relay Related Articles

IntroductionThermal relay is a protective device.  are protective devices. It is used in conjunction with a contactor to protect electric motors. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts. Getting to know more about the thermal basics just check the following note. CatalogIntroductionⅠ The Working Principle and Structure of Thermal Relay  1.1 The Role and Classification of Thermal Relay  1.2 Protection Characteristics and Working Principle of Thermal RelayⅡ Selection and Setting Principle of Thermal Relay  2.1 Thermal Relay Selection Overview  2.2 Type Selection of Thermal Relay  2.3 Selection of Rated Current of Thermal Relay  2.4 Selection of Thermal Element Setting Current  2.5 Reliable and Reasonable Protection Characteristics of The Thermal Relay  2.6 Other ConsiderationsⅢ Other Matters Needing Attention  3.1 Installation Direction  3.2 Selection of Connecting Wires  3.3 Use Environment  3.4 Adjustment of Thermal RelayⅣ Frequently Asked Questions about Thermal RelayⅠ The Working Principle and Structure of Thermal Relay1.1 The Role and Classification of Thermal RelayIn the electric drag control system, when the three-phase AC motor runs under abnormal conditions such as long-term under-load and under-voltage operation, long-term overload operation, and long-term single-phase operation, it will cause the motor winding to overheat and even burn out. In order to give full play to the overload capacity of the motor, to ensure the normal start and operation of the motor, and once the motor is overloaded for a long time, it can automatically cut off the circuit, so that there are electrical appliances that can change the operating time with the degree of overload and that is thermal relay.  Obviously, the thermal relay is used for overload protection of the three-phase AC motor in the circuit. It must be pointed out that, due to the thermal inertia of the heating elements in the thermal relay, instantaneous overload protection cannot be done in the circuit, and short circuit protection cannot be done either. Therefore, it is different from overcurrent relays and fuses. According to the number of phases, there are three types of thermal relays: single-phase, two-phase, and three-phase. Each type has different specifications and model numbers according to the rated current of the heating element. Three-phase thermal relays are often used in three-phase AC motors for overload protection. Divided by function, there are two types of three-phase thermal relay. One is without phase failure protection and the other one is with phase failure protection.1.2 Protection Characteristics and Working Principle of Thermal Relay1) Protection Characteristics of Thermal Relay Because the contact action time of the thermal relay is related to the overload of the motor being protected, before analyzing the working principle of the thermal relay, the relationship between the motor's overload current and the motor's energizing time must be clarified under the condition that the motor does not exceed the allowable temperature rise. This relationship is called the overload characteristic of the motor. When an overload current occurs during the motor operation, it will inevitably cause the winding to heat up. According to the thermal equilibrium relationship, it is not difficult to draw the conclusion that under the condition of allowable temperature rise, the motor energizing time is inversely proportional to the square of its overload current. According to this conclusion, it can be concluded that the motor's overload characteristics have inverse time characteristics, as shown by curve 1 in Figure 1. Figure 1. Overload Characteristics of the Motor and Protection Characteristics of the Thermal Relay and Their Coordination In order to adapt to the overload characteristic of the motor and play the role of overload protection, it is required that the thermal relay should also have the inverse time characteristic as the motor overload characteristic. For this reason, the thermal relay must have a resistance heating element. The thermal effect generated by the overload heating current through the resistance heating element causes the sensing element to act, thereby driving the contact to complete the protection function.  The relationship between the overload current passed in the thermal relay and the action time of the thermal relay contact is called the protection characteristic of the thermal relay, as shown by curve 2 in figure 1. Considering the effects of various errors, the overload characteristics of the motor and the protection characteristics of the relay are not a curve, but a belt. Obviously, the larger the error, the wider the belt; the smaller the error, the narrower the belt. It can be known from the curve 1 in the figure that when the motor is overloaded, it is safe to work below the curve 1. Therefore, the thermal relay's protection characteristics should be close to the motor's overload characteristics. In this way, if an overload occurs, the thermal relay will operate before the motor reaches its allowable overload limit, cutting off the power to the motor to prevent damage. 2) Working Principle of Thermal Relay The heat-generating heating element in the thermal relay should be connected in series with the motor circuit. In this way, the thermal relay can directly reflect the overload current of the motor. The sensing element of a thermal relay generally uses a bimetal. The so-called bimetallic sheet is to mechanically roll two metal sheets with different linear expansion coefficients into one body. The larger the expansion coefficient is called the active layer, the smaller the expansion coefficient is called the passive layer. The bimetallic sheet undergoes linear expansion when heated. Because the linear expansion coefficients of the two layers of metal are different and the two layers of metal are closely attached together, the bimetallic sheet is bent to the passive layer side, and the mechanical force generated by the bending of the bimetallic piece drives the contact action. There are four types of bimetal heating methods, namely direct heating, indirect heating, composite heating, and current transformer heating. The direct heating type uses the bimetal as a heating element and allows current to pass through it directly; the heating element of the indirect heating type is made of resistance wire or tape, is wound around the bimetal and is insulated from the bimetal; the composite heating type is between the above two methods; the heating element of the current transformer heating type is not directly connected to the motor circuit, but is connected to the secondary side of the current transformer. This method is mostly used in situations where the motor current is relatively large to reduce the current passes through the heating element. Figure 2. Structural Schematic of the Thermal Relay The thermal element 3 is connected in series to the motor stator winding, and the motor winding current is the current flowing through the thermal element. When the motor is running normally, although the heat generated by the thermal element can bend the bimetal 2, it is not enough to make the relay operate; when the motor is overloaded, the heat generated by the thermal element increases, causing the bending displacement of the bimetal to increase.  After a certain period of time, the bimetal is bent to push the guide plate 4, and the contacts 9 and 6 are separated by the compensating bimetal 5 and the push rod 14, the contacts 9 and 6 are normally-closed contacts in which the thermal relay is connected to the contactor coil circuit, and the contactor is de-energized after being disconnected. The normally-open contacts of the contactor disconnect the power supply of the motor to protect the motor. The adjusting knob 11 is an eccentric wheel, which constitutes a lever with the support 12, and 13 is a compression spring. Turning the eccentric wheel and changing its radius can change the contact distance between the compensating bimetal 5 and the guide plate 4 so that the purpose of adjusting the setting action current is achieved. In addition, the position of the normally-open contact 7 is changed by adjusting the reset screw 8 so that the thermal relay can work in two working states: manual reset and automatic reset. When debugging the manual reset, after the fault is excluded, button 10 must be pressed to restore the movable contact to the contact position of the static contact 6. 3) Thermal Relay with Open Phase Protection One of the main reasons for a three-phase asynchronous motor to burn out is that wiring of a three-phase motor is loosened or a phase fuse is blown. If the motor protected by the thermal relay is Y connection method when one phase power failure occurs in the line, the current of the other two phases will increase a lot. Since the line current is equal to the phase current, the current flowing through the motor windings and the current flowing through the thermal relay are increased by the same proportion, so ordinary two-phase or three-phase thermal relays can protect this.  If the motor is △ connection method, the phase current and line current of the motor will not be the same when the phase failure occurs, the current flowing through the motor windings and the current flowing through the thermal relay will increase in different proportions, and the thermal element is connected in series with the power supply line of the motor, and it is set according to the rated current of the motor, that is, the line current, and the setting value is relatively large. When the fault line current reaches the rated current, in the motor winding, the fault current of the phase winding with the larger current will exceed the rated phase current, and there is a danger of overheating and burning. Therefore, the △ connection method must use a thermal relay with phase failure protection. The thermal relay with phase failure protection is a differential mechanism added to the ordinary thermal relay to compare the three currents. The structural principle of the differential phase-open protection device is shown in figure 3. The guide plate of the thermal relay is changed to a differential mechanism, which is composed of an upper guide plate 1, a lower guide plate 2 and a lever 5. They are connected by a rotating shaft. Figure 3a shows the positions of the components of the mechanism before power is applied. Figure 3b shows the position during normal energization. At this time, the three-phase bimetals are bent to the left by heating, but the bending deflection is not enough. Therefore, the lower guide plate is moved to the left for a short distance, and the relay does not operate. Figure 3c shows the situation when the three phases are overloaded simultaneously. The three-phase bimetal is bent to the left at the same time, and the lower guide plate 2 is pushed to move to the left. The normally-closed contact is immediately measured by lever 5. Figure 3 shows the disconnection of phase C.  