When selecting automotive relays, you can carry out analysis and research one by one according to the following key points: appearance and installation method; input parameters; output parameters; environmental conditions; electromagnetic compatibility; installation and use requirements. The input parameters of automobile relays include: 12VDC input parameters, 24VDC input parameters, 12VDC pulse input parameters, and 24VDC pulse input parameters. Consider the following parameters when selecting: Coil rated voltage Coil power consumption Operating voltage, release voltage Maximum continuous energization current Coil resistance Coil temperature rise Pulse width of pulse input parameter (magnetic latching relay).
Input parameter selection should focus on:
1. Ambient temperature: The influence of the temperature of the operating environment and the temperature rise of the coil on the operating voltage, generally divided into engine compartments (the maximum temperature requirement is 125℃ ) and the cockpit (the maximum temperature requirement is 85°C); the relay coil resistance changes with temperature changes, which has an obvious impact on the relay action and release voltage. For every 1°C increase in temperature, the coil resistance will increase by 4‰. When the relay coil is energized for a period of time, the coil heats up. At this time, the relay contact switching action is performed, and its operating voltage is higher than the cold state operating voltage.
2. Action voltage: When using transistors and integrated circuits to drive relays, pay attention to the voltage drop of the transistors and integrated circuits and the destructive effects of the relay coil back electromotive force on the transistors and integrated circuits.
3. Coil rated voltage: After the normally open contact of the relay is closed, it is generally required that a voltage above the minimum operating voltage should be applied to the coil. Low holding voltage is not recommended for automotive relays because it will weaken the product's vibration resistance. nature, malfunction may occur when the car is violently bumpy.
4. Maximum working voltage of the coil: In order to meet the requirements of low operating voltage (60% rated voltage), automotive relays are generally designed to consume high power. The voltage value applied to the coil for a long time should generally be less than 120 % of the rated voltage. If you need to reach 130% of the rated voltage or above, you need to contact the relay manufacturer for technical support. Especially when used at high temperatures, the coil temperature will be too high and the aging will be accelerated---eventually the coil insulation layer will be damaged and the turns will be short-circuited and fail.
5. Release voltage: The release voltage of automobile relays is generally 10% of the rated voltage. When the remaining voltage on the line is too large, the relay will not release.
Output parameters
The following parameters should be considered when selecting relay output parameters: Number of contact groups Contact form Contact load Contact material Electrical life, mechanical life 1. Load type Domestic largest The load capacity of most relays only indicates the maximum pure resistive load. This gives users two misunderstandings when selecting relay loads, leading to selection errors. One of the misunderstandings is that what users use is often not a pure resistive load, but an inductive, lamp, motor or capacitive load. The load size is equal to or close to the resistive load. The second misunderstanding is that the load can be changed from a low voltage to a resistive load. It can adapt to everything from flat to rated load. It should be pointed out that a relay that can reliably convert a 10A resistive load cannot convert a 10A inductive load and may not reliably convert a 10mA load. Because the failure mechanisms of electrical contacts under different nature of load conditions are completely different. The automotive system power supply uses DC. The DC voltage has no zero-crossing point. An arc is generated the moment the contacts are opened. And because the external voltage is maintained continuously, only the arc is stretched and cannot sustain itself and is extinguished. The arc heat energy will cause serious burning of the contacts, and the DC current always flows in one direction, which will cause the contact material transfer to increase. The load capacity of most automotive relays is only nominal resistive load. However, the actual use of automotive relays is often not resistive loads, but inductive loads, lamp loads, and motor loads. Due to the high inrush current, the steady-state load of the contacts The size should be derated according to the size of the inrush current. It should be emphasized that contact failure is the main cause of relay failure. The electrical contact characteristics, failure phenomena and failure mechanisms of contacts are different under different load types and load size conditions.
2. Contact material Contact material is the most critical material used in relays, and its performance determines the quality level of the relay.
The following parameters should be considered when selecting the time parameters of the relay: Suction time, release time, suction bounce time, release bounce time, the relay time parameters are defined as follows:
During the time test, the typical waveform diagram on the oscilloscope ① is often Open contact ② Normally closed contact ③ Break first and then make contact O b s Things to note when selecting: Action time Rebound time Bridge time
④ Make first and then break contact r t c Release time Conversion time reaches stable closing Time
1) When using automotive relays, generally no attention is paid to the time parameter.
2) Pay attention to the time of combining car relays, such as flash frequency. The following environmental parameters should be considered when selecting relays:
1. Temperature
1) Under high temperature conditions, the insulating material softens and melts; under low temperature conditions, the material cracks, and the insulation resistance performance decline, resulting in failure. However, choosing engineering plastics with excellent performance can meet the requirements.
2) Under the alternating action of high and low temperatures, the structure becomes loose and the position of the moving parts changes, resulting in out-of-control suction and release, and poor or no contact at the contacts.
3) At low temperatures, water vapor condenses and freezes inside the relay, resulting in a decrease in insulation performance.
4) Under high temperature conditions, the coil resistance increases, and the suction voltage increases accordingly, causing no suction or seemingly suction but not suction, resulting in relay failure.
5) Under high temperature conditions, when the contacts switch power loads, the arc breaking ability is reduced, contact corrosion and metal transfer are intensified, the possibility of failure is increased, and the service life is shortened.
