A Complete Guide to the Switches

Switches are one of the most basic components in electronic circuits. Their core function is to control the on and off of the circuit to conduct or block the current. As the “gatekeeper” of the circuit system, switches play a vital role in modern electronic devices. You can find switches in many places, from the power buttons on home appliances to the intricate control systems in factories.

1. What is a Switch?

A switch is nothing but a device that is used to turn a device on and off. Such a device could be an appliance like a fan, TV, etc. For the current to flow out of a circuit, a closed path (loop) is required. If the switch is in the off state, it means the circuit is open and the current cannot flow through the conductor and the device is de-energized (off state). To energize it, we have to turn on the switch, which creates a complete circuit and a closed path. Therefore, the current can flow through the device and be turned on. Therefore, the function of a switch is to make (switch on) and break (switch off) the circuit.

2. What are the Types of Switches?

In control system engineering, switches play an important role. There are two main types of switches: mechanical switches and electrical switches. Mechanical switches require physical or manual contact with the switch to operate. Electrical switches do not require physical or manual contact, and it has the ability to perform operations. Electrical switches operate under the action of semiconductors.

2.1 Mechanical Switches

Mechanical switches are further divided into different types of switches based on the number of poles and the number of paths. The number of poles refers to the number of input circuits (power circuits) available to the switch. The number of paths refers to the number of output circuits (the number of paths that current can flow) available to the switch.

(1) Single pole single throw (SPST)

This switch consists of two terminals; one input terminal is called the pole and one output terminal is called the throw. Hence, the name of this switch is single pole single throw. This switch is the simplest example of a switch. Usually, this switch is used in a single loop, meaning that the circuit only needs to control one closed path. The symbol of a single pole single throw switch is shown in Pic-1a. This switch is connected in series with a device, source, or component as shown in Pic-1b.

(2) Single pole double throw (SPDT)

This switch consists of three terminals; one input terminal (pole) and two output terminals (throws) as shown in Pic-2a. By using this switch, we can provide current or signal to two loops as shown in Pic-2b. Sometimes this switch is called a selector switch.

(3) Double pole single throw (DPST)

This switch consists of four terminals; two input terminals (pole) and two output terminals (throw) as shown in Pic-3a. This switch is very similar to two SPST switches. Both switches are connected to a single pole, so both switches operate simultaneously. These switches are used when we want to control two circuits at the same time as shown in Pic-3b.

(4) Double pole double throw (DPDT)

This switch consists of six terminals; two input terminals (poles) and two terminals per pole, so a total of four output terminals (throws). This switch works like two separate SPDT switches that function at the same time. In this switch, the two terminals (poles) of the input are connected to one set (of two) outputs (throw 1) in switch position 1. If we change the position of the switch, it will connect this input with the second set of outputs (terminal 2) as shown in Pic-4a. Here as an example, let us assume that in position 1, if the motor rotates clockwise, if we change to position 2, the motor will rotate counterclockwise.

(5)Two poles six throws (2P6T)

This switch has fourteen terminals. It includes two input terminals (or poles) and six terminals for each pole. So, there are twelve output terminals (throws) in total, as shown in Pic-5a. Generally, this type of switch is used to switch in circuits that have a common input terminal.

(6)Momentary operation switch / Momentary control switch

Push button switch

When you press the switch, it closes the circuit and lets current flow. When you release the button, the switch opens, cutting off the circuit. This switch is therefore a momentary contact switch, capable of controlling a circuit by making and breaking contacts. In a push button switch, a spring opens the contacts when the pressure on the switch is removed.

Pressure switch
This type of switch consists of a C-shaped diaphragm. Depending on the pressure, this diaphragm indicates the pressure. These switches are used to sense the pressure of air, water, or oil in industrial applications. This switch operates when the system pressure increases or decreases from the set point.

Temperature switch

This type of switch consists of a temperature sensing device like RTD (Resistance Temperature Device). This switch operates based on the measured temperature value.

Toggle switch

This type of switch is commonly used to turn household appliances on and off. A toggle switch has a lever that we can move up or down to turn the appliance on or off.

Rotary switch

This type of switch is used to connect one line to one of the multiple lines. A multimeter knob and the selectors for channels, ranges, and bands in communication devices are all examples of this type of switch. This switch is the same as the SPMT switch. But the arrangement of this switch is different.