At this time, the phase C bimetal gradually cools down, the end moves to the right and pushes the upper guide plate 1 to the right. While the temperature of the other two-phase bimetals rises, the ends are bent to the left, pushing the lower guide plate 2 to continue to move to the left. Because the upper and lower guide plates move left and right, a differential function occurs, and the normally-closed contacts are opened by the amplification of the lever. Due to the differential function, the thermal relay is accelerated to protect the motor when the phase failure occurs. Figure 3. Schematic Diagram of Differential Relay Phase Failure Protection Mechanism of Thermal Relay Ⅱ Selection and Setting Principle of Thermal Relay2.1 Thermal Relay Selection OverviewThe thermal relay is mainly used to protect the motor from overload. In order to ensure that the motor can obtain both necessary and sufficient overload protection, it is necessary to fully understand the performance of the motor, and assign it with a suitable thermal relay to perform the necessary settings. Generally, conditions related to the motor are the working environment, starting current, load nature, working system, allowable overload capacity and so on. In principle, the ampere-second characteristic of the thermal relay should be as close as possible or even overlap the motor's overload characteristic, or under the motor's overload characteristic, and at the same time, the thermal relay should not be affected (not actuated) at the moment when the motor is temporarily overloaded and started. The correct selection of the thermal relay is closely related to the working system of the motor. When the thermal relay is used to protect the motor for long-term or intermittent long-term operation, it is generally selected according to the rated current of the motor. For example, the setting value of the thermal relay may be equal to 0.95-1.05 times of the rated current of the motor, or the median value of the setting current of the thermal relay is equal to the rated current of the motor, and then adjust. When the thermal relay is used to protect a motor that is repeatedly operated for a short time, the thermal relay has only a certain range of adaptability. If there are many operations per hour, a thermal relay with a speed saturation current transformer must be selected. For special working motors with frequent forward and reverse phase on and off, it is not appropriate to use thermal relays as overload protection devices. Instead, use temperature relays or thermistors embedded in the motor windings to protect them.2.2 Type Selection of Thermal RelayThe thermal relay can be divided into the two-pole types and three-pole types from the structural type. The three-pole type is divided into phase-open protection and no phase-open protection, which should be selected according to the stator wiring of the protected motor. When the motor stator winding is in delta connection, a three-pole thermal relay with phase failure protection must be used; for a motor using the star connection method, a thermal relay without phase failure protection is generally used. Because the general motor does not have a neutral wire when using the star connection method, the two-pole or three-pole type of the thermal relay can be used. However, if the motor is set to use a star connection method with a neutral wire, the thermal relay must use a three-pole type. In addition, generally a two-phase structure thermal relay should be selected for light-load starting, long-term working motors or intermittent long-term working motors; when the current and voltage balance of the motor is poor, the working environment is poor, or there are fewer people to look after, three-phase thermal relay can be used.2.3 Selection of Rated Current of Thermal Relay1) Ensure the normal operation and starting of the motorIn the case of normal starting current and starting time and infrequent starting, it must be ensured that the starting of the motor does not cause the thermal relay to malfunction. When the starting current of the motor is 6 times the rated current, the starting time does not exceed 6s, and rarely starts continuously, the thermal relay can generally be selected according to the rated current of the motor. (In practice, the rated current of the thermal relay can be slightly larger than the rated current of the motor) 2) Consider the object of protection-the characteristics of the motorModels, specifications, and characteristics of motors. The insulation materials of motors are classified into A, E, and B grades. Their allowable temperature rises are different, so their ability to withstand overload is also different, which should be paid attention to when selecting a thermal relay. In addition, the open-type motor is easier to dissipate heat, while the closed-type motor is much more difficult to dissipate heat. With a slight overload, its temperature rise may exceed the limit. Although the selection of the thermal relay is based on the rated current of the motor in principle, the rated current of the thermal relay (or thermal element) that it is equipped with should be appropriately small for the motor with poor overload capacity. In this case, the rated current of the thermal relay (or thermal element) can also be taken as 60% -80% of the rated current of the motor. 3) Consider load factorsIf the nature of the load is not allowed to stop, even if the overload will shorten the life of the motor, the motor should not be allowed to trip unexpectedly, so as not to suffer a huge loss many times higher than the price of the motor. At this time, the rated current of the relay can be selected to a larger value (of course, the selection of the motor under this working condition generally also has a strong overload capacity). In this case, it is best to use the protective measures of the organic combination of thermal relays and other protective appliances, and only consider tripping when a very dangerous overload occurs. In short, this is not a dogmatic formula and should be considered comprehensively. 2.4 Selection of Thermal Element Setting CurrentAccording to model number of the thermal relay and the rated current of the thermal element, the adjustment range of the setting current of the thermal element can be found out. Generally, the setting current of the thermal relay is adjusted to the rated current of the motor; for motors with poor overload capacity, the setting value of the thermal element can be adjusted to 0.6-0.8 times of the rated current of the motor; when the motor starts for a long time, drags the impact load or is not allowed to stop, the setting current of the thermal element can be adjusted to 1.1-1.15 times of the rated current of the motor. 2.5 Reliable and Reasonable Protection Characteristics of The Thermal RelaySpecifically, it should have an inverse time characteristic similar to the allowable overload characteristic of the motor, and it should be below the allowable overload characteristic of the motor, and it should have high accuracy to ensure the reliability of the protective action. 2.6 Other Considerations1) Operating frequency: When the operating frequency of the motor exceeds the operating frequency of the thermal relay, such as the motor's reverse braking, reversible operation, and dense on-off, the thermal relay cannot provide protection. At this time, you can consider using a semiconductor temperature relay for protection. 2) It is not necessary to set overload protection for motors with short working hours and long intervals (such as rocker lifting motors for rocker drilling machines, etc.), and motors that have little possibility of overload despite long-term work(such as exhaust fans, etc.). 3) Thermal relays are generally not suitable for motors with a jog, heavy load starting, continuous forward and reverse rotation, and reverse braking. 4) It should have a certain temperature compensation: due to the change in the temperature of the surrounding medium, under the same overload current, the operation of the thermal relay will cause an error. To eliminate this error, temperature compensation measurements should be set up. 5) In general, the principle that the protected motor should not restart automatically even after the thermal relay is automatically reset after the thermal relay protection action, otherwise, the thermal relay should be set to the manual reset state. This is to prevent the motor from being repeatedly restarted several times to damage the equipment before the fault is eliminated.  For example: Generally, for the control circuit using button control to manually start and stop, the thermal relay can be set to the automatic reset form; for the automatic start circuit using automatic component control, the thermal relay should be set to the manual reset form; Any thermal relay that can be automatically reset should be able to be automatically reset reliably within 5 minutes after operation, while the manual reset one should be reset reliably when the manual reset button is pressed by hand within 2 minutes after the operation. Most products generally have both manual and automatic reset methods and can be adjusted to any method with screws to meet the needs of different occasions. 6) The operating current value should be adjustable to meet the needs in production and use, and reduce the specification-grade, so the thermal relay of a certain specification should be able to be realized by adjusting the cam. 7) Since it takes time for the thermal element to deform due to heat, the thermal relay can only be used as overload protection for the motor, not as short circuit protection. Therefore, when using a thermal relay, a fuse should be installed as short circuit protection. For heavy load, frequent-starting large-capacity important motors, overcurrent relays (time-delay action type) can be used for its overload and short circuit protection. Ⅲ Other Matters Needing Attention3.1 Installation DirectionThe installation direction of the thermal relay is easily overlooked. In the thermal relay, there is a current that generates heat through the heating element, which promotes the action of the bimetal. There are three ways of heat transfer: convection, radiation and conduction. Convection is directional, and heat is transferred from the bottom up. During the placement, if the heating element is under the bimetal, the bimetal will heat up quickly and the action time will be short; if the heating element is next to the bimetal, the bimetal will heat slowly and the action time of the thermal relay will be long.  When the thermal relay is installed with other electrical appliances, it should be installed below the electrical appliances and away from other electrical appliances by more than 50 mm to avoid the influence of other electrical appliances. The installation direction of the thermal relay should be in accordance with the specifications of the product manual to ensure that the thermal relay's operating performance is consistent during use.3.2 Selection of Connecting WiresThe connecting wires at the output end should be selected according to the rated current of the thermal relay. Too thick or too thin will also affect the normal operation of the thermal relay. If the connecting wire is too thin, the heat generated by the connecting wire will be transferred to the bimetallic sheet, and the heat-generating component will dissipate less heat along the wire, which shortens the trip time of the thermal relay.  On the contrary, if the connecting wire is too thick, this will extend the trip time of the thermal relay. For thermal relays with a rated current of 10 A, the cross-sectional area of the connecting wire at the output end is preferably 2.5 mm2 (single-strand copper-core plastic wire), the one of 20 A is preferably 4 mm2 (single-strand copper-core plastic wire), 16 mm2 is suitable for the one of 60 A(multi-strand copper-core rubber flexible wire), and the one of150 A is preferably 35 mm2 (multi-strand copper-core rubber flexible wire).  