Relay FAQ
  • To get an idea of what relays are, think of a children's athletic carnival. Image
    Little A holds on tightly to the baton and passes it to the Big B. This is a relay.

    Now lets look at a more technical example. Think of turning on a television with a remote control. Image
  • A relay consists of an electromagnet that receives an electric signal and converts it to a mechanical action and a switch that open and closes the electric circuit.
    image Schematic Diagram Showing the Principle of Relays
    General Relays

    Principle of Operation
    In this example, we will turn ON a lamp using switch S1 and a relay.
    1. Press S1 to turn it ON.
    2. Current I flows to the operating coil and magnetizes the core.
    3. The armature is drawn to the core by the electromagnetic force.
    4. When the armature reaches the core, the moving and fixed contacts make contact and the lamp lights.
    5. When S1 is released to turn it OFF, current no longer flows to the operating coil, the electromagnetic force no longer exists, and the armature returns to its original position by the force of the release spring.
    6. When the armature has returned to its original state, the contacts become separated and the lamp turns OFF.
  • There are different ways to classify relays. The following groupings will be used in this technical guide.
  • By Definition:
    RELAY: An electromagnetic device for remote or automatic control that is actuated by variations in conditions of an electric circuit and which, in turn, operates other devices (such as switches) in the same or a different circuit.

    CIRCUIT BREAKER: A switch that automatically interrupts an electric circuit under an infrequent abnormal condition; e.g., a fault condition such as an overload or rupture of either high voltage or high current or both.

    SWITCH: A device for making, breaking, or changing the connections in an electrical circuit. Usually mechanical and operated by hand.
  • It is a relay that is set (ON) or reset (OFF) by the input of a pulse voltage. Even after the input voltage is interrupted, this relay maintains its set or reset condition until it receives the next inverting input. It is also called a keep relay.

    There are two types of mechanisms for maintaining the set and reset conditions: a magnetic holding type and a mechanical holding type.

    There are also two types of coils for applying the set and reset pulse voltages: a single-winding type and a double-winding type.

    Basic Operation:
    In these Relays, the input pulse of the set coil causes the operating condition to be maintained magnetically or mechanically, whereas the input pulse to the reset coil side puts the Relay into the reset condition.
    Image Outline
    In these Relays, the set input pulse causes the operating condition to be maintained magnetically, whereas the reset input pulse (input with inverse polarity of set input) puts the Relay into the reset condition. Image
  • SSRs (Solid State Relays) have no movable contacts. SSRs are not very different in operation from mechanical Relays that have movable contacts. SSRs, however, employ semiconductor switching elements, such as thyristors, triacs, diodes, and transistors. Furthermore, SSRs employ optical semiconductors called photocouplers to isolate input and output signals. Photocouplers change electric signals into optical signals and relay the signals through space, thus fully isolating the input and output sections while relaying the signals at high speed. SSRs consist of electronic parts with no mechanical contacts. Therefore, SSRs have a variety of features that mechanical relays do not incorporate. The greatest feature of SSRs is that SSRs do not use switching contacts that will physically wear out.

    SSRs are ideal for a wide range of applications due to the following performance characteristics.
    1. They provide high-speed, high-frequency switching operations.
    2. They have no contact failures.
    3. They generate little noise.
    4. They have no operation noise.
  • Relays are used in most machines and devices that use electricity.
  • The load types and their characteristics are as follows. We must express the load as a contact rating, which is the electrical load-handling capability of relay contacts under specified conditions and for a prescribed number of operations or life cycles.

    RESISTIVE LOAD: A resistive load usually consists of some sort of resistance in the circuit; e.g., heaters, resistors, etc.

    INDUCTIVE LOAD: An inductive load consists of a load created by a wire wound coil, such as in a relay or solenoid, a transformer, or any load which uses a winding over a magnetic iron core. Breaking an inductive load is usually more severe than breaking a resistive load and will generally produce heavy arcing.

    MOTOR LOAD: A motor load can be referred to as a rotating inductive load, generally with a high inrush of six times the normal load. The breaking of the load is much the same as a resistive load.

    LAMP LOAD: There are many types of lamp loads such as tungsten filament, fluorescent, mercury-vapor, and other exotic gas lamps. The loads we normally concern ourselves with are tungsten filament. Tungsten filament lamps, when first turned on, will draw an inrush current of 10-15 times of the steady-state current. The inrush is similar to a motor load inrush and is caused by the cold filament in the lamp. After the lamp filament has heated up, the current will drop to its normal level. Most tungsten filament lamp load ratings are 20% of a resistive load.

    AC Load Types and Inrush Current:

    DC Load Types and Inrush Current:
  • For a DC load, a diode is generally the most effective, and the next most effective is CR elements. For an AC load, a varistor or CR elements are the most effective.

    Typical Surge Suppressor Examples:
    Do not use the following kind of surge suppressor application.
    It is commonly believed that it is more difficult to switch DC inductive loads than resistive loads, but the performance can be improved to about the same level as resistive loads by using a surge suppressor.
  • No, it will not. Because the two contacts do not necessarily turn ON and OFF simultaneously in actual operation (there is some lag in their timing), the entire load will be applied to a single contact for an instant.
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