Chapter 12
Fundamentals of Electricity & Electronics
can be made between one contact and the other.
Double-Pole, Single-Throw (DPST)
The double-pole, single-throw (DPST) switch connection can be made between one set of contacts and either of two other sets of contacts. [Figure 12-73]
Double-Pole, Double-Throw (DPDT)
The schematic symbol for the double-pole, double-throw (DPDT) switch is shown in Figure 12-74. This type of switch makes a connection from one set of contacts to either of two other sets of contacts.
A toggle switch that is spring-loaded to the OFF position to rest at either of two positions, opening the circuit in one position and closing it in another, is a two-position switch. A toggle switch that comes to rest at any one of three positions is a three-position switch.
A switch that stays open, except when it is held in the closed position, is a normally open switch (usually identified as NO). One that stays closed, except when it is held in the open position is a normally closed switch (NC). Both kinds are spring loaded to their normal position and return to that position as soon as they are released.
Locking toggles require the operator to pull out on the switch toggle before moving it in to another position. Once in the new position, the switch toggle is release back into a lock, which then prevents the switch from inadvertently being moved.
Microswitches
A microswitch opens or closes a circuit with a very small movement of the tripping device (1⁄16 inch or less). This is what gives the switch its name, since micro means small.
Microswitches are usually pushbutton switches. They are used primarily as limit switches to provide automatic control of landing gears, actuator motors, and the like. Figure 12-75 shows a normally closed microswitch in cross-section and illustrates how these switches operate. When the operating plunger is pressed in, the spring and the movable contact are pushed, opening the contacts and the circuit. Figure 12-76 shows a pushbutton microswitch.
Rotary Selector Switches
A rotary selector switch takes the place of several switches. When the knob of the switch is rotated, the switch opens one circuit and closes another. Ignition switches and voltmeter selector switches are typical examples of this kind of switch. [Figure 12-77]
Pushbutton Switches
Pushbutton switches have one stationary contact and one designed for momentary contact.
