Electrical Switch Overview
Electrical Switch Overview
An electrical switch is a binary device, which means that a switch is either on (closed contacts) or off (open contacts). A switch can be any device that opens or closes an electrical circuit, and switches can range in from a simple on-off toggle switch to the large contactors, used to switch high power loads like electric motors, to more complex solid-state switches like a NPN or PNP bipolar transistor switch.
Though there are numerous switch designs, all provide the same basic function of opening and closing a circuit with a set of contacts. The following sections describe the basic components of mechanical switches, introduce important switch terms (e.g. normally-open and normally-closed contacts), and go over types of switches. The last section gives general information on how to select switches.
Basic Components in Electrical Switches
A force must act on a switch in order to open or close the circuit, and the device that applies this operating force is called the actuator.
Do not think actuating a switch means you are turning something on. Actuation just means changing the state of a switch's contacts from either open to closed (making a connection) or closed to open (breaking a connection). Thus, a switch can be actuated to turn a device ON or OFF.
In mechanical switches the moving part of a switch that conducts the current between contacts is called the armature. Levers are commonly used as armatures.
Wherever you have two pieces of conductive material separated by an insulated gap and connected to a circuit, you have electrical contacts. To be of any industrial value electrical contacts must be composed of metals that have a high resistance to corrosion and mechanical wear. There are only two possible states for any set of contacts:
Open / Break / Off - Contacts CANNOT conduct electricity (i.e. there is an insulated gap separating the contacts and preventing current flow)
Closed / Make / On -- Contacts CAN conduct electricity (i.e. contacts are connected by some conductive material)
Some switches come with attached cables, strain relief, and connectors, while other switches are sold where you just get the switch by itself. Browse our product guides for general information about the selection, function, and use of wires and cable, strain relief for flexible cables, and electrical connectors.
One simple way to check the number of poles for an electromechanical switch is to count the number of armatures. In an electromechanical relay, the number of