The contact is the most important structural part of the relay. The service life of the contract is affected by the contact material, the voltage and current value on the contact (especially the voltage and current waveform when it is on and off), the type of load, and the switching Due to the influence of frequency, environmental conditions, contact form, contact bounce phenomenon, etc., contact failures are mostly caused by material transfer, adhesion, abnormal consumption, and contact resistance increase of the contact. Pay attention when using it.
In order to better use the relay, please refer to the precautions for contacts described below.
Generally, the size of the resistive load is recorded in the product manual, but these are not enough, and the actual contact circuit should be tested and confirmed.
The minimum load recorded in the product specification is not the standard lower limit that the relay can reliably switch. This value is different due to the on-off frequency, environmental conditions, changes in the required contact resistance, and absolute values.
1.1.1 Voltage
The voltage of the contact circuit has a reverse voltage that is greater than the circuit voltage when the inductive circuit is disconnected. The higher the voltage, the greater the maximum energy, resulting in increased contact consumption and material transfer. Therefore, it is necessary to pay attention to the position of the relay contact. Control the type and size of the load. Under the same current, the direct current (DC) voltage value that the relay can reliably switch is much lower than the alternating current (AC) voltage value, because the alternating current has a zero point (the point at which the current is zero), and the generated arc is easily extinguished. , The generated arc can only be extinguished after the gap between the contacts reaches a certain value, making the arc last longer than the AC situation, and intensifying the consumption of contacts and material transfer.
1.1.2 Current
When the contact is closed and opened, the impact current has a great influence on the contact. For example, when the load is a motor or an indicator light, the greater the inrush current when closing, the more the contact consumption and the amount of material transfer, and the more likely it is to cause the contact to stick and fail to open. Please confirm during actual use.
1.2.1 Prevent the same relay from making and breaking large loads and small loads
Because it is easy to produce contact spatters when switching on or off a large load, they will attach to the contacts of the small load on and off, resulting in contact failure. Therefore, please avoid the same relay switching on and off the large load and the small load. If you have to use it in this way, place the contacts that make and break the small load above the contacts that make and break the larger load during installation, but the reliability of the relay will be affected.
1.2.2 Precautions for the parallel connection of two sets of contacts
When the contacts of group C are connected in parallel, the reliability of the connection can be improved, but the capacity of the load cannot be improved, because the two groups of contacts cannot be opened or closed at the same time.
1.2.3 About the synchronization of contact action and AC load phase
When the relay contact action is synchronized with the AC power phase of the switched load, if the contact is always turned on or off when the load voltage is high, as shown in Figure 4, it will increase contact bonding or material transfer, which will cause the relay Premature failure, please confirm in actual use whether to use random phase on and off. When the relay is driven by a timer, a microcomputer, etc., the power phases may be synchronized.
1.2.4 Electrical durability at high temperatures
When the relay is used at high temperatures, its electrical durability will be lower than when used at room temperature, so please confirm it in actual use.
1.2.5 Connection of multiple sets of contacts and load
When there are multiple sets of contacts, please arrange the contacts on the same pole of the power supply as much as possible, and the load is on the other pole of the power supply, as shown in Figure 5(a), so as to prevent the contact caused by the voltage difference between the contacts If there is a possibility of short-circuit, avoid wiring as shown in Figure 5(b)).
1.2.6 Avoid short circuits due to contact bonding and arcing
In the circuit, the following points should be considered (see Figure 6):
1) Generally, the contact gap of relays is relatively small, and the possibility of short circuit caused by arc between contacts should be considered. Please do not use the circuit shown in Figure 6(b). It is recommended to use the circuit shown in Figure 6(a) and set a certain interval between the actions of the contacts Con1 and Con2.
2) It should be considered that when a short circuit is caused by bonding between contacts or wrong action, overcurrent should not be generated, which may cause circuit overload or burnout.
3) Be careful not to use the two sets of changeover contacts shown in ©) to form the motor forward and reverse circuit. It is recommended to use the circuit shown in Figure 6(c), and set a certain interval between the actions of the contacts Con1 and Con2.
1.2.7 Avoid short circuits between contact groups
Due to the miniaturization of electrical control equipment, the control components tend to be miniaturized. Therefore, when using relays with multiple sets of contacts, please pay attention to the type of load and the voltage difference between the sets of contacts. Each set of contacts is recommended. It is best not to have an excessive voltage difference between them to avoid short circuits between contact groups.
1.2.8 Points to note when using long wires
In the relay contact circuit, when using a long wire longer than tens of meters, due to the presence of parasitic capacitance in the wire, an inrush current will be generated. Please connect a series resistance (about 10Ω~50Ω) to the contact circuit, as shown in the figure.
1.2.9 Precautions for magnetic latching relay contacts
When leaving the factory, the general magnetic latching relay is set to the reset state, but during transportation or installation of the relay, it may become the operating state due to shocks, etc., so it is recommended to set it to the required state when using (when the power is connected) Necessary state.
1.3.1 Inrush current and reverse voltage
When the motor, capacitor, solenoid and lamp load are switched on, it will cause an inrush current several times the steady-state current. When inductive loads such as solenoids, motors, and contactors are disconnected, reverse voltages of hundreds to thousands of volts will be caused. Generally, the critical insulation breakdown voltage of air at normal temperature and pressure is 200V~300V, so if the reverse voltage exceeds this value, discharge will occur between the contacts.
Both inrush current and reverse voltage will cause great damage to the contacts and significantly shorten the service life of the relay. Therefore, proper use of contact protection circuits can increase the service life of the relay.
1.3.2 Material transfer phenomenon of contacts
The material transfer phenomenon of the contact refers to the transfer of one contact material to the other contact. When the material transfer is severe, the unevenness of the contact surface can be seen with the naked eye, as shown in Figure 8. Such unevenness can easily cause contact bonding.
Generally, the material transfer of the contact is caused by the unidirectional flow of a large current or the impact current of a capacitive load, which mostly occurs in a DC circuit, and generally exhibits a convex anode and a concave cathode. Therefore, proper use of contact protection circuits or AgSnO2 contacts with good resistance to material transfer can alleviate the material transfer phenomenon of the contacts. For large-capacity DC loads (a few A to tens of A), it must be tested in actual applications confirm.
1.3.3 Contact protection circuit
Generally, inductive loads are more likely to cause damage to the contacts than resistive loads. If you use an appropriate protection circuit, the impact of the inductive load on the contacts can be basically equivalent to that of the resistive load. However, please note that if it is used incorrectly, it may cause adverse effects. effect. The table is a representative example of a contact protection circuit.
1.3.3 Precautions when installing protective components
When installing protective components such as diodes, C-Rs, and varistors, they must be installed next to the load or contacts. If the distance is too far, the protection effect will be unsatisfactory. It is recommended to install within 50cm.
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