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Mechanical Relays Fundamentals and their applications
Definition of Relays Terminology COIL Nominal Coil Voltage (Rated Coil Voltage) -- Voltage intended by design to be applied to the coil to operate it. Pick-Up Voltage -- As the voltage on an unoperated device is increased, the value at or above which all contacts must function. Drop-Out Voltage -- As the voltage on an operated device is decreased, the value at or below which all contacts must revert to their unoperated position. Maximum Continuous Voltage -- The maximum voltage that can be applied continuously to the coil. Nominal Operating Current -- The current flow in the coil when nominal voltage is impressed on the coil. Nominal Operating Power -- The power used by the coil at nominal voltage. DC coils(watts); AC coils(VA). Coil Resistance -- The DC resistance of the coil in DC type at the temperature conditions listed. Typical temperature conditions is 20° Celcius or 68° Fahrenheit.
Contact Symbols
Form A contacts are also called N.O. contacts or make contacts. Form B contacts are also called N.C. contacts or break contacts. Form C contacts are also called changeover contacts or transfer contacts. Rated Switching Power -- The design value in watts (DC) or volt amperes (AC) which can safely be switched by the contacts. This value is the product of switching voltage x switching current, and will be lower than the maximum voltage and maximum current product.
Maximum Switching Voltage -- The maximum open circuit voltage which can safely be switched by the contacts. Maximum Switching Current -- The maximum current which can safely be switched by the contacts. Maximum Switching Power -- The upper limit of power which can be switched by the contacts. Maximum Carrying Current -- The maximum current which after closing or prior to opening, the contacts can safely pass without being subject to temperature rise in excess of their design limit, or the design limit of other temperature sensitive components in the relay (coil, springs, insulation, etc.). This value is usually in excess of the maximum switching current. Minimum Switching Capability -- This value is a guideline as to the lowest possible level at which it will be possible for a low level load to allow switching. The level of reliability of this value depends on switching frequency, ambient conditions, change in the desired contact resistance, and the absolute value. Please use a relay with AgPd contacts if your needs analog low level loads, control, or a contact resistance of 100 mohm or less. Maximum Switching Capacity -- This is listed in each type of relay as the maximum value of the contact capacity and is an interrelationship of the maximum switching power, maximum switching voltage, and maximum switching current. The switching current and switching voltage can be obtained from this graph. For example, if the switching voltage is fixed in a certain application, the maximum switching current can be obtained from the intersection between the voltage on the axis and the maximum switching power. Maximum Switching Capacity (TX type) Example: Using TX type at a switching voltage of 60V DC, the maximum switching current is 1A.
(Maximum switching capacity is given for a resistive load. Be sure to carefully check the actual load before use.)
Capacitance -- This value is measured between the terminals at 1kHz and 20°C or 68°F. Terminal Configuration
Typical Applications of Relays
Driven by a Transistor The voltage impressed on the relay is always full rated voltage, and in the OFF time, the voltage is completely
zero for avoidance of trouble in use.
If the coil current is suddenly interrupted, a sudden high voltage pulse is developed in the coil. If this voltage exceeds the voltage resistance of the transistor, the transistor will be degraded, and this will lead to damage. It is absolutely necessary to connect a diode in the circuit as a means of preventing damage from the counter emf. As suitable ratings for this diode, the current should be equivalent to the average rectified current to the coil, and the inverse blocking voltage should be about 3 times the value of the power source voltage.
Driven by a SCR For SCR drive, it is necessary to take particular care with regard to gate sensitivity and erroneous operation due to noise.
Relays for PC board use have high sensitivity and high speed response characteristics, and because they respond sufficiently to chattering and bouncing, it is necessary to take care in their drive. When the frequency of use is low, with the delay in response time caused by a condenser, it is possible to absorb the chattering and bouncing.
PCB Design Considerations Since relays affect electronic circuits by generating noise, the following points should be noted. Keep relays away from semiconductor devices. Design the pattern traces for shortest lengths. Place the surge arrester (diode, etc.) near the relay coil. Avoid routing pattern traces susceptible to noise (such as for audio signals) underneath the relay coil section. Avoid through-holes in places which cannot be seen from the top (e.g. at the base of the relay). Solder flowing up through such a hole may cause damage such as a broken seal. Even for the same circuit, pattern design considerations which minimize the influence of the on/off operations of the relay coil and lamp on other electronic circuits are necessary.
Tips for hand soldering of Relay
~ Keep the tip of the soldering iron clean.
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