relay Related Articles

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

IntroductionThe relay is an electrical device regarded as a switch in the circuit. That is, the current in the control circuit depends on the "open" and "close" of relay contacts. Therefore, the reliability and service life of the relay depend on the quality and performance of the contacts greatly. The performance of the contact is affected by factors such as contact material, contact voltage, load type, operating frequency, atmospheric environment, contact configuration and bounce.  If any of these factors cannot meet the predetermined value, contact problems such as electrochemical corrosion of the metal between the contacts, contact welding, contact wear, and contact resistance may occur. The volume of the load determines the size of the voltage and current that the relay can control (The rated load of the contact refers to the voltage and current that the electromagnetic relay allows to break.). If you not pay attention to it when use, it is easy to damage the relay contacts.Relay ContactCatalogIntroductionⅠ Relay Contact Form ConfigurationⅡ Relay Contact SymbolⅢ Relay Contact Fault Analysis3.1 Terminology3.2 Contact Bonding and Fusion Welding3.3 Contact Erosion3.4 Contact Metal Migration3.5 Contact Loose and Crack3.6 Contact DustⅣ Contact Protection MethodsⅤ Frequently Asked Questions about Relay ContactⅠ Relay Contact Form Configurationa. Normally Opened ContactIt would mean the contacts are normally open when the coil of the relay is not energized or there is no magnetic field nearby in a reed switch. b. Normally Closed ContactIt would mean the contacts are normally closed when the coil of the relay is not energized or there is no magnetic field nearby in a reed switch. c. Common ContactIt would have 3 leads and would have one normally open and one normally closed circuit. This is also called a “changeover” because the common contact changes from the normally closed position to the normally open position when the coil is energized in a relay or a magnetic field is nearby in a reed switch. Ⅱ Relay Contact Symbol Ⅲ Relay Contact Fault Analysis3.1 TerminologyThere is something in which the relay contact seems to be closed, but the circuit works abnormally sometimes. This is due to the existence of the contact resistance of the relay contacts. When the current passes through the closed contact, the contact resistance will consume a certain amount of power, which will increase the temperature of the contact. If the current is large, the contact material will soften and deform, resulting in greater contact resistance, and even having welding failure in severe cases, making the closed contact unable to be disconnected. Another form of contact resistance is "membrane resistance". Because the contacts of the relay are exposed to the air for a long time, there will always be compounds produced by dust, water vapor, and chemical gas, which will adhere to the contacts to form a thin film. Because of it, the conductivity of the contacts will become worse, and even become non-conductive in severe cases. 3.2 Contact Bonding and Fusion WeldingContact bonding usually occurs when the contacts are in a static connection. Contact resistance making the temperature of the conductive spots and nearby materials increase, which leads to a great increase in the diffusion rate and a large expansion of the contact area. The molecular force formed by the mutual extrusion and penetration of metal molecules at the contact point is the internal factor leading to the contact bonding, in addition, the sliding friction between the contacts is a necessary condition for accelerating the molecular extrusion penetration and accumulating bonding force. The size of the bonding force depends on the rigidity of the contact material and the physical conditions that cause molecular extrusion and penetration. Whether the contacts are bonded depends on the bonding force is greater than the return force of the reed. Fusion welding refers to the phenomenon that the contact areas of two electrodes are united together by metal welding. According to the reasons for formation, welding can be divided into static welding and dynamic welding. The Joule heat generated by the contact resistor melts the contacts part, and the phenomenon that they are combined and cannot be disconnected is called static welding. In the process of the contacts controlling the external circuit, the contact pressure of the contacts is near zero or above, and meanwhile, the liquid metal bridge between the contacts made. The welding phenomenon that occurs owing to the arc heat flow melting the contacts is called dynamic welding. 3.3 Contact ErosionA load of contact switching is mostly inductive. When the inductive load is disconnected, its accumulated magnetic energy will generate a high back electromotive force at both ends of the contact, which will break up the air gap between the contacts to form sparks and cause electrical corrosion. Cause the contact surface to dent or stick and cannot be separated, all of them belong to poor contact, which will result in a short circuit. The main factors that affect arc erosion include the characteristics of the arc and its effect on the heat flow and force of the electrode and the response of the contact material to the heat and force of the arc. In general, there are two main forms of arc erosion: 1) Vaporization and evaporation: Under the action of arc energy, the surface material of the contact changes from solid to liquid, and then into a gaseous state to leave the contact. Except that, in certain conditions, the contact material also has a sublimation process from a solid-state to a gas state. 2) Liquid splashing: Under the action of arc energy, a certain area of the surface of the contact melts. The liquid metal splashes out in the form of tiny droplets under the action of various forces, resulting in a larger material loss. These forces include spot pressure, electrostatic field force, electromagnetic force, force and reaction force of material movement, contact surface tension, etc. The form of arc erosion varies with the contact material and load current conditions. When the load current is small, the erosion of the contact material is dominated by vaporization and evaporation. When the current is increased, not only the vaporization and evaporation of the contact material but also the splashing phenomenon of liquid metal will occur. When the current is further increased, the metal liquid splashing becomes the main form of contact erosion. Preventing electrical corrosion between the contacts can be obtained by setting up a resistance spark extinguishing circuit and a resistance-capacitance spark extinguishing circuit. Therefore, when choosing a relay, you should consider the voltage applied to the contact and the load capacity of the contact. For example a relay with a contact load of 28V(DC)×10A means that the relay’s contact can only work at a DC voltage of 28V, and the contact current is 10A. If these two ratings are exceeded, the service life of the relay will be affected, and even the contacts will be burnt and damaged. In addition, the number of circuits that the relay needs to control should be determined according to actual requirements. In the same model series of relays, there are generally a variety of contact forms for selection, and each group of contacts should be fully utilized when using. 