A comprehensive introduction to the principle and function of charge relay



A charge relay is an electronic control device that has a control system (also known as an input loop) and a controlled system (also known as an output loop).


Usually used in automatic control circuits, it is actually a kind of "automatic switch" that uses a smaller current to control a larger current. Therefore, it plays the role of automatic adjustment, safety protection, conversion circuit and so on in the circuit.

1. What is charge relay

A charge relay is an electrical control device that can give a specified amount of input and hold it for a long enough time to cause a predetermined step change in the controlled quantity in the electrical output circuit. When the input drops to a certain level and remains for a sufficient period of time, it returns to the initial state.

Relays and contactors work on much the same operating principle. The main difference is that the main contact of the contactor can pass through a large current. The relay's volume and contact capacity are small, the number of contacts is large, and only a small current can be passed. Relays are usually used in control circuits. It usually has no primary contact because the overload capacity is relatively small. So all of them use auxiliary contacts, and the number is relatively large.

  • Current relays: for undercurrent or overcurrent protection and control of the battery.
  • Voltage relays: mainly used as overvoltage, undervoltage and zero voltage protection of the battery.

The safety of high voltage is an important factor to ensure the safety of the lifepo4 battery, and the relay must be disconnected in an emergency to ensure that the power battery will not output power.

2. The role of the charge relay

● Charge relay

  • Expand control

When the charge relay control signal reaches a certain value, it can be in different forms according to the contact group. At the same time, change, disconnect and turn on multiple circuits.

  • Magnify

Sensitive relays, intermediate relays, etc., with a very small amount of control, can control a large power circuit. Intermediate relays are commonly used to transmit signals and control multiple circuits simultaneously. It can also be used directly to control small capacity motors or other electrical actuators. The intermediate relay contact capacity is small and the number of contacts is large, which is used to control the line.

The role of the charge relay

  • Integrated signals

When multiple control signals are input to the charge relay in the prescribed form, the predetermined control effect is achieved after comparison and synthesis.

  • Remote control and monitoring

The relays on the automatic device, together with other electrical appliances, can form a program control line to achieve automated operation.

● Contactors

Contactors are divided into AC contactors (AC voltage) and DC contactors (DC voltage), which are used in power supply, distribution and power supply occasions. In a broad sense, contactors refer to electrical appliances that use the current flowing through the coil to generate a magnetic field and close the contacts to control the load.

  • The main role of the contactor

When the contactor coil is energized, the coil current will generate a magnetic field, and the magnetic field generated will cause the static iron core to generate electromagnetic attraction to attract the iron core, and drive the AC contactor point action. The normally closed contact is open, the normally open contact is closed, and the two are connected.

When the coil is deenergized, the electromagnetic attraction disappears, and the armature is released under the action of the release spring, so that the contact is restored, the normally open contact is disconnected, and the normally closed contact is closed. The working principle of a DC contactor is somewhat similar to that of a temperature switch.

3. The difference between the charge relay and the contactor

The charge relay and contactor is electrical components of the relay class. It is an important control element for the realization of automatic control in the electrical control system. It mainly plays the role of indirect control in the system, so many people often can't tell the difference between the two.

The difference between the charge relay and the contactor


● Different effect

The primary function of charge relay is for signal detection, transmission, conversion or disposal. The circuit current it switches on and off is generally small, and is generally used in control circuits to control weak signals. Its primary function is to turn on or off the main circuit.

The main circuit refers to whether a circuit is working or not and is marked by whether the circuit is on or off. The main circuit concept corresponds to the control circuit. Generally, the current passing through the main circuit is greater than that of the control circuit.

● Opening and closing contacts

Contactors are used to turn on or off loads with high power, and are used in (power) main circuits, and the main contact may have interlocking contacts to indicate the opening and closing of the main contact. The charge relay is generally used in electrical control circuits to extend the battery capacity of miniature or small relays to drive larger loads.

