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Basic knowledge of relay

  • Summary of relay
    • Relay

      Relay is an automatic control device that will change by leaps and bounds in output when the input quantity (electrical, magnetic, acoustic, light, thermal) reaches a certain value.
      Power relays: The relays powered by electromagnetic.
      Direct-current relays: The relays need input of direct current.
      Alternating-current relays: The ralays need input of alternating current.
      Polar relays: The control of the relay is dependent on the direction of the input current.

  • Explanation of relays’terms
    • Coil Resistance:
      The rated resistance value of the relay design。
      CR(Contact resistance)::
      The formation of electrical contact with a much larger than the original conductor resistance after a conductor is divided into two parts, this resistance is called the contact resistance.
      Contact Resistance test is not suitable for millivolt test. The international standard is the voltage drop method (DC6V 1A).
      Operation Voltage
      The minimum value of the voltage required for all contacts of the relay from the return state to the operating state.
      Reset Voltage:
      The minimum value of the voltage required for all contacts of the relay from the operating state to the return state.
      Operation Time
      The time required for the relay coil to apply the rated voltage to the start of all contact actions. (Excluding bounce time).
      Reset Time
      The time required for the relay coil to be powered off until all contacts are released (excluding bounce time).
      Bounce Back:
      Intermittent opening and closing of contacts between the active parts of the relay (armature and core, contact, armature and push card).
      Action Bounce Back: When the relay is operating, the rebound time between contacts.
      Reset Bounce Back Time: When the relay is released, the rebound time between contacts.

      Insulation Resistance:
      The resistance value exhibited when measured with a predetermined DC voltage between mutually insulated conductive portions (DC500V condition) .
      Withstand Voltage:
      Also known as dielectric strength, refers to the dielectric material in the case of no failure can withstand the maximum voltage gradient (1mA, or 3mA, or other).
      Mainly: 1) between the coil and the contact, 2) between the same pole, 3) between the different contacts, 4) set the coil and reset coil.
      Impact Withstand Voltage (lightning withstand voltage):
      For abnormal conditions (such as lightning, inductor reverse voltage, etc.) generated by the impact voltage pulse durability value. Usually a few thousand volts to ten thousand volts.
      The vibration caused by the vibration generated during transportation and installation, the durability vibration specified by the breakage, and the vibration malfunction caused by vibration during use. Usually specify the amplitude and vibration frequency.
      the impact of the transport caused by the installation, the characteristics caused by changes in the damage specified by the impact of durability; In addition, the use of the impact caused by the impact of false action. Usually provided: Acceleration.
      Mechanical Life:
      In the case of no load, the life after the fatigue test of structure of the relay and parts.
      Electrical Life:
      The electrical durability life of the relay in the case of the specified load conditions and opening and closing frequencies.
      Inpact Current (Surge Current):
      When the contact is turned on, there is a current greater than the steady-state current. Mainly in the capacitive load, lamp load, motor load and so on.
      Rated voltage and current:
      Rated value of the relay design, the use of the relay under normal conditions, the voltage applied to the coil (current) as a standard.
      Permissible voltage:
      In the use of relays, due to the actual circuit voltage or the use of the environment may produce fluctuations, resulting in the voltage applied to the relay coil voltage may also fluctuate, in order not to affect the normal use of the relay, the provisions of the relay can be used Of the voltage range.(Each product has an allowable voltage range, refering ro the specification when used, if not specified, the allowable range is rated +/- 5%).
      Contact structure:
      Normally open contact (H or A contact), normally closed contact (D or B contact), conversion contact (Z or C contacts).
      Normally open contacts: the normal state is open contacts.
      Normally closed contact:s: the normal state is closed contacts.
      Rated load:
      Nominal load reference value for relay design, expressed in voltage and current, eg AC250V10A.
      Maximum permissible voltage of the contact:
      The maximum voltage allowed at the time of contact opening and closing, can not exceed this voltage when used.
      The maximum permissible current of the contact:
      The maximum allowable current value when the contact is open, and can not exceed the current when used.
      Contact Capacity:
      The product of the allowable voltage and current when the contact is open, can not exceed the capacity when used.
      Coil Rated Power:
      Power consumption of the coil when relay desinged, DC = mW, AC = VA.
      The frequency of opening and closing:
      The opening and closing times of relay in unit time.