3.4 Contact Metal MigrationDuring the working process, there is usually a mutual transfer of materials between two contacts. If this mutual transfer cannot be offset, a net transfer of materials occurs. The significant contact metal migration is a big net transfer. The asymmetry of various factors in the contact operation is the main reason for the metal migration of the contact. These factors include arc, contact material characteristics and various external forces. Details are as following: 1) The arc has various forms of energy input to the contacts. For the contact at the cathode, the kinetic energy of the ion current colliding with the cathode after being accelerated by decompression, the potential energy released by the ion current on the cathode surface and the electrons, the arc column radiation or the energy conducted to the cathode surface, and the cathode Joule heat generated by the current in the body. All of these energies will increase the temperature of the contact material, resulting in contact material melting and evaporation. 2) The contact has various forces in the working process, including electronic force, electrostatic force, electromagnetic force, the reaction force of material movement, plasma flow force, these forces may cause the metal in the molten pool on the surface of the contact Liquid splashing occurs. 3) The material properties that affect the migration of the contact metal include electrical conductivity, specific heat capacity, latent heat of melting and vaporization, melting point and boiling point, metallurgical dynamics, and so on. In addition, the size, shape, and connection form of the contacts will also affect the metal migration. 3.5 Contact Loose and CrackContacts are electrical contact parts for relays to switch loads. Some products have contacts that are press-fitted by riveting. The main drawbacks of this installing method are loose contacts, cracks in the contacts, or excessive size and so on. They will affect the contact reliability of the relay. The loosening of contacts is caused by the improper size of the mating part of the reed and the contact or the improper adjustment force by the operator. Contact cracking is caused by too high material hardness or too much pressure. Different crafts should be used for contacts of different materials, and some contact materials with higher hardness should be annealed before contact manufacturing, riveting, or welding. 3.6 Contact DustSometime after use, dust and dirt will deposit on the contacts of the relay, which will cause a black oxide film on the surface, resulting in poor contact. Therefore, the contacts need to be cleaned regularly. For example, carbon tetrachloride liquid can be used to ensure good contact performance.  Ⅳ Contact Protection MethodsFigure 1. Contact Oscillogram (contact action time, release time, rebound time and stabilization time)We know that the relay contact protection needs to be more careful than MOSFET. Generally, the load of the relay is much larger than MOSFET. Common DC motors, DC clutches and DC solenoid valves with large DC loads, these inductive load switches are often closed, because surges caused by hundreds of or even thousands of back electromotive force will shorten the life of the contacts or even completely damage them. On the contrary, if the current is small, such as around 1A, the back electromotive force will cause arc discharge, which will cause metal oxides to contaminate the contacts, leading to failure of the contacts and increasing contact resistance. Protect contacts mainly to extend the use time of the relay, because the contacts will always accumulate carbon and age, and the surface is not as clean as it was originally. What’s more, when the relay life is approaching the end, its contact resistance will increase rapidly. Generally, under normal temperature and pressure, the breakdown voltage of the key dielectric in the air is 200~300V. Therefore, our goal is generally to control the voltage below 200V or less.Figure 2. Breakdown VoltageThere generally have the following methods to do it:MethodCircuitCharacteristicComponent SelectionResistor and CapacitorIf the load is related to time, the initial leakage current may cause the load to malfunction.R: The contact voltage is 1VC: The contact current is 1A, and the value of RC varies with the relay and load.The function of the capacitor C is to suppress the excessive voltage when the inductor is discharged.The value of resistance R is determined by the test needs.The breakdown voltage of the capacitor C is 200~300V.If the load is a relay or solenoid valve, the release time will be extended. When the contact power supply voltage range is 24V~ 48V, the voltage across the load is 100 ~ 200V.DiodeThe diode (regarded as a freewheeling diode) acts as a channel for the coil to release energy and a way to dissipate heat. Compared with the RC circuit, it significantly changes the release time of the relay (2~5 times).The reverse breakdown voltage is at least 10 times the power supply voltage, and the forward current is equivalent to the load.Zener DiodeThis circuit effectively prevents the diode from affecting the release time of the relay.The breakdown voltage of the Zener diode must be consistent with the power supply voltage of the relay.VaristorBased on the characteristics of the varistor to stabilize the voltage, this circuit can prevent the contact voltage from being too high, and also slightly delay the relay release time. When the load contact power supply voltage is 24V or 48V, and the voltage across the load is 100 to 200V, the varistor is very effective. * Standard diodes can significantly extend the rebound time. Connecting conventional diodes in series with Zener diodes will affect it lightly. If it is an inductive load, when the contacts are separated, a longer rebound time prolongs the arc generation time and shortens the life of the contacts. For example, a relay with a diode connected to the coil needs 9.8ms to release the contact. Combining the Zener diode with the small signal diode can shorten the time to 1.9ms. In addition, the return time of the relay without a diode connected to the coil is 1.5ms. Although the inductive load is not easy to handle than the resistive load, the use of effective protection will make the performance better. There are two methods that can’t be used.Figure 3. Capacitor and Relay CircuitIn the actual circuit, the protection device (diode, resistor, capacitor, varistor, etc.) and the load should have a certain distance. If the two are too far apart, the effect of the protective device may be weakened. Generally, the distance between the two should be within 50cm. DC loads at higher frequencies will cause abnormal switch corrosion (electric spark generation). When the DC solenoid valve or clutch is controlled at a higher frequency, the contacts may have corrosion. The reason for this is that when an electric spark (arc discharge) is generated, the reaction between nitrogen and oxygen causes contact corrosion. Ⅴ Frequently Asked Questions about Relay Contact1. How do relay contacts work?A relay is an electrically operated switch. They commonly use an electromagnet (coil) to operate their internal mechanical switching mechanism (contacts). When a relay contact is open, this will switch power ON for a circuit when the coil is activated. 2. What is a relay contact output?A relay contact output works basically like an on/off switch. To simplify, if the output is "off" the circuit will be broken (open). If the output is "on" the contact will be made, completing the circuit. Therefore, the controller does not supply any current or voltage itself. 3. Why do relay contacts weld?Consequently, when the contacts are ON again, short-circuited current from the capacitance may cause contact weld. This circuit effectively suppresses arcs when the contacts are OFF. When the contacts are ON again, however, charge current flows to the capacitor, which may result in contact weld. 4. How do I protect my relay contacts?Various ways to protect relay contacts from the effects of switching an inductive load – from left to right: a diode, a spark quench capacitor, Zener diodes or a transil, a varistor. 5. What is a contact form relay?Contact Form: The arrangement of the contacts in the relay. This determines how many circuits the relay can operate. Form 1A (or “1 Form A): One circuit is opened and closed with the contacts in a Normally Open position. 