Because the material and thickness of the wire will affect the heat conduction from the termination of the thermal element to the external heat, if the wire is too thin, the axial thermal conductivity is poor, and the thermal relay may act in advance; if the wire is too thick, the axial heat conduction is fast, and the thermal relay may lag behind. The connecting wire at the output end of the thermal relay is generally a copper core wire. If an aluminum core wire is used, the cross-sectional area of the wire should be increased by 1.8 times, and the end of the wire should be tinned. Reference table for selection of cross-section of connecting wire:Setting current of thermal relay I / ACross-sectional area of connecting wire MM20 < IN ≤81.08 < IN ≤121.512 < IN ≤202.520 < IN ≤254.025 < IN ≤326.032 < IN ≤5010.050 < IN ≤6516.065 < IN ≤8525.085 < IN ≤11535.0115 < IN ≤15050.0150 < IN ≤16070.03.3 Use EnvironmentThis mainly refers to the ambient temperature, which has a greater impact on the speed of the thermal relay. The temperature of the medium surrounding the thermal relay should be the same as the temperature of the medium surrounding the motor, otherwise, the adjusted fit will be destroyed. For example: when the motor is installed in an environment of high temperature and the thermal relay is installed in an environment of lower temperature, the action of the thermal relay will lag (or the action current is large); otherwise, its action will be advanced (or the action current is small). For thermal relays without temperature compensation, they should be used at the place where there is little difference in an ambient temperature between the thermal relay and the motor. For the thermal relay with temperature compensation, it can be used in the place where the environmental temperature of the thermal relay and the motor is different, but the influence caused by the environmental temperature change should be minimized as much as possible. The ambient temperature of the thermal relay and the protected motor should be considered. When the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15℃, a thermal relay with a larger rated current rating should be used; when the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15℃, a thermal relay with a smaller rated current rating should be used. In addition, the load of the motor and the adjustment range that the thermal relay may require should also be considered.3.4 Adjustment of Thermal RelayBefore putting it into use, the setting current of the thermal relay must be adjusted to ensure that the set current of the thermal relay matches the rated current of the protected motor. Before the thermal relay is used in the circuit, the specific current of the thermal relay must be adjusted according to the rated current of the motor to meet the requirements of corresponding occasions. For example, for a 10kW, 380V motor with a rated current of 19.9A, a XX20-25 thermal relay can be used. The setting current of the thermal element is 17 ～ 21 ～ 25A. First, set it at 21A according to the general situation. If it is found that it often moves in advance and the temperature rise of the motor is not high, you can change the setting current to 25A and continue to observe; if the motor temperature rises at 21A, and the thermal relay lags, you can change the setting current to 17A and observe to get the best fit. It is used to adjust the rated current when overload protection of the motor is repeatedly operated for a short time. Multiple tests and adjustments in the field can get more reliable protection. The method is: first adjust the rated current of the thermal relay to be slightly smaller than the rated current of the motor. If it is found that it often moves during operation, then gradually increase the rated value of the thermal relay until it meets the operating requirements. There should be motor protection during the special operations. Motors with forward, reverse, and frequent on-off operations should not be protected by thermal relays. The ideal method is to protect it with a temperature relay or thermistor embedded in the winding.  Ⅳ Frequently Asked Questions about Thermal Relay1. What is a thermal relay?A relay that opens or closes contacts with a bending mechanism as a result of the difference in the expansion coefficients of a bimetal, which is heated by the current. ... The thermal relay is combined with a magnetic contactor because it cannot switch the main circuit by itself. The operating point can be changed. 2. How does a thermal relay work?A thermal relay works depending upon the above-mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying overcurrent of the system, it bends and makes normally open contacts. 3. What is the purpose of thermal overload relay?Thermal overload relays are economic electromechanical protection devices for the main circuit. They offer reliable protection for motors in the event of overload or phase failure. The thermal overload relay can make up a compact starting solution together with contactors. 4. What are the two types of thermal overload relays?Thermal Overload RelaysThermal overloads can be divided into two types: solder melting type, or solder pot, and bimetal strip type. Because thermal overload relays operate on the principle of heat, they are sensitive to ambient (surrounding air) temperature. 5. How do you use a thermal overload relay?Overload relays protect a motor by sensing the current going to the motor. Many of these use small heaters, often bi-metallic elements that bend when warmed by the current to the motor. When the current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor. 6. How do you test a thermal overload relay?CEP7 Overload Relay test procedures:Measure 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. 7. What is thermal overload relay how it functions?The function of a thermal overload relay, used in motor starter circuits is to prevent the motor from drawing excessive current which is harmful to motor insulation. It is connected either directly to motor lines or indirectly through current transformers. 8. What is the purpose of a thermal overload?Thermal overload relays are protective devices. They are designed to cut power if the motor draws too much current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed (NC) relay. 9. What does a thermal overload relay consist of?Bimetallic thermal overload relays (sometimes referred to as heater elements) are made of two metals, with different coefficients of thermal expansion, that is fastened or bonded together. A winding, wrapped around or placed near the bimetallic strip, carries current. 10. How do I know if my overload relay is bad?Unplug the start relay from the compressor and give it a shake. If you can hear rattling on the inside of the start relay, then the part is bad and will have to be replaced. If it's not rattling and appears to be in good condition, you may have a problem with the actual compressor. 

Warm hints: The word in this article is about 2500 words and reading time is about 12 minutes.   This paper mainly introduces that how to use a solid state relay to drive a thermostat.    As we all known,relay is an electrical control device, an electrical appliance that makes the predetermined step change in the electrical output circuit when the input (excitation) changes reach the required requirements. Catalog   I. Solid State Relay Basics 1.1    What is solid state relay 1.2    Solid state relay working principle 1.3    Solid state relay appliances II. Thermostat Basics 2.1    What is thermostat 2.2     Types of thermostat 2.3    Features of thermostat 2.4    Applications of thermostat III. Drive Thermostat by Using Solid State Relay 3.1    Power thermostat 3.2    Case of driving thermostat FAQ   I. Solid State Relay Basics   1.1 What is Solid State Relay   A solid-state relay(SSR) is a contactless switch consisting of microelectronic circuits, discrete electronic devices and power electronic power devices. The isolation between the control end and the load side is realized by isolation devices. The input of the solid-state relay is controlled by a tiny control signal to directly drive the large current load.   SSR takes advantage of the switching characteristics of electronic components, such as switch triode, bidirectional thyristor and other semiconductor devices, to achieve the purpose of connecting and disconnecting the circuits without contact and sparkless, and therefore is also called "contactless switch".    A solid-state relay is a four-terminal active device, of which two terminals are input control terminal, and the other ends are output controlled ends. It has both amplification and isolation function. It is suitable for driving high power switching actuator, which is more reliable than electromagnetic relay and has no contact, long life, fast speed and interference to the outside. Because of its small size, it has been widely used.   1.2  Solid State Relay Working Principle   SSR can be divided into two types: AC type and DC type according to the use occasions.    They can switch loads on AC or DC power supply, and they can not be mixed. The following is an example of the AC type SSR as an example of its working principle. The following diagram is a block diagram of its working principle. The components in the diagram constitute the main body of the AC SSR. From the whole, SSR has only two inputs (A and B) and two output terminals (C and D), and is a four-terminal device.   Working principle block diagram of solid state relay   When a certain control signal is added to the A and B, the "switch" and "break" between the two ends of C and D can be controlled and the function of "switch" can be realized. The function of the coupling circuit is to provide a channel between the input/output terminal of the control signal input from the A and B ends, but it disconnects the input and output terminals of the SSR in the electrical circuit.    In order to prevent the effect of the output end on the input end, the coupling circuit used the "optical coupler", which is sensitive, responsive, and high in the input/output insulation (voltage resistance) level; because the input terminal load is a light-emitting diode, this makes the input end of the SSR easily matched with the input signal level. When used, it can be directly connected to the output interface of the computer, that is, the logical level control of "1" and "0".    The function of the trigger circuit is to generate the required trigger signal, drive the switch circuit 4, but because the switch circuit does not add the special control circuit, it will produce the radio frequency interference and pollute the power grid such as the high order harmonic or the peak, so the zero-crossing control circuit is set up. The "zero crossings" means that when the control signal is added and the AC voltage is over zero, the SSR is a passing state, and when the control signal is broken, the SSR is to wait for the junction point (zero potential) of the positive half of the alternating current and the negative half of the half-week (zero potential), and the SSR is broken.    This design can prevent high-order harmonic interference and pollution to the power grid. The absorption circuit is designed to prevent the shock and interference (or even misoperation) of the peak, surge (voltage) transmitted from the power supply to the bidirectional thyristor in the switch device, usually using an "R-C" series absorption circuit or a nonlinear resistor (varistor).   1.3  Solid State Relay Appliances   The special solid-state relay can have the function of short circuit protection, overload protection and overheating protection. With the combined logic curing package, the intelligent module can be realized by the user. It is directly used in the control system.   Solid-state relay has been widely used in computer peripheral interface equipment, thermostat system, temperature regulating, electric furnace heating control, motor control, CNC machine, remote control system, industrial automation device, signal light, light adjustment, scintillator, lighting stage lighting control system, instruments, medical instruments, duplicator, automatic laundry. Machine, automatic fire protection, security system, and power capacitor switching switch as power factor compensation for the power grid, and so on, in addition to the chemical, coal mine, explosion-proof, anti-corrosion, corrosion prevention and so on. The logical curing encapsulation can realize the intelligent modules that users need and is directly used in the control system.     II. Thermostat Basics   2.1  What is Thermostat   The thermostat is a device that directly or indirectly controls one or more hot and cold Yuanlai to maintain the desired temperature. In order to achieve this function, a thermostat must have a sensitive element and a converter. The sensitive element can measure the change of temperature and produce the function required for the converter. The converter converts the function from the sensing element to the proper control of the device that changes the temperature. Thermostat 2.2 Types of Thermostat The types of the thermostat are generally the following: （1）Insert thermostat is installed on the pipe and sensitive element is inserted into the pipeline. （2）Immerse sensitive elements immersed in liquid in pipes or containers to control liquids. （3）Surface sensitive elements installed on the surface of pipes or similar surfaces.   