● The arc extinguishing device 

In fact, looking at contactors and charge relay, one of the most important differences is that contactors have arc extinguishing devices, while relays do not. Have you learned the above three differences?

4. Component of charge relay

The general circuit is often divided into two parts: the main circuit and the control circuit. The charge relay is mainly used for the control circuit, and the contactor is mainly used for the main circuit.

Through the charge relay, the function of controlling one or more signals with one control signal can be realized, and the start, stop, linkage and other controls can be completed, and the main control object is the contactor.

The contact of the contactor is relatively large, the bearing capacity is strong, through which the control of weak current to strong current is realized, and the control object is electrical appliances. Intermediate charge relay composition: An charge relay is a relay that consists of a fixed core, spring, dynamic and static contacts, coils, terminal blocks and housing.

5. Failure judgment of the charge relay

charge relay, mainly used in the control loop, can increase the number and capacity of contacts. Generally, an charge relay has a specific function. So what are the faults of general charge relays?

Failure judgment of the charge relay


Since the structure of the charge relay is basically the same as that of the contactor, the common battery discharge warning, diagnosis and countermeasures of the contact part and the electromagnetic system can be found in the relevant part of the contactor. It is only necessary to mention in particular. The contacts of charge relays are prone to false connection faults, which often occur during the operation of electrical control.

It does not necessarily occur frequently or fixedly, making it difficult to catch and making mistakes difficult to judge. Occasionally, it can cause major accidents. The reason for this failure is that due to the change in the contact resistance of the control loop, the actual voltage at both ends of the electromagnetic coil is lower than 85% of the rated voltage, so that the core cannot be absorbed, causing the circuit to lose control.

6. Specific solutions to the problem

The charge relay body is visibly damaged. For example, the relay has been blackened. Even if it can still operate, it is also a hidden danger. Or the shell of the charge relay has been deformed, then there is no need to think. It is said that it has been changed.

The body of the charge relay is damaged invisibly, and maintenance personnel need to use a multimeter to measure the coil of the charge relay: If the power is generated but does not operate, then the body part of the charge relay needs to be replaced. If there is power and it is turned on (the indicator is on), but the contacts are not active.

This is the intermediate relay stuck, and the intermediate relay body also needs to be replaced. It has been energized, and the action (intermediate relay indicator is on), and contacts are also active. However, a pair of contacts is poorly contacted, which is externally manifested as one or several contacts in hundreds of intermediate relays that are not in place.

Specific solutions to the problem


This is an obvious soft fault, and it is a relatively difficult fault to troubleshoot. Generally, all wires on this control loop will be tightened first. The components on this control loop are then replaced one by one. The base is damaged invisibly, and damage to the base is generally invisible.

If the fault point is at the coil terminal post, the intermediate relay will not operate. And no matter how you replace the intermediate relay body, the intermediate relay will not operate. This is when the base needs to be replaced. If the fault point is at the contact terminal post, one of the intermediate relay contacts will not operate.

And it is useless to replace the intermediate relay, at this time it is enough to replace the base. Regularly measure the temperature of the intermediate relay coil and contact, and replace the intermediate relay with a sudden increase in temperature or greater than 90 °C in advance.

The hidden dangers of the intermediate relay can be detected in advance to avoid expansion into faults or accidents. The best way to eliminate failure is: Try to avoid using low voltages of 12V and below as the control voltage, because in such low voltage circuits, false contact connection failures are most likely to occur.

When the control loop uses 24V as the control voltage, parallel contact should be used to improve its working reliability. When the control loop must be controlled with low voltage, it is best to use 48V. The control loop is best to use 110V and above voltage as the rated control voltage, which can effectively prevent the occurrence of contact virtual connection.

7. Conclusion

Charge relays are used in relay protection and automatic control systems to increase the number and capacity of contacts and to transmit intermediate signals in the control circuit. It is also widely used in the automatic fire alarm system for fire fighting.


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