On relay contact

Contact is an important part of relay. Their performance is greatly affected by the following factors, such as contact material, applied voltage and current value (especially the voltage and current waveform when the contact is excited and de excited), type of load, working frequency, atmospheric environment, contact configuration and jump. If any of these factors can not meet the predetermined value, problems such as metal deposition between contacts, contact welding, wear, or rapid increase of contact resistance may occur.

 

Contact voltage (AC, DC)

 

When the relay is disconnected and the inductive load is applied, a high back emf will be generated in the contact circuit of the relay. The higher the back EMF, the greater the contact damage. This will result in a serious reduction in the switching capacity of the DC conversion relay. This is because unlike AC conversion relays, DC conversion relays do not have zero crossing points. Once the arc is generated, it is not easy to weaken, thus prolonging the arcing time. In addition, the unidirectional flow of the current in the DC circuit will also cause the contacts to accumulate and wear quickly.

 

Although the information about the approximate switching power of the relay is specified in the catalog or data sheet, the actual switching power shall be determined by tests under the actual load conditions.

 

Contact current

 

The current through the contact directly affects the performance of the contact. For example, when the relay is used to control inductive load, such as motor or electric lamp, the contact wear will be faster, and due to the increase of the surge current of the contact, metal electrodeposition will be generated more frequently between the mating contacts. So in some places, the contact will not open.

 

Contact protection circuit

 

It is recommended to use contact protection circuits designed to extend the life expectancy of relays. The other advantage of this protection is to suppress noise and prevent the generation of carbides and nitric acid, otherwise when the relay contacts open, they will be generated on the contact surface. However, in addition to the correct design, the protection circuit will have the following adverse effects: such as extending the release time of the relay.

 

1、 Contact composition

 

The so-called contact structure refers to the contact mechanism. For example: B contact (break contact), a contact (make contact),

 

C contact (transfer contact), etc.

 

2、 Contact number

 

The so-called contact series is the number of contact circuits.

 

3、 Contact mark

 

Each contact mechanism is represented in the following ways:

 

A contact (normally open) B contact (normally closed) C contact (conversion) MBB contact

 

4、 Specification load

 

The standard value that determines the performance of the switch unit (contact) is represented by the combination of contact voltage and current.

 

5、 Specification energized

 

Current value that can be continuously energized to the contact without exceeding the upper temperature limit without switch contact (JIS c4530)

 

6、 Larger value of switching capacity (VA max, Wmax)

 

The larger value of the load capacity that can be switched on and off. When used, the circuit design shall not exceed this value.

 

7、 Failure rate

 

Failure occurs within unit time (action times) when the relay is continuously opened and closed under the type and load of individual specified test

 

The proportion of obstacles. This value sometimes changes with the switching frequency, the surrounding environment, and the expected level of trust. In practice

 

Use, please confirm under the actual use condition.

 

8、 Contact impedance

 

The so-called contact impedance refers to the inherent impedance of the conductor and the mutual connection of the contacts that make up the circuit of movable piece, terminal, contact, etc

 

The impedance at contact and the combined value of the concentrated impedance. The general measurement condition of contact impedance is the voltage drop as shown in the following figure

 

The following method (four terminal method) measures the current according to the following table. Test current (JIS c5442)

 

Contact load or switching power supply (a) test current (MA)

 

0.01 below 1

 

Above 0.01 to below 0.1 10

 

Above 0.1 to below 1 100

 

1 above 1000

 

9、 Larger value of contact voltage

 

The larger value of the contact voltage that can be switched. This value must not be exceeded when used.

 

10、 Larger value of contact current

 

The larger value of the contact current that can be switched. This value must not be exceeded when used.