2.3  Features of Thermostat This thermostat pressure gauge setting range (5~35 C) This thermostat measurement accuracy: plus or minus 1 DEG C The thermostat. Size: 86 x 86 (mm) Power supply: AC220V thermostat. This thermostat using ultra-thin design, electrical interface It has a large LCD screen with an LCD thermostat (backlight green, Lan Beiguang) You can display the thermostat in international language (Chinese + English) The thermostat has the function of automatic and manual. This thermostat for refrigeration heating and ventilation three working modes This thermostat high low-speed automatic selection Thermostat timing shutdown function. This thermostat control fan coil end of the fan, water valve, air valve You can also set the password on the thermostat setting temperature and wind speed according to the requirements of users.   Features of thermostat 2.4  Applications of Thermostat   The most common use of the thermostat is to control the room temperature.   Typical uses include: control the gas valve; control the fuel furnace regulator; control the electric heating regulator; control the refrigeration compressor; control the gate regulator.   A room temperature regulator can be used to provide a variety of control functions, such as heating control, heating - cooling control, day and night control (at night at lower temperatures), multistage control, primary or multistage heating, primary or multistage cooling, or multistage heating and cooling control.       III. Drive Thermostat by Using Solid State Relay   3.1  Power Thermostat   There are two kinds of power supply for the thermostat: battery and 24VAC power.    The thermostat needs battery power to run without interruption. It is very important that these batteries consume as low energy as possible, but even if you minimize the power consumption, the users are still inconvenient because the battery needs to be replaced from time to time. In order to reduce the replacement frequency, you can use a 24 VAC power supply. When the C line in the system is not available, the bridge rectifier shown in Figure 1 can convert the AC (AC) voltage to a DC (DC) voltage by the load. Single thermostat signal relay connection with HVAC load   3.2  Case of Driving Thermostat When the HVAC load (compressor, fan, gas valve, etc.) is turned off, the contact of the signal relay is broken. When the contacts are open, the terminals of the rectifier bridge see the voltage of the HVAC transformer is 24VAC, and convert the AC power to DC power, as mentioned earlier. The resulting DC voltage is used to drive the thermostat or subcircuit.   During the HVAC load conduction, the contacts of the signal relay are closed. When the contact is closed, the voltage across the bridge terminal is reduced to zero. This eliminates the need to use 24VAC as a power supply, so the thermostat battery power must be controlled. The range of current required for operating electromechanical relays ranges from tens to hundreds of Ma, which can have a significant impact on battery life.   If there is a way to drive a relay without using a thermostat battery, what will happen? Battery life will increase and replacement frequency will be further reduced. One way is to turn on the relay and charge the control system briefly during the HVAC load conduction (signal relay contact closure).  Compared with the turn off time of the power relay, the time required during charging is very short, which can stimulate the power relay and its corresponding load. Unfortunately, electromechanical (signal) relays are not likely to achieve this goal due to their switching speed limits. The time taken by the contact to the desired location is in milliseconds and will interrupt the HVAC load.   Fortunately, a device can achieve the appropriate switching speed: solid-state relay (SSR). SSR is a semiconductor repeater based on a thyristor or power transistor to perform on / off control.   This recharge method requires a dual MOSFET SSR because it can turn off MOSFET based SSR when necessary. Besides, body diodes of each MOSFET can assist in 24VAC rectification. A full-wave rectifier bridge is built with two diode MOSFET diodes, as shown below.   A power supply for SSR in a HVAC system The following figure shows the rectified waveform corresponding to the color coded diode in the above figure. The voltage ripple of the final waveform can be eliminated by connecting a suitable capacitor to the output of the rectifier bridge. Then, you can reduce the DC voltage of the control system to the desired voltage. Full wave rectifying waveform The use of SSR enables the HVAC system to fully supply the thermostat and reduce the power utilization rate of the battery. When SSR closes, the HV1 and HV2 pipelines will see the full 24VAC voltage and provide a constant 33VDC voltage at the output of the rectifier bridge. When SSR is connected, it may still be circulated through a short-time on/off state to recharge the power supply capacitor. This design can greatly reduce the energy requirements of the thermostat battery and reduce the battery replacement frequency.   FAQ   1. What is solid state relay and how it works? 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. It serves the same function as an electromechanical relay, but has no moving parts and therefore results in a longer operational lifetime.   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. How fast is a solid state relay? The SSR output is activated immediately after applying 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 application where a fast response time is desired, such as solenoids or coils.   4. Do solid state relays get hot? All solid state relays develop heat as a result of a forward voltage drop through the junction of the output device. Beyond a point, heat will cause a lowering (or derating) of the load current that can be handled by the SSR. ... Loads greater than 4 Amps will require heat sinks.   5. What causes solid state relay failure? What are the main causes and solutions of the Solid-state Relays (SSR)'s failures? If an inrush current exceeds the rated making current of the SSR due to the high inrush current of loads such as motors and lamps, SSR output elements are damaged. Consider using an SSR with a higher capacity.   6. Can a solid state relay switch DC? Solid state relays can be designed to switch both AC or DC currents by using an SCR, TRIAC, or switching transistor output instead of the usual mechanical normally-open (NO) contacts.   7. How do you test a solid state relay with a multimeter? Using Multimeter:  1. Set the multimeter in continuity test mode. 2. Place the probes of the multimeter on the coil terminals. 3. If the multimeter beeps (or show any sign of continuity), the coil is electrically closed (good). 4. If the multimeter does not beep, the coil is open & damaged. The relay needs to be replaced.   8. How reliable are solid state relays? Solid-state relays are the preferred choice for system reliability because they have no moving parts or contacts. Over time, the plating on the contacts inside EMRs can erode. This erosion can cause the contacts to weld shut; therefore they no longer open/close properly, and the relay has to be replaced.   9. Is a solid state relay a transistor? Solid-State Relay: A sort of hybrid between a conventional relay and a transistor, these relays switch a load using an LED activated by the control circuitry. The LED activates a light-activated MOSFET that controls the load.   10. 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.   11. How do I choose a solid state relay? When selecting a Solid State Relay, consider: Current rating, as a general rule consider using the relay at no more than 70% of its rated current. Electrical environment,. i(In harsh electrical environments, consider a relay with an line voltage rating above the application line voltage.)   12. Do solid state relays need a diode? 2 Answers. The control side of solid state relays is usually just a LED, sometimes two LEDs back to back, and sometimes with integrated resistor. ... If the relay is on the same board as whatever is driving it, then no inductive kickback diode is needed. It's no different than driving any other on-board LED.   13. 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. ... The SSR control is attached to the atmega328 through a 200ohm resistor.   Relevant information about "How  to Drive Thermostat by Using Solid State Relay " About the article "How  to Drive Thermostat by Using Solid State Relay", If you have better ideas, don't hesitate to write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.   Making a Arduino Variable Timer Relay Comprehensive Introduction of the Time Delay Relays

SummarySome electornic or electrical appliances needs time limited power supply,or usage of some devices are depends on limitted time.To automate electrical devices depends on time simple and robust solution given based on arduino.Today let's make a arduino variable time relay together.By using this arduino variable timer relay we can control high voltage electrical appliances or electronic devices.Now let me share the process. To indicate the time duration and status 16×2 LCD display is included in this design, once the program uploaded to the Arduino then it can work independent with some external battery power source. Connection DiagramConstruction and Working In this project arduino uno board is used to control SPDT (Single pole double throw) Relay and 16 x 2 character LCD indicates the time duration status. Digital pins D2 to D7 are connected to the LCD display. VR1 varible resistor helps to control the contrast of LCD display, Transistor Q1 BC547 reacts as a Switching device and controls the power supply to the Relay coil depends on arduino output. There are three push buttons are placed to set different time durations, S1 Switch makes the count start, S2 changes the Hours and S3 changes the Minutes of time duration. Output signal from the Arduino is taken from D8 pin and it drives the Relay through transistor. After making the connection, upload the following arduino sketch and pretest the operation with real timer clock.Note:- Candle with extreme care if you using High voltage supply at the Relay end. Arduino Code #include <LiquidCrystal.h>LiquidCrystal lcd(7,6,5,4,3,2);const int set = 9;int hours=10;int start=11; int relay=8;int b=0,h=0,t=0;int buttonState = 0; int lastButtonState = 0; void setup() {    pinMode(set,INPUT);  pinMode(hours,INPUT);  pinMode(relay,OUTPUT);  pinMode(start,INPUT);  lcd.begin(16,2);  lcd.setCursor(0,0);  lcd.print("Adjustable Timer");  }int timer( int b,int h){         if(b<=9)           {            lcd.setCursor(3,1);            lcd.print(0);            lcd.setCursor(4,1);            lcd.print(b);          }     else{lcd.setCursor(3,1);lcd.print(b);}         lcd.setCursor(2,1);         lcd.print(":");     if(h<=9)           {            lcd.setCursor(0,1);            lcd.print(0);            lcd.setCursor(1,1);            lcd.print(h);          }     else{lcd.setCursor(0,1);lcd.print(h);}    }void loop()       {           buttonState = digitalRead(set);                         if (buttonState != lastButtonState)       {                 if(buttonState == HIGH)         {                       lcd.clear();           lcd.print("Set time in min:");                      ++b;           timer(b,h);                                   }                   lastButtonState = buttonState;          }       if (digitalRead(hours)== HIGH)          {              lcd.clear();              lcd.print("Set time in hours");              ++h;              timer(b,h);              while(digitalRead(hours)==HIGH);                                    }             if(digitalRead(start)==HIGH)          {             lcd.clear();             t=((h*60)+(b))*1000;             lcd.print("Timer is set for");             timer(b,h);             digitalWrite(relay,HIGH);             delay(t);             digitalWrite(relay,LOW);             while(digitalRead(start) == HIGH );                                   }                        } Have you make it successfully?  

