The selection of automotive relays can be analyzed and studied item by item according to the following points: appearance and installation mode; Input parameters; Output parameters; Environmental conditions; Electromagnetic compatibility; Installation requirements. The input parameters of automobile relays are: 12VDC input parameters, 24VDC input parameters, 12VDC pulse input parameters, 24VDC pulse input parameters. The following parameters are considered in the selection: rated voltage of coil coil power consumption operating voltage, release voltage maximum continuous current coil resistance coil temperature rise pulse input parameter pulse width (magnetic holding relay).
Input parameter selection Attention:
1, ambient temperature: the use of ambient temperature and coil temperature rise on the effect of the operating voltage, generally divided into the engine compartment (maximum temperature to be 125℃) and the cockpit (maximum temperature requirements of 85℃); The resistance of relay coil varies with temperature, which has obvious influence on relay operation and release voltage. For every 1℃ increase in temperature, the coil resistance will rise 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 carried out, and the operating voltage is higher than the cold operating voltage.
2, operation voltage: When using transistors and integrated circuits to drive relays, pay attention to the voltage drop of transistors and integrated circuit voltage and the damage of the relay coil back potential to transistors and integrated circuits.
3, coil rated voltage: After the normally open contact of the relay is closed, it is generally required that the voltage above the minimum operating voltage should be applied on the coil, and low holding voltage is not recommended for automotive relays, because it will weaken the product's vibration resistance, and misoperation may occur when the car is violently jolting.
4, the maximum working voltage of the coil: in order to meet the requirements of low operating voltage (60% rated voltage), the general design power consumption is high, the voltage value applied on the coil for a long time should generally be less than 120% rated voltage, if it needs to reach 130% rated voltage and above, it is necessary to contact the relay manufacturer to obtain technical support. Especially used at high temperatures, it will cause the coil temperature to be too high, the aging is accelerated - the final coil insulation layer is damaged, and the turn short circuit fails.
5, release voltage: the release voltage of the automobile relay is generally 10% of the rated voltage, when the residual voltage on the line is too large, it will cause the relay to not release.
Output parameter
The following parameters should be considered when the relay output parameters are selected: contact group number contact form contact load contact material electrical life, mechanical life 1, load type Most of the domestic relay load capacity, only the maximum pure resistance load, which gives the user in the selection of relay load, two misunderstandings, resulting in selection errors. One of the misunderstandings is that the user's practical load is often not pure resistance, but inductive, lamp, motor or capacitive load, the load size is equal to or close to the resistance load; The second misconception is that the load can be adapted from low level to rated load. It should be noted 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 electrical contact failure mechanism under different load conditions is completely different. The automotive system power supply is DC, DC voltage does not cross zero, the contact is broken at the moment, that is, arc is generated, and because the applied voltage continues to maintain, only the arc is elongated, can not be self-sustaining and extinguished. The arc heat energy will cause the contact to burn seriously, and the direct current always flows in one direction, which will cause the contact material transfer to intensify. The load capacity of most automotive relays is only nominal resistive load, but the actual use of automotive relays is often not resistive load, but inductive load, lamp load, motor load, because of the existence of a high impact current, the steady state load of the contact should be derated according to the impact current size. It should be emphasized that contact failure is the main cause of relay failure. The electrical contact characteristics, failure phenomena and failure mechanism of contact are different in different load types and load sizes.
2, contact material Contact material is the most critical material used in relays, and its performance determines the quality level of relays. The following parameters should be considered when selecting the time parameters of the relay: suction time Release time Suction rebound time Release rebound time The time parameters of the relay are defined as follows:
When the time test, the typical waveform diagram on the oscilloscope ① normally open contact ② normally closed contact ③ first break and then close contact O b s when selecting precautions: action time bounce time bridge time
④ First make and then break contact r t c release time conversion time to stable closing time
1) In the use of automotive relays generally do not pay attention to time parameters.
2) Pay attention to the time of the combined car relay, such as the flash frequency. The following environmental parameters should be considered when selecting relays:
1. Temperature
1) Under high temperature conditions, the insulation material softens and melts; At low temperature, the material cracks and the electrical resistance of the insulation decreases, resulting in failure. However, the selection of engineering plastics with excellent performance can meet the requirements.
2) Under the alternating action of high and low temperature, the structure is loosened, the position of the moving parts changes, resulting in out-of-control suction and release, and poor contact or no contact.
3) At low temperature, the water vapor inside the relay condenses and freezes, resulting in decreased insulation performance.
4) Under high temperature conditions, the coil resistance increases, and the suction voltage increases correspondingly, resulting in non-suction or non-suction, resulting in relay failure.