2.2 Electrical Switches

An electrical switch is nothing but a semiconductor device. These switches are more useful due to their low cost, small size, and reliability. In this switch, semiconductor materials like Silicon (Si), and Germanium (Ge) are used. This type of switch is common in Integrated Circuits (ICs), motor drives, and HVAC systems. It’s also often used as a Digital Output (DI) for controllers.

(1) Relay

The working principle of a relay is electromechanical, so this type of switch is also called an electromechanical switch. When current passes through a coil, it creates a magnetic field around the coil. The size of the magnetic field depends on the amount of current passing through the coil. The contacts are arranged in such a way that if the current increases, the curtain limit contact is energized and changes its position. Sometimes, relays use bimetallic strips to sense temperature for safety. Relays have a wide range of voltage and current. In power systems, relays play an important role in fault identification. In industry, relays are also used as protection devices.

(2) Bipolar transistor

Bipolar transistors have three parts: the collector, emitter, and base. Think of them as two diodes placed back to back, capable of switching on or off or even amplifying signals in a device circuit. Bipolar transistors are essential for fast switching and amplifying device tasks. They come in two main types: NPN and PNP, each with unique properties. NPN transistors use a small current to control a large current and make devices like radios or computer parts work. However, PNP transistors are used when we need the current to flow in the opposite direction.

(3) Power diode

A power diode has two terminals: an anode and a cathode. A diode is made of p-type and n-type semiconductor materials, forming a pn junction, called a diode. When a diode is forward biased, current can flow through a circuit, while when the diode is reverse biased, the current is blocked. If the anode is positive with respect to the cathode, the diode is forward biased and acts as a switch ON. Similarly, if the cathode is positive with respect to the anode, the diode is reverse biased and acts as a switch OFF. Power diodes serve in many power electronics applications. They are found in rectifiers, voltage multipliers, and voltage clamping circuits, to name a few.

(4) MOSFET

A MOSFET, or Metal Oxide Silicon Field Effect Transistor, is a four-terminal device. It is controlled by voltage and is used for switching and amplifying in circuits. They have three main parts: drain, source and gate.

There are two basic forms of MOSFET: depletion mode and enhancement mode. If the gate-source voltage (VGS) is insufficient, the MOSFET will operate as a depletion mode and the depletion mode of the MOSFET is like an off switch. If the gate-source voltage (VGS) is sufficient, the MOSFET will operate as an enhancement mode and the enhancement mode of the MOSFET is like an on switch. The switching range of MOSFET is from tens of neon seconds to hundreds of microseconds. MOSFET is used in linear regulators, choppers and audio power amplifiers, etc.

(5) IGBT

IGBTs (Insulated Gate Bipolar Transistors) are key to handling high-voltage circuits. The device mixes easy gate-drive features of a MOSFET with the high current and low saturation voltage strengths of a bipolar transistor.

IGBTs control power in many industrial, commercial, and energy applications. IGBTs are popular in commercial applications. Their fast switching speeds and low saturation voltage make them ideal for devices like solar power systems and uninterruptible power supplies (UPS). In consumer electronics, they are used for temperature control in induction cooktops and digital camera stroboscopes.

(6) SCR

When we need a reliable way to control high-voltage AC, we often use silicon controlled rectifiers (SCRs). These devices ensure that current flows smoothly in one direction at the right time. SCRs function like diodes. They begin conducting when forward biased, meaning the cathode is negative and the anode is positive. Additionally, they need a positive clock pulse at the gate to operate. In forward bias, when the clock pulse to the gate is zero, the SCR turns off due to forced commutation. In reverse bias, the SCR stays off like a diode. SCRs are used in motor control, power regulators, and lamp dimming.

(7) TRIAC

TRIAC is a bidirectional switching device and an important member of the thyristor family of devices. TRIAC stands for Triode AC Switch. The “TRI” means it has three terminals. The “AC” shows it controls alternating current, allowing it to conduct in both directions. They are most commonly used in motor speed controllers, AC circuits, pressure control systems, dimmers, and other AC control devices.

(8) DIAC

A DIAC (Diode Alternating Current) is a semiconductor switch. It allows electricity to flow in both directions after reaching a specific voltage. DIACs are great for managing power in devices like motor speed controllers and thermal control circuits.