In this article, we will provide you the basic information of relay: what is a relay? What types of relays are there? What are their characteristics? How to maintain the common faults of the relay? With these questions, let us find the answers in the article together.     Catalog     I. What is a Relay?     1.1 Electrical Symbol     1.2 Contact Form     1.3 Functions of Relay II. Relay Classification III. Main Types of Relay IV. How to Test a Relay V. Influencing Factors of Relay Reliability VI. Maintenance of Common Faults of Relay VII. Example Explanation: Delay Relays FAQ   I. What is a Relay? This video will explain what is a relay, and how does a relay works with basic information about construction and different types of relay.   Relay is a kind of electric control device. When the change of input quantity (excitation quantity) reaches the prescribed requirement, the controlled quantity will be changed step by step in the electric output circuit. It has an interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop). Usually used in an automatic control circuit, it is a kind of automatic switch which uses a small current to control the operation of a large current. Therefore, it can play the role of automatic regulation, safety protection, conversion circuit, and so on.   1.1 Electrical Symbol A relay is composed of two parts: coil and contact group, the graphic symbol of the relay in a circuit diagram also includes two parts: a box for coil and a set of contact symbols for contact combination. When the contact circuit is relatively simple, the contact group is often drawn directly on one side of the circle frame, which is called centralized representation.   Relay coils are represented by a rectangular symbol in the circuit, and if the relay has two coils, draw two side-by-side boxes. The contacts of relays are represented in two ways: one is to draw them directly on the side of the box, which is more intuitive. The other is to draw each contact point into its own control circuit according to the need for circuit connection. Usually, the contact of the same relay is marked with the same text symbol, and the contact group is numbered to show the difference.   1.2 Contact Form There are three basic forms of contact for relays: 1. The two contacts are disconnected when power off, and the two contacts are closed when power on.   2. The two contacts are closed when power off and the two contacts are disconnected when the power on.    3. The contact group has three contact points, that is, a moving contact in the middle and a static contact in the upper and lower parts of the contact group, respectively. When the coil is power-off, the dynamic contact is disconnected from one of the static contacts and connected with the other. After the coil is power-on, the dynamic contact moves, the connecting contacts state is opposite to the power-off state to achieve the purpose of conversion. Such contact groups are called switching contacts.      1.3 Functions of Relay Relay is an automatic switching element with an isolation function. It is widely used in remote control, telemetry, communication, automatic control, electromechanical integration, and power electronic equipment. It is one of the most important control components.   The relay generally has induction parts (input section) that can reflect a certain input variable (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); an executing part (output section) capable of realizing power-on and power-off state of the controlled circuit. Between the input port and the output port of the relay, there is also an intermediate mechanism (driving section) for coupling the input, the function processing, and the driving of the output.   As a control element, relays generally have the following functions:   1) Expand the control range: for example, when the control signal of the multi-contact relay reaches a certain value, the multi-circuit can be switched on and off at the same time according to the different connecting forms of the contact group.   2) Amplification: a very small volume can control a large power circuit, such as sensitive relays, intermediate relays, etc.   3) Synthesis signal: when a plurality of control signals input multiple winding relays in the prescribed form, a predetermined control effect can be achieved by comparing and synthesizing.   4) Automatic, remote control, monitoring: relays on the automatic device, together with other electrical appliances, can form a program control circuit to achieve automatic operation.   II. Relay Classification   1. According to the working principle or structural characteristics of relays: 1) Electromagnetic relay: an electrical relay that is driven by the suction of an input circuit between an electromagnet core and an armature. 2) Solid relay: a relay in which electronic components perform their functions without mechanical movement, and the input and output are isolated. 3) Temperature relay: a relay that operates when the external temperature reaches a certain value. 4) Reed relay: a relay operates by a reed that is sealed in a tube and has a dual action caused by the electricity action on the reed and the armature. 5) Time relay: When the input signal is added or removed, the output part needs to be delayed or limited to a specified time to close off. 6) High-frequency relay: a relay used for switching a high frequency, a radio frequency circuit which had a minimum loss. 7) Polarization relay: a relay driven by magnetic field synthesis caused by a polarized magnetic field and a controlled current acting through the magnetic field generated by the control coil. The direction of the operating relay depends on the direction of the current flowing through the control coil. 8) Other types of relay: optical relay, sound relay, thermal relay, instrument relay, Hall effect relay, differential relay, etc.   2. According to shape size of relays: 1)miniature relay  2)subminiature relay 3)small miniature relay Note: for sealed or enclosed relays, the size is the maximum of the three vertical dimensions of the relay body, excluding the dimensions of mounting, leading end, rib pressing, edge pressing, flanging, and sealing solder joint.   3. According to the load of the relays: 1) micro power relay 2) small power relay 3) medium power relay 4) high power relay    4. According to the protective characteristics of relays: 1) sealed relay  2) enclosed relay 3) unenclosed/ open relay   5. According to the principle of action: 1) electromagnetic type 2) induction type 3) rectifier type 4) electronic type 5) dig type   6. According to the physical quantity of the reaction: 1) current relay 2) voltage relay 3) power relay 4) impedance relay  5) frequency relay 6) gas relay   7. According to the role of relays in the protection circuit: 1) starting relay  2) measuring relay  3) time relay 4) auxiliary/intermediate relay 5) signal relay 6) exit relay    III. Main Types of Relay   1) electromagnetic relay As long as a certain voltage is added at both ends of the coil, a certain current will flow through the coil, producing an electromagnetic effect, and the armature will contact the iron core under the action of electromagnetic force attraction, thus driving the armature dynamic contact and static contact (normally open contact) suction. When the coil is powered off, the electromagnetic suction also disappears, and the armature will return to its original position in the reaction force of the spring and release the dynamic contact from the original static contact (normally closed contact). In this way to achieve the purpose of switching on and off. In addition, it can be distinguished the "normal open and closed contacts of the relay.   Note: The static contact in the broken state when the coil is powered off is known as the "normal open-contact"; the static contact in the on-state is called the "normal closed-contact". Relays generally have two circuits, a low-voltage control circuit, and a high-voltage working circuit.   2) solid-state relay  A solid-state relay is a kind of four-terminal device with two connection terminals as input and the other two as output. In the middle, an isolation device is used to realize the electrical isolation of the input and output.   Solid-state relays can be divided into AC type and DC type according to the type of load power supply. According to the switch, type can be divided into normal open type and normally closed type. According to the isolation type, it can be divided into hybrid type, transformer isolation type, and photoelectric isolation type, and the photoelectric isolation type is the most.   3) thermal reed relay A thermal reed relay is a new type of thermosensitive switch which uses thermosensitive magnetic material to detect and control temperature. It consists of a temperature-sensitive magnetic ring, constant magnetic ring, reed tube, heat conduction mounting sheet, a plastic substrate, and other accessories. Thermal reed relays do not use coil excitation but are driven by magnetic forces generated by constant magnetic rings. In addition, whether the constant magnetic ring can provide magnetic force to drive the reed tube is determined by the temperature control characteristic of the temperature sensing magnetic ring.     4) reed relay Reed relay is a kind of coil sensing device, which uses a coil to produce a magnetic field to drive a magnetic reed tube. In addition, the characteristics of reed relays include small size, lightweight, fast reaction speed, short jump time, and so on.   When a whole piece of ferromagnetic metal or other conductive material is close to it, turn on or turn off the circuit. Reed relay consists of a permanent magnet and a reed tube. Both of them fixed to a bracket without magnetism or magnetic conduction. Take the line of the permanent magnet's north-south pole as the axis, which should be coincident or basically coincident with the axis of the reed. From far to near, adjust the distance between the permanent magnet and the reed tube, and fix the position of the magnet when it happens to move (turning off for normally open reed tube and turning on for normally closed reed tube). At this point, when there's a whole piece of magnetic material, and when the iron plate is close to the magnet and the reed tube at the same time, the reed tube will move again and return to the state without magnetic field action; when the iron plate leaves, the spring tube will move in the opposite direction. The reed relay has a strong structure, sealed contact, and high durability. It can be used as a position limiting switch for mechanical equipment, and can also be used to detect whether iron doors, windows, etc.    5) optical relay An optical relay is a semiconductor relay used in AC/ DC, refers to the integration of light-emitting and light-receiving devices. The input side and the output side are electrically insulated, but the signal can be transmitted through the optical. Its characteristics are semi-permanent, micro-current, high impedance, insulating, voltage-resistance, ultra-small, optical-transmission, contact-free, and so on.   It is mainly used in measuring equipment, communication equipment, security equipment, medical equipment, and so on. 6) time relay Time relay is a kind of control apparatus that uses electromagnetic principle or mechanical principle to realize time delay control. There are many kinds of it, such as air damping type, power-driven type, and electronic type.   Air damping time relay is often used in AC circuit, which uses the principle of air throttling through orifice compensation to obtain delay action. It consists of an electromagnetic system, delay mechanism, and contact. Time relay can be divided into two types: power-on delay type and power-off delay type.   