2. Hot and humid
Moisture and heat pose a threat to relay performance, which is manifested as follows:
1) Long-term moisture and heat will directly lead to a decline in the level of insulation resistance, resulting in complete failure. In particular, when the relay insulation is polluted by sand and dust and then subjected to moisture and heat during long-term exposed storage or use, it will cause insulation failure.
2) Under hot and humid conditions, the coil is broken due to electrochemical corrosion or mildew, and the electrochemical corrosion and oxidation of the contact are intensified; The corrosion rate of metal parts increases significantly, the performance of relays deteriorates, and the working reliability deteriorates, leading to complete failure.
3) Under hot and humid conditions, when the contact is charged to switch the load, the arc pulling phenomenon is intensified, resulting in shortened electrical life. In the tropical, sub-tropical use of electronic products, product design, material selection must fully consider the problem of humidity and heat.
3. Sand dust
The failure of relays caused by sand and dust pollution has not attracted enough attention from users. Under natural environmental conditions or general industrial workshop environmental conditions, especially the electronic devices used in automobiles, sand and dust often penetrate into the relay through the heat sink and crack parts, after a long time, boot to see, you can find dirt accumulation, resulting in moving parts rotating (sliding) not work, stuck; Contact electrical contact failure; Under the action of moisture, the corrosion of metal parts is intensified, and the insulation performance of insulating parts is reduced, resulting in failure. Some power protection relays, automotive relays before the factory inspection qualified, after one or two years of operation, relays continue to fail. The hazard of sand and dust pollution must be fully considered when designing and using. Users put forward specific requirements according to 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 and failure of contacts; As a result, coil leads rust and break, and the insulation level decreases. Chemical harmful gases are common in nature, but in different occasions, the types of harmful gases (vapors) are different. Adopting process measures can reduce and eliminate its erosion, but the cost will rise significantly. For example, military sealed relays, through long-term high temperature vacuum baking, filling the relay cavity with high purity N2, using electron beam (or laser) sealing welding, the leakage rate can reach 10-8pa.c m3\/s; ; The contact is plated with 1 ~ 3u gold. Civil relays are limited by price, generally just add plastic sealing shell to alleviate the erosion of harmful gases (vapors) in the atmosphere, when used, according to the size of the relay load and the advantages and disadvantages of the environment, the process hole can be opened as appropriate to improve the heat dissipation capacity, reduce the internal organic vapor and sulfur dioxide pollution on the contact surface.
5. Mechanical vibration
The relay will encounter vibration of certain frequency range and acceleration value around strong power equipment and during transportation. The on-board vibration can represent the field vibration stress effect of missiles, high-thrust jets and rocket engines. The effect of vibration on relays is manifested in:
a. Vibration may cause mechanical structural parts to loosen, fatigue, fracture failure;
b. The closed contact fails due to the instantaneous disconnection caused by vibration greater than the standard specified time;
c. The break contact fails because the vibration produces a momentary closure greater than the standard specified time;
d. Causes relative movement between moving parts, resulting in noise, wear and other physical failures.
6. Impact
Relays are often subjected to mechanical impact during transportation, handling and use. The impact of shock on the relay is manifested in:
1) Due to impact, resulting in structural loosening, damage, fracture and loss of working ability.
2) Due to the impact, the closed contact produces an instant break greater than the specified requirements and fails; Break contact produces more than the required instant closure and failure. Therefore, in view of (1), the relay should be required to have impact strength resistance, and the measurement results of the specified items before and after the test should meet the requirements of the product standard. For (2), the relay should have the performance of shock stability, and the contact state should be dynamically monitored. Relay safety requirements consider the following parameters when selecting:
1. The insulating materials used in insulating materials products should have good temperature resistance, and the long-term working temperature should reach 125 ° C.
2. The voltage resistance of the insulated voltage horizontal relay is divided into voltage resistance between contacts and insulation resistance; Voltage resistance and insulation resistance between contact coils. Typical values for automotive relays are voltage 500 VAC and insulation resistance 100 MΩ.
3, Electromagnetic compatibility Electromagnetic compatibility (EMC) is the ability of automotive relays to operate in an electromagnetic environment without interference or interference. EMC has become an important criterion for product quality. Electromagnetic compatibility (EMC) is divided into electromagnetic interference (EMI) and electromagnetic interference interference (EMS). Because automotive relays use a unified power supply, the relay coil will form a high voltage when disconnected, interfering with other systems and modules, so plug-in automotive relays usually have a parallel resistor or diode for transient suppression, so that the coil backpotential is less than 100V. When the relay contact is broken, an arc is generated and electromagnetic waves are emitted, which will affect the IC operation. If this is the case, an arc extinguishing circuit can be added to the contact. The distance between the relay and the IC can also be appropriately increased.
According to the national standard GB7251.10-2008 \"Low-voltage switchgear and control equipment Part 10: Special purpose switchgear and control equipment\