General principles in the selection of relay

  • A、According to the application of load and control to determine the relay contact capacity and the number of contact group composition:
    • Under the normal conditions, the nominal load rating of the relay is based on the pure resistance load (cos = 1.0) as the standard load.
      1、Control the load / circuit type: mainly consider the changes of the current or voltage’s parameters when the load on or off , power factor, surge current, reverse voltage, etc. such as AC / DC circuit / R / L / C circuit / motor / Solenoid / electromagnet / contactor coil / tungsten lamp / fluorescent lamp / car light / halogen lamp / heating wire / microwave tube / etc. The parameters are not the same in the different load, it needs to confirm the actual parameters when designing, and to select matching relay according to the different load parameters.
      2、Control the number of load / circuit groups: under the normal circumstances, a relay controls a group, but in some special equipment, it needs to simultaneously control multiple sets of load / circuit, such as audio circuits. Generally it controls two or more at the same time. In addition, A lot of industrial equipment also need to control multiple groups at the same time.
      3、The need of on-off frequency of control circuit: With the same relay, the faster breaking frequency, the shorter life in the different on-off frequency and all other conditions are the same circumstances.
      4、Life expectancy: when designing, the life expectancy of the product and the life of relay keep primary balance, or relay life slightly surplus.

  • B、According to the design parameters of drive circuit to determine the relay coil parameters:
    • The decision of the relay drive circuit design requires the parameters of the relay coil, and the circuit parameters of the relay corresponding to the different circuits are different.
      1、According to the circuit design, the rated value of the relay coil and the coil power consumption are selected. When used, it is desirable to be able to apply the rated voltage. When using the transistor to drive the relay coil, consider the voltage drop of the transistor itself as well. In fact, I recommend the best practical voltage is the rated voltage of 100% -110%.
      2、Determine the operation voltage. By the influence of the voltage fluctuation range (power ripple) of the drive coil, when the fluctuation range is large, it may cause the relay to be unstable or damage the coil insulation, causing the relay to fail. Under the normal conditions,when the coil voltage fluctuates greatly, the relay with lower operating voltage should be used. When using the product,refer to product specifications, if no special instructions, the range of coil voltage fluctuation is +/- 5%.
      3、Determine the operation time: For some specific circuits, it needs to control the operating time, such as: MWO microwave tube, it needs phase control to reduce the inrush current. Audio circuit also need to control the time of bounce back of relay to ensure good sound quality.

  • C、Determine the ambient conditions around the relay according to the applicable environmental conditions:
    • In different external environments, the impact on the relay is not the same. For example, high temperature / humidity / dust / gas / vibration impact, etc., so select the appropriate product according to the actual environmental conditions.
      1、The temperature of the relay directly affects the life of the relay. The higher the temperature, the shorter life. The general use of the ambient temperature ot relay is 70℃,please refer to the product recommended temperature when using.
      2、Humidity generally affects the contact performance of the relay, which has the greatest impact on the circuit of the tiny current. The wet environment accelerates the oxidation of the contact and the internal metal parts, creating a layer of oxide on the contacts to increase the contact resistance. Under the normal conditions, select the sealed relay when the humidity is greater than 90%.
      3、Dust, gas (including S, NO, HCl, etc.) and more serious environment generally affect the contact performance of the relay. Dust, or gases with S and Ag generated Ag2S attached to the contact surface, resulting in increased contact resistance. Select sealed relay in the case of bad environment.
      4、The environment with serious vibration and impact will affect the reliability of the relay, prone to malfunction or coil disconnection. In general, the vibration resistance of relay is 55HZ (international standard), automotive relay is larger, about 400HZ or 500HZ. So it should give priority to the use of automotive relays if it is used for the equipment with greater impact of vibration.

  • D、According to the circuit design and component layout to determine the relay appearance:
    • In some design and layout of the circuit board, it may be subject to space constraints.Therefore , it needs to consider the components of the size, installation size and the ways of connection.

  • E、Safety performance:
    • In some special equipment, it needs to consider the safety of components, such as the generally consider of relay: the insulation resistance and withstand voltage.

