How to choose a relay?

How to choose a relay?

When selecting a model, you can conduct analysis and research according to the following key points:

1. Appearance and installation method, installation foot position

2. Input parameters

3. Output parameters

4. Environmental conditions

5. Safety requirements

6. Installation and usage requirements

Outline, installation trial type, installation foot position relay selection principle

1. Relays come in various shapes, installation methods, and pin configurations. When selecting a relay, it is essential to consider the specific requirements of the entire system, including the relay's height and footprint, installation method, and pin arrangement. Generally, the following principles are adopted: products that meet the same load requirements may have different dimensions, so choose a product with a lower profile or smaller footprint based on the available installation space. However, smaller products may be limited in terms of contact load capacity and sensitivity.

2. Relay installation methods include PC board mounting, quick-connect, flange mounting, and socket mounting. Quick-connect relays can have connectors such as 187# or 250#. For small relays that are not frequently replaced, PC board mounting is generally preferred. For relays that need to be replaced often, socket mounting is chosen. For relays handling main circuit currents over 20A, quick-connect is used to prevent damage to the circuit board due to heating from high currents. For larger relays, flange mounting is selected to prevent damage to the mounting pins under impact or vibration conditions.

3. Installation pin positions should consider the convenience of circuit board layout and the separation between high and low voltage (creepage distance). The universality of the pin positions should be particularly considered. Some companies have unique product designs with special pin positions, mostly designed for specific users. Other manufacturers are reluctant to develop such products due to market considerations, making supply difficult after selection.

Input parameter

The input parameters of different types of electromagnetic relays are divided into: AC input parameters, DC input parameters, and pulse input parameters.

When selecting, consider the following parameters:

(1) Coil parameters: pull-in voltage, release voltage, coil power, coil resistance;

(2) Contact parameters: contact capacity, contact resistance, maximum switching current and voltage;

(3) Characteristic parameters: pull-in time, release time, environmental temperature, environmental humidity; withstand voltage, insulation, impact resistance, vibration resistance; mechanical life, electrical life.

(4) Working current of AC relay;

(5) Coil temperature rise;

(6) Frequency of AC input parameters;

(7) Pulse width of pulse input parameters.

General selection considerations for various input parameters

1. The resistance of a coil changes with the ambient temperature, affecting the pull-in and release voltages of relays to varying degrees. Without considering structural effects, the pull-in voltage at 70°C is generally about 20% higher than at 20°C.

2. After the normally open contacts of a relay close, it is generally required that a voltage above the minimum operating voltage be applied to the coil, preferably the rated voltage. It is not recommended to use a low holding voltage as this can reduce the product's vibration resistance and load-bearing capacity.

3. The voltage value applied to the coil over a long period should generally be less than 120% of the rated voltage. If it needs to reach 130% of the rated voltage or more, consultation with the manufacturer is advised. Especially under high temperatures, this can cause excessive coil temperature and accelerate aging.

4. When using a switch to control the relay coil's on/off status, the impact of switch contact bounce should be considered.

5. Using a thyristor to control the AC load relay coil may cause it to open at the same phase of the load each time. If this coincides with the peak of the load voltage, the relay's lifespan will be significantly shortened. Misfiring of the thyristor should also be avoided.

6. The release voltage for DC relays is generally 5%-10% of the rated voltage, and for AC relays, it is generally 10%-30% of the rated voltage. Excessive residual voltage in the circuit can prevent the relay from releasing.

7. The selection of voltage specifications should preferably use common standards, such as 12VDC and 24VDC for DC, and 110VAC and 220VAC for AC.

8. After the relay coil has been energized for a period, it heats up. At this point, if a relay contact switching action is performed, its pull-in voltage will be higher than the cold pull-in voltage, potentially causing the relay to fail to operate.

