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Aug 12 2020

Electrical Relay: Relay Contact Overview

Introduction

The relay is an electrical device regarded as a switch in the circuit. That is, the current in the control circuit depends on the "open" and "close" of relay contacts. Therefore, the reliability and service life of the relay depend on the quality and performance of the contacts greatly. The performance of the contact is affected by factors such as contact material, contact voltage, load type, operating frequency, atmospheric environment, contact configuration and bounce. If any of these factors cannot meet the predetermined value, contact problems such as electrochemical corrosion of the metal between the contacts, contact welding, contact wear, and contact resistance may occur. The volume of the load determines the size of the voltage and current that the relay can control (The rated load of the contact refers to the voltage and current that the electromagnetic relay allows to break.). If you not pay attention to it when use, it is easy to damage the relay contacts.

Relay Contact

Catalog

Introduction

Ⅰ Relay Contact Form Configuration

Ⅱ Relay Contact Symbol

Ⅲ Relay Contact Fault Analysis

3.1 Terminology

3.2 Contact Bonding and Fusion Welding

3.3 Contact Erosion

3.4 Contact Metal Migration

3.5 Contact Loose and Crack

3.6 Contact Dust

Ⅳ Contact Protection Methods


Ⅰ Relay Contact Form Configuration

a. Normally Opened Contact

It would mean the contacts are normally open when the coil of the relay is not energized or the there is no magnetic field nearby in a reed switch.

 

b. Normally Closed Contact

It would mean the contacts are normally closed when the coil of the relay is not energized or the there is no magnetic field nearby in a reed switch.

 

c. Common Contact

It would have 3 leads and would have one normally open and one normally closed circuit. This is also called a “changeover” because the common contact changes from the normally closed position to the normally open position when the coil is energized in a relay or a magnetic field is nearby in a reed switch.

 

Ⅱ Relay Contact Symbol

relay contact symbol

 

Ⅲ Relay Contact Fault Analysis

3.1 Terminology

There is something in which the relay contact seems to be closed, but the circuit works abnormally sometimes. This is due to the existence of the contact resistance of the relay contacts. When the current passes through the closed contact, the contact resistance will consume a certain amount of power, which will increase the temperature of the contact. If the current is large, the contact material will soften and deform, resulting in greater contact resistance, and even having welding failure in severe cases, making the closed contact unable to be disconnected.

Another form of contact resistance is "membrane resistance". Because the contacts of the relay are exposed to the air for a long time, there will always be compounds produced by dust, water vapor, and chemical gas, which will adhere to the contacts to form a thin film. Because of it, the conductivity of the contacts will become worse, and even become non-conductive in severe cases.

 

3.2 Contact Bonding and Fusion Welding

Contact bonding usually occurs when the contacts are in static connection. Contact resistance making the temperature of the conductive spots and nearby materials increases, which lead to a great increase in the diffusion rate and a large expansion of the contact area. The molecular force formed by the mutual extrusion and penetration of metal molecules at the contact point is the internal factor leading to the contact bonding, in addition, the sliding friction between the contacts is a necessary condition for accelerating the molecular extrusion penetration and accumulating bonding force. The size of the bonding force depends on the rigidity of the contact material and the physical conditions that cause molecular extrusion and penetration. Whether the contacts are bonded depends on the bonding force is greater than the return force of the reed.

Fusion welding refers to the phenomenon that the contact areas of two electrodes are united together by metal welding. According to the reasons of formation, welding can be divided into static welding and dynamic welding. The Joule heat generated by the contact resistor melts the contacts part, and the phenomenon that they are combined and cannot be disconnected is called static welding. In the process of the contacts controlling the external circuit, the contact pressure of the contacts is near zero or above, and meanwhile, the liquid metal bridge between the contacts making. The welding phenomenon that occurs owing to the arc heat flow melting the contacts is called dynamic welding.

