Otis Singleton
Control Circuit Devices are used to regulate and manage the operation of an electrical circuit by turning it on or off. They control the voltage and current within the circuit. Devices such as switches, solenoids, and relays are used to control, signal, and switch the circuit on and off. A relay is a remote-control switch that is electrically activated, allowing a trigger input or current flow in the relay to control a larger current flow. There are many types of relays, including safety relays, which are used for safety functions.
| Characteristics | Values |
|---|---|
| Control Circuit Devices | Regulate and manage the operation of an electrical circuit by turning it on or off |
| Switches | Have conductive pieces called contacts, which connect to the external electrical circuit |
| Normally open switches have contacts that close when activated | |
| Normally closed switches have contacts that open when activated | |
| Relays | Electromechanical and remote-control switches that are electrically activated |
| Allow a trigger input or current flow in the relay to control a larger current flow | |
| Types of Relays | Control relays, fan duty or service duty relays, and pilot duty control relays |
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What You'll Learn
Transformers transfer electrical power between circuits
Transformers are electrical devices that transfer power between circuits by changing the voltage level. They are essential for transmitting, distributing, and utilising alternating current (AC) electric power.
The basic principle behind transformers is electromagnetic induction, specifically Mutual Induction. Mutual induction involves a coil of wire inducing a voltage in another coil located nearby. This process creates a magnetic circuit that links two or more electrical circuits, allowing electrical energy to be transferred between separate coils without a metallic connection.
The varying current in a transformer coil produces a varying magnetic flux in its core, which induces a varying electromotive force (EMF) in any other coils wound around the same core. The EMF of a transformer increases with frequency, so higher frequencies allow for a more compact design as a given core can transfer more power without reaching saturation. Transformers can either increase or decrease voltage levels, known as step-up or step-down transformers, respectively.
An example of an early transformer was exhibited in 1882 by Lucien Gaulard and John Dixon Gibbs, who called it a 'secondary generator'. Their device used an open-core design with a fixed 1:1 ratio to supply a series circuit for lamps. However, the voltage was controlled by moving the iron core in or out, and such open-core transformers are inefficient at transferring power to loads.
Today, transformers are integral to daily life, enabling various applications such as baking pizzas, drying hair, melting snow, and powering data centres. They are also used to provide galvanic isolation between circuits and couple stages of signal-processing circuits.
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Switches control circuits
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting or diverting the electric current. Switches have conductive pieces called contacts, which connect to the external electrical circuit. The most common type of switch is an electromechanical device consisting of one or more sets of movable electrical contacts connected to external circuits. When a pair of contacts is touching, the current can pass between them; when the contacts are separated, no current can flow.
Switches are made in many different configurations. They may have multiple sets of contacts controlled by the same knob or actuator, and the contacts may operate simultaneously, sequentially, or alternately. A switch may be operated manually, for example, a light switch or a keyboard button, or it may function as a sensing element to sense a machine's position, liquid level, pressure, or temperature, such as a thermostat.
There are many specialized forms of switches, including the toggle switch, rotary switch, mercury switch, push-button switch, reversing switch, relay, and circuit breaker. A common use is controlling lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures. Switches in high-powered circuits must be specially constructed to prevent destructive arcing when they are opened.
Some switches are more complex, containing electronic circuits that can turn on or off depending on some physical stimulus (such as light or a magnetic field) being sensed. In any case, the final output of any switch will be a pair of wire-connection terminals that will either be connected together ("closed") or not connected ("open").
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Relays control circuits
Relays are used to control a circuit by an independent low-power signal and to control several circuits by one signal. The traditional electromechanical relay uses an electromagnet to open or close the contacts, but relays can also work without moving parts, such as solid-state relays, which use semiconductor properties for control.
A relay switch circuit can be used to control motors, heaters, lamps, or AC circuits, which can draw a lot of electrical voltage, current, and power. The electro-mechanical relay is an output device (actuator) and comes in various shapes, sizes, and designs, with many uses and applications in electronic circuits. Electrical relays can be used to allow low-power electronic circuits to switch high currents or voltages on or off.
A relay switch circuit typically has the coil driven by an NPN transistor switch, depending on the input voltage level. When the Base voltage of the transistor is zero or negative, the transistor acts as an open switch, and no Collector current flows, de-energizing the relay coil. If a large enough positive current is driven into the Base to saturate the NPN transistor, the current flowing from Base to Emitter controls the larger relay coil current flowing through the transistor from the Collector to Emitter.