  • F、Other special requirements:
    • Such as safety specifications, as well as other special issues that need to be considered in the design of the circuit or use.


  • A、Load type and control mode:
    • The majority of the failure of the relay in the application is caused by the load. Due to the actual parameters of the load or application circuit is not fully confirmed, it results in the use of relays with no match, and resulting in the relay contact adhesion, contact wear and tear depletion, contact instability, control circuit occurs to short circuit, performance deterioration of relay insulation, and more severe cases may cause like security risks. Therefore, it is very important to use the relay correctly and safely, the introduction of some of the larger impact of the load and the precautions when using relays as following:
      1DC load: the capacity when relay control DC load is much smaller than the AC load.
      When open or close the load, it will produce arc discharge. Since the DC has no zero crossing, the arc is not easy to extinguish. Time of Arc burning is much longer. Under the normal circumstances, the majority of failure phenomenon of relay in the DC load is contact metal transfer, leading to contact stick together. The temperature of the arc is very high (about 6000C), so that the contact has metal melting and adhesion, or because part of the metal was torn, so when the relay pull up again, it is prone to mechanical chuck. Therefore, in applications of the DC load, please be sure to use under the conditions of referring to the design specifications of products, generally should not use high-capacity DC load.
      2Inductive load: a high reverse voltage occurs when switching the inductive load.
      In control of electromagnets / contactors / relays / solenoids / transformers / motors (motors) etc., when the inductive load is cut off, a very high reverse voltage (several hundred to several thousand volts) is generated, and the arc is easily generated too, which will make the relay contacts stick together, make life shorter and so on. Meanwhile, a large inrush current (5 to 20 times) will generate when the above-mentioned load is turned on. Under the normal conditions, when open or close the same load, the lower the cos, the shorter the relay life .In the application of the circuit, it should be added the reverse voltage absorption circuit (contact protection circuit)
      3、Capacitive load: a high inrush current will generate when conducting capacitive loads, and the inrush current is very short (about 8-400us).
      At the moment of turning on condensive load, because the capacitor needs to be charged, it will produce a high surge current, easily lead to contact adhesion. Therefore, when controlling the capacitor circuit, it should choose high current surge relay; or in series with a resistance of about 1Ω in the capacitor to reduce the inrush current.
      4Lamp load: at the moment of turning on the lamp load, it will produce a high surge current, and the time of surge current is (about 50-300ms).
      It’s easy to make the contact stick together, and the surge current of different types of lamps is also different. For example: tungsten lamp / mercury lamp / halogen lamp / fluorescent lamp and so on. They all have their own features. It needs to confirm the size of the inrush current in practical use. In the control of such loads, the general selection is inrush current relay. In the UL standard, the TV certification reflects the ability of the pressure of surges. (TV certification is the use of tungsten bulb as the standard load).
      5Motor load and solenoid load: at the moment of turning on the above-mentioned load, it will produce a higher surge current, which will easily lead to contact adhesion.
      In the actual control of such loads It needs to confirm the size of the surge current to select the matching relay in the actual control.
      6、Power contactor load: the generation of inrush current about 3-10 times at the moment of turning on contactor load will easily lead to contact adhesion.
      It needs to confirm the size of the surge current to select the matching relay in the actual control of such loads.
      7Control high frequency signal
      The frequency of the signals controlled in some areas is very high (30-3000 MHz), so high frequency relays capable of switching high frequency signals are required. The high frequency insulation, insertion loss, reflection loss, standing wave ratio and other characteristics of such relays are completely different from those of DC load and low frequency AC load.
      8、Control small load:
      As for relays, the capacity of load is from a few mAmps to dozens of Amps (even hundreds of A also). According to the different application of load to select matching relays. Generally speaking, the smaller the load of the relay, the longer the span of life, but there are also certain restrictions, if it is too small, the life is not long. Because the contact materials used for different capacity of the relays is different, the contact surface will produce a layer of oxide and black carbon under the action of a small current, which will result in increased contact resistance so that affect the normal operation of the circuit.
      9、Open and close (on and off) frequency:
      Under the normal circumstances, there will be a strong arc when open or close the larger current load, and there is a close relationship between burning time of arc and the on-off frequency. If the on-off frequency of relays is too fast, the time of arc will be longer, and the faster the relay will fail. Generally there is the specified opening and closing frequency in the product specifications. Under the normal circumstances, the load capacity and the frequency of opening and closing as following:


      Recommended frequency

      Reference frequency

      UL frequency

      IEC frequency


















      Note: In the case of a long time without opening and closing, it is usual to use magnetic holding relay or the normally closed end of the relay to control the normally closed side.

