electronic equipment in the shell or the net from external electrical interference, or prevent the electrical equipment in the shell or the net from causing interference to external electronic equipment.Signal earthing: A earthing method set to ensure that the signal has a stable reference potential.8. What is working ground?In order to ensure the safe operation of the electrical device, the earthing of any point (usually the neutral point of the power supply) of the device conductive part is called the working ground.9. What is the relationship between the safety voltage and the use environment?The safety voltage is to prevent personal electric shock. The degree of electric shock is related to the impedance of the human body, and the impedance of the human body has a great relationship with the contact condition. Under different conditions, it is different.The relationship between human body impedance and contact conditions is usually divided into three categories:1) High impedance: dry skin, dry environment, high impedance ground2) Low impedance: moist skin, humid environment, low impedance ground3) Zero impedance: for example, the human body is immersed in water10. What is the difference between short circuit and ground fault?The electrical connection between mutually insulated live conductors due to insulation damage is called a short circuit. For example, between phase wires of different phases, or between a phase wire and a neutral wire, exist an electrical connection, there may be a short circuit. The electrical connection error between the live conductor and the earth is called a ground fault. In addition, live conductors refer not only to the phase line, but also the neutral line. The ground refers to the metal shell of grounded electrical equipment, non-electrical metal pipes and the earth.11. What parts of the earthing device consist of?earthing device is a general term for earthing electrode and earthing wire.The earthing electrode is a conductor buried in the soil or concrete foundation for dissipating current. It can be divided into two types: natural earthing electrode and artificial earthing electrode.There are several types of natural earthing electrodes: the underground metal plumbing systems, the metal structure of the building and the reinforced concrete structure.The artificial earthing electrode should adopt horizontally laid round steel, flat steel, metal earthing plate, and vertically laid angle steel, steel pipe, round steel, etc.12. What are the measures to prevent direct electric shock?Insulate charged objectsUse shields or barriers to block the human body from charged objectsUse leakage switch as additional protection13. What are the measures to prevent indirect electric shock?Set up automatic power-off deviceEquipment with double insulationTake ungrounded local potential connectionElectrical isolation14. What are the types of earthing systems for high-voltage systems?1) Direct earthing, that is, the neutral point of the transformer or generator is connected to the earthing device directly or through a small resistance (such as a current transformer). This kind of earthing method has a large earthing current when a single-phase earthing short circuit occurs, so it is also called the large current earthing system.2) Ungrounded, the neutral point of the transformer in this system is not grounded or connected to earthing equipment such as arc suppression coils, large resistances, and the earthing device.15. Can the natural earthing electrode be used for the earthing of DC electrical devices?The earthing of AC electrical installations should make full use of the natural earthing electrode buried in the ground. For the earthing of DC electrical installations, it is not allowed to use the natural earthing electrode as the PE wire, earthing wire and earthing electrode of the current pattern. The earthing device is connected to the natural earthing. The distance between earthing devices and AC electrical devices shall not be less than 1m to avoid electrical corrosion.16. What is the function of total equipotential bonding?The function of total equipotential bonding (MEB) is to reduce the contact voltage of indirect contact electric shock in the building and different metal parts with different potential, which eliminate the dangerous fault voltage introduced from outside the building through electrical lines and various metal pipes.17. What is supplementary bonding?The two conductive parts are directly connected with wires to make the contact voltage of the fault drop below the contact voltage limit, which is called supplementary or additional equipotential bonding (earthing). When the earthing device fails, the indirect contact protection conditions for automatically cutting off the power supply cannot be met, supplementary bonding should be set. It should also be installed in places with special requirements such as