There are many types of relays for different purposes. Control relays operate control circuits and light-duty loads, or even control the coil of another relay in the case of pilot duty control relays. Fan duty or service duty relays are used to control fan motors. Latching relays require only a single pulse of control power to operate the switch persistently. Magnetic latching relays are useful when interrupted power should not affect the circuits being controlled.
DPDT relays are common in industry due to their versatility. DPDT relays have two electrically isolated "poles" or Form-C contact sets, offering a choice of either normally-open or normally-closed contacts.
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Flow switches control liquid and gas flow
Flow switches are an essential component of electrical circuits that control the flow of liquids and gases through a channel. They are designed to monitor and control the flow rate of fluids within industrial process systems. When the flow rate deviates from the pre-established level—either increasing or decreasing—the flow switch activates and performs two functions: closing its contacts and turning on a specific piece of equipment, or opening its contacts and turning off the equipment.
Flow switches are engineered with specialised components to ensure precise flow monitoring and measurement. They incorporate sensing mechanisms such as magnetic paddles, vanes, or diaphragms, strategically placed within the flow path of a liquid or gas. These sensors enable automation and reliable system monitoring by detecting changes in flow rate, safeguarding equipment, and triggering alarms in critical situations.
The type of medium being monitored—whether liquid, gas, or air—is crucial to the operation of a flow switch. For example, solid-state flow switches, which operate on thermal dispersion or calorimetric principles, are suitable for clean liquids, air, or inert gases. They feature two temperature sensors, one of which acts as a reference while the other is heated by a built-in element. As the flow rate increases, the thermal energy is dissipated more rapidly, resulting in a narrower temperature differential. This makes solid-state switches highly sensitive and accurate.
Flow switches are available in various designs to cater to different applications. Paddle flow switches, for instance, are constructed with a hinged or spring-mounted paddle that makes direct contact with the flowing medium. The paddle shifts when the flow rate deviates, moving a lever to activate an internal microswitch. This action can signal pumps, alarms, or control processes, making paddle flow sensors widely adopted in HVAC, fire protection, and water treatment systems.
Another type of flow switch is the piston or shuttle flow switch, which uses a free-floating or spring-loaded magnetic piston to react to changes in the flow rate. As the piston moves within its housing, it brings a magnet close enough to activate a reed switch, automating actions such as system shutoff, pump activation, or triggering alarms. Piston flow switches are ideal for clean liquid flow detection in hydraulic, lubrication, and cooling circuits where reliable and repeatable actuation at preset threshold rates is critical.
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Transistors, capacitors, and connecting wires form circuits
Transistors, capacitors, and connecting wires are all essential components of an electronic circuit. An electronic circuit is a structure that controls electric current to perform functions such as signal amplification, computation, and data transfer.
Transistors act as electronic switches, preventing current from passing through them until a small amount of power is applied to the base pin. They are used to control the flow of current in a circuit. Capacitors, on the other hand, store electrical energy and are used to smooth out voltage fluctuations in power supplies. They consist of two plates that store energy in the form of an electric charge. When a battery is connected, capacitors charge up and store electrons, and when the battery is removed, they release the electrons to power the circuit. Connecting wires or traces are used to connect the various components of the circuit together, forming a conducting path for the electric current to flow through.
The simplest circuit consists of three elements: a conducting path, a voltage source, and a load. A load is an element in the circuit that consumes power to perform a particular function, such as a light bulb. More complex circuits may have different types of loads, such as resistors, capacitors, and transistors. Resistors are passive two-terminal devices that resist the flow of current and are commonly used in circuits to control the amount of current flowing through.
Overall, transistors, capacitors, and connecting wires work together to form and control electronic circuits, allowing for the performance of various functions through the manipulation of electric current.
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Frequently asked questions
A control circuit device regulates and manages the operation of an electrical circuit by turning it on or off. It controls the voltage and current within the circuit. Examples include switches, solenoids, and relays.
A relay is a type of electromechanical and remote-control switch that is electrically activated. It allows a trigger input or current flow in the relay to control a larger current flow.
A solenoid is a soft iron core wound with wire that comprises the control circuit. An electric current flowing through the solenoid creates an electromagnetic field that pulls an armature down to close contacts in the circuit.
Switches have conductive pieces called contacts that connect to the external electrical circuit. Normally open switches have contacts that close when activated, while normally closed switches have contacts that open when activated.