 

3.3 Contact Erosion

The load of contact switching is mostly inductive. When the inductive load is disconnected, its accumulated magnetic energy will generate a high back electromotive force at both ends of the contact, which will break up the air gap between the contacts to form sparks and cause electrical corrosion. Cause the contact surface to dent or stick and cannot be separated, all of them belong to poor contact, which will result in a short circuit.

The main factors that affect arc erosion include the characteristics of the arc and its effect on the heat flow and force of the electrode, and the response of the contact material to the heat and force of the arc. In general, there are two main forms of arc erosion:

1) Vaporization and evaporation: Under the action of arc energy, the surface material of the contact changes from solid to liquid, and then into a gaseous state to leave the contact. Except that, in some certain conditions, the contact material also has a sublimation process from a solid state to a gas state.

2) Liquid splashing: Under the action of arc energy, a certain area of the surface of the contact melts. The liquid metal splashes out in the form of tiny droplets under the action of various forces, resulting in a larger material loss. These forces include spot pressure, electrostatic field force, electromagnetic force, force and reaction force of material movement, contact surface tension, etc.

The form of arc erosion varies with the contact material and load current conditions. When the load current is small, the erosion of the contact material is dominated by vaporization and evaporation. When the current is increased, not only the vaporization and evaporation of the contact material, but also the splashing phenomenon of liquid metal will occur. When the current is further increased, the metal liquid splashing becomes the main form of contact erosion. Preventing electrical corrosion between the contacts can be obtained by setting up a resistance spark extinguishing circuit and a resistance-capacitance spark extinguishing circuit.

Therefore, when choosing a relay, you should consider the voltage applied to the contact and the load capacity of the contact. For example: a relay with a contact load of 28V(DC)×10A means that the relay’s contact can only work at a DC voltage of 28V, and the contact current is 10A. If these two ratings are exceeded, the service life of the relay will be affected, and even the contacts will be burnt and damaged. In addition, the number of circuits that the relay needs to control should be determined according to actual requirements. In the same model series of relays, there are generally a variety of contact forms for selection, and each group of contacts should be fully utilized when using.

 

3.4 Contact Metal Migration

During the working process, there is usually a mutual transfer of materials between two contacts. If this mutual transfer cannot be offset, a net transfer of materials occurs. The significant contact metal migration is a big net transfer. The asymmetry of various factors in the contact operation is the main reason for the metal migration of the contact. These factors include arc, contact material characteristics and various external forces. Details are as following:

1) The arc has various forms of energy input to the contacts. For the contact at the cathode, the kinetic energy of the ion current colliding with the cathode after being accelerated by decompression, the potential energy released by the ion current on the cathode surface and the electrons, the arc column radiation or the energy conducted to the cathode surface, and the cathode Joule heat generated by the current in the body. All of these energy will increase the temperature of the contact material, resulting in contact material melting and evaporation.

2) The contact has various forces in the working process, including electronic force, electrostatic force, electromagnetic force, the reaction force of material movement, plasma flow force, these forces may cause the metal in the molten pool on the surface of the contact Liquid splashing occurs.

3) The material properties that affect the migration of the contact metal include: electrical conductivity, specific heat capacity, latent heat of melting and vaporization, melting point and boiling point, metallurgical dynamics, and so on. In addition, the size, shape, and the connection form of the contacts will also affect the metal migration.

 

3.5 Contact Loose and Crack

Contacts are electrical contact parts for relays to switch loads. Some products have contacts that are press-fitted by riveting. The main drawbacks of this installing method are loose contacts, cracks in the contacts, or excessive size and so on. They will affect the contact reliability of the relay. The loosening of contacts is caused by the improper size of the mating part of the reed and the contact or the improper adjustment force by the operator. Contact cracking is caused by too high material hardness or too much pressure. Different crafts should be used for contacts of different materials, and some contact materials with higher hardness should be annealed before contact manufacturing, riveting or welding.

 

3.6 Contact Dust

Some time after use, dust and dirt will deposit on the contacts of the relay, which will cause a black oxide film on the surface, resulting in poor contact. Therefore, the contacts need to be cleaned regularly. For example, carbon tetrachloride liquid can be used to ensure good a contact performance.