6. What is a relay contact?Relays control one electrical circuit by opening and closing contacts in another circuit. ... When a relay contact is Normally Closed (NC), there is a closed contact when the relay is not energized. In either case, applying electrical current to the contacts will change their state. 7. How many contacts does a relay have?Two. A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke that provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts. 8. What is the difference between relay and contactor?A contactor joins 2 poles together, without a common circuit between them, while a relay has a common contact that connects to a neutral position. Additionally, contactors are commonly rated for up to 1000V, while relays are usually rated to the only 250V. 9. What is the purpose of contactor or relay?A contactor is a large relay, usually used to switch current to an electric motor or another high-power load. Large electric motors can be protected from overcurrent damage through the use of overload heaters and overload contacts. 10. What are the three major parts of a contactor or relay?There are three major parts of a contactor or relay: the coil, mechanical linkage and contacts. The coil is used to create a magnetic field and is rated based on voltage (24 V, 120 V, 208/204 V, 480 V). The mechanical linkage connects the armature to the contacts when the coil is energized, completing the circuit. Recommended ReadingBasic Knowledge of Relay Electronics Tutorial with VideoThe Role of the Relay and Its Working PrincipleHow Relays Work? Relay Functions and Applications

IntroductionA relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. Because of unique characteristics, it is widely used in many fields. How does a relay work? What the functions of relays, let’s check the following details.How Does A Relay Work?CatalogIntroductionⅠ Working Principle  1.1 Operation Example: Relay as a Switch  1.2 Working Principle of Different RelaysⅡ What is the Function of a Relay?  2.1 Summary  2.2 Types of RelayⅢ Relay Applications  3.1 Automotive Field  3.2 Household Appliance  3.3 Industrial Relays  3.4 Example Analysis: JYB-714 Liquid Level RelayⅣ Relay Selection RulesⅤ One Question Related to DC RelayⅥ Frequently Asked Questions about How Relay WorksⅠ Working PrincipleThe relay is generally composed of an iron core, coil, armature, contact reed and so on. As long as both ends of the coil having a voltage, a certain current will flow through the coil, which will produce electromagnetic effects. Under the action of the electromagnetic force, the armature will overcome the pull force of the return spring and attract to the core, thereby the movable contact and the static contact (normally opened terminal) are in the state of pull-in.Figure 1. Relay StructureWhen the coil is power-off, the electromagnetic attraction will disappear. The armature will move back to its original position under the reaction force of the spring, making the movable contact and the static contact (normally closed terminal) contract together. Under the actions of pull-in and release, achieve the purpose of conducting and cutting off in the circuit. 1.1 Operation Example: Relay as a SwitchThe following figure is the circuit diagram of the relay controlling the light. The relay has normally open contacts and normally closed contacts. The movable contact is a common terminal. This is a DC relay powered by a battery. When the coil of the relay is powered by a DC power supply, the coil with the iron core will generate the corresponding magnetic field to adsorb the armature, and the movable contact will move from the normally closed contact side to the normally open contact side, which is equal to the normally open contact being pulled in. We can see that the start/stop button, battery, and relay coil form a control loop. As long as this loop is closed, the current will flow through the coil and a magnetic field will be generated. The normally open contact, the lamp, and the control power supply (the other battery in the picture) form a loop. When the normally opened contact is closed, the loop is closed and the current will flow from the positive of control power supply to the bulb, passing through the closed normally opened contact to the negative pole, so that the light will on.Figure 2. A relay as a SwitchWhen the start/stop button disconnects, the coil has no current. So that the armature will not be attracted by the magnetic force, and will be reset by the spring. So that the other end of the moving contact will go from the normally opened contact to the normally closed contact. The circuit of the bulb is forcibly disconnected and does not turn on.Figure 3. Relay Controls a Light1.2 Working Principle of Different Relays1) Electromagnetic RelayIt works by using the suction force generated by the circuit in the input circuit between the electromagnet core and the armature.2) Solid State RelayElectronic components have their functions without mechanical moving parts, and the input and output are isolated.3) Temperature RelayIt will act when the outside temperature reaches a given value.4) Reed RelayUsing the reed action sealed in the tube, open, close, or switch the circuit with the function of the electric contact reed and the armature magnetic circuit.5) Time RelayWhen the input signal is added or removed, the output part needs to be delayed or time-limited before closing or opening its controlled circuit until the specified time.6) High-frequency Relay It is used to switch high-frequency and radiofrequency lines with minimal loss.7) Polarized RelayA polarized magnetic field and a control current are combined to act by the magnetic field generated by the control coil. Ⅱ What is the Function of a Relay?2.1 SummaryRelay is an automatic switching element with isolation function, which is widely used for remote control, telemetry, communication, automatic control, electronic equipment, etc. It is the most important control element in the circuits.Relays generally have a sensing mechanism (input part) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); there is a mechanism (input part) that can realize "switch on" and "switch off" to the controlled circuit. Between the input end and the output end of the relay, there is also an intermediate mechanism for coupling isolation of the input quantity, functional processing and driving the output part (driving part). As a control element, relays have the following functions:1) Expanding control rangeFor example, when the control signal of a multi-contact relay reaches a certain value, multiple circuits can be switched, disconnected, and connected at the same time from different forms of contact groups.2) AmplificationFor example, using a very small control quantity can control a large power circuit, such as sensitive relays, intermediate relays and so on.3) Integrated signalFor example, when multiple control signals are input to a multi-winding relay in a prescribed form, they will be relatively integrated to achieve a predetermined control effect.4) Automatic control, remote control, and monitoringFor example, the relay on the automatic device and other electrical appliances can form a program control circuit to realize automatic operation. 