bathrooms, hospitals, and swimming pools.18. What is local equipotential bonding?Local equipotential bonding (LEB) refers to the connection of multiple supplementary equipotential bonding through the bonding terminals in a local board, which is called local equipotential bonding.19. How to check the conductivity of equipotential bonding?1) Welding quality inspection2) Bolt connection quality inspection3) Measure resistance between branch and trunk20. What are the characteristics of arc short circuits?There are two forms of short circuit and ground fault: metallic and arc short. The current of metallic short circuit is very large, which can make the overcurrent protector (circuit breaker or fuse) act in time and the fault is not easy to go on. The short circuit point of arc short circuit has arc or electric spark and the impedance is large, therefore, the short circuit current is small. So overcurrent protection will not take effect. However, the temperature of the arc short-circuit point is very high, which can reach thousands of degrees Celsius locally. It is very easy to ignite the substances around the short-circuit point and cause a fire.Arcing short circuit not only occur in electrical and earthing faults, but poor connections between wires can also cause it. For example, cause flickering of incandescent lamps or interference for TV sets. At this time, you must check whether the connection point of the line is reliable. Frequently Asked Questions about Electrical Earthing System Basics1. What is elecrical earthing and types of earthing?Earthing is the first step towards electrical safety. ... Earthing is done to provide safety to user from electric shock. It is a set of conductors connected in series or in parallel in order to dissipate the potential difference immediately into the ground. The wire connected from equipment to earth called earthing wire. 2. What is difference between earthing and grounding?The key difference between earthing and grounding is that the term “Earthing” means that the circuit is physically connected to the ground which is Zero Volt Potential to the Ground (Earth). Whereas in “Grounding” the circuit is not physically connected to ground, but its potential is zero with respect to other points.Difference between Earthing and Grounding 3. Is grounding the same as earthing?The key difference between earthing and grounding is that the term “Earthing” means that the circuit is physically connected to the ground which is Zero Volt Potential to the Ground (Earth). Whereas in “Grounding” the circuit is not physically connected to ground, but its potential is zero with respect to other points. 4. What is the purpose of earthing?Earthing is used to protect you from an electric shock. It does this by providing a path (a protective conductor) for a fault current to flow to earth. It also causes the protective device (either a circuit-breaker or fuse) to switch off the electric current to the circuit that has the fault. 5. Which wire is used for earthing?copper wiresEarthing Lead or Earthing JointEventhough copper wires are generally used as earthing lead, copper strips are preferred for high installation as it can carry higher values of fault current due to its wider area.

Ⅰ IntroductionThis article focuses on the electronic component known as the Optocoupler. (For the fiber-optic networking component, please refer to Optical Isolators). This guide covers the fundamentals of optocouplers, their working principles, specifications, and practical examples of how to implement them in your circuits.Optocoupler Related VideoVideo: How an Optocoupler Works and Example CircuitⅡ Photocouplers, Opto-couplers & Opto-isolatorsThese devices are known by a variety of names, including optoisolator, photocoupler, and optocoupler.An optocoupler is a semiconductor device that transmits an electrical signal between two isolated circuits using light. This process ensures there is no direct electrical connection between the input (source) and the output (load), effectively protecting sensitive low-voltage components.While often used interchangeably, there is a technical distinction in the industry:Optocoupler: Typically refers to devices used to transfer analog or digital information between circuits with voltage differentials below 5,000 Volts.Optoisolator: Often refers to devices specifically designed to withstand very high voltage differentials (5,000V to 50,000V+) for safety isolation in power systems.Optocouplers are typically housed in small packages ranging from standard DIP (Dual Inline Package) to tiny SMD (Surface Mount Device) packages. Despite their small size, they play a massive role in linking data, optical encoding, and detecting position transitions on encoder wheels.They are also the core technology inside Solid-State Relays (SSR), allowing low-power logic signals to switch high-power AC or DC loads without any mechanical parts.Figure 1: Typical Photocouplers in DIP packagingⅢ Photocoupler / Optocoupler BasicsAn optocoupler consists of two main internal elements encased in a light-tight body:The Emitter: Usually a Near-Infrared LED (Light Emitting Diode) that converts the electrical input signal into light.The Detector: A photosensitive device (such as a phototransistor, photodiode, or TRIAC) that detects the light and generates an electrical output.These two components are separated by a transparent dielectric barrier (glass, plastic, or air gap). Because the connection is made via light photons rather than electrons, the input and output sides are electrically isolated. This isolation prevents high voltages or rapidly changing voltage spikes on one side from damaging components on the other.Ⅳ Optocoupler SymbolIn circuit diagrams, the optocoupler symbol illustrates its internal functionality. The left side typically shows the LED (Emitter), and the right side shows the receiver (Detector).Figure 2: Optocoupler circuit symbol (Phototransistor output)Common Variations:Phototransistor: The most common type for DC signal switching (shown above).Photo-Darlington: Uses a Darlington pair transistor for much higher gain (sensitivity) but slower switching speed.Photo-TRIAC / Photo-SCR: Used for controlling AC power mains.Figure 3: Photo-TRIAC circuit symbol (used for AC control)Ⅴ Optocoupler Specifications to WatchWhen selecting a component, consult the datasheet for these critical parameters:1. Current Transfer Ratio (CTR)This is the equivalent of "gain" (Beta) in a standard transistor. It is the ratio of the output collector current ($I_C$) to the input LED forward current ($I_F$), expressed as a percentage.Standard Phototransistor: CTR ranges from 10% to 100%.Photodarlington: CTR can range from 500% to 5000% (high sensitivity).Design Note - CTR Degradation: The efficiency of the internal LED decreases over time (aging). A good engineering practice is to design your circuit assuming the CTR will drop by 50% over the product's lifespan.2. Bandwidth and SpeedThis determines the maximum data rate.Phototransistors: Generally limited to about 250 kHz.Photodarlingtons: Slower, often limited to < 20 kHz due to long turn-off times.High-Speed Optocouplers: Devices like the 6N137 use a photodiode + logic amplifier architecture and can handle 10 MHz or more.3. Input Current ($I_F$)This is the current required to light up the internal LED. You must calculate a series resistor to limit this current, typically between 5mA and 20mA for standard devices.4. Isolation Voltage ($V_{iso}$)The maximum voltage difference the component can withstand between the input and output pins without electricity jumping the gap. Common ratings are 2500V to 5000V RMS.Ⅵ How It WorksThe operation is straightforward:Current is applied to the input side, flowing through the internal infrared LED.The LED emits infrared light inside the package. The intensity of this light is proportional to the input current.The light strikes the photosensitive base of the output transistor (or Triac).The photosensitive device "turns on" and conducts current.Figure 4: The internal light pathWhy is the Base pin unconnected?In many 6-pin optocouplers (like the 4N25), the base of the transistor is broken out to a pin (Pin 6). However, in most applications, this pin is left floating (unconnected) because the light serves as the base current. Connecting a resistor from the base to the ground can reduce sensitivity but increase switching speed.Figure 5: Effective isolation between Input and OutputⅦ Benefits and TypesPrimary Benefits:Ground Loop Elimination: Breaking the ground path between two circuits prevents hum and noise (critical in audio and instrumentation).Safety: Protects low-voltage microcontrollers (3.3V/5V) from high-voltage spikes (110V/220V).Level Shifting: Allows a 3.3V signal to switch a 24V or 48V circuit effortlessly.Common Types:Photo-Transistor: General-purpose DC switching.Photo-Darlington: High gain for very low input currents.Photo-SCR / Photo-TRIAC: Designed for interfacing with AC power mains.Logic Gate Output: (e.g., 6N137, H11L1) Includes internal logic buffers for high-speed digital communications.Figure 6: Common output configurationsⅧ Typical ApplicationsMicroprocessor I/O: Protecting GPIO pins on Arduinos or PLCs.Switch Mode Power Supplies (SMPS): Used in the feedback loop to maintain voltage regulation while keeping the mains side isolated from the low-voltage side.Motor Driving: Isolating the control logic from the noisy high-current motor drivers.Example: Triac Optocoupler for AC LoadsBy using a device like the MOC3020, a 5V digital signal can trigger a large external Triac, which in turn controls an AC motor or lamp. Many Triac optocouplers feature Zero-Crossing Detection, which ensures the device only switches when the AC voltage is at zero, significantly reducing Electromagnetic Interference (EMI).Figure 7: A basic DC switching configurationⅨ Differences Between Optocouplers and Solid State Relays (SSR)While they operate on the same principle, the distinction lies in power capability and integration.Figure 8: Solid State Relays (SSRs)Optocouplers: Low power. Used for signal transmission. Usually requires external components (external Power Triacs or MOSFETs) to switch heavy loads.Solid State Relays: High power. They contain an optocoupler plus the high-power switching components and protection circuitry inside a single, larger block. They can switch tens of Amps directly.