(9) GTO

GTO stands for Gate Turn-Off Thyristor. It is a semiconductor device that controls unidirectional switching. It has three terminals: gate, cathode, and anode. A unique feature of a GTO is that it can be switched on and off using the same gate terminal. GTOs are used in inverters, AC and DC drives, induction heaters, and SVC (Static Var Compensation). GTOs cannot be used to switch off inductive loads without the help of a snubber circuit.

3. How do Switches Work?

Mechanical Switch: It turns on and off by closing and opening metal contacts. You need to think about the conductivity and wear resistance of the contact material, like silver alloy.

Electrical Switch: It controls the gate voltage of the semiconductor. This adjusts the conduction state between the source and drain.

4. What are the ‌Core Parameters of Switches?

1) ‌Electrical Characteristics‌

‌Rated Voltage/Current‌: The maximum working conditions that the switch can withstand.

‌Contact Resistance‌: Affects energy loss during conduction.

‌Insulation Resistance: The ability to prevent leakage current in the disconnected state.

2) ‌Mechanical Characteristics‌

‌Operating Force and Stroke‌: The tactile design of buttons or toggles.

‌Life Cycle: The typical life of a mechanical switch is thousands to millions of times.

5. Where are Switches Used for?

1) Consumer Electronics Products

Power Management: mobile phone power button, laptop sleep wake-up;

Human-computer Interaction: game controller buttons, touch screen gesture control;

Function Switching: camera mode dial, headphone volume adjustment.

2) Industrial Automation

PLC Control System: control motor start and stop through relay module;

Safety Interlock: emergency stop button (E-Stop) and access control switch;

Sensor Interface: limit switch for robot arm position detection.

3) Automotive Electronics

Driving Control: turn signal switch, cruise control lever;

High Voltage System: contactor of electric vehicle battery management system;

Smart Cockpit: capacitive steering wheel buttons, gesture control central control screen.

4) Communication Equipment

RF Switch: switch antenna signal path in 5G base station;

Fiber Optic Switch: used for routing selection of optical modules in the data center.

6. How to Test the Reliability of Switches?

Functional Testing: verification of on/off status, measurement of contact resistance.

Life Testing: evaluation of performance degradation after repeated operation.

Environmental Testing: stability under conditions of high temperature, high humidity, vibration, etc.

7. How to Know Whether the Switches Have Failed?

Relay: Use a multimeter to check whether the coil resistance is normal. The resistance should be close to zero when the contact is turned on.

MOSFET: Measure the gate threshold voltage and drain source on-resistance. If abnormal, it may break down.

SCR: If it cannot maintain conduction after triggering, it may be due to gate damage or excessive junction temperature.

8. Common Causes of Switches Failure

Over-voltage/Over-current: Breakdown occurs when the device tolerance is exceeded. This can happen with MOSFET avalanche breakdown.

Thermal Stress: Poor heat dissipation causes high-temperature burning (common in high-power scenarios).

Mechanical Wear: Oxidation or fatigue of relay contacts leads to poor contact.

9. Recommendations for Selecting Switches

Low Power Consumption Scenarios: electronic switches are preferred to reduce energy consumption.

High-Frequency Operation Requirements: Choose mechanical switches with long life, like gold-plated contacts, or solid-state relays.

High Voltage/Current Scenarios: relays or high-power MOSFETs are used.

10. Future Development Trends of Switches

1) Intelligent and Integrated‌

With the development of the Internet of Things (IoT), switches are transforming from passive components to intelligent nodes. For example:

Intelligent switches with integrated MCUs can report status data;

MEMS (microelectromechanical system) switches achieve nanoscale control.

2) ‌Application of New Materials‌

Graphene Contacts: reduce resistance and improve arc resistance;

Flexible Electronics: curved switches for foldable devices.

3) ‌Green and Energy-saving Design‌

Ultra-low power switches (standby current <1μA);

Energy recovery technology (such as piezoelectric switches convert mechanical energy into electrical energy).

4) ‌Wireless and Passive‌

RFID-based passive switches do not require an external power supply;

Ultrasonic or Li-Fi technology enables air touch.

11. Typical Brands for Switches

Schneider

Siemens

Omron

Phoenix

MOSO

And so on…

12. Switches FAQs

13. Summary

Switches play an irreplaceable role in electronic systems. Switches directly control the circuit’s on/off state. This allows for precise control of electronic devices or system functions. It can also reduce the risk of overload or short circuits. This is key for the safe, efficient, and smart operation of electronic systems. So knowing the different switch types and their uses is very important.