The time delay range of the air-damping time relay is large (0.4~60s and 0.4~180s). Its structure is simple, and its accuracy is low. When the coil is electrified (voltage specification is ac380v, ac220v or dc220v, dc24v, etc.), the armature and bracket are attracted by the iron core and moved down instantly, so that the instantaneous action contact is turned on or off, and meanwhile, the piston rod and lever cannot fall with the armature at the same time, because the upper end of the rod is attached to the rubber film in the air chamber, and when the rod begins to move downward under the action of the released spring, the rubber film falls downward. Air becomes thin in the upper air chamber and the damping piston rod drops slowly. After a certain period of time, the piston rod drops to a certain position, then pushes the delay contact action through the lever, causing the dynamic break contact to break and the dynamic close contact turned off. From the coil to the delay contact to complete the action, this time is the delay time of the relay. The delay time can be changed by adjusting the size of the air chamber inlet hole. After the suction coil is powered off, the relay is restored by the action of the recovery spring, and the air was ejected quickly through the vent hole. 7) auxiliary relay a. characteristics of auxiliary relay:   The relay is composed of several high-quality sealed small relays with low coil voltage, which is damp-proof, dust-proof, non-breaking, high reliability, and overcomes the shortcomings of the electromagnetic auxiliary relay wire which is too thin and easy to break. Low power consumption, low-temperature rise, no need to attach high-power resistance, easy installation and connection, large capacity of a relay contact, long working life, easy to observe on the spot, and so on. The delay only needs to be adjusted by the dial switch on the panel, the delay precision is high, and the delay range can be set freely in 0.02S~ 5.00S.   Purpose of intermediate relay: auxiliary relay is used in various protection and automatic control lines to increase the number of contacts and the capacity of contacts in the protection and control loop.   b. classification of auxiliary relays:   static auxiliary relay   delay auxiliary relay   electromagnetic auxiliary relay   elevator auxiliary relay   rail auxiliary relay   c. auxiliary relay principle   When the coil is electrified, the moving iron core is absorbed under the action of the electromagnetic force, and the moving contact action is driven so that the normally closed contact is separated and the normally open contact is closed. When the coil is powered off and the moving iron core drives the dynamic contact to reset under the action of the spring. The working principle of the relay is that when a certain input (such as voltage, current, temperature, velocity, pressure, etc.) reaches a predetermined value, it operates. In order to change the working state of the control circuit, so as to achieve the established purpose of control or protection. In this process, the relay mainly plays a role in the transmission of the signal.   d. function of the auxiliary relay   The general circuit is often divided into two parts of the main circuit and a control circuit. The relay is mainly used for a control circuit, and the contactor is mainly used for the main circuit. Through the relay, using one control signal can control the other one or several signals, and the control of starting, stopping, linkage, and so on. The main control object is a contactor, the contact of the contactor is relatively large, and the carrying capacity is strong, but the control of weak current too strong electricity can be realized through the contactor, and a control object is an electric appliance.   1. Replace small contactor   The contact of the auxiliary relay has a certain load capacity. When the load capacity is small, it can be used instead of the small contractor, such as the electric shutter and the control of some small appliances. This advantage is that not only can play the purpose of control, but also can save space so that the electrical control part of the more refined.   2. Increase the number of contacts   This is the most common use of auxiliary relays, for example, in a circuit control system where a contact needs to control multiple contactors or other components, adding an auxiliary relay to the line.   3. Increase contact capacity   We know that although the contact capacity of the auxiliary relay is not very large, it also has a certain capacity with load, and the current required for its drive is very small. Therefore, the auxiliary relay can be used to expand the contact capacity. For example, it is not possible to use induction switches directly and the output of the transistor to control the heavy load of electrical components. In fact, the auxiliary relay is used in the control line, and the other load is controlled by the auxiliary relay to enlarge the control capacity.   4. Convert pin type   In the industrial control circuit, it is often necessary to use the normally closed contact of the contactor to achieve the control purpose. However, the normally closed contacts carried by the contractor are not enough to achieve the control task. At this time, an auxiliary relay can be parallel to the original contactor coil, and the corresponding components can be controlled by the normally closed contact of the auxiliary relay, and the contact type can be transformed to achieve the desired control purpose.   5. As a switch In some control circuits, intermediate relays are often used to turn on and off some electrical components, such as automatic demagnetization circuits common in color televisions or displays, and transistor controls the on and off of intermediate relays, which are controlled by the opening and closing of their contacts, such as color televisions or displays. So as to control the demagnetization coil on-off action.   6. Switching voltage   7. Eliminating interference in the circuit   8. Power direction relay   An electrical appliance that causes the controlled output circuit to be switched on or off when the input (such as voltage, current, temperature, etc.) reaches a specified value. It can be divided into two categories of electrical volume (such as current, voltage, frequency, power, etc.) relay and non-electrical volume (such as temperature, pressure, speed, etc.) relay. It has the advantages of fast movement, stable work, long service life, small volume, and so on. Widely used in power protection, automation, motion, remote control, measurement, and communication devices.       Common Types   1. overcurrent relay The overcurrent relay is a relay that operates from the current beyond its set value and can be used as a system line and overload protection. The most commonly used is an induction type overcurrent relay, which is opposite to the rotating disk of aluminum or copper by an electromagnet. The rotating disc is rotated by means of the electromagnetic induction principle so as to achieve the protective effect.   action principles:   The inductive overcurrent relay uses the secondary current of the current transformer to generate a magnetic field in the relay to cause the disk to rotate, but the current flowing through the relay must be greater than the current value of a certain current to rotate.   2. overvoltage relay  Overvoltage relay, its main purpose is that when the abnormal voltage of the system rises to more than 120% rating, the overvoltage relay operates so that the circuit breaker can jump off and protect the electric equipment from damage. The construction and operation principle of induction overvoltage relay are similar to those of overcurrent relay, except the main loop.   3. under voltage relay The under voltage relay is constructed in the same way as the overvoltage relay, except that the inner contact and the turntable turn immediately when the voltage is applied.   4. ground overvoltage relay The grounding overvoltage relay has the same structure as the overvoltage relay, and uses a three-phase three-wire non-grounding system, and is connected to the earthing transformer with an open triangle earthing to detect zero-phase voltage.   5. grounding overcurrent relay Grounding overcurrent relay, abbreviated as GCR, is a kind of high-voltage line earthing protection relay.   Main uses:   1) grounding overcurrent protection of high resistance grounding system.   2) grounding protection of generator stator winding.   3) layer short circuit protection of phase-separated generator.   4) overheat protection of grounding transformer.   6. selective grounding relay Selective grounding relay, also called directional grounding relay, is used in non-grounding systems to protect distribution lines. In addition, it can also be used in overhead lines and cable systems.   Selective grounding relay: if a zero-phase sequence current is detected by a grounding voltage transformer when a line is grounded, the selective grounding relay can accurately detect the fault line and alert it and disconnect it according to the requirement. And then continue to send electricity to the normal operating line.   7. free-phase relay In the three-phase line, phase-failure relay or phase-failure protection relay will burn out the single-phase operation of the motor if it does not cut off the line immediately when there is a one-wire break in the power supply end and causes the single-phase.   8. percentage differential relay A percentage differential relay is used as the AC motor of the transformer. Alternator with differential protection and over-current protection relay used as the protection devices, and when abnormal current generated by external fault flows over protection equipment, if current on the transformer is unbalanced or inconsistent with the characteristics of the current transformer, in these cases, this phenomenon will extend several times and cause failure operation to the relay.     IV. How to Test a Relay Relay is the key device in the intelligent prepaid electric energy meter, the life of the relay determines the life of the meter to a certain extent, thus the performance of the relay is very important to the operation of the intelligent prepaid electric energy meter. There are many manufacturers of relays around the world. Their production scale is quite different, the technical level and performance parameters are very different. Therefore, the manufacturers of electric energy meters must have a set of perfect testing devices when detecting and selecting relays. To ensure the quality of the meter. At the same time, the national power grid has also strengthened the sampling detection of the relay performance parameters in the intelligent electric energy meter, which also needs the corresponding testing equipment to check the quality of the meter produced by different manufacturers. However, at present, relay testing equipment is not only a single test item, but detection process also can not be automated completely, the detection data needs manual processing and analysis, the detection results are random, artificial, and the detection efficiency is low, in addition, there is no guarantee of safety.   According to the test requirements of relay performance parameters, the test items can be divided into two categories: one is the test items without load current, such as operating value, contact resistance, service life; the other, test items with load current, such as contact voltage, electrical life, overload capacity.   1.Measuring coil resistance: the multimeter R×10Ω barrier can be used to measure the resistance value of the relay coil, so as to judge whether there is an open circuit phenomenon in the coil. The resistance value of the relay coil is closely related to its working voltage and current. And the service voltage and working current can be calculated by the resistance value of the coil.   2. Contact resistance measurement: using the resistance barrier of the multimeter, the resistance value of the normally closed contact and the moving point resistance should be 0, and the resistance value of the normally open contact and the moving point shall be infinitely large. From this, you can distinguish between the normally closed contact and the normally open contact.   