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▶ The Impact of Temperature Changes
As the temperature changes, the coil resistance of the relay changes as well, causing a remarkable impact on the voltage of pull-in and release of the relay. When the temperature reaches its maximum, the release voltage tends to reach its maximum as well, causing the operating voltage to rise. When the temperature reaches its minimum, the release voltage tends to reach its minimum, causing the pull-in voltage to drop. Non-pull-in or unreliable pull-in under the maximum temperature or non-release or slow release under the minimum temperature will cause the relay to fail. Current-mode relays are not subject to temperature change because the suction and release of ampere-turn are not affected by the resistance change of the coil. It should be noted that some customers of current-mode relays actually use the voltage source instead of the constant-current source as the driving source. In this case, the effect the temperature has on the coil resistance must be taken into consideration.
▶ Excitation of the Switch of Solid-state Devices
a. The load capacity of the switch of solid-state devices must be compatible with the coil of the exciter relay, and a sufficient tolerance must be allowed (generally double).
b. When the switch of solid-state devices is on, the voltage distribution of the excitation circuit must be sufficient to ensure that the actual excitation voltage in the coil of the relay conforms to rated operating voltage requirement.
c. When the switch of solid-state devices is off, the leakage current of the excitation circuit must be less than the minimum release current of the relay.
d. The reverse withstand-voltage of the switch of solid-state devices must be compatible with the 50-80V peak voltage; additionally, necessary allowance must be established. A considerable surge voltage up to 1500V is created at the moment when the coil of the relay is de-energized; therefore, it is necessary to take suppression measures in order to maintain the peak voltage in the range of 50-80V.
▶ Low-voltage Voltage and High-voltage Output Isolation
In modern industrial auto control systems, switches of solid-state devices with low-voltage circuit are widely used to control the input of miniature intermediate relays followed by the use of the relay contacts to transfer 220VAC or 380VAC inductive load circuit (e.g. electromagnet, contactor coil, etc.) to perform auto control and protection. The intermediate relay actually performs the low- and high-voltage isolation and inductive load transfer. Intermediate relays of this nature can be considered when the following conditions are in place: good insulation and dielectric strength and good resistance to extreme temperatures, moisture, dust and noxious gas. In general, the capability of withstanding ambient impact can be improved by air-tight sealing and other necessary protective measures while the insulation and dielectric strength can be enhanced by strictly controlling and maintaining insulation spacing interval and power distribution distance.
▶ Mutual Interference and Operation Error
The high-density assembly of various relays, especially relays with large electromagnets or contactors, on the printed circuit board may give rise to electromagnetic mutual inductance which will result in operation error of the relay. Operation error can also result from impact on or vibration of the moving parts of the relay. Special caution must be exercised when arranging for the related positions for the installation of sensitive compact general-purpose relays.
▶ Long-distance Wired Excitation Mode
The excitation mode for the ringing circuit of automatic telephones and door-type wiring falls into this category. As the conducting wire for excitation is usually long, the effect of the voltage drop of the conducting wire on the actual excitation value must be taken into full consideration to ensure actual excitation value added to the relay coil conforms to the prescribed rated operating voltage.