The role and classification of relays
A relay is a component that produces a predetermined jump in one or more of its output when the input reaches the specified conditions. The main functions of the relay in the automotive circuit are as follows:
(1) Current amplification: allow the use of low current switches (or internal MOSFET drive of the module) to control the on and off of large currents;
(2) Electrical isolation: Provide electrical isolation between loads (for example, eliminate hidden loops);
(3) Logic reversal: convert low-level control signals into high-level control signals;
There are many ways to classify automotive relays, which are divided into:
(1) Electromagnetic relay: make use of the principle of electromagnetic induction to turn on/off the circuit through the closing/opening of the contacts;
(2) Solid state relay: a new type of non-contact switch device composed of microelectronic circuits and discrete electronic devices;
(3) Combined relays: PCB relays and other logic electronic circuits are used to form relays with specific functions.
National standard QC/T 695-2018—"Automotive Relays" divides automotive automotive relays into 7 categories according to installation type, shape and load capacity (see Table 1). This article mainly focuses on automotive power distribution systems (insurance/relay boxes) The selection and use principles of commonly used plug-in electromagnetic relays are introduced.
3. The structure and key parameters of automotive plug-in electromagnetic relays
The plug-in electromagnetic relay is mainly composed of windings (coils), cores, yokes, armatures, moving reeds, moving break contacts NC (contact groups that are in the open state after the relay is actuated, and are in the closed state after being released), moving and Contact NO (the contact group that is in the closed state after the relay is actuated, and is in the open state after the release), the lead-out feet and the base (as shown in Figure 1), when the coil is not energized, it relies on the pulling force of the moving reed to make the contact break. When a certain voltage or current is input at both ends of the coil, the magnetic field generated by the coil overcomes the pulling force of the movable reed and attracts the armature, so that the movable contact is opened, moved and touched. Click to close.
4. Selection method of relay
The following analyzes the selection method of plug-in electromagnetic relay through the selection process of the cooling fan relay of a pure electric light truck drive system:
4.1 Choose the relay model according to the load type and load size
The load type of the whole vehicle electrical appliances can be roughly divided into resistive load (such as exterior mirror heating, rear window heating, etc.), inductive load (such as cooling fan, etc.), capacitive load (headlights, fog lights, etc.), For inductive loads and capacitive loads, in addition to the continuous current after stable operation of the load, the pulse current and surge current at the moment when the contact is turned on should also be considered. The continuous current for stable operation of the load should be less than that of the selected relay contact parameters. The maximum continuous current limit, the peak value of pulse current and surge current should be less than the maximum switching current limit in the selected relay contact parameters. Figure 2 shows the starting current waveform collected by the cooling fan oscilloscope used in this pure electric light truck, and it works stably The current is 19.4A, and the instantaneous peak current is 64A. According to the company’s platformization and generalization rules, the Hongfa HFV15 type relay is selected. The maximum continuous current of the normally open contact of the relay is 40A (85°C), and the maximum switching current is 150A (NO contact Point) to meet the needs of the load.
4.2 Determine the voltage of the relay
The voltage of the relay is selected according to the vehicle voltage platform. For example, there are 12V and 24V platforms in commercial vehicles. Therefore, the coil voltage of the relay should be matched with the voltage platform of the vehicle. The contact gap of the relay used to switch the 24V load is much larger than Switch the contact gap of the 12V load. If the relay marked with 12V is used to switch the 24V load, it may not be able to disconnect the arc and cause the relay to burn out. Because the pure electric light truck low-voltage system is a 12V platform, the Hongfa HFV15/12 type is adopted Relay.
4.3 Determine the form of the contact
According to the electric principle of the whole vehicle, select a set of normally open (only the moving contact NO) or a set of conversion forms (including the moving break contact NC and the moving contact NO). The cooling fan of the pure electric light truck drive system is controlled by the VCU according to the motor The controller IGBT temperature and the temperature of the drive motor set thresholds to control the contact closure of the relay, so a set of normally open forms is selected.
4.4 Choose the packaging method according to the use environment of the relay
The plug-in electromagnetic relays commonly used in automobiles are generally placed in the relay box. The dust-proof type is usually selected. For those with waterproof requirements, the plastic-encapsulated type is selected. The base, terminal and shell are sealed with a little glue, and the inside of the relay is sealed. In the shell and the bottom plate, the heat dissipation fan relay of this model is arranged in the chassis fuse box, so the dust cover type can be selected.
4.5 Choose contact material
For the same type of relay, different contact materials are applicable to slightly different load types or ranges. The commonly used contact material is AgSnO2 silver-tin alloy oxide contact material, which has excellent adhesion resistance and less material transfer. It is suitable for resistive load, lamp load, inductive load and capacitive load, so when choosing a relay The contact material usually defaults to AgSnO2 silver-tin alloy oxide contact material.
4.6 Whether to connect transient suppression components in parallel at both ends of the coil
Since the coil of the relay is an inductive load, when the relay coil circuit is disconnected, the energy stored in the coil will generate an overvoltage on both sides of the switch. The peak value of the overvoltage can even reach several KV. The higher the voltage peak is, if the withstand voltage of the semiconductor component of the control coil circuit is not high enough, it will most likely be broken down. At the same time, the inverse peak voltage conducts along the power line and will cause serious electromagnetic disturbance. Transient in parallel at both ends of the coil The suppression element (resistor, diode, TVS) can form a conductive loop between the coil and the suppression element after the control switch of the coil is turned off, thereby effectively reducing the reverse peak voltage of the coil disconnection, but at the same time slowing down the attenuation speed of the coil current , Causing the moving contact of the relay to return to the normally closed position slower and time increase, causing the life of the relay to decrease. Parallel transient suppression diodes at both ends of the coil have the best suppression effect on the anti-peak voltage, but at the same time the reduction of the relay is the greatest. Therefore, it is usually not recommended to use it. Usually, parallel transient suppression resistors at both ends of the relay coil are used to reduce the anti-peak voltage. Peak voltage, so choose 680Ω transient suppression resistance in parallel at both ends of the heat dissipation relay to reduce the conduction disturbance caused by the relay.