Output parameters

When selecting relay output parameters, consider the following:

1. Number of contact sets

2. Contact form

3. Contact load

4. Contact material

5. Electrical and mechanical lifespan

Environmental conditions

1. High Temperature

(1) Under high temperature conditions, insulating materials soften and melt; under low temperature conditions, materials crack, and the insulation's electrical resistance decreases, leading to failure.

(2) Alternating high and low temperatures cause structural loosening, changes in the position of moving parts, leading to uncontrolled attraction and release, poor or no contact at all.

(3) Under high temperature conditions, coil resistance increases, correspondingly increasing the activation voltage, causing failure to activate or a condition of seeming activation without actual action, leading to relay failure.

(4) Under high temperature conditions, when contacts switch power loads, arc quenching ability decreases, contact corrosion and metal transfer intensify, increasing the likelihood of failure and shortening lifespan.

(5) Under low temperature conditions, internal moisture condensation and freezing within the relay lead to decreased insulation performance, rusting of parts, etc. Therefore, in environments below zero degrees Celsius, it is advisable to use fully sealed relays as much as possible.

(6) For relays required to operate under extreme high or low temperatures, consultation with the relay manufacturer is necessary for necessary modifications and tests before use. When designing circuit boards, keep them as far away from heat-generating components as possible.

2. Humidity and Heat Humidity and heat pose threats to relay performance, specifically manifested as follows:

(1) Long-term humidity and heat directly lead to a decrease in insulation resistance levels, leading to complete failure.

(2) Non-sealed relays under humid and hot conditions suffer from broken wires due to electrochemical corrosion or mold, increased electrochemical corrosion and oxidation of contacts; metal parts corrode significantly faster, degrading relay performance, reliability worsens, leading to complete failure.

(3) Under humid and hot conditions, when contacts switch on load, arcing intensifies, leading to a shorter electrical lifespan.

(4) Avoid storing or using non-sealed relays in humid and hot environments. Excessive humidity can cause relay failure due to plastic moisture absorption.

3. Low Atmospheric Pressure Under low atmospheric pressure conditions, the following adverse effects on relays will occur:

(1) Insulation resistance of insulating parts and dielectric withstand voltage decrease, contact arc quenching ability declines, reducing lifespan.

(2) Relay heat dissipation worsens, temperature rises. This effect is particularly evident for relays with high power consumption, while for domestic relays, the impact of low atmospheric pressure is not significant.

4. Shock and Vibration Under shock and vibration conditions, the following adverse effects on relays will occur:

(1) Causing structural loosening, damage, fracture, and loss of working capability.

(2) Closing contacts produce momentary disconnections greater than specified requirements.

5. Selection Considerations:

(1) Product usage conditions generally require being within the standard + test condition range. For harsh usage conditions, the manufacturer must be notified.

(2) When operating at higher temperatures, the voltage applied across the coil ends should be appropriately increased, and the load to be switched off should be reduced.

(3) When used under humid (humidity exceeding RH85%) and corrosive atmosphere conditions, plastic-sealed relays should be used.

(4) As an electromechanical component, relays have poorer vibration and shock resistance compared to other electronic products. During product use, strong impacts, collisions, and drops should be avoided.

(5) When the product may be subjected to vibrations greater than the specified amplitude or frequency, corresponding tests should be conducted.

(6) During assembly and use, relays should not be exposed to prolonged soldering heat, which may cause the relay's leads to become loose, rotate, pull out, press in, etc., leading to relay failure.

Safety requirements

When using relays, consider the following safety requirements:

1. The insulating materials used should have good flame-retardant properties and sufficient temperature resistance, generally meeting the 94V-0 flame retardant level, with a long-term use temperature of up to 120°C.

2. The voltage resistance of the relay is divided into contact-to-contact, contact-to-coil, and inter-contact group. When selecting, determine whether it meets the requirements based on the different demands of each part of the circuit. The insulation resistance between each part of the relay is generally the same value, typically 100MΩ or 1000MΩ.

3. To prevent electric shock and fire, relay products must comply with relevant national safety regulations, such as the United States UL, Canada CSA, Germany TUV, VDE, China CQC's CCC certification, etc.