Ⅹ How to Use an Optocoupler with ArduinoConnecting a load directly to an Arduino is risky. If the load is a motor or a solenoid, "flyback" voltage spikes can destroy the microcontroller. Using an optocoupler like the 4N25 or PC817 resolves this.The Circuit Concept:The Arduino drives the internal LED of the optocoupler. The optocoupler's output transistor acts as a switch for the secondary circuit.Figure 9: 4N25 OptocouplerWiring Guide (4N25 to Arduino):1. Input Side: Connect Arduino Pin -> 220Ω Resistor -> Optocoupler Pin 1 (Anode). Connect Pin 2 (Cathode) to Arduino GND.2. Output Side: Connect the device you want to control. Important: If you are using the optocoupler to send a signal into another digital pin, you must use a Pull-up Resistor on the collector (Pin 5) because the phototransistor can only pull voltage down to ground; it cannot "source" voltage effectively.Figure 11: Basic wiring diagram for isolating a signalⅪ FAQ1. What are the disadvantages of an optocoupler?The main disadvantages are speed and power handling. Standard optocouplers have a relatively slow frequency response compared to digital isolators. Also, the output phototransistor cannot handle high currents directly; it usually requires an external transistor or relay to switch heavy loads.2. Is an optocoupler the same as a relay?Not exactly. While both isolate circuits, a mechanical relay uses a physical electromagnet and moving contacts (clicking sound). An optocoupler uses light and has no moving parts. Optocouplers are faster and last longer but handle much less current than relays.3. How do you use an optocoupler for analog signals?While mostly used for digital switching, linear optocouplers exist. To send audio or analog data, you set up a specific bias current (standing current) through the LED and modulate that current with your signal. Specialized "Linear Optocouplers" use feedback photodiodes to linearize the output.4. How do I ensure the optocoupler switches fully (Saturation)?To use an optocoupler as a solid switch, you must drive it into "saturation." This means ensuring the input current ($I_F$) is sufficient and the output collector load resistor is high enough so that the phototransistor turns completely on. Always check the CTR curve in the datasheet.5. Are optocouplers analog or digital?They are fundamentally analog devices (light intensity varies with current), but they are most commonly used in digital applications (On/Off switching). Specialized high-speed digital optocouplers (logic-output) are available specifically for data transmission. ul { margin-bottom: 20px; } li { margin-bottom: 10px; } .caption { text-align: center; font-size: 14px; color: #7f8c8d; margin-top: -15px; margin-bottom: 25px; font-style: italic; } .note-box { background-color: #e8f6f3; border-left: 5px solid #1abc9c; padding: 15px; margin: 20px 0; font-size: 16px; } .warning-box { background-color: #fff3cd; border-left: 5px solid #ffc107; padding: 15px; margin: 20px 0; } strong { color: #d35400; } .faq-item { margin-bottom: 20px; background: #fff; padding: 15px; border: 1px solid #eee; border-radius: 5px; } .faq-question { font-weight: bold; color: #e67e23; font-size: 18px; display: block; margin-bottom: 10px; }

A Relay is an electrically operated switch. It allows a low-power signal (like one from a microcontroller or dashboard switch) to control a high-power circuit (like an electric motor, headlights, or industrial machinery). In essence, it provides complete electrical isolation between the control system (input loop) and the controlled system (output loop).Used extensively in automotive systems, industrial automation, and modern Smart Home setups, the relay acts as an "automatic switch." It uses a small current to control a much larger one, offering crucial benefits like automatic adjustment, safety protection, and circuit conversion.As of 2025, while Solid State Relays (SSRs) are gaining popularity for their silence and longevity, the traditional electromechanical relay remains the industry standard for high-current and cost-effective switching. This guide covers how to wire these essential components effectively.