In order to correctly select relays, customers need know the characteristics of the relays to ensure whether these characteristics meet with the practical requirements. It will be more reliable if these characteristics can be tested in the practical environment. The principles of selecting relays can be seen in table 4. In table 4, in the column “must be confirmed” the item with mark is confirmed and a type of relay can be selected. If there is further requirement, the correspondent items with the mark are required to be further confirmed.

Table 4

Item The considered points Confirmed Reference Influence factors
Contact Contact load AC, DC, size and types

(inductive or resistive)

ü •   the ambient temperature

•   as for AC load, is the operation and the load synchronous or not

•   Does the contact material match the load?

Contact arrangement NO or NC or switching?

how many pairs of the contacts?

Electrical endurance The frequency and the expected operation times? ü
Contact material Which material? ü
Contact resistance How much and the testing conditions? ü
Coil Rated voltage How much, direction, AC, DC? ü •   the ambient temperature

•   the power fluctuation

•   the voltage drop driven by semiconductor

Coil resistance How much? The input power consumption? ü
Operate voltage How much? The influence of the power wave? ü
Release voltage How much? The influence of the power fluctuation? ü
Max. allowable voltage How much? How long? ü
Coil temperature rise How much? Insulation level? ü
Performance Enclosure type Open type, dust cover, flux proofed, or plastic sealed? ü •   the ambient atmosphere

•   the safety requirements

Dielectric strength How much? where? ü
Insulation resistance How much where? ü
Vibration resistance How much? Functional or destructive? ü
Shock resistance How much? Functional or strength? ü
Practical Environment Ambient temperature High or low? How long? ü •   insulation level

•   method of encapsulation

•   the life

Atmosphere Humidity? Harmful gases? ü

And mounting

Outline Size and dimension ü •   the required mounting size

•   mounting method

Type Of Terminals PCB, QC, plug-in or screw fixed model? ü
Welding mode Manual solder, wave solder, reflow solder ? is cleaning needed or not? ü
Mounting gap Cling or with gap? ü
Others Safety approval UL, VDE, TUV, CQC etc? ü •   zone

•   the customers’ requirements

Special requirements and conditions The requirements of the customers ü

The following will give the further explanation about the items in the table above.




  1. Contact Parameters

1.1      Contact load

Before ensuring whether the load the relay can carry in order to meet with the application, we should confirm the type of the real load except for confirming the load value for different loads have different steady state value and inrush value. Seen in table 5. The load given in the instructions are generally the resistive load, unless otherwise stated.

Table 5

The Type of load Inrush Current
Resistive Load once steady state current
Motor Load 5-10 times steady state current
Capacitive Load 20-40 times steady state current
Transformer Load 5-15 times steady state current
Solenoid Load 10-20 times steady state current
Incandescent Lamp Load 10-15 times steady state current
Mercury Lamp Load 3 times steady state current
Sodium Vapor Lamp Load 1-3 times steady state current

Figure 3 shows the relations between the representative load and the inrush current. In addition, according to the characteristics that the polarity of different moving and stationary contacts will influence the electrical endurance. Please check in the practical application or consult the technician of TIANBO company.


Incandescent lamp load

l/lo=10 to 15





Mercury lamp load




Fluorescent lamp load

l/lo=5 to 10



Motor load

l/lo=5 to 10

Solenoid load

I/Io=10 to 20

Electromagnetic contact load

l/lo=3 to 10

Capacitive load

I/Io=20 to 40

                                                                                               Figure 3


1.2   Contact Material

For the same type of relay, different contact materials are applicable to different load types or ranges. Seen in table 6.

Table 6

Material Feature Typical Application
AgNi+Au (gold plating) •  gold plating with good resistance to erode in the air

•  by contrast to other material, lower contact resistance and better consistency in low load

•  high electrical conductivity and thermal conductivity

•  Small load: gold plating almost not eroded, from 10mW(5V, 2mA) to1.5W (24V,62.5mA) (resistive load)

•  Middle load: gold plating is eroded after seve operations and AgNi functions mainly, from 2.4W (24V, 100mA) to 60W (30V, 2A) (resistive load) Note: Break the low load, the typical value is 1mW (0.1V 1mA) (eg. in the testing devices); Suggest to use two pairs of the contacts in parallel.

AgPd •  good resistance to erode and sulfur in room temperature

•  low contact resistance and good consistency

•  expensive

•  the same as the above
AgNi •  the standard material of most contact material

•  high electrical conductivity and thermal conductivity

•  high resistance to burn

•  average resistance to solder

•  easily produce the sulfured film in the atmosphere with sulfid.

•  resistive load and low inductive load

•  rated current below 12A

•  surge current below 25A

AgCdO •  high AC load

•  high electrical conductivity and thermal conductivity

•  good resistance to burn

•  great resistance to welding

•  easily produce the sulfured film in the atmosphere with sulfid

•  resistive load, motor load and inductive load

•  rated current below 30A

•  surge current below 30A

AgSnO2 •  great resistance to welding

•  the materials transferred less than those above 3 in DC load

•  easily produce the sulfured film in the atmosphere with sulfid.