Electromagnetic Compatibility

Electromagnetic Compatibility (EMC) is the ability of electrical devices or systems to operate in an electromagnetic environment without causing or suffering from interference. EMC has become an important criterion for product quality. Electromagnetic Compatibility (EMC) is divided into Electromagnetic Interference (EMI) and Electromagnetic Susceptibility (EMS). Since general-purpose electromagnetic relays have a low probability of failure in terms of EMI and EMS, there are no specific standards worldwide, but some explanations are still needed:

1. When the interference source on the line causes a sudden change in the relay coil voltage, it may cause the relay to misoperate.

2. When there is a strong magnetic field around the relay, it may also cause the relay to misoperate. It should be avoided to arrange closely with large transformers, speakers and other devices.

3. When the relay coil is disconnected, there will be a reverse voltage, which can be paralleled with a freewheeling diode to reduce the reverse voltage.

4. When the relay contacts are opened, an electric arc is generated, emitting electromagnetic waves, which will affect the operation of ICs. If this occurs, an arc extinguishing circuit can be added to the contacts. The distance between the relay and the IC can also be appropriately increased.

5. Attention should be paid to the influence between strong and weak currents during circuit board design.

Installation and usage requirements

1. Installation, Storage

1) The position of the lead-out terminals should match the holes on the printed circuit board; any improper fit may cause dangerous stress to the relay, impairing its performance and reliability. Please refer to the manufacturer's sample for hole drilling. When using machine insertion, request the manufacturer for special pin perpendicularity.

2) Do not apply excessive pressure to the relay housing during insertion to avoid cracking or changes in operational characteristics.

3) After inserting the relay into the circuit board, do not bend the lead-out pins to prevent affecting the relay's seal or other performances.

4) The plugging and unplugging force for quick-connect pins is between 3~7 kilograms-force, while for PCB lead-out pins, it is generally 0.2~0.5 kilograms-force. Too much force can damage the relay, and too little can affect contact reliability.

5) Avoid touching the lead-out pins when installing the relay to prevent impacting soldering performance.

6) Adjacent Installation Impact: Installing many relays close together can cause heat accumulation, potentially leading to abnormal high temperatures. There should be sufficient gaps between them to prevent heat buildup and ensure the actual operating temperature of the relay does not exceed the specifications.

7) It is especially emphasized that if a relay accidentally falls or is struck during installation, although the electrical parameters may be qualified, its mechanical parameters could significantly change, posing serious hidden dangers, and it should preferably not be used.

8) Relays should be stored and installed in a clean environment.

9) Pay attention to monitoring storage temperature and try to avoid prolonged storage of relays.

2. Apply flux

Non-sealed relays are highly susceptible to flux contamination. It is recommended to use flux-resistant or sealed relays to prevent flux gas from infiltrating through gaps between the leads, base, and housing. These types of relays are suitable for processes involving multiple foam-coated fluxes and sprayed fluxes. For flux-resistant relays, preheating and drying (100°C/1 minute) can further prevent flux penetration.

3.Welding process

When using flux or automatic welding, care should be taken not to damage the performance of the relay. Relays resistant to flux or those with plastic encapsulation can be suitable for dip soldering or wave soldering processes, with a soldering temperature of about 250°C and a duration of 5~10 seconds. However, the solder must not exceed the circuit board. The temperature for manual soldering is about 350°C, with a duration of 2~3 seconds.

4. Cleaning process

After welding, cool down first before cleaning. Avoid overall cleaning for non-sealed relays. For sealed relays, use appropriate cleaning agents, preferably water or alcohol. If using other solvents, check for logo discoloration on the casing surface. Avoid ultrasonic cleaning to prevent contact cold welding and other damages. After cleaning and drying, ventilate immediately to reduce the relay to room temperature. If overall and ultrasonic cleaning are required, discuss with Sanyou Technical Department before ordering for special manufacturing processes.