Ⅰ Electrical Relay Structure & BasicsFigure 1. Electrical Relay StructureTo understand how to wire a relay, you must first understand its internal architecture:Core Components: A relay consists of four primary parts: the coil, the magnetic circuit (core/yoke), the spring, and the contacts.The Coil: When energized, the coil generates an electromagnetic field. This attraction pulls the armature, changing the state of the contacts.Magnetic Circuit: Comprising an iron core, choke, and armature, this establishes the path for magnetic flux.Air Gap: This is the critical distance between the armature and the core. When the coil is off, the gap is at its maximum (contacts in initial state). When on, the gap closes (contacts switched).The Spring: Provides the resetting force. When the coil is de-energized, the spring pushes the armature back to its original position.Contacts: These execute the control. They are divided into Normally Closed (NC) and Normally Open (NO).Energized: NC opens, NO closes.De-energized: Contacts reset to initial state.Common Types of Relays:Voltage Relays: High coil turns, thin wire. Connected in parallel with the load. (Most common).Current Relays: Few turns, thick wire. Connected in series with the load.Intermediate Relays: Used for signal transmission and controlling multiple secondary circuits.Ⅱ How Do Relays Work?An electromechanical relay is a switch operated by an electromagnet. When the coil receives current, the magnetic force pulls the "Common" (COM) contact arm from the "Normally Closed" (NC) position to the "Normally Open" (NO) position. When power is cut, a spring snaps it back.In short: When a specific input (voltage, current, temperature) hits a set value, the relay changes the state of the output circuit to control or protect the system.Example Analysis: Controlling a LightFigure 2. 8 Pin Relay Wiring ConnectionFigure 3. Relay Controls One LightWiring Logic:To control a lamp using a relay, the power circuit is wired through the relay's contacts. The Neutral wire connects directly to the lamp. The Live (Hot) wire connects to the relay's Normally Open (NO) contact. When the relay is triggered, the circuit closes, and the light turns on.Figure 4. Relay Controls Two Lights (Toggle)Dual Light Setup: By using both NC and NO contacts, you can toggle between two loads. When the coil is OFF, the NC light is ON. When the coil is ON, the NC light turns OFF and the NO light turns ON.Ⅲ Relay Wiring with Different Pins3.1 3-Pin RelayWhat is a 3-Pin Relay?These are commonly found in automotive applications as Flasher Units (for turn signals) or simplified horn relays. They work on electromechanical or thermal principles to cycle power on and off.How to Wire a 3-Pin Relay:Figure 5. 3-Pin Relay Wiring DiagramStandard configuration for a horn or load:Pin 1 (Load): Connected to the device (e.g., horn).Pin 2 (Battery/Power): Connected to the 12V power source (Common).Pin 3 (Switch/Coil): Connected to the button (e.g., steering wheel button).3.2 4-Pin Relay (SPST)What is a 4-Pin Relay?The 4-pin relay is the most common Single Pole Single Throw (SPST) relay used in automotive and general electronics to switch a single circuit on or off.How to Wire a 4-Pin Relay:Figure 6. 4-Pin Relay Wiring DiagramPins 85 & 86 (Coil): These control the magnet. Connect one to ground and the other to your switch (+12V).Pin 30 (Common): Connected to the high-power source (Battery +).Pin 87 (Normally Open): Connected to the load (Fan, Light, Motor).When the coil (85/86) is energized, Pin 30 connects to Pin 87.Figure 7. Standard 12V 40A 4-Pin RelayFigure 8. Coil Pins (85 & 86)Figure 9. Contact Pins (30 & 87)3.3 5-Pin Relay (SPDT)What is a 5-Pin Relay?This is a Single Pole Double Throw (SPDT) relay. It allows you to switch power between two circuits (e.g., High Beam vs. Low Beam) or simply use the "Normally Closed" feature.How to Wire a 5-Pin Relay:Figure 10. 5-Pin Relay Wiring DiagramPins 85 & 86: Coil (Control).Pin 30: Common (Power In).Pin 87a: Normally Closed (Power flows here when relay is OFF).Pin 87: Normally Open (Power flows here when relay is ON).3.4 6-Pin RelayWhat is a 6-Pin Relay?A 6-pin relay often functions similarly to a 5-pin but includes an extra terminal for internal bridging or specific DPDT signal configurations. In some automotive wiper relays, the extra pin handles parking logic.Wiring Overview:Figure 11. 6-Pin Relay Wiring DiagramTypically, two pins act as the coil, and the remaining four form two pairs of switching contacts (or one complex changeover). Always check the specific datasheet, as 6-pin configurations vary more than standard 4/5-pin types.3.5 8-Pin Relay (DPDT)What is an 8-Pin Relay?This is usually a Double Pole Double Throw (DPDT) relay. It effectively houses two 5-pin relays inside one shell, controlled by a single coil. It is ideal for reversing polarity on motors.How to Wire an 8-Pin Relay:Figure 12. 