  • B、The drive mode of relay coil:
    • The control of the relay coil directly affects the performance of the relay.
      The usage of reasonable control methods can not only reduce the inconvenience and harm caused by the failure of the relay, but also can effectively save energy.
      The several control methods of coil as following:
      1、Ideal relay control voltage source - constant voltage power supply:
      In the control of relays, the ideal coil supply voltage is a constant voltage, without any fluctuations, but it is difficult to do that in the practical application of circuit. If the drive voltage of relay coil is rectified with alternating current, the voltage waveform should be smooth, and the smaller fluctuations the better. The fluctuations should be controlled within +/- 5%. In addition, using full-wave rectifier as far as possible, and it is inadvisable to use half-wave rectifier.
      3、  Pulse voltage drive:
      In some devices, the driving voltage of relays is not supplied by the constant voltage power or rectified voltage, but by pulse generator issued by the pulse voltage. When using a pulse voltage to drive the relay, it should use a pulse generator with a large pulse frequency, otherwise it may cause to malfunction of relays (a hissing sound). Pulse frequency: Under the normal circumstances, the stability of the environment to be greater than 10KHZ.
      4、Short drive:
      In some applications, it is necessary to select a magnet-held relay in order to conserve power when the relay contact is switched on for a long time, and it is often used for short-time driving, so that the relay is held and not driven. In addition, in some stable environments (the temperature does not change, the vibration is not the basic), In addition, in some stable environment (temperature change is not, the impact of vibration is basically no), in order to save electricity, it is often to use maintain pressure. In this case, the magnitude of the buck is generally not less than 50% of the rated voltage. (Such as power monitoring circuit, alarm circuit.)
      5、 Transistor drive:
      5.1 Driving by adopting Schmidt circuit
      The slow boost or buck of DC relay coil voltage will make the relay action unstable, which easily leads to failure. Therefore, it is desirable to rapidly increase the signal waveform to instantaneously increase the relay coil voltage.
      5.2 Prevent leakage current in the use of transistor drive
      In the following figure T1 : There is still a current flowing through the relay coil at the cut-off moment, causing the relay to release more slowly, which may cause the relay to fail.
      5.3 Try not to use Darlington connection
      Vcesat will be bigger, please note it. Under the normal conditions, although it not directly lead to bad, but long hours of work or the increasing of the number of these cases will make it possible to fail. The relay may be malfunctioning or not moving.
      6、When the same AC power is used for both load and drive relays, the coil is driven by full wave rectification.
      If use half-wave rectifier, the relay will be in phase with the situation of the opening and closing, equivalent of DC load, which will greatly reduce the life of the relay and make the early failure occurred.
      7、Avoid the phenomenon that the relay contacts are open and closed in the same phase in AC load.
      When open and close the relay contacts, the phase of the load AC should be random. When the opening and closing of the relay and the phase of the load power are synchronized, it will cause contact welding, locking and other contact obstacles. When using the TIMER / microprocessor / thyristor rectifier drive and other relay coil drives, it is easier to occurs the phenomenon of the opening and closing at the same phase.

