Relay Contact

Relay Contact

Contacts are the most important component of a relay. Their performance is greatly influenced by factors such as the material of the contacts, the voltage and current values applied (especially the voltage and current waveforms that excite and de-excite the contacts), the type of load, the operating frequency, the atmospheric conditions, the contact configuration and runout. If any of these factors do not meet the predetermined values, problems such as metal build-up between the contacts, contact welding, wear, or a rapid increase in contact resistance may occur.

Contact voltage (AC, DC)

　　When a relay is opened with an inductive load, a fairly high counter-electromotive force is generated in the contact circuit of the relay. The higher the counter-electromotive force, the greater the damage to the contacts. This causes a serious reduction in the switching capacity of the DC changeover relay. This is because, unlike AC changeover relays, DC changeover relays do not have a zero crossing point. Once an arc has been generated, it cannot be easily reduced, thus extending the arcing time. In addition, the unidirectional flow of current in the DC circuit causes the contacts to build up electrically and wear out quickly.

　　Although information is specified in catalogues or data sheets as an approximate switching power for relays, tests under actual load conditions are always required to determine the actual switching power.

Contact current

　　The amount of current flowing through the contacts directly affects the performance of the contacts. For example, when a relay is used to control inductive loads such as motors or lamps, the contacts will wear out more quickly and, due to the increased inrush current in the contacts, metal deposits will occur more frequently between the mating contacts. As a result, the contacts will not open in certain areas.

Contact protection circuits

　　A contact protection circuit designed to extend the life expectancy of the relay is recommended. This protection is additionally good for suppressing noise and preventing the formation of carbide and nitric acid, which would otherwise be generated on the contact surfaces when the relay contacts are opened. However, without the correct design, the protection circuit can have adverse effects such as prolonging the release time of the relay.

1. Contact composition

　　By contact composition, we mean the contact mechanism. Examples: b contact (Break contact), a contact (Make contact), c contact (Transfer contact).

c contact (Transfer contact), etc.

2. The number of contact levels

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

3. Contact markings

　　Each contact mechanism is indicated by the following.

a contact (normally open) b contact (normally closed) c contact (Transfer) MBB contact

4. Specification load

The standard value that determines the performance of the switch section (contact) is expressed as a combination of contact voltage and current.

5. Specification energisation

　The value of current that can be continuously energised to the contact without exceeding the upper temperature limit without a switching contact (JIS C4530).

6. Maximum switching capacity (VA max, Wmax)

　The maximum capacity of the load that can be switched. When used, the circuit should be designed not to exceed this value.

7. Failure rate

　The percentage of failures per unit time (number of operations) when switching relays continuously for each type of test and load specified.

The percentage of failures per unit time (number of operations) when switching a relay continuously for the type of test and load specified. This value may vary according to the frequency of switching, the ambient environment and the expected level of reliability. For practical use, please refer to the actual operating conditions.

In practice, please check under actual operating conditions.

8. Contact impedance

　Contact impedance refers to the inherent impedance of the conductors forming the circuit of movable plates, terminals, contacts, etc., the impedance of the contacts when they are in contact with each other, and the concentrated impedance.

The contact impedance is a composite value of the impedance of the conductors forming the circuit of the movable piece, terminal and contact, the impedance of the contacts when they are in contact with each other and the concentrated impedance. The general contact impedance is measured by the voltage drop shown in the following diagram

The four terminal method is used to measure the current as specified in the table below. Test current (JIS C5442)

Contact load or switching power supply (A) Test current (mA)

0.01 or less 1

Above 0.01 to below 0.1 10

Above 0.1 to below 1 100

1 or more 1000

9. Maximum value of contact voltage

　The maximum value of the contact voltage that can be switched. When used, this value must never be exceeded.

10. Maximum contact current

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

继电器触点

触点是继电器的最重要组成部分。它们的性能受以下因素的很大影响，诸如触点的材料，所加电压及电流值（特别是使触点激励和去激励时的电压及电流波形），负载的类型，工作频率，大气环境，触点配置及跳动。如果其中任何因素不能满足预定值，可能就要发生诸如触点间的金属电积，触点焊接，磨损，或触点电阻快速增加等问题。

电压（交流，直流）

　　当继电器断开，感性负载时，在继电器的触点电路中便产生相当高的反电动势。反电动势越高，触点的损坏便越大。这会造成直流转换继电器开关容量的严重降低。这是因为和交流转换继电器不同，直流转换继电器没有零交叉点。一旦产生电弧，它就不容易减弱，从而延长了发弧时间。此外，直流电路中电流的单向流动也会使触点产生电积，并很快磨损。

　　尽管在商品目录或数据表中规定有作为继电器近似开关功率的资料，但总还要在实际负载条件下进行试验来确定实际的开关功率。

接触电流

　　通过触点的电流量直接影响触点的性能。例如当继电器用来控制感性负载，诸如电动机或电灯时，触点的磨损将更快，并且由于触点的浪涌电流增加，在配合触点间，便会更经常地产生金属电积。因此在某些部位，触点会不能打开。

触点保护电路

　　推荐使用设计用来处长继电器期望寿命的触点保护电路。这种保护另外的好外是抑制噪声，并防止产生碳化物及硝酸，否则当继电器触点打开时，它们将产生在触点表面。但是除去正确设计，保护电路会产生以下不利影响：诸如延长继电器释放时间。

一、触点构成

　　所谓触点构成，就是指接触机构。例如：b触点（Break触点），a触点（Make触点），

c触点（Transfer触点）等。

二、触点级数

　　所谓触点级数就是触点回路数。

三、触点记号

　　各接触机构分别以下列方式表示：

a触点（常开） b触点（常闭） c触点（转换） MBB触点

四、规格负载

　　决定开关部（触点）性能之标准值，以触点电压及电流的组合来表示。

五、规格通电

　电流无开关接点的情况下,未超过温度上限而持续可以通电至触点的电流值(JIS C4530)

六、开关容量的数值（VA max,Wmax)

　可以开关之负载容量的顶 值。使用时，回路设计上应不超过此值。

七、故障率

　个别规定之试验的种类及负载下，连续开关继电器时之单位时间（动作次数）内发生故

障的比例。此值有时会随着开关频度、周围环境、及期待的信赖度水准而变化。在实际使

用上，请在实际使用条件下进行实际确认。

八、接触阻抗
　所谓接触阻抗，就是指构成可动片、端子、触点等回路之导体固有的阻抗、触点互相接
触时的阻抗，以及集中阻抗的合成值。一般接触阻抗的测量条件，是以下图所示之电压降

下法（四端子法）通过下表规定的测量电流。试验电流（JIS C5442）

触点负载或开关电源（A） 试验电流（mA）

0.01以下 1

0.01以上 to 0.1以下 10

0.1以上 to 1以下 100

1以上 1000

九、触点电压的数值

　可以开关之触点电压的顶值。使用时，不能超过此值。

十、触点电流的数值

　可以开关之触点电流的顶值。使用时，不能超过此值。