▶ How to Choose Relays
Relays are generally required for circuit function changes in communication equipment, automatic devices, household appliances, electronic devices in automobiles, etc. Relays have a wide range of applications, anddifferent customers may have different requirements regarding
To cater to different usage requirements in different sectors, relay manufacturers
have produced a good variety of relays in different types, specifications and performances. With the advancement of science and technology, relays with new structures, high performance and high reliability have hit the market. How to choose the right relay from the huge pool of relays and use it properly is crucial for the performance and reliability of the whole appliance of which the relay is a part. How to choose the right relay? First of all, conduct an in-depth analysis of the usage conditions and technical requirements of the whole appliance, and based on "Value Engineering" principles, come up with a list of technical characteristics the relay must possess.
Our and sales personnel can offer assistance in selecting right provide excellent pre- post-sales
service by tapping expertise this field. analysis be based on following major points :
physical dimensions, installation methods, input parameter, output ambient conditions, safety requirements, reliability requirements. is discussion above
▶ Physical Dimensions, Installation Methods and Installation Dimensions
Relays have widely differing physical dimensions, installation methods and installation dimensions. The customer must take into consideration the installation area and permissible height, installation methods and installation dimensions. These issues should be the primary consideration when selecting the right relay. We would also like to bring your attention to the following issues:
1. The Leading-out of the PC Board
The pin spacing intervalis generally 2.54Xn (n=1, 2, 3 …, the same below) or 2.5Xn; there are also relays with non-standard spacing interval, e.g. HA1, HA2. The leading-out pin is usually 3.5±0.2 in length. The weldability of the leading-out pin, the welding heat resistance of the relay, and the non-perpendicularity of the bed plate corresponding to the leading-out pin must meet high standards.
2. Quick-connecting Relays
Quick-connecting leading-out pins are generally in two types : #250 (6.35X0.8) and #187 (4.75X0.5) The inserting and pulling force of the leading-out pin must meet the following requirements : #250 Leading-out Pin : torque > 10kg.cm; #187 Leading-out Pin : torque > 5kg.cm
▶ Input Parameters
Input parameter is an essential consideration when selecting the right relay. Input parameters frequently used include :
1. AC Input Parameter
When the input parameter is AC voltage (current), an AC relay must be chosen. The following factors should be considered when choosing relays of this kind:
(1) AC Frequency : The frequency of the input voltage (current) of the AC relay is generally 50Hz or p; 60 Hz. Since the coil inductive impedance of the two types are different, there is a remarkable difference in the operating voltages. This should be clearly provided for in the contract.
(2) Ambient Temperature : Because the AC relay suffers an eddy current loss and magnetic hysteresis loss, the relay has a high temperature rise, generally in the range of 70-80°C. Ideally, the operating ambient temperature should be in the range of 40-65°C. The calculation formula for determining the ambient temperature is : t1￡t2-t3-15°C.
t1 : the maximum ambient temperature ;
t2 : the maximum permissible long-term operating temperature of the enamel wire and the insulation material (130°C for Grade B; 155°C for Grade F);
t3 : the average temperature rise . It is obvious that when the ambient temperature is elevated, the level of resistance to elevated temperatures of the enamel wire and insulation material must be raised accordingly, which will result in a much higher cost .
(3) AC Noise : When the relay is in operation, an AC noise will be created. The initial noise should be less than 45 dB, however, in real operation, the AC noise may increase as a result of the dirt accumulated between the magnetic poles and the change of mechanical parameters.
(4) Operating Voltage : In general, the operating voltage of the AC relay is less than 80%VH (rated operating voltage; the same below);
the maximum permissible operating voltage < 90%VH.
Relays driven directly by power supply voltage may fail when the power supply voltage fluctuates beyond ±10% or when excessively low voltage causes the pull-in operation to be unsteady and unreliable. Relays may also fail when excessively high voltage results in a high temperature rise which in turn damages the insulation. If the power supply voltage is likely to fluctuate beyond ±10% (the voltage of the power grid in the rural areas usually suffers a big fluctuation), it must be specified in the contract to reduce the impact of the operating voltage accordingly; enamel wires and insulation materials with high resistance to extreme temperatures must be used in this case.
2. DC Input Parameter
Relays of this kind have an extensive range of application and are discussed here in different circumstances.
(1) The main issue in selecting DC relays is the sensitivity L (coil rated power consumption), which is closely related to the output power, physical dimensions, ambient conditions (ambient temperatures, vibration, impact, etc.) Extreme caution must be exercised in establishing the sensitivity of the relay. The sensitivity should not be emphasized at the expense of other performance features. When the customer does not have a high requirement of the sensitivity, DC relays with a general sensitivity may be chosen. When the customer has a high requirement of the sensitivity, the output power is heavy current, and the ambient conditions are tough, a solid-state relay with a medium-level sensitivity may be chosen. If a high sensitivity is required (e.g. lower than 0.2W), hybrid relays or electropolarized relays can be used. However, hybrid relays are usually expensive and bulky while electropolarized relays have a poor ambient adaptability and load capacity.
(2) If the input voltage is to last for a considerable length of time, for example, a few hours, days or months, a magnetic latching relay is recommended for the following reasons : to save input electric energy, lower the temperature rise of the relay and improve its ambient adaptability. However, the input must be impulse, there is an additional polarity requirement, and the input circuit is quite complex. In the case of the relays in magnetic-card kilowatt-hour meters and relays for controlling satellite power, the contacts of the relays usually work continuously for scores of hours or a couple of months in a conducting state; therefore, it is economical to use magnetic latching relays. Where electric energy consumption must be strictly limited, magnetic latching relays are frequently used.
(3) Where the input parameter frequency is 10Hz or more and the relay must be quick acting, reed relays, polarized relays or solid-state relays may be considered. Reed relays have a motion frequency up to 50 times/second and are cheap, but their contact load capacity is low, usually having a maximum of 50mA and 28VDC. Polarized relays and solid-state relays have a switching speed up to 100 times/second and are reliable in performance, but are expensive and bulky.