2.2 Types of RelayIntermediate RelayIt is the function of converting and transmitting the control signal. That is, its input signal is the power off/on the signal of the coil, and the output signal is the contact action of the intermediate relay. In essence, it belongs to one of the voltage relays and has the characteristics of multi-contacts (six pairs or even more). The contact can withstand a large current (rated current is 5A~10A), and its action is more sensitive (response time less than 0.05s). Voltage RelayIts main principle uses the voltage signal, and determines the action of the contact according to the coil voltage, in addition, the coil needs to be connected in parallel with the load during circuit design. The voltage relay can be divided into AC and DC type according to the coil voltage, and can be divided into overvoltage and Undervoltage according to the operating voltage. Therefore, their functions are also different. As for the overvoltage relay, when the coil voltage is within the rated value range, the armature will not make any pull-in action. On the contrary, the action will execute if the coil voltage is exceeded. The AC overvoltage relay plays the role of overvoltage protection in the circuit. When the coil voltage reaches or exceeds the rated value of the coil, the armature will make a pull-in action, and the coil voltage will be lower than the rated value. The Undervoltage relay mainly plays a role of Undervoltage protection in the circuit when the armature is released immediately. Current RelayIt works according to the current signal and determines the contact action according to the current of the coil. The current relay coil needs to be connected in series with the load when having the installation. According to the coil current, it can be divided into two types AC and DC. According to the action current, it can be divided into overcurrent and undercurrent types.Since the load current will pass through the coil when having overcurrent, the coil rated current (that is, the setting current) is usually chosen to be equal to the maximum load current. When the load current is not higher than the setting value, the armature will not act. On the contrary, if it exceeds, a pull-in action will occur. The main function of the overcurrent relay is to play overcurrent protection in the circuit, especially in some occasions where impulsive over-current occurs. Because it has a good protective effect.  The principle of the undercurrent relay is that when the current in the coil reaches or exceeds the operating current value, the armature will perform a pull-in action. On the contrary, the armature will be released immediately when the coil current is less than the operating current value. In a normal state, if the load current exceeds the working current of the coil, the armature will also perform pull-in. When the load current drops below the coil current, the armature will be released. Time RelayIt belongs to a relay that starts with the input signal (that is when the coil is powered on or off) and will output the signal (contact closed or disconnected) after a preset delay in advance. Time relays are generally used in relatively low voltage or current circuits to turn on and off higher voltage or current, just as an electric switch device in the circuit used for automatic control.Figure 4. Electrical Relay Symbol (SPST/SPDT/DPST/DPDT)Ⅲ Relay ApplicationsRelays are employed in a wide range of fields, and their environmental conditions and technical requirements vary greatly. What’s more, in the same application field there are different requirements. Here are some examples and a brief description.3.1 Automotive FieldThe automotive industry is increasingly using relays. The more common relays are: starting relays to start motors, horn relays, open circuit relay of motor or generator, regulating relays for charging voltage and current, flashing relays, control relays fro light brightness,control relays for air conditioning, and so on. The power supply in the car now mostly uses 12V, and the coil voltage is mostly set to be 12V. Due to the battery power supply, the voltage is unstable.  The environmental conditions are not good, for example, the suction voltage is less than 60VH (rated working voltage), and the overvoltage of the coil is required to 1.5VH. What’s more, the power consumption of the coil is relatively large, generally 1.6~2W, and the temperature rise is relatively high. Their environmental requirements are also quite harsh: the ambient temperature range is -40℃～100℃; the relay used in the engine box must be able to withstand the damage of sand, dust, water, salt, and oil; vibration and shock are undoubtedly affecting normal operation. 3.2 Household Appliance1) Air conditioning relays are mainly used to control compressor motors, fan motors and cooling pump motors to have control functions. Owing to the moment when the load starts, a large inrush current appears, which is about 6 times the full-load operating current. It takes a long time for the compressor motor to reach full speed (the power of the home appliance compressor motor is generally 1 to 3 horsepower, where the fan motor and cooling pump motor are 1/4 to 2 horsepower.), which is a serious threat to the relay contacts to eliminate as much as possible the contact bounce when the relay is sucked.  Because the relay is required to release fast, minimize contact bounce as much as possible. The safety requirements are also strict and must be recognized by a safety certification agency. For example, as for product environmental conditions, the ambient temperature requires -40 to 55℃, relative humidity up to 40%, 90RH, and have rainwater infiltration. Because weight and size are not important indicators, the relay is required to be robust and impact resistant.2) Relay used in washing machines, microwave ovens, electric heaters, etc. Relay contact load: the large load can reach 220V, 5000W heater (or 1 horsepower motor), and the small load can be as small as driving solenoids load, other relay coils load, indicator light load, etc. The expected life span of the relay is required to reach 5 to 10 years. That is to say, the electrical life of the relay is required to reach 105 times to 2×105 times. Ambient temperature: -40 to 55°C (85°C for microwave ovens and electric heaters); relative humidity 20 to 95%/RH. 3.3 Industrial RelaysIn industrial control, the main control function is completed by the universal AC relay. The relay is usually driven by a button or limit switch. It is also used in traffic signal controllers, temperature controllers, etc. The contacts of the relay can control solenoid valves, larger start motors, and indicator lights.  What’s more, the field of digital control has expanded the application of relays. Copy milling and coordinate boring are operated by data programming, and the signals are sent to the machine tool controller, memory unit and other logic elements to control 2 to 5 axes of the coordinate servo motor. With this mechanical control method, it is easy to control drilling machines, hexagonal lathes, ordinary lathes and automatic profiling machines.The digital control system requires the relay to have the ability to adapt to low-level signals, medium sensitivity, fast action and high switching reliability. The environmental conditions for the installation of industrial machinery must be considered. For instance, operating industrial machinery and surrounding equipment always transmit some shocks and vibrations to the control cabinet, and they also have the influence of splashing cutting coolant. So that these unfavorable environmental conditions must still be considered when selecting and designing relays. With strict safety requirements, high requirements are needed for electrical insulation, voltage resistance, and flame retardants. 3.4 Example Analysis: JYB-714 Liquid Level RelayLiquid level relay is a kind of relay that uses liquid level to control the circuit. To be specific, this is a relay with electronic circuits inside. Based on the conductivity of the liquid, when the liquid level reaches a certain height, the relay will act to cut off the power; when the liquid level is lower than a certain position, turn on the power to make the pump work.  