 

 

Ⅳ Contact Protection Methods

Contact Oscillogram

Figure 1. Contact Oscillogram (contact action time, release time, rebound time and stabilization time)

We know that the relay contact protection needs to be more careful than MOSFET. Generally, the load of relay is much larger than MOSFET. Common DC motors, DC clutches and DC solenoid valves with large DC loads, these inductive load switches are often closed, because surges caused by hundreds of or even thousands of back electromotive force will shorten the life of the contacts or even completely damage them. On the contrary, if the current is small, such as around 1A, the back electromotive force will cause arc discharge, which will cause metal oxides to contaminate the contacts, leading to failure of the contacts and increasing contact resistance.

Protect contacts mainly to extend the use time of the relay, because the contacts will always accumulate carbon and age, and the surface is not as clean as it was originally. What’s more, when the relay life is approaching the end, its contact resistance will increase rapidly.

Generally, under normal temperature and pressure, the breakdown voltage of the key dielectric in the air is 200~300V. Therefore, our goal is generally to control the voltage below 200V or less.

Voltage

Figure 2. Breakdown Voltage

There generally have the following methods to do it:

Method

Circuit

Characteristic

Component Selection

Resistor and Capacitor

RC Circuit

If the load is related to time, the initial leakage current may cause the load to malfunction.

R: The contact voltage is 1V

C: The contact current is 1A, and the value of RC varies with the relay and load.

The function of the capacitor C is to suppress the excessive voltage when the inductor is discharged.

The value of resistance R is determined by the test needs.

The breakdown voltage of the capacitor C is 200~300V.

RC Circuit

If the load is a relay or solenoid valve, the release time will be extended. When the contact power supply voltage range is 24V~ 48V, the voltage across the load is 100 ~ 200V.

Diode

diode circuit

The diode (regarded as a freewheeling diode) acts as a channel for the coil to release energy and a way to dissipate heat. Compared with the RC circuit, it significantly changes the release time of the relay (2~5 times).

The reverse breakdown voltage is at least 10 times the power supply voltage, and the forward current is  equivalent to the load.

Zener Diode

zener diode circuit

This circuit effectively prevents the diode from affecting the release time of the relay.

The breakdown voltage of the Zener diode must be consistent with the power supply voltage of the relay.

Varistor

Varistor circuit

Based on the characteristics of the varistor to stabilize the voltage, this circuit can prevent the contact voltage from being too high, and also slightly delay the relay release time. When the load contact power supply voltage is 24V or 48V, and the voltage across the load is 100 to 200V, the varistor is very effective. 

*

 

Standard diodes can significantly extend the rebound time. Connecting conventional diodes in series with Zener diodes will affect it lightly. If it is an inductive load, when the contacts are separated, a longer rebound time prolongs the arc generation time and shortens the life of the contacts. For example, a relay with a diode connected to the coil needs 9.8ms to release the contact. Combining the Zener diode with the small signal diode can shorten the time to 1.9ms. In addition, the return time of the relay without a diode connected to the coil is 1.5ms.

Although the inductive load is not easy to handle than the resistive load, the use of effective protection will make the performance better. There are two methods that can’t be used.

capacitor and relay contact

Figure 3. Capacitor and Relay Circuit

In the actual circuit, the protection device (diode, resistor, capacitor, varistor, etc.) and the load should have a certain distance. If the two are too far apart, the effect of the protective device may be weakened. Generally, the distance between the two should be within 50cm.

DC loads at higher frequencies will cause abnormal switch corrosion (electric spark generation). When the DC solenoid valve or clutch is controlled at a higher frequency, the contacts may have corrosion. The reason for this is that when an electric spark (arc discharge) is generated, the reaction between nitrogen and oxygen causes contact corrosion.

 

Recommended Reading

Basic Knowledge of Relay Electronics Tutorial with Video
The Role of the Relay and Its Working Principle
How Relays Work? Relay Functions and Applications

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