8-Pin Relay Wiring DiagramPins 2 & 7: Coil terminals (Power these to activate).Pins 1 & 8: Common terminals (COM).Pins 3 & 6: Normally Open (NO).Pins 4 & 5: Normally Closed (NC).3.6 Intermediate (Auxiliary) RelayWhat is an Intermediate Relay?Often used in industrial control panels (DIN Rail mounted), these relays transmit signals to control multiple larger contactors or actuators simultaneously. They are the backbone of classical automation logic.Wiring and Safety (Flyback Diodes):Figure 13. Intermediate Relay Wiring DiagramStandard industrial numbering (IEC):13 & 14: Coil (A1/A2).Contacts: Arranged in groups (e.g., 5-6-7-8 as NC, 9-10-11-12 as NO).⚠️ 2025 Safety Tip: When using intermediate relays with DC currents, always install a Freewheeling (Flyback) Diode across the coil (Reverse biased: Cathode to Positive). When the coil turns off, the collapsing magnetic field creates a high-voltage spike (back EMF) that can destroy sensitive control electronics (like PLCs or Arduino boards).Ⅳ FAQ: Relay Wiring in 20251. What is the difference between a Solid State Relay (SSR) and a Mechanical Relay?Mechanical relays use moving parts (magnets/contacts) and make a "click" sound. They are cheaper and handle high surge currents well. SSRs use semiconductors (light/optical isolation), have no moving parts, are silent, and last much longer, but they generate heat and are generally more expensive.2. What do the numbers on a standard automotive relay mean?These are DIN standard numbers: 30: Common (Main Power Input) 85: Coil Ground 86: Coil Positive (Trigger) 87: Normally Open (Output when ON) 87a: Normally Closed (Output when OFF)3. Does a Smart Home relay switch require a Neutral wire?Yes. Unlike older mechanical switches that just cut the Live line, most modern 2025 Smart Relays (WiFi/Zigbee) need a Neutral wire to power their internal WiFi chip so they can stay connected even when the light is off.4. What happens if I wire Pins 85 and 86 backwards?On a standard mechanical relay without a diode, nothing happens—it will still work because the coil is not polarized. However, if the relay has a built-in suppression diode (common in modern cars), wiring it backwards will cause a dead short and blow your fuse.5. What is an SPDT Relay?SPDT stands for Single Pole Double Throw. It has one input (Common) and two outputs (NC and NO). It can route power to Circuit A when off, and switch to Circuit B when on.6. Can I use a 12V relay on a 24V circuit?No. You must match the Coil Voltage to your control system (e.g., 12V car vs. 24V truck). However, the contacts (switch part) can often handle higher voltages than the coil. Always check the rating printed on the case.

Our everyday electrical devices are powered by plug-ins. Some plugs only have two prongs, while others have three prongs. So, what is the purpose of the third prong in the plug? The third prong is a grounding safety feature that guards against malfunctions. According to the Consumer Product Safety Commission, Ground Fault Circuit Interrupters, or GFCIs, have prevented 50% of home electrocutions. Now we will take a closer look at three-prong plugs and what they do in our electrical systems. CatalogIntroductionRelated VideoHow Does the Third Prong on a Plug Work?Is It Safe to Cut or Use an Adapter on the Ground Prong?Why Do Some Plugs Have Three Prongs While Others Don't?What is the Difference Between Two- and Three-pronged Plugs?How to Change a Two-prong Outlet to ThreeFAQ IntroductionWhy does an electrical plug have a third prong? The ground connector is found on the third prong of a plug, in case you didn't know. It's circular and sits below the two blade-style plug protrusions. It is a necessary component for outdoor extension cords and any device that is not completely shielded. Consider it an emergency path for electricity to take if the electrical device develops a short circuit or other fault. Related Video Video: Why some power plugs have 3 prongs instead of 2Video Description:Have you ever looked at your iPhone cable and then looked at your computer charger and thought, "why does one of these have 2 prongs, but the other one has 3?" Well, the answer all boils down to your personal safety. Here's why. How Does the Third Prong on a Plug Work?The majority of outlets in North America have two vertical slots. The hot wire is in the right slot, and the neutral wire is in the left. To power your device, electricity first travels through the hot wire. The current then continues to flow by returning to the outlet via the neutral wire. The circuit would be incomplete without this loop. In other words, only two prongs are required for an outlet or device to function. A ground prong or grounding wire is the rounded third prong on a plug. Electricity is constantly looking for a way to reach the ground. If the circuit is damaged or malfunctions, the ground wire serves as an emergency path that protects you and your devices from electric shock. A loose wire or a malfunction in a two-prong outlet could cause the electrical current to connect with the device or outlet's outer parts. The outer layers of two-prong plugs are insulated, but they