3. Measure the pull-in voltage and current: using an adjustable voltage stabilizing power supply and ammeter, input a set of voltages to the relay, and connect the ammeter in the power supply circuit to monitor. Slowly raise the power supply voltage and note down the pull-in voltage and current when the relay absorbs sound. To be accurate, you can try a few more times and get the average value. Measurement of release voltage and discharge current: it is also like the above-mentioned connection test, when the relay suction, then gradually reduce the power supply voltage, when heard the relay again release sound, note the voltage and current at this time, in addition, you can also try more than a few times to get an average release voltage and current. Generally, the release voltage of the relay is about 10% of the pull-in voltage, and it will not work properly if the release voltage is too small (less than 1/ 10 of the pull-in voltage), which will affect the stability of the circuit and the device operation.   1. Understand the necessary conditions firstly.   1) The power supply voltage of the control circuit can provide the maximum current.  2) Voltage and current in the controlled circuit. 3) The requiring contacts on the controlled circuit. When the relay is selected, the power supply voltage of the general control circuit can be used as the basic factor for selection. The control circuit should provide sufficient working currently for the relay, otherwise, the relay absorption is unstable.   2. After consulting the relevant information to determine the applying conditions, you can find out the type and specification number of the relays required. If you already have a relay on hand, you can check whether it can be used against the data. Finally, consider whether the size is appropriate.   3. Pay attention to the volume of the apparatus. For general electrical appliances, consider the volume of the chassis and the layout of the circuit board installation. For small electrical appliances, such as toys, remote control devices should select ultra-small relay products.     The main test items are briefly described as follows:    (1) Operating value: The voltage required for relay action.    (2) Contact resistance: When electric contact closes, the resistance value between two contacts.   (3) Mechanical life: In the case of the mechanical part without damage, the relay switching times.   (4) Contact voltage: When the electric shock is closed, a certain load current is applied in the electric shock circuit, at this time, the voltage value between the contacts.    (5) Electric life: When the rated voltage is applied on both ends of the relay drive coil and the rated resistive load is applied in the contact circuit, the reliable operation times of the relay under the condition of duty cycle 1:4 less than 300 cycles per hour.    (6) Overload capacity: When the rated voltage is applied on both ends of the relay drive coil and 1.5 times rated load is applied in the contact circuit, the reliable operation times of the relay under the condition of (10 ±1) times/ minute (operation frequency). V. Influencing Factors of Relay Reliability   1.The influence of environment on relay reliability: the average fault interval time of relay working in GB and SF is the highest, reaching 820000h, while in the NU environment, it is only 60000h.   2.The effect of quality grade on relay reliability: the average failure interval of the A1 relay is 3660000h, while that of the C class relay is 110000, the difference between them is 33 times. It can be seen that the quality level of the relay has a great impact on its reliable performance.   3.The effect of the contact form on the reliability of relay: the contact form of the relay will also affect its reliability. The reliability of the single-throw relay is higher than that of the double-throw relay with the same number of tools, and the reliability decreases gradually with the increase of tool number, in addition, the reliability of a single-throw relay is higher than that of the double-throw relay with the same number of cutters. The average failure interval of a single-pole, single-throw relay is 5.5 times that of a four-pole double-throw relay.   4.The influence of structures on relay reliability: there are 24 types of relay structures, and all of them have an influence on the reliability of the relay.   5.Effect of temperature on the reliability of relay: the operating temperature range of the relay is between -25℃ and 70℃. With the increase of temperature, the average time between failures of the relay gradually decreases.   6.The effect of operating rate on relay reliability: with the increase of relay operating rate, the average fault interval time decreases exponentially. Therefore, if the designed circuit requires the relay to operate at a very high speed, it is necessary to carefully detect the relay in order to replace it in time for circuit maintenance.   7.The effect of the current ratio on the reliability of the relay: the so-called current ratio is the ratio of the operating load current of the relay to the rated load current. The current ratio has a great influence on the reliability of the relay, especially when the current ratio is greater than 0.1, the average fault interval time is rapidly reduced, and the current ratio is less than 0.1, the average fault interval time is basically unchanged, therefore, the load with a larger current rating is selected to reduce the current ratio when the circuit is designed because this ensures that the relay and even the entire circuit are not reduced in reliability due to the fluctuation of the operating current. VI. Maintenance of Common Faults of Relay   a. Maintenance of the sensing mechanism For electromagnetic (voltage, current, intermediate) relay, its sensing mechanism is the electromagnetic system. The fault of the electromagnetic system is mainly focused on the coil and the moving and static iron core.   1) coil fault Coil faults are usually caused by coil insulation damage; mechanical injuries form a turn-to-turn short circuit or grounding. Because the power supply voltage is too low, and dynamic, static core contact does not connect tightly, resulting in the current through the coil is too large, the coil heated to burn. The coil should be rewound during the repair. If the armature is not sucked after the coil is electrified, it may be that the wire connection of the coil is removed, so that the coil is short-circuited, therefore, the joint should be re-welded.   2) iron core fault The main fault of the iron core is that the armature can not be absorbed after the power on, which may be caused by the broken coil, having impurities between the moving and static iron core, and the low voltage of the power supply, thus repair should be differentiated. After the power on, the armature noise is big, this may be due to moving or static core contact surface is not smooth, or there is oil on the surface. During repair, the coil should be removed, filing or flattening the contact surface, and oil should be cleaned. Noise may be due to short-circuit or ring fracture, replacing new short-circuit ring to repair. After power loss, if the armature cannot be released immediately, possibly because the moving armature is stuck, the air gap of the iron core is too small, and the spring strain and the contact surface of the iron core have been polluted by oil. Taking maintenance should be differentiated according to the cause of the fault, or adjust the size of the air gap, or replace the springs, or use gasoline cleaning oil.   For the thermal relay, the sensing mechanism is the thermal component, and the common fault is that the thermal component burns out, or operation failures of the thermal element and does not operate.   (1) Thermal component burnout. This may be due to a short circuit on the load side or the high frequency of action of the thermal element. The thermal components should be replaced during maintenance and the setting value should be adjusted again. (2) Operation failure of thermal component. This may be due to the setting value is too small, the operation without overload, or the strong impact and vibration influence, make its action mechanism loosening and tripping. (3) No operation of thermal component. This may be due to the setting value is too small to lose the thermal element overload protection function. During maintenance, the setting current should be adjusted according to the overload working current.   b. Inspection and repair of executing parts Most relay actuators are contact systems. Through its "power on" and "power off" to complete a certain control function. Contact system faults generally caused by contact overheating, wear, melting soldering, and so on. The main reasons for contact overheating are insufficient capacity, insufficient contact pressure, surface oxidation or uncleanliness, etc. The main cause of wear is that the contact capacity is too small, the arc temperature is too high to cause contact metal oxidation, and so on. The main cause of contact melting soldering is that the arc temperature is too high, or the contact is seriously moved, and so on.   The order of maintenance of the contacts is as follows:   1) Open the outer cover and check the contact surface. 2) If the contact surface is oxidized, it is not necessary for the silver contact to be processed, and the oxide layer on the surface of the Cu contact may be lightly scraped with a file or a knife with a knife. 3) If the contact surface is not clean, clean it with gasoline or carbon tetrachloride. 4) If there is a burning trace on the surface of the contact, it is not necessary to repair the silver contact, and the copper contact should be repaired by a file or with a knife. Sand cloth or sandpaper is not allowed to be used for refurbishment, to avoid poor contact due to the residual stand. 5) Contact should be replaced if it welded. If the contact capacity is too small, replace the relay with a larger capacity. 6) If the contact pressure is insufficient, adjust the spring or replace the spring to increase the pressure, if the pressure is insufficient, the contact should be replaced.   c. Maintenance of intermediate part   1) In that air-type time relay, the intermediate part is mainly an airbag. The common faults are time delays. This may be because the airbag is not tight or air-leak, the action delay is shortened, and even the delay is not delayed; it is also possible that the air passage of the airbag is blocked so that the action delay is prolonged. In terms of repair, the former shall reassemble or replace the new airbag, and the latter should open the air chamber and remove the blockage. 2) For the speed relay, its rubberwood pendulum belongs to the intermediate part. If the motor can not stop braking during reverse braking, it is possible that the tilting rod of rubberwood is broken, and it should be replaced when overhauled.     VII. Example Explanation: Delay Relays RF Cafe has said "Relays are a topic that never goes out of date even with the advent of fully solid state relays that use semiconductors in the conduction path,there are still many applications that only mechanical contacts can satisfy." in April 1967 electornics world. It is true that there are switching diode arrays that can handle very high powers,but they are typically expensive compare with relays. Today, let's talk about something about time-delay relays.     