2. Damp and heat
Damp and hot poses a threat to the performance of the relay. The specific manifestations are as follows:
1) Long-term damp and heat will directly lead to a decrease in the insulation resistance level, resulting in Completely ineffective. Especially if the relay insulation is contaminated by sand, dust, etc. during long-term exposed storage or use, and then is exposed to moisture and heat, it will cause insulation failure.
2) In non-sealed relays, under hot and humid conditions, the coils are disconnected due to electrochemical corrosion or mildew, and the electrochemical corrosion and oxidation of the contacts are intensified; the corrosion rate of metal parts increases significantly, and the performance of the relay deteriorates. Operational reliability deteriorates, leading to complete failure.
3) Under hot and humid conditions, when the contacts are charged to switch loads, the arcing phenomenon intensifies, resulting in shortened electrical life. For electronic products used in tropical and subtropical areas, moisture and heat issues must be fully considered in product design and material selection.
3. Sand and dust
Sand and dust pollution causes relay failure, which has not attracted enough attention from users. Under natural environmental conditions or general industrial workshop conditions, especially electronic devices used in automobiles, sand and dust often penetrate into the interior of the relay through heat dissipation holes and cracks. Over time, dust accumulation can be found when the power is turned on, causing movement. The parts rotate (slide) poorly and get stuck; the electrical contact of the contacts fails; under the action of moisture, the corrosion of metal parts increases, and the insulation performance of the insulating parts decreases, leading to failure. Some power protection relays and automotive relays passed factory inspections, but after one or two years of operation, the relays continued to malfunction. The hazards of sand and dust pollution must be fully considered during design and use. Users put forward specific requirements based on practical needs.
4. Chemical atmosphere pollution
Organic vapor, oxygen, sulfur dioxide, salt spray, etc. in the ambient atmosphere have corrosive effects on relay contacts, metal parts, coils, and insulating parts. , resulting in poor electrical contact of the contacts, resulting in failure; causing the coil leads to be corroded and broken, and the insulation level to decrease. Chemically harmful gases are ubiquitous in nature, but the types of harmful gases (steam) are different in different situations. Taking process measures can reduce and avoid corrosion, but the cost will increase significantly. For example, military sealed relays are baked at high temperature for a long time, the inner cavity of the relay is filled with high-purity N2, and electron beam (or laser) is used for sealing welding. The leakage rate can reach 10-8pa.c m3/s; Point plated with 1~3u gold. Civilian relays are limited by price, and generally just add a plastic shell to alleviate the erosion of harmful gases (steam) in the atmosphere. When in use, depending on the load of the relay and the quality of the environment, the process holes can be opened as appropriate to improve heat dissipation and reduce internal heat dissipation. Contamination of the contact surface by organic vapor and sulfur dioxide.
5. Mechanical vibration
Relays will encounter vibrations in a certain frequency range and acceleration value around high-power equipment and during transportation; random vibration can represent missiles and high-thrust jets and on-site vibration stress effects generated by rocket engines. The impact of vibration on relays is as follows:
a. Vibration may cause loosening, fatigue, fracture and failure of mechanical structural parts;
b. The closed contact will vibrate for a period longer than the time specified in the standard. Failure due to instantaneous disconnection;
c. Failure of the disconnected contact due to vibration that exceeds the instantaneous closing time specified in the standard;
d. Causes relative movement between movable parts, Produces noise, wear and other physical failures.
6. Impact
Relays are often subject to mechanical impact during transportation, handling, and use. The impact of impact on relays is as follows:
1) Due to the impact, the structure is loose, damaged, broken and loses its working ability.
2) Due to impact, the closed contact produces an instant opening greater than the specified requirements and fails; the open contact produces an instant closing greater than the prescribed requirements and fails. Therefore, for (1), it is required that the relay should have impact resistance performance, and the measurement results of the specified items before and after the test should comply with the product standard requirements. Regarding (2), the relay should have the performance of impact resistance and stability, and the contact status of the contacts should be dynamically monitored. Consider the following parameters when selecting relay safety requirements:
1. Insulating material The insulating material used in the product should have good temperature resistance, and the long-term operating temperature should reach 125°C.
2. Insulation withstand voltage level The withstand voltage of the relay is divided into withstand voltage and insulation resistance between contacts; voltage withstand and insulation resistance between contact coils. Typical values ??for automotive relays are 500 VAC withstand voltage and 100 MΩ insulation resistance.
3. Electromagnetic compatibility Electromagnetic compatibility (EMC) is the ability of automotive relays to operate without interference or interference in an electromagnetic environment. EMC has become an important criterion for judging product quality. Electromagnetic compatibility (EMC) is divided into electromagnetic interference (EMI) and electromagnetic anti-interference (EMS). Since automotive relays use a unified power supply, high voltage will be formed when the relay coil is disconnected, interfering with other systems and modules. Therefore, plug-in automotive relays usually have parallel resistors or diodes for transient suppression, so that the coil back electromotive force is less than 100V. When the relay contacts are opened, an arc is generated and electromagnetic waves are emitted, which will affect the operation of the IC. If this happens, an arc extinguishing circuit can be added to the contacts. You can also increase the distance between the relay and IC appropriately.