can wear down over time or with continuous use. Is It Safe to Cut or Use an Adapter on the Ground Prong?Although using a ground plug adapter is common, it is not safe for your home or electronics. Older electrical outlets only had two slots: a narrow "hot" slot and a wide "neutral" slot. A round "ground" slot is included in a three-slot receptacle for safety. If there is a problem with the outlet, cord, or appliance, the ground provides a path for the electricity to return to the breaker box. Without a ground, electricity will seek the shortest path, which may be through your body. Using a ground plug adapter for a three-prong cord and a two-slot outlet with older electrical systems that lack a dedicated grounding wire may result in shock. Certainly, a plug adapter can be purchased at almost any store for very little money and requires very little effort to use, allowing you to use electrical devices in places where you would normally be unable to. They do not, however, provide any type of security. Surges, malfunctions, and even fire are all protected by the third prong on devices. Using an adapter disables that functionality, leaving them vulnerable to damage. A grounded circuit is not the same as a grounded plug adapter. Because they are sold in stores, most people believe they are safe. If an electrical surge occurs while using one of these adapters, it can cause electrocution or a fire. Why Do Some Plugs Have Three Prongs While Others Don't?A ground prong provides an emergency path for electricity to travel through if the device experiences a short circuit or fault. Other electrical components, such as wires, are present in all appliances with plugs. If one of these wires breaks or becomes loose, the electrical current will not flow properly through the device. At best, this will destroy your device. In the worst-case scenario, the misdirected current could deliver a painful and potentially lethal shock. To avoid this problem, the third prong grounds the device. If your appliance fails, the grounding prong creates a new, low-resistance grounding path down to the main electrical panel. This trips the circuit breaker, halting the electrical current and preventing damage to your appliance, a house fire, or an electrical shock. Some electronic devices are designed to prevent an electric overload or surge in the absence of a ground prong. Even if an appliance fails, it may have an internal voltage adapter that prevents it from emitting a lethal voltage. Other devices have plastic cases or other insulation to protect them from short-circuiting. Shielding, on the other hand, does not protect against water. To prevent electrocution, devices or appliances designed for outdoor or wet-area use must include a ground prong. Devices with metal housing—metal that you would touch when handling the appliance—will have a third ground prong to prevent electrical shocks. What is the Difference Between Two- and Three-pronged Plugs?Let us begin by explaining what the holes in an outlet do. In the United States, a standard 120-volt outlet has two vertical slots and a round hole centered below them. The slot on the left is slightly larger than the slot on the right. The left slot is referred to as "neutral," the right slot as "hot," and the hole beneath them as "ground." The prongs on a plug are designed to fit into these slots in the outlet. You already know that electricity must flow in a circuit if you've read How Batteries Work. Electricity flows from one terminal of the battery to the other in a battery. Power flows from hot to neutral in a household outlet. When you plug an appliance into an outlet, it completes the circuit from the hot slot to the neutral slot, and electricity flows through it to run a motor, heat some coils, or whatever. Assume you plug a light bulb into an outlet. Power will flow from the hot prong to the filament, then back to the neutral prong, producing light in the process. What if you plugged a thick strand of the wire straight from an outlet's hot slot to its neutral slot? Unlike an appliance, which is limited to 60 watts (for a light bulb) or 500 watts (for a toaster), the wire would allow an enormous amount of electricity to flow through it. Back in the breaker box, the circuit breaker for the outlet would detect the massive surge and shut down the electricity flow. The circuit breaker keeps the wires in the wall or the outlet from overheating and catching fire. An outlet's ground slot and neutral slot are the same. That is, if you return to the breaker box, you will notice that the neutral and ground wires from all of the outlets are connected to the same location. They are all connected to the ground (see How Power Distribution Grids Work for details on grounding). Why do you need both if they both go to the same place? If you look around your house, you'll notice that almost every appliance with a metal case has a three-prong