What is time delay relays? Time delay relays are simply control relays with a time delay built in. Their purpose is to control an event based on time. The difference between relays and time delay relays is when the output contacts open & close: on a control relay, it happens when voltage is applied and removed from the coil; on time delay relays, the contacts can open or close before or after some time delay. Time delay relays have an important influence in industrial contor logic circuits. There are some examples following:   Flashing light control (time on, time off): two time-delay relays are used in conjunction with one another to provide a constant-frequency on/off pulsing of contacts for sending intermittent power to a lamp. Motor soft-start delay control: Instead of starting large electric motors by switching full power from a dead stop condition, reduced voltage can be switched for a “softer” start and less inrush current. After a prescribed time delay (provided by a time-delay relay), full power is applied. Furnace safety purge control: Before a combustion-type furnace can be safely lit, the air fan must be run for a specified amount of time to “purge” the furnace chamber of any potentially flammable or explosive vapors. A time-delay relay provides the furnace control logic with this necessary time element.   How does time delay relay work? Time delay relays can provide simple, reliable, and economical control. Adjusting the delay time is often as simple as turning a knob. Providing time-delayed switching to start a motor, control a load, or affect a process, TDRs are typically used in industrial applications and OEM equipment. Additionally, they play an important role for targeted logic needs, such as in a small panel or in sub-panels. They have a variety of features and operating characteristics, such as compactness, economy, simplicity, and ease-of-use.Time delay relays not only can be available as plug-in devices but aslo as single-function,single-time-range devices traditionally.   All in all, with an on-delay timer, timing begins when voltage is applied. When the time has expired, the contacts close — and remain closed until voltage is removed from the coil.   Time delay relays circuit and working     See the above circuit diagram, time delay relay circuit contains an electromechanical relay and driver circuit, this circuit decides the time delay to give power supply to the electromechanical relay coil by the way to the load connected to the relay.   This circuit is divided into two sections. The first section is time delay elements such as voltage divider resistor series and two electrolytic capacitors. The second section is a relay with an indicator LED. Resistor R1, potentiometer, and R2 connected in series and across to the DC input supply, the output of the variable resistor (potentiometer) is connected to the C1 capacitor and reverse-biased Zener diode then C2 capacitor finally to the base of transistor SL100. 12V Relay is connected with the collector terminal of SL100 transistor and Bicolor LED terminal green is connected with the emitter of Q1 and terminal Red is connected across collector.   When the supply given to this circuit depends on the value of the Potentiometer small level voltage passed to C1 and it gets charged when its completed and above the cutoff limit of the Zener diode, Voltage passed to the C2 capacitor and it gets charge, finally the base-emitter voltage limit of Q1 transistor reached by the C2 then Q1 gets turn ON and Relay coil gets complete DC supply then Relay energized for to complete the above process it takes some time delay depends on Potentiometer value, C1-C2 charge time and Zener diode breakdown voltage hence we can achieve few seconds to few minutes time delay.   By changing the Potentiometer value or C1-C2 value we can achieve different time delay levels. We can use this circuit to turn ON or turn OFF some sensitive time delay required electrical applications.     How to select a delay relay? Selecting a relay, there are many factors that need to consider including data on thermal,motor-driven, pneumatic, RC, slugged, hydraulic, escapement, and solid-state types.   The fantastic growth of the field of automatic industrial control has increased the demand for new and more versatile devices to perform the basic electrical switching functions required. The use of time-delay relays has grown rapidly to keep pace with the demand for the basic function which they can perform: that of obtaining a predetermined delay from one switch operation to another.     (A) Delay on energization. (B) Delay on de-energization. Time-delay relays perform in a manner quite similar to a standard relay in that they have contacts that open and close when power is applied and removed from the input terminals. The basic difference is that a delay is incorporated into the contact opening or dosing. Time-delay relays are used in a wide range of applications: from determining how full your coffee cup will be when you put a dime in a vending machine, to shutting off the cutting oil on a milling machine. The most popular time-delay relay is the delay on operation, or de-energization, in which the normally open load switching contacts transfer at a predetermined time after power is applied to the input. The contacts drop out immediately upon the removal of the input power   Often a time delay on release, or de-energization, is required. In this case, the normally open load switching contacts operate immediately when the input power is applied and remain in this position as long as the input power remains "on". Upon removal of this power the timing begins, and after a predetermined delay, the contacts drop out. Several variations on these two basic timing modes are used, such as interval "on", automatic recycle, combined "on" and "off" timers, and sequence timers. Many of these can be made by simple connections of the two basic types.   FAQ   1. What is Relay and its uses? Relays are switches that open and close circuits electromechanically or electronically. Relays control one electrical circuit by opening and closing contacts in another circuit. ... In addition, relays are also widely used to switch starting coils, heating elements, pilot lights and audible alarms.   2. What is the relay device? Relay is an asynchronous, screen-free walkie talkie system that allows parents to stay in touch with their kids at the push of a button. Relay is a Republic Wireless product, and makes use of the carrier's cell phone network (via T-Mobile and Sprint).    3. What is Relay and its types? Relays are electrically operated switches. They are used to control a circuit by a separate low-power signal or to control several circuits with one signal. ... The three main types of relays are electromechanical, solid-state, and reed. This overload protection relay reacts to overheating.   4. What is the working principle of relay? Relay works on the principle of electromagnetic induction. When the electromagnet is applied with some current it induces a magnetic field around it. Above image shows working of the relay . A switch is used to apply DC current to the load.   5. Does relay important? Converting a small electrical input into a high-current output is no easy feat, but this task is necessary to efficiently operate a wide range of standard appliances and vehicles. Many circuits achieve these conversions through the use of relays, which are indispensable in all kinds of electronic equipment.   6. What are the 5 applications of relay? Applications of Relays in Electronic Circuits: Relay Drive by Means of a Transistor. Relay Drive by Means of SCR. Relay Drive from External Contacts. LED Series and Parallel Connections. Electronic Circuit Drive by Means of a Relay. Power Source Circuit. PC Board Design Considerations. 7. What is difference between relay and circuit breaker? The Relay is a switching and sensing device, but the Circuit breaker is an isolating or disconnecting device. Relays operate on low power input voltage. ... The Relay is used to control or select one among many circuits, whereas Circuit Breaker is one per circuit. Relay acts an electrical amplifier for discrete signal.   8. How fast can a relay switch? 5 to 15 ms. While the mechanical construction of electromechanical relays allows for much flexibility in switching capability, they have one important limitation: speed. When compared to other relays, electromechanical relays are relatively slow devices -- typical models can switch and settle in 5 to 15 ms.   9. Why do I need a relay for LED lights? Relays can be used to switch a low-current trigger to high current, switch a circuit on or off, reverse polarity, and much more. When adding LED lights, such as off-road light bars, driving/work lights, or other auxiliary lights to a vehicle, you must add a circuit to power the light adequately.   10. What is the major application of relays in our daily lives? The typical applications of electromechanical relays include motor control, automotive applications such as an electrical fuel pump, industrial applications where control of high voltages and currents is intended, controlling large power loads, and so on.   You May Also Like: Making a Arduino Variable Timer Relay How to Drive Thermostat by Using Solid State Relay Product Recommendation: CMRD6055 CB-1001B-70 G6K-2F-Y-TR DC24

Today I want to share a project of making a simple automatic street light controller using relay and LDR I found in circuitdigest with you.   You have seen street light which automatically gets turned on in the night and gets turned off in the morning or day time, there are sensors who senses the light and control the light accordingly. These Street lights are an important project in smart cities.   So here in this project, we are going to make a Simple Automatic Street Light Controller Using Relay and LDR. This circuit is very simple circuit and can be built with Transistors and LDR, you don’t need any op-amp or 555 IC to trigger the AC load. Here we have used an AC bulb as street light. Some applications of this circuit are street light controlling, home/office light controlling, day and night indicators, etc.   Components Required:   Transistor BC547 -2 LDR (Light Dependent Resistor) Relay Resistor 1k 100k Potentiometer Power Supply 12v -1 Connecting wires Jumper wires Screw terminal Block 2 pin or 3 pin Bread Board or Perf Board 1n4007 Diode AC supply AC Load or Bulb   Here you may want to know: what is LDR? LDRs are made from semiconductor materials to enable them to have their light sensitive properties. There are many types but one material is popular and it is cadmium sulphide (CdS). These LDRs or PHOTO REISTORS works on the principle of “Photo Conductivity”. Now what this principle says is, whenever light falls on the surface of the LDR (in this case) the conductance of the element increases or in other words the resistance of the LDR falls when the light falls on the surface of the LDR. This property of the decrease in resistance for the LDR is achieved because it is a property of semiconductor material used on the surface. LDR (Light Dependent Resistor)   Circuit Diagram and Explanation:   Below is the circuit diagram of this Light sensing Street Light:       In this project, we have used an LDR (Light Dependent Resistor) which is responsible for detecting light and darkness. The resistance of LDR increases in darkness and reduces in presence of light. This circuit is same as a Dark Detector or Light Detector Circuit, only here we have replaced simple LED with a AC load, using a Relay. Two BC547 NPN transistors are used to drive the relay. Automatic Street Light circuit using LDR and relay   Whenever light falls over LDR its resistance get decreased and transistor Q1 turns ON and collector of this transistor goes LOW, and this makes the second transistor turns OFF due to getting a LOW signal at its base, so relay also remain turned OFF due to second transistor.   Now whenever LDR senses Darkness, mean no light, then transistor Q1 turned ON due to increase in the resistance of LDR which is responsible for voltage drop at the base of Q1. Due to a LOW signal at the Q1 base, Q2 transistor gets a HIGH signal from the collector of Q1 and turns ON the relay. Relay turned ON the AC load that is connected to relay. A 10K pot is also used for setting up the sensitivity of the circuit.   So this is how automatic Street Lights turns on in the night and turn off in the day, and below is the effect pictures. >>>>>                                                       > >>>>   Ref. KY56-BC547A KY32-1N4007