5. Apply glue

Sometimes, to ensure the moisture resistance and high insulation of the circuit board, it is necessary to apply glue treatment to the circuit board. A softer glue that does not contain silicon should be chosen as much as possible. Avoid using high-temperature glue sealing for the entire relay.

6. Usage requirements

The so-called product reliability usually refers to the operational reliability of a product, which is defined as the ability to perform specified functions under specified conditions and within a specified timeframe. It consists of inherent reliability and usage reliability; the former is determined by the product's design and manufacturing process, while the latter is related to the user's correct usage and the manufacturer's pre-sales and after-sales services. Users should pay attention to the following points when using it.

(1) The coil usage voltage should ideally be selected according to the rated voltage in design. If not possible, refer to the temperature rise curve for selection. Using any coil voltage less than the rated working voltage will affect the operation of the relay. Note that the coil working voltage refers to the voltage applied between the leads of the coil, especially when using an amplifier circuit to excite the coil, it is essential to ensure the voltage value between the two leads of the coil. Conversely, exceeding the maximum rated working voltage will also affect product performance. Excessive working voltage will cause excessive temperature rise in the coil, especially at high temperatures, excessive temperature rise will damage the insulating material, and also affect the safe operation of the relay. For magnetic latching relays, the excitation (or reset) pulse width should not be less than 3 times the pull-in (or reset) time, otherwise, the product will be in a middle state. When using solid-state devices to excite the coil, its device withstand voltage should be at least 80V or higher, and the leakage current should be sufficiently small to ensure the release of the relay. Excitation power supply: At 110% rated current, the power supply regulation rate ≤10% (or output impedance <5% of the coil impedance), the ripple voltage of the DC power supply should be <5%. The AC waveform is a sine wave, the waveform factor should be between 0.95~1.25, the waveform distortion should be within ±10%, and the frequency variation should be within ±1Hz or ±1% of the specified frequency (take the larger value). Its output power should not be less than the coil power consumption.

(2) Transient suppression When the relay coil is disconnected instantaneously, a reverse peak voltage more than 30 times higher than the rated working voltage of the coil can be generated on the coil, which is extremely harmful to electronic circuits. It is usually suppressed by parallel transient suppression (also called peak clipping) diodes or resistors, so that the reverse peak voltage does not exceed 50V, but parallel diodes will extend the release time of the relay by 3~5 times. When a high release time is required, a suitable resistor can be connected in series at one end of the diode.

(3) Parallel and series power supply of multiple relays When multiple relays are powered in parallel, the relay with a high reverse peak voltage (i.e., large inductance) will discharge to the relay with a low reverse peak voltage, extending its release time. Therefore, it is best to control each relay separately before parallel connection to eliminate mutual influence. Relays with different coil resistances and power consumption should not be powered in series, otherwise, the relay with a large coil current in the series circuit cannot work reliably. Only relays of the same specification and model can be powered in series, but the reverse peak voltage will increase and should be suppressed. Series resistors can be connected according to the voltage division ratio to bear the part of the supply voltage that exceeds the rated voltage of the relay's coil.

(4) Contact parallel and series Contact parallel use cannot increase its load current because the absolute non-simultaneity of action of multiple sets of contacts of the relay, that is, it is still one set of contacts switching the increased load, which can easily damage the contacts without contact or welding and cannot be disconnected. Contact parallel can reduce failure rate for "off" failures, but it is opposite for "stick" failures. Since contact failures are mainly "off" failures as the main failure mode, parallelism should be affirmed to improve reliability and can be used in critical parts of equipment. However, the use voltage should not be higher than the maximum working voltage of the coil, nor lower than 90% of the rated voltage, otherwise, it will endanger the lifespan and reliability of the coil. Contact series can improve its load voltage, and the increase multiple is the number of series contact groups. Contact series can improve its reliability for "stick" failures, but it is opposite for "off" failures. In short, when using redundancy technology to improve contact reliability, it is essential to pay attention to the nature, size, and failure mode of the load.