•  lamp load, inductive load and capacitive load

•  excessively high surge current load (up to 120A)

AgSnO2 (with other oxide matter) •  the same as the above •  lamp load, inductive load and capacitive load

•  excessively high surge current load (up to 120A)

•  with different oxide matter, the different applicable load


1)        Consider the maximum current value specified in different relays.

2)        It would be better to be checked and tested in application when the conditions are catalogue allowable.

3)        Gold plating of the contacts shows good performance for the low loads. However, for the high load, it can only keep the initial contact performance of the contacts before the relays are used.

1.3    Electrical Endurance

Unless otherwise stated, electrical endurance listed in the catalogue is the nominal value in the rated load, the certain temperature, load and operation frequency. Therefore for other types of load and switching frequency, electrical endurance is different.

For the load above 2A and for the same mode relays electrical endurance of the flux proofed type and the dust cover type is longer than that of the sealed type washable. Therefore in the conditions allowable, to the extent that is possible, The relays of the flux proofed type and of the dust cover type and of the plastic sealed are used to increase the life of the relays.

  1. Coil

2.1    Voltage

To make the relay work reliably, be sure that work circuit can supply the rated voltage to the coil.

In the case of transistor drive circuit, that the voltage on the coil is less than the normal voltage of the transistor drive circuit because of the voltage drop on the transistor, it is recommended to use 4.5V type relay which in 5V transistor circuit and 2.4V type relay in 3V transistor circuit.

Sometimes to shorten the operating time, the coil can be applied to maximum allowable voltage to the coil in the short time. However it should be ensured that the relay will not overheat or even be damaged.

For polarized relays, please check the polarity of the coil voltage.

2.2      Coil Resistance

To make the relay work reliably, be sure that work circuit supplies the nominal coil power consumption to the relay. Therefore please select the suitable coil resistance.

  1. Performances

3.1      Enclosure Type

To ensure the reliability of the relay, different ways of encapsulation will require different post-processing (table 7).

Table 7

Type Construction Features Automatic


Automatic Cleaning Dust Resistance Liquid


Harmful Gas Resistance
Open type Without the protective case × × × × ×
Dust cover With the dust protective case; the case and the base are fitted together and their joint is close to PCB. × × ü ×
The terminals are plastically sealed on the base or the base and the terminals are fitted with sealing epoxy; the fitted joint is far from PCB.

Without exceeding the scheduled position, the flux will not penetrate the relay.

ü × ü ×


Base, terminals and case are fitted with sealing epoxy; there is ventilating hole far from PCB.

Without exceeding the scheduled position, the flux will not penetrate the relay.

ü × ×



Base, terminals and case are fitted with sealing epoxy; The internal of the relay is sealed in the case and base. Washable in limited condition. ü ü ü ü ü
Sealed or Hermetically Metal case and metal base are sealed; terminals and base are sealed with glass.

The leakage rate of the air in the internal of the relay meet with the requirements.

ü ü ü ü ü




1)    “ü” means good; “×” means not good; “” means to notify

2)        Because the plastic has the certain leakage, please use hermetic relays in the conditions that there are harmful gases or the explosive proof is required.

3.2      Dielectric Strength And Insulation Resistance

Please confirm that these two parameters can meet the application requirement and will not lead to such conditions as the breakdown of the circuit, short circuit.

3.3      Vibration Resistance And Shock Resistance

Please confirm that these two parameters can meet the application requirement and will not lead to the failure of the relay in the course of the application.

  1. Temperature

4.1      Ambient Temperature

Generally speaking, when the temperature does not exceed temperature range speculated in the catalogue, the relay can normally work. When the temperature in application is higher than the temperature speculated in the instructions, please contact TIANBO to ensure whether the relay can be normally used according to the loads.

4.2      Atmosphere

In the atmosphere with high humidity, even freezing dew and much dust, recommend to use sealed type washable relays. Under high humidity, it would easily accelerate the rust of the relay parts and the dust easily result in the failure of the relay contacts.

In the atmosphere with organic silicon, recommend to use hermetic products for the organic silicon will accelerate the failure of the contacts. In the atmosphere with such harmful gases as H2S, SO2, NO2 etc., the flux proofed and dust cover products can not be applied while the sealed type washable products can be used and tested in application.

In application, if the ambient atmosphere is better, recommend to use the dust cover and flux proofed and plastic sealed relays for they can get the longer electric endurance than sealed type washable relays.

  1. Outline And Mounting

5.1      Outline And Mounting Gap

The outline sizes of the relays usually have a certain tolerance. Therefore when the circuit and the mounting gap are designed, the design is suggested to be done according to the maximum size in the instructions.

5.2      Welding Methods

The suggested welding temperature and time are respectively 240°C to 260°C, 2s to 5s.

If reflow solder is required, it should be confirmed the relay can be reflow soldered according to the instructions. If you have questions, please contact TIANBO.

  1. Others

6.1      Safety Approval

Generally UL/CUL approvals are applicable in North America and VDE&TüV approvals are applicable in Europe. However, due to the international authority of these approvals, most of countries also accept them. If you have questions, please contact TIANBO.

6.2      Special Requirements

Except for normal products, we accept the customer’s order for the products with special specifications Please contact TIANBO when required.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes:

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>