2.13: Signal state of the normally open switch
Figure 2.14: A basic switch
Figure 2.15: Switch configuration
Figure 2.16: Various throw-pole combinations
The number of throws implies the number of possible actuated positions. Switches may have one, two, or any number of throws with one and two being the most common. Figure 2.16 shows single pole-single throw (SPST), single pole-double throw (SPDT), double pole-double throw (DPDT), and triple pole-double throw (TPDT) configurations.
2.3.2Switch State
Switches are designed to have either open or closed contacts when not actuated. Figure 2.17 shows the pole-contact schemes when a switch is in its normal position (not actuated). Figure 2.18 shows the same contact as in Figure 2.17, but in its actuated state. In some drawings, a side arrow is used to indicate that the switch is held (actuated) by a cam or any other devices (Figure 2.19).
Figure 2.17: Contacts in normal state
Figure 2.18: Contacts in actuated state
Figure 2.19: Contacts in actuated state showing with a side arrow
Figure 2.20: Various switch-contact configurations
A normally open contact changes to closed state when the switch is actuated. Inversely, a normally closed contact changes to open when the switch is actuated. Figure 2.20 shows a variety of switch contact configurations.
2.3.3Types of Switches
There are a great number of switches available for users to choose from. In this section, only those switches commonly used in electrical control systems are discussed. All switches work on the same principle — that the actuation occurs when the switch is actuated (made). The switch returns back to its normal, unactuated state when the actuation is released. Table 2.1 summarizes the switch types, their operation and graphic symbols.
2.4 Logic Elements
Logic elements are those devices that can perform logic decisions for the output devices based on the input signals. Commonly used logic elements include relays, contacts, timers, and counters.
2.4.1Electromechanical Relays
Electromechanical relays, also known as relays, are electrically operated switches that actuate electric contacts in making or breaking the connections in electric circuits. A relay consists of five parts: iron core, coil, contacts, armature, and bias spring as shown in Figure 2.21. In its de-energized state (Figure 2.21a), the bias spring holds the armature with the common contact in contact with the normally closed (NC) contact. When the coil is energized, it produces a magnetic flux, which in turn creates an attractive force between the core and armature. This force overcomes the bias spring tension and pulls down the armature, which in turn makes the common contact touch the normally open (NO) contact (Figure 2.21b). The relay remains energized as long as sufficient current flows through the coil. When the coil is de-energized, the magnetic force vanishes to return the armature back to its original position, which in turn makes the common contact touch the normally closed (NC) contact.
Table 2.1: Various types of switches
| Switch Type | Operation | Graphic Symbol |
|---|---|---|
| Toggle switch | The switch has two positions, On and Off. It is mechanically actuated from one position to the other. It has no normal state. | |
| Selector switch | The switch has two or more positions. It is mechanically actuated to the desired position. It has no normal state. | |
| Push button switch | This switch has two types: momentary button and maintained button. The momentary type uses springs to provide the return action. Actuation occurs when the button is depressed, and is returned back to its original state when the button is released. The maintained type is mechanically interlocked, one with the other. Depress one button to lock it in while unlatching and releasing the opposite button. | |
| Limit switch | The switch is activated by physical contact of the actuator. It is used to sense the travel distance of actuator. | |
| Pressure switch | Pressure-actuated devices such as cylinders, diaphragms, or Bourdon tubes operate the switch. It is actuated when the preset pressure is reached. The switch returns to its normal state when the pressure drops below the preset value. | |
| Temperature switch | The switch is actuated when the temperature reaches a pre-set level. It returns to its normal state when the temperature drops below the pre-set level. | |
| Liquid level switch | The switch is used to measure the liquid level in reservoirs, fuel tanks, etc. As the liquid rises or falls, the switch mechanism makes or breaks the contacts of the switch. | |
| Proximity switch | The switch senses the presence or absence of an object without requiring physical contact. It is actuated when an object approaches in close proximity to it. The switch returns to its normal state when the object’s presence is removed. | |
| Photodetector | The switch is used to detect the presence or absence of an object without requiring physical contact. It can sense a much longer distance than a proximity switch. |
Figure 2.21: Electromechanical relay
The switching function in a relay is accomplished by the contacts. A relay can be made with virtually any number of moving contacts. The number of moving contacts that are attached to the armature is called the number of poles of the relay. A normal relay can have two throws, fixed contacts per pole. Figure 2.22 shows schematic symbols of a variety of relay contact configurations in ANSI standard and ASA standard, respectively.
Relays are often used in electrical circuits. Figure 2.23 shows a relay used in a simple electrical