outlet. This may also include some items, such as your computer, that have a metal-encased power supply inside, even if the device itself is plastic. Grounding is intended to protect people who use metal-encased appliances from electric shock. The casing is directly connected to the ground prong. What happens if the ground prong is removed or a cheater plug is used to plug a three-prong appliance into a two-prong outlet? Nothing, really; the appliance will continue to function. However, you have disabled an important safety feature that protects you from electric shock if a wire becomes loose. How to Change a Two-prong Outlet to ThreeTo learn how to ground an outlet, follow the steps below. Step 1: Examine the ground.Insert one prong of a circuit tester into the hot slot of the receptacle (the shorter one) and touch the other to a screw that holds the cover plate in place.The tester should illuminate. If it doesn't, the box isn't properly grounded. You have two options: install a GFCI  (see steps below) or hire an electrician to repair the wiring. Step 2: Disconnect the old receptacle.Turn off the electricity at the breaker panel or fuse box.Remove the old receptacle from the box and disconnect the wires. Step 3: Install the new receptacle.Connect the black (hot) and white (neutral) wires to the brass and silver terminals, respectively.Use the terminals on a GFCI that correspond to the "line" label on the back of the receptacle. (Skip to Step 6 if your box is not grounded.) Step 4: Screw in the ground screw.This green screw, available at hardware stores, fits into a threaded hole in the box's back.Tighten the screw with one end of an 8-inch green grounding wire or pigtail (also available at hardware stores). Step 5: Connect the receptacle to the ground.Connect the opposite end of the 8-inch grounding pigtail to the green grounding terminal on the three-prong or GFCI receptacle.Place the new receptacle in the box. Step 6: Turn on the electricity.To ensure that the circuit is operational, use a circuit tester. FAQ1.Should I upgrade a two-prong outlet to a three-prong outlet?Yes. If you have an older home (built before 1962) with two-prong outlets, you should have them rewired to a grounded three-prong outlet. 2.Is it safe to remove the third prong or use a "cheaper plug"?A three-pronged plug will only fit into three-pronged outlets. The third prong, however, cannot be removed because it completes the ground circuit. It's a critical safety feature that guards against flaws. 3.What can I do with outlets that only have two prongs?Assume the device you want to use has a three-prong plug but you only have a two-prong outlet. An adapter is a useful short-term solution, but it should not be used for long-term purposes or with outdoor devices. An electrician can also convert your ungrounded outlets to three-prong outlets. 4.Why do some electronic devices only have two prongs?Surge protection is built into some devices that do not require a ground prong. When using a two-prong or ungrounded outlet, this is a must-have safety feature. Instead of a ground wire, these devices employ an internal voltage adapter or insulation to prevent a short circuit. The holes found in 98 percent of flat two-prong plug appliances are not for grounding. They're a safety feature that grips the inside of the socket to keep it from slipping. 5.Why do some plugs have a third prong?The added security of 3-prong plugs is critical for any appliance or cable intended for use in an outdoor or wet area. Moisture and other external factors can cause plugs to wear and cause dangerous malfunctions. 6.Are two-prong outlets legal and code-compliant?Existing two-prong receptacle outlets are legally allowed to remain in place and can be replaced with another two-prong receptacle where a ground connection (what a third prong plugs into) does not exist, according to the National Electrical Code. 7.Why are three-prong plugs used?A three-prong plug is designed to safely supply electricity to electrical appliances. The third prong grounds the electricity, preventing electric shock to anyone who uses the metal-encased appliance. 8.Can you remove the third prong?If you remove the third prong, nothing happens technically. You have, however, disabled an important safety feature that protects you from electric shock if a wire becomes entangled. 9.Are plug adapters safe to use?No, not in particular. While many people use ground plug adapters, they are not safe for you, your home, or whatever you are plugging in. Using an adapter disables the ground prong's safety function, leaving it vulnerable to damage. 10.What wires are connected to a three-prong plug?The left slot is "neutral," and the white wire connected to it should be connected to the silver screw. The right slot is "hot," and the black wire connected to it should be connected to the brass screw. Connect the green "grounding" wire to the green screw.