To achieve the role of automated control, this control is composed of sensors and control actuators. According to the conductivity of water, but it is poor and cannot directly drive the relay. Therefore, there must be an electronic circuit to amplify the current to drive the relay to work. So the sensor of the liquid level controller is generally a wire. The line is divided into three types, high and low, and the middle line. The high is the water level overflow point to control the water level freely, in addition, the water will stop to fill in automatically. At the low water level, the low point is the automatic water filling point. Where the middle is constant contact.JYB-714 Liquid Level Relay①, ⑧ are the working power connecting terminals of the relay. ① is connected to L1, ⑧ is connected to N.②, ③, and ④ output the automatic control signal, and the working voltage of the output terminal is AC220V. ③ is the output signal common end, the level control signal of the water supply pump is output between ② and ③, and the drainage pump level control signal is output between ③ and ④.  ⑤, ⑥, ⑦ are the wiring terminals corresponding to the liquid level electrodes A, B, C in the pool.  ⑤ is connected to the high water level electrode A, ⑥ is connected to the low water level electrode B, and ⑦ connect to the lowest common electrode C. Note that in the experiment, the water inlet electrode uses a copper hard insulated wire of 1 to 1.5mm2, and the water inlet end is stripped of 5mm insulation. In addition, the safety voltage between the liquid level electrode terminals is DC24V. Tech Note1) Drainage type liquid level relay instructions"High" is the upper limit liquid level control point of the pool. When the water level rises to a high level, the water contacts the probe (electrode), and the controller automatically turns on the pump and starts to drain."Middle" is the lower limit liquid level control point of the pool. When the water level drops below the midpoint level, the water and the probe (electrode) are out of contact, and the controller automatically turns off the pump and stops draining."Low" is the ground line of the pool, the lowest point of the pool. 2) The difference between water-supply type liquid level relay and drainage-type liquid level relay:Water-supply type liquid level relay works in water shortage and stops when the water is full.The drainage-type relay works when water is full and stops in a water shortage.Figure 5. A RelayⅣ Relay Selection RulesTo use the relay well, the correct selection is very important. First of all, you must get a thorough understanding of characteristics and requirements of the controlled object, and have careful consideration. The principle, purpose, technical parameters, structural characteristics, specifications and models of the selected relays should be analyzed. On this basis, the relay should be correctly selected according to the actual situation and specific conditions of the project.1. The necessary conditions① The power supply voltage of the control circuit, the maximum current that can be provided.② Voltage and current in the controlled circuit. ③ Contact: When selecting a relay, on the one hand, you should consider whether the control circuit can provide enough working current, otherwise the pull-in of the relay is unstable. When the pull-in and release time of the relay cannot meet the requirements, the time constant of the coil loop can be changed to solve the problem. On the other hand, there is the elimination of electric sparks. Due to the small on-off current of the relay contacts, there will be no arc between the contacts, but "spark discharge" will occur. This is due to the presence of inductance in the contact circuit, and an overvoltage will appear on the inductance when it is disconnected. Together with power supply voltage on the contact gap, so that the contact gap will break down and discharge. Because of energy limitation, only spark discharge will generate. The alternating energy conversion between the capacitance and inductance existing between the contacts makes the spark looming and becoming a high-frequency signal. In addition, spark discharge will cause damage to the contacts, resulting in short service life. 2. After consulting the relevant materials to determine the conditions of use, you can find the relevant materials to find out the specific relay. If you already have a relay on hand, you can check whether it can be used according to the datasheet, and finally consider whether the size is appropriate. 3. Pay attention to the size of the appliance. If it is employed in general electrical appliances, in addition to the cabinet volume, small relays mainly consider the circuit board installation layout. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be used. 4. Rated load and service life are reference values, which will vary greatly according to different environmental factors, load properties and types. So it is better to confirm in actual or simulate actual use. 5. Try to use rectangular wave control for DC relays, and use sine wave control for AC relays. 6. In order to maintain the performance of the relay, please be careful not to drop the relay or subject it to strong shocks. 7. Do not use the relay in an environment with much dust and harmful gas. Harmful gases include gas containing sulfur, silicon, nitrogen oxides, etc. 8. As for the magnetic latching relay, it should be placed in the action or reset position as needed before use. 9. For polarized relays, please pay attention to the polarity of the coil voltage. 10. The relay is a heat-resistant component. High temperature can speed up the aging of the internal plastic and insulating materials of the relay. Contacts are oxidized and corroded, making it difficult to extinguish the arc. The technical parameters of the electrical components decay and the reliability reduces. So that good ventilation conditions should be maintained.And meanwhile, the low temperature cannot be ignored. Low temperature can aggravate the cold adhesion of the contacts and expose the contact surface. Many manufacturers’ relays indicate that the minimum temperature is -25°C, but high-voltage switches are also used in extreme cold. So it is recommended to leave the room when selecting the model to avoid the relay being unreliable due to low temperature. If circumstances permit, add heaters in the high cold area to ensure that the relay operates reliably and ensure the stability of the entire system. 11. Under the condition of low air pressure, the heat dissipation condition of the relay goes bad,  and the temperature of the coil rises, which changes the given pull-in and release parameters of the relay, affecting the normal operation of the relay. The low air pressure can also reduce the insulation resistance of the relay. It is difficult to extinguish the arc and is easy to melt the contacts and affect the reliability of the relay. It can be used normally at an altitude of fewer than 2000 meters, and it needs capacitance derated used at an altitude of more than 2000 meters. 12. Reduce the impact of mechanical stress on the relay. Mechanical force mainly refers to stress such as vibration, impact, and collision on the control system. The self-vibration of the circuit breaker in the high-voltage switch and the vibration caused by the opening and closing operations has a greater impact on the relay. An intermediate relay with a balanced armature mechanism should be selected. Electromagnetic relays have cantilevered beam structure, the natural frequency is low, oscillation and impact will cause resonance, resulting in the relay contact pressure to drop and contact instant disconnection or contact vibration, which will affect the reliability of the relay. It suggests that vibration measures should be taken to prevent resonance. Ⅴ One Question Related to DC Relay5.1 QuestionHow Does a DC relay work?5.2 AnswerA DC relay uses a single coil of wire wound around the iron core to make the electromagnet. When the DC coil is energized, the magnetism generated in the core is steady because the DC just keeps going. The steady magnetism keeps the lever attracted as long as the DC is flowing. Ⅵ Frequently Asked Questions about How Relay Works1. What is a relay and how it works?A relay is an electrically operated switch. They commonly use an electromagnet (coil) to operate their internal mechanical switching mechanism (contacts). When a relay contact is open, this will switch power ON for a circuit when the coil is activated. 