  • C、The ambient environment when using the relay:
    • 1、The influence of temperature and humidity:
      The temperature affects the operating performance of the relay. In general, the higher the temperature, the higher the operating voltage of the relay. Therefore, in order to ensure the smooth operation of the relay, please use the recommended temperature. If the ambient temperature exceeds the temperature specified in the product specifications, you need to evaluate the relay to determine whether it can use or not. Humidity will affect the contact performance of the relay. In general, the greater the humidity, the relay contact surface is more likely to produce oxide film, which may leads to the failed phenomenon of poor contact.
      Refer to the product specifications when using it. Usually humidity> 90%, the general selection is sealed relays.
      2、The influence of vibration shock:
      Vibration and shock usually affect the performance parameters of the relay. If the vibration is more serious, it will directly lead to mechanical damage to the relay. The relay will be affected by the impact of vibration in the processing of transportation and usage. In general, in the processing of transportation, only through the packaging to minimize the extent of the impact. It can’t be completely avoided. In the processing, if the vibration is very serious, it is generally possible to adjust the direction of installation of the relay, or irrigation hardening resin reinforcement, or use vibration-resistant products with strong impact (such as: car relays). Therefore, please refer to the specifications of product when using.
      3、 External strong magnetic field interference:
      Electromagnetic relays are controlled by electromagnets. Therefore, external magnetic field will directly affect the function of electromagnetic relay, parameter changes, or no action, or malfunction, etc. For example: transformers, loudspeakers, electromagnets, relays stray magnetic field and other components. In addition, the interference of high-frequency power supply will make the relay coil induction heating, so that burned. Try not to use the relay around the above components, and it needs to add magnetic shielding if necessary.
      4、Gas and dust in the environment:
      If the harmful gases (H2S, HCl, NOx, etc.) in the surrounding environment of the relay are serious, these gases react with the silver in the contact under the action of the arc, and Ag2S is formed on the surface of the contact, it will result in poor contact of the relay. It may also be attached to the surface of contacts in the serious environment with lots of dust, which results in poor contact.
      In the more serious areas with lots of harmful gases and dust, the general selection is sealed relays.
      5、 Silicon and its compounds
      Silicon and its compounds are natural enemies of relays.
      Silicon-containing products are widely used for a large number of electrical products because of its insulation and good temperature resistance. The insulating layer of the enameled wire of the coil manufactured by the part of the relay manufacturer contains Si.

  • D、The appearance and installation of the relay:
    • 1、Consider the relay dimensions according to the actual space.
      In the process of circuit design, due to the layout of the components and the installation of space constraints, the choice of the relay should also be considered its appearance size.
      2、The installation method of relay:
      2.1 The fixed installation of PCB solder
      2.2 The fixed installation of socket inserting
      2.3 The fixed installation of screw
      3、 The precautions of relay installation:
      3.1 The arrangement of the relays should not be too dense, and if it must be installed together, the gap between the relays should be at least 5 mm. (To avoid the effect of temperature rise / stray magnetic field).
      3.2 Terminals and socket type relays must be mounted vertically to prevent damage to the relay.
      3.3 The relay is generally no longer used after landing. In addition, the PCB board with a relay to prevent the impact of severe vibration.

  • E、Derating use of the relay:
    • For the relay load, generally it regards pure load as the rated load, and sometimes it will increase the conditions of some inductive load and TV load conditions. It is usually satisfied to use it according to the rated load given by us. But in order to optimize and improve the reliability of the letter patience, we need to use the actual use of derating.
      Reference of derived for several typical loads:
      A:Resistive load rating: usually derating 75% of rated capacity
      B:Capacitive load rating: usually derating 75% of the rated load, and it needs to series a resistor to reduce the impact current.
      C:Inductive load rating: usually derating 40% of the rated load, and it needs to use an arc suppression of the absorption circuit.
      D:Motor load rating: Normally the derating is 20% of the rated load, and it needs to adopt a circuit that suppresses the impulse current and the arc is also required.
      E:Tungsten lamp load rating: usually derating 10% of the rated load, and it needs a series of resistance to reduce the impact current.

  • F、The difference of DC relay and AC relay:
    • DC RELAY coil goes through the DC (U or I), resulting in a constant magnetic field, the power loss is only caused by the heat of the coil. Since the current is constant, the flux is constant and the attraction is constant, so there is no trembling phenomenon, unless the coil voltage fluctuates significantly.
      The coil of AC RELAY goes through the AC, resulting in a changed magnetic field (the main flux, leakage flux).
      In addition to the coil resistance itself, the heat loss (copper loss), the AC coil also has iron loss (including hysteresis loss, eddy current loss).
      AC-RELAY current is alternating, which results that the magnetic flux is also alternating, and the attraction is also changing. So AC-RELAY will have 'hissing' sound. If the voltage fluctuates, the relay will continue to open and close.


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