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    2021-10-21 17:39:45
    A User Guide to Automotive Relay


    Ⅰ Introduction
    Ⅱ Working principle
    Ⅲ Precautions for the use of automotive relays

    Ⅰ Introduction

    A relay is an electrical control device that jumps in the output when the input reaches a certain value. It's an "automatic switch" that employs a little current to control the functioning of a large current. It's usually seen in automated control circuits. The automotive relay is also based on this principle, so it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit. The difference between the automotive relay and the relay is that the relay has a large switching load power, high shock resistance, and vibration resistance.

    Automotive Relay 2.jpg

    Automotive Relay

    Automotive relays are mainly used in automotive headlights, automotive flashlights, automotive horns, automotive ignition, automotive blowers, automotive defoggers, etc.

    Ⅱ Working principle

    The automotive relay is composed of a magnetic circuit system, a contact system, and a restoration mechanism. Iron core, yoke, armature, coil, and other pieces make up the magnetic circuit system. Static reeds, moving reeds, contact bases, and other components make up the contact system. The restoration mechanism is composed of a restoration reed or a tension spring.

    When a certain voltage or current is applied to the two ends of the electromagnetic relay coil, the magnetic flux generated by the coil passes through the magnetic circuit composed of the core, yoke, armature, and the working air gap of the magnetic circuit. The armature is drawn to the pole surface of the core due to the magnetic field. As a result, the typically closed contact is forced to open, while the normally open contact is forced to close. The armature returns to its initial condition when the voltage or current at both ends of the coil is less than a particular value and the mechanical reaction force is higher than the electromagnetic attraction. The typically open contact is switched off, while the normally closed contact is activated.

    The automobile relay can then be thought of as an assembly made up of two parts: the coil's control circuit and the contact's primary circuit. There is only a little working current in the relay's control circuit. This is because the contact capacity of the control switch is small, and it cannot be used to directly control the load with large power consumption. It can only be controlled by the contact on and off the relay.

    The relay is a control item as well as a control switch (actuator). As an example, consider the fuel pump relay. It is the fuel pump control switch, however, when the driving transistor in the electronic control unit is turned on, the coil of the fuel pump relay can only form a loop through the grounding point of the electronic control unit.

    Five-pin relay 1.jpeg

    The above is a typical five-pin relay. Let’s take a look at its composition. In fact, it’s easy to understand. First, 85# and 86# are connected to the excitation coil. The top of the coil is flexible shrapnel, many of which are also springs. But the principle is the same. 87a# and 30# are through when there is no power. This situation will change when 86# and 5# are connected to the positive and negative poles. Let’s look at the picture below:

    Five-pin relay 2.jpeg

    After the power is turned on, 30# and 87# are connected and 87a# is disconnected. If it is a four-pin relay, please ignore the normally-on contact 87a#. The work of the relay is three wires, one power supply, and two outputs corresponding to different use environments.

    In fact, the four-pin relay is missing the normally-on closed contact, or closed contact. For example, the well-known anti-theft device is a type that retains the normally-on contacts. Why not keep the closed contacts? Because the power-off relay is closed most of the time, it needs to be energized only when the vehicle enters the anti-theft and needs to be turned off. Therefore, this design can reduce current consumption and increase the stability of the relay.

    Ⅲ Precautions for the use of automotive relays

    Precautions for coil input

    1. The rated voltage is the guarantee of the reliability of the relay. Although the relay can work when the coil voltage exceeds the operating voltage, it will malfunction under strong impact. The coil voltage exceeding the maximum operating voltage will cause the coil insulation to drop, turn-to-turn short-circuits, and burns.

    2. The coil resistance value of the relay will change by about 0.4% ℃ due to the change in the ambient temperature and the heating of the relay itself. Therefore, if the coil temperature rises, the operating voltage and disconnection voltage will also become higher.

    3. Automotive relays are driven by batteries. The power supply voltage will be reduced when a large load is connected, which will affect the life of the relay. Pay attention to the influence of power supply voltage fluctuations on the reliability of the relay.

    4. The maximum continuous applied voltage of the coil: In addition to the stability of the relay, the maximum continuous applied voltage of the coil is mainly limited by the insulation performance of the enameled wire. You should understand the insulation level of the enameled wire of the product. In actual use, when the ambient temperature of Class F insulation is 40°C, the temperature rise can be considered to be limited to the maximum 115°C measured by the resistance method. The optimum temperature is 105°C due to the unevenness of the inner and outer rings.

    5. Electric corrosion of the coil: The automotive relay works in a temperature and humidity cycle environment for a long time. When the coil is continuously connected to the positive electrode of the power supply (disconnect the negative electrode), the coil will be corroded by electricity and cause disconnection, so the relay coil cannot be connected to high potential. It must be ensured that the relay coil, moving reed and the positive pole of the power supply are disconnected.

    Precautions for the use of contacts

    The contact is the most important part of the relay. The working reliability of the contact is affected by the contact material, contact voltage and current (especially on and off voltage, current waveform), load type, on-off ratio, and environmental conditions. Contact voltage: The inductive load will produce a very high reverse voltage. The higher the voltage, the greater the energy, which will accelerate the contact's electrical corrosion and metal transfer, so attention should be paid. Contact current: The current when the contact is closed and opened has a great influence on the contact. When the load is a motor or a headlight, the inrush current when closed is larger, the loss of the contact and the amount of metal transfer will be greater, and the transfer of the contact will cause contact adhesion failure, and a confirmation test should be carried out.

    Matters needing attention in the use of relays

    1. In order to prevent contamination of the lead-out surface, the lead-out should not be directly touched, otherwise, the solderability may decrease.

    2. The position of the lead-out should match the hole position of the printed board. Any mismatch may cause dangerous stress on the relay and damage its performance and reliability. Please refer to the punching diagram in the catalog to make holes.

    3. After the relay is inserted into the circuit board, do not bend the lead pins, so as not to affect the sealing or other performance of the relay.

    4. Do not apply excessive pressure to the relay shell during the plug-in process, so as to prevent the shell from cracking or changing the action characteristics.

    5. The insertion and extraction pressure of the quick connect pin is 10 kgf. Too much insertion force will damage the relay, and too little pressure will affect the contact reliability and current carrying capacity.

    6. If the relay is accidentally dropped or impacted during installation, although the electrical parameters are qualified, the mechanical parameters may change greatly and there are serious hidden dangers.

    7. Do not use silicon-containing resins and preservation agents, which may cause contact failure, even for plastic-encapsulated relays.

    8. Pay attention to connect the coil power supply and the contact power supply according to the specified polarity. The contact is usually the positive pole (+) of the moving spring.

    9. Avoid the coil applied voltage exceeding the maximum allowable voltage or the coil temperature rise exceeding the insulation level of the enameled wire.

    10. The rated load and life are under the specified standard conditions, and it is impossible to cover the various requirements for the use of automotive relays.

    The load and life of the actual application contact will be significantly different due to the type of load, environmental conditions, operating frequency or other various conditions. Please carry out a test or contact the relay manufacturer for technical support.

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