2. Why relay is used?The switch may have any number of contacts in multiple contact forms, such as making contacts, break contacts or combinations thereof. Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. 3. How do you know if a relay is working?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 4. What is the main function of the relay?Relays are electric switches that use electromagnetism to convert small electrical stimuli into larger currents. These conversions occur when electrical inputs activate electromagnets to either form or break existing circuits. 5. What is the difference between relay and switch?The main difference between Relay and Switch is that the Relay is an electrically operated switch and Switch is an electrical component that can break an electrical circuit. ... Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. 6. Are Relays AC or DC?A Dc relay coil has a resistance that limits the dc current. An AC coil relies on its impedance for governing the current. An AC relay will remain contact closed due to mechanical inertia and a little mechanical hysteresis and, the fact that an alternating north and south pole both attract the relay armature. 7. How a relay works in a car?Although there are various relay designs, the ones most commonly found in low voltage auto and marine applications are electro-mechanical relays that work by activating an electromagnet to pull a set of contacts to make or break a circuit. These are used extensively throughout vehicle electrical systems. 8. What happens when a relay fails?If the ignition relay shorts burns out or otherwise fails while the engine is operating it will cut off power to the fuel pump and ignition system. ... In some instances of a faulty relay the vehicle will be able to restart once the relay cools off, only to stall out once again after the relay overheats. 9. Does a relay need constant power?The answer to that one is No. Relays have a finite lifetime in terms of how many times they can open and close. And limit to how much current they can handle. But keeping a relay constantly energized does not wear it out. 10. Why is a relay better than a switch?Relays are a better choice for switching large currents (> 5A). Relays can switch many contacts at once. Disadvantages of relays: • Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. Recommended ReadingBasic Knowledge of Relay Electronics Tutorial with VideoThe Role of the Relay and Its Working PrincipleThe Types of Common Relay and How to Choose Relay?

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

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

Ⅰ IntroductionFlyback Diodes, which are also known as freewheeling diodes, generally refer to diodes that are inversely paralleled across the ends of energy storage elements such as inductors, relays, and thyristors. When a voltage or current changes suddenly in a circuit, it protects other components in the circuit. When using a flyback diode, the circuit current can be changed more gently to avoid the occurrence of voltage spike. This article will introduce in detail what is flyback diode, how freewheeling diode works, flyback diode selection and the flyback diode function.How Freewheeling Diode WorksCatalogⅠ IntroductionⅡ DesignⅢ How It Works?Ⅳ SelectionⅤ Applications5.1 Summary5.2 In Forward Switching Power Supply5.3 In Converter Technology5.4 In Unidirectional Half Wave Silicon Control Rectifier Circuit5.5 In BUCK CircuitⅥ Something Has to CareIn electronics, a flyback voltage or an inductive flyback is a voltage spike created by an Inductor when its power supply is removed abruptly. The reason for this voltage spike is the fact that there cannot be an instant change to the current flowing through an Inductor.In addition, time constant of the inductor determines the rate at which the current can change through an inductor. This is similar to the time constant of a capacitor, which determines the rate at which its voltage can change.The freewheeling diode is named because it plays the role of freewheeling in the circuit. It is generally used in the circuit to protect components from being damaged or burned out by voltage breakdown, connected in parallel to both ends of the elements that generate the induced electromotive force(EMF), and form a loop with them, so that the high electromotive force generated in the loop is consumed by the continuous current method, thereby protecting the components in the circuits.Flyback diodes are connected in parallel at both ends of the coil. When the current passes through the coil, it will generate induced electromotive force at both ends. When the current disappears, its induced electromotive force generates a reverse voltage to the components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the elements, it will cause damage to the elements such as triode and thyristor. When the current flowing through the coil disappears, the induced electromotive force generated by the coil is consumed by the work formed by the diode and the coil, thereby protecting the other elements in the circuit.Ⅱ DesignIn the following figure, it is showed that a flyback diode is placed across the inductor. An ideal flyback diode will have a very large peak forward current; capacity which helps in handling the voltage transients from damaging the diode, and inductor’s power supply is suited for reverse breakdown voltage and low forward voltage drop. Voltage spike can be 10times to the voltage of power supply which depends on the equipment involved and the application. So it is understood that not to underestimate the energy which contain within an energized inductor. Figure 1. Flyback DiodeFor an ideal flyback diode selection, a diode which has very large peak forward current capacity (to handle voltage transients without burning out the diode) should be selected, moreover, low forward voltage drop, and a reverse breakdown voltage fitted the inductor's power supply. Depending on the application and equipment in real requirement, some voltage surges can be upwards of 10 times the voltage of the power source, so it is critical not to underestimate the energy contained within an energized inductor.Flyback Diode Selection Note You Should KnowWhen used with a DC coil relay, a flyback diode can cause delayed drop-out of the contacts when power is turned off, due to the continued circulation of current in the relay coil and diode. When rapid opening of the contacts is important, a small value resistor can be placed in series with the diode to help dissipate the coil energy faster, at the expense of higher voltage at the switch.Schottky diodes are preferred in flyback diode applications as switching power converters, because they have the lowest forward drop (~0.2V rather than >0.7V for low currents) and are able to quickly respond to reverse bias (when the inductor is being re-energized). They therefore dissipate less energy while transferring energy from the inductor to a capacitor.When the flyback diode is used to simply dissipate the inductive energy, as with a solenoid or electric motor, cheap 1N540x and 1N400x general-purpose diodes are used instead. Ⅲ How It Works?Flyback diodes are often used with energy storage elements to prevent sudden changes in voltage and current to provide a pathway. The inductor can provide continuous current to the load through it to avoid sudden changes in load current and smooth the current. In the switching power supply, you can see a freewheeling circuit composed of a diode and a resistor connected in series, which is connected in parallel with the primary side of the transformer. When the switch is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too large and breakdown the switch. Generally, it is often to choose the fast recovery diode or the Schottky diode as flyback diode.Circuit Expressions Figure 2. Flyback Diode in Switching Power Supply CircuitIn Figure 2(c), when KR is turned on, the upper is positive voltage and the lower is negative voltage,  and the current direction is from top to bottom. When the VT is turned off, the current in the KR is suddenly interrupted and an induced potential is generated. The current direction is kept constant, that is, keeping the KR current direction from the top to bottom, which based on the Lenz's law. The induced potential and the power supply voltage are superimposed and applied across the VT, making it easy for the VT to breakdown. To avoid it, VD is used to short-circuit the induced potential generated by KR, that is, The current flows clockwise in the small circuits of the diodes and relays to protect the VT. R and C in Figure 2(b) also use the principle that the voltage on C cannot be abruptly changed to absorb the induced potential.In short, the flyback diode is connected in parallel to the relay or the inductor at both ends of the circuit. When the inductor is powered off, the electromotive force at both ends does not disappear immediately. At this time, the residual electromotive force is released through a freewheeling diode to reverse the reverse generated by the coil (the EMF is consumed in the form of current). It can be seen that the freewheeling diode is not a substantial component, but plays a "freewheeling" role in the circuit.For example, reversely connect a flyback diode at both ends of a relay coil or at both ends of a unidirectional thyristor. In practice, electromagnetic relays are usually controlled by triodes or MOS tubes to achieve automatic control of electrical loads (such as through a single-chip microcomputer), and the coil of the relay is a large inductance, which can store electrical energy in the form of a magnetic field. So when it pulls in, it stores a lot of magnetic field. When the triode controlling the relay changes from on to off, the coil is powered off, but there is a magnetic field in the coil. At this time, the back electromotive voltage can be as high as 1000v to destroy other circuit components. This is because the access of the diode is exactly the same as the direction of the reverse electromotive force. So that the reverse potential is neutralized by the freewheeling diode in the form of current to protect other circuit components. In addition, it is generally a diode with a fast switching speed.  Figure 3. Freewheeling Diode CircuitBecause the relay coil exists inductive load, which will absorb the self-inductive voltage of the relay coil when the triode is turned off. According to Lenz's law, when the current on the inductor decreases, a self-inductive voltage is generated. The direction of this voltage is that the forward terminal is negative and the collector of the driving tube is positive. This voltage will break through the triode, so an freewheeling diode is connected in parallel with the relay to absorb this self-inductive voltage.1) The influence of the time parameter of the circuit below the ms level on the mechanical contact is ignored.2) Even the 1N4000 reverse recovery time is far below the ms level, and the forward conduction time is shorter.3) Capacitance between the driving tubes and parasitic capacitance of the relay is enough to disable the high-speed diode.4) The consumption of inductive energy storage mainly depends on the winding resistance, which is generally in an overdamped state.It is general to use transistors as switches. As shown in Figure, a transistor TR1 is used to control the conduction of the relay coil, and the relay contact is used to control the load circuit.In a thyristor circuit, the thyristor is generally used as a contact switch, if a large inductive load is controlled, a high-voltage back electromotive force will be generated, and the principle is the same as that of a relay.Flyback diode also used on displays coils commonly used in relays. It is often used with energy storage elements to prevent sudden changes in voltage and current and provide a path. The inductor can provide continuous current to the load to avoid sudden changes in load current and smooth the current. In the switching power supply, it is common to see a freewheeling circuit composed of a diode and a resistor connected in series. The following circuit is connected in parallel with the primary side of the transformer. Figure 4. Flyback Diode in Relay CircuitThe freewheeling diode is added to both ends of the inductive load, and the inductive here is to have an inductive characteristic. The characteristic of the inductive load is that the current cannot be abruptly changed, in other words, it can't be all of a sudden. Common inductive loads include relay coils and solenoid valves.Figure 5.  Typical Freewheeling CircuitThe Figure 5 shows the typical application circuit of the flyback diode, where the resistor R determined whether it is needed or not. When the energy storage element VT is turned on, the upper voltage is positive, and the lower voltage is negative, and the current direction is from top to bottom. When the VT is turned off, the current in the energy storage element is suddenly interrupted, and an induced potential is generated at this time. This induced potential and the power supply voltage are superimposed and applied to both ends of the VT, which can easily cause VT to break down. VD can be added for this purpose, so that the induced potential generated by the energy storage element can be short-circuited to achieve the purpose of protecting the VT. Ⅳ Selection1) Based on working voltage2) Based on working current1N4007 is a not bad choice but not the best, because the PLC may be damaged before the diodes have time to play the freewheeling effect. Therefore, it is best to use FR107 to protect the freewheeling circuit, which can better protect the PLC output interface, and the cost will not rise too much. It is also possible to choose IN5819 or IN5817, which has better performance than FR107, but the cost is a little higher. Ⅴ Applications5.1 SummaryFlyback diodes are usually used with energy storage elements, and their role is to prevent sudden changes in voltage and current in the circuit and provide a power-consuming path for reverse electromotive force. The inductive coil can provide continuous current to the load through EMF, so as not to change the load current and smooth the current. In the switching power supply, a freewheeling circuit always composed of a diode and a resistor connected in series. This circuit is connected in parallel with the primary side of the transformer. When the switch is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too large and breakdown the switch.5.2 In Forward Switching Power SupplyIn the forward switching power supply, when the MOS is turned off, the secondary side of the transformer provides current to the outside by the energy stored in the inductor. In order to make the inductor play this role under load, a freewheeling diode is added on the secondary side of the transformer. The inductor, load, and freewheeling diodes create paths to transfer the energy in the inductor to the outside.5.3 In Converter TechnologyIn the electronic converter circuit, the single-phase bridge rectifier in the rectification section is the single-phase rectifier circuit with the most practical applications. And three-phase bridge rectification is the most widely used method for power systems, especially generator excitation systems. Both of these circuits must be connected to a flyback diode. Its function is almost the same. Take a single-phase bridge circuit as an example: When the rectifier bridge is connected to an inductive load, because the inductor current cannot be abruptly changed, during the thyristor off time, it must connect freewheeling diode at both ends of the load to provide a smoothing path  to prevent dangerous overvoltages across the inductive load, and also the thyristor can be commutated to conduct.The three-phase bridge rectifier circuits used in generator excitation systems are divided into three-phase half-control bridges and three-phase full-control bridge circuits. Therefore, in order to ensure reliable commutation of the rectifier components, the half-control bridge needs to connect flyback diodes in parallel at both ends of the inductive load, while the full-control bridge does not need to do so. In addition, when the conduction angle is changed, the average voltage and line current of the half-controlled bridge change more slowly than the full-controlled bridge.At present, current converters such as rectifiers and inverters are now used in a large number of devices, in which flyback diodes are typically added to the internal DC bus of the converter. Because if the load is an inductive element, when a large-capacity inverter on the bus fails, the DC bus will generate huge reverse surge energy. At this time, it is necessary to provide a discharge channel for this energy, otherwise it will break down or burn the converter. This channel needs a diode to form, that is a flyback diode.5.4 In Unidirectional Half Wave Silicon Control Rectifier CircuitFor unidirectional half-wave silicon control rectifier circuit with large inductive load, when the  silicon control is turned off in the negative half cycle, the inductive load will generate a high reverse induced electromotive force. This reverse electromotive force is sufficient to cause the silicon control to break down and burn. After that, the reverse electromotive force can be discharged into the forward voltage drop of the diode (about 0.7V), thereby effectively protecting the circuit components. 5.5 In BUCK Circuit Figure 6. BUCK CircuitIn the BUCK circuit, fast recovery diodes or Schottky diodes are generally selected as freewheeling diodes. It is generally used in the circuit to protect components from being broken down or burned by induced voltage. The two ends of the element form a loop with it, so that the high electromotive force generated in the loop is consumed in a continuous current manner, thereby protecting the elements in the circuit.In theory, the diode is selected at least 2 times the maximum current. In actual use, due to the strong transient overload resistance of the diode, an ultra-fast diode with a maximum current of 50A can also be used. In addition, a reasonable heat sink generally has little damage in actual use. The total impedance when conducting is the internal resistance of the motor plus the equivalent internal resistance of the drive tube. And the total impedance during freewheeling is the internal resistance of the motor plus the equivalent internal resistance of the freewheeling diode. In general, the AC equivalent internal resistance of the freewheeling diode is smaller than the AC equivalent internal resistance of the driving transistor. Therefore, in conventional design, the maximum current of the freewheeling diode is generally doubled to the maximum current of the motor.The transient current is only a moment, and the anti-overload capability of the surface-contact diode is enough, as long as it is not used in overvoltage, if necessary, a small resistor can be connected in series to limit the current. The flyback diode is to protect the switching device. The transient current during freewheeling is related to the working voltage of the motor and the internal resistance of the winding, and has nothing to do with the power of the motor. If necessary, the peak value of the transient current is the reverse self-inductance voltage minus diode junction voltage drop and then divided by the loop resistance. The reason why a diode with a certain current used is because the internal resistance of the winding of the low-voltage high-power motor is low, so the transient current will be relatively large. A series of small resistors can suppress the peak current, the transient voltage of the switch tube rises slightly because the operating voltage is not high, and now the current withstand voltage of transistors is at least 50V or more. Ⅵ Something Has to CareFreewheeling diodes are commonly used in switching power supplies, relay circuits, thyristor circuits, IGBTs, and other circuits. They are widely used, so it is necessary to pay attention to the following points when using them: 1) Fylback diode is an effective method to prevent the high voltage generated by self-inductive potential from causing damage to related components when the DC coil is powered off.2) The polarity of the flyback diode must not be connected wrongly, otherwise a short circuit situation will be caused.3) The flyback diode is always reversed to the DC voltage, that is, the negative pole of the diode is connected to the positive pole of the DC power supply.4) The flyback diode works in the forward conduction state, not in the breakdown state or the high-speed switching state, that is, the flyback diode does not used in electrical breakdown, recoverable situation, but its unidirectional conduction effect is the key point.5) Zener diodes can't be regarded as flyback diode. Because the zener diodes use reverse characteristics, and the flyback diodes use forward characteristics. Frequently Asked Questions about Flyback Diode or Freewheeling Diode1. What is a flyback diode?A flyback diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. 2. What is the role of freewheeling diode?A Flyback diode is also called as freewheeling diode. ... Here catch diode is used to eliminate flyback, when the abrupt voltage spike is witnessed across the inductive load when the supply current abruptly reduced. It helps the circuit from damaging. 3. What is a flyback diode used for?A flyback diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. 4. How does a flyback diode work?The Flyback diode makes inductor to draw current from itself in a loop until the energy is dissipated in diode and wires. When the current flow to an AC induction motor is suddenly interrupted, then the inductor tries to maintain increasing the voltage and the current by reversing polarity. 5. How do you choose a freewheeling diode?The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. A diode in your application having 50 volts would be more than adequate. Again 1N4001 will do the job. 6. How do I choose a flyback diode for a relay?Specify a diode for at least 79.4 mA current. In your case, a 1N4001 current rating far exceeds the requirement. The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. 7. What are the advantages of freewheeling diode?What are the advantage of free wheeling diode in a Full Wave rectifier? It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. 8. Why freewheeling diode is used in controlled rectifier?When the inductive circuit is switched off, this diode gives a short circuit path for the flow of inductor decay current and hence dissipation of stored energy in the inductor. This diode is also called Flywheel or Fly-back diode. circuits, inverter circuits, and chopper circuits by making it continuous. 9. What is the effect of adding free wheeling diode?It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. 10. What is the use of freewheeling diode in converter circuit?A free wheeling diode is used in converter circuits . It is connected across the load. During positive cycle of input it is reverse biased. During negative cycle of input the diode conducts and the energy stored in the circuit inductor during the previous half cycle is delivered to the load itself.