5.5: Circuit Control Devices
Controlling a circuit is an essential part of the electrical system. The control devices will turn circuits on and off and limit the conditions within the circuit, including amperage and voltage.
Circuit control devices include:
- resistors
- switches
- relays
- solenoids
Resistors
Resistors are used in circuits where full current flow may not be required. One side effect of resistors is that they produce heat. There are three common resistors found in most circuits:
- fixed
- stepped
- variable
Fixed resistors
Fixed resistors (Figure \(\PageIndex{1}\)) restrict current flow or voltage and are connected into the circuit or built right into a component. In some cases, this resistance is provided by wires. These special wires can be identified by markings that state “resistor wire—do not cut or splice.”
Stepped resistors
Stepped or tapped resistors (Figure \(\PageIndex{2}\)) have more than one fixed value and are connected by wiring and a switch into the circuit. The control system on a fan is a good example of one use for this type of resistor. The current will flow through different resistances and provide different fan motor speeds by selecting different fan speeds with the switch.
Variable resistors
Variable resistors (Figure 8) include potentiometers and rheostats. A variable resistor is called a potentiometer when it is used to control voltage or is called a rheostat when it is used to control current.
A rheostat has two connections and a movable wiper that contacts a winding. If the wiper is close to the power connection, there is a high current flow, and as the wiper is moved away from the power connection, the resistance in the circuit increases, and the current drops.
A potentiometer has a winding that is in series rather than being open ended. Each end of the winding has a terminal for connection into a circuit, as does the movable arm. The movable arm provides an output voltage that varies from full source voltage to zero voltage as it moves along the winding.
Switches
Switches are the most common control device for electrical circuits. Switches operate a circuit by opening or closing the circuit. Switches are operated manually, hydraulically, electrically, or pneumatically.
Switches have different numbers of inputs, called poles, and different numbers of outputs, called throws. The most common switch is the single-pole, single-throw switch (SPST).
A switch that can make more than one contact is called a single-pole, double-throw switch (SPDT). A slide switch, rocker switch, and a toggle are shown in Figure \(\PageIndex{4}\).
If the same control operates more than one switch, it is called a gang switch. The arrangement of the fixed contact positions (called throws) and the moving contact (the pole), can be seen in the schematic symbol (Figure \(\PageIndex{5}\)). It is called a double-pole, double-throw switch (DPDT).
Another type of switch is the pushbutton switch. This type of switch can be used in different configurations, including SPST, SPDT, and DPDT.
A rotary switch, shown in Figure \(\PageIndex{6}\), has a single pole and two positions.
Momentary contact switches (Figure \(\PageIndex{7}\)) require the operator to maintain pressure to keep the circuit open or closed, because the operating button is spring-loaded and will return to its at-rest position when released. A typical application is a horn switch or blower door safety on a furnace.
Temperature, pressure, and motion switches
A temperature-sensitive switch operates on the same principle as a cycling circuit breaker to make and break an electrical connection. These switches are called flashers and can be used to operate turn signals or emergency lights. In this type of switch, the heater is included as part of the switch.
Other temperature-sensitive switches can be used to turn current flow on or off for operations such as cooling fans or electric defrosters. They sense the temperature of the engine or outside air and control the circuit by using a bimetal strip and contacts (Figure \(\PageIndex{8}\)).
Pressure-sensitive switches can sense the pressure in a pneumatic or hydraulic system and turn electrical devices on and off. These are used for electrical clutches in air conditioning, air pumps, and other similar systems.
Relays and Contactors
Relays and Contactors are electrically activated switches that allow a small current flow to control a large current flow. Every relay and contactor has a power circuit and a control circuit. An external switch energizes the control circuit. The control circuit consists of a soft iron core with a wire winding, called a coil or solenoid.
When current flows through the coil, an electromagnetic field is created that pulls an armature down to close contacts in the circuit. A typical application is a starter relay (Figure \(\PageIndex{9}\):) on a car or contactor that starts an AC compressor.
Solenoids
A solenoid (Figure \(\PageIndex{10}\)) is an electromagnet with a movable core that changes electrical energy into mechanical force. Some solenoids may activate contacts and act as a relay, such as in a starter solenoid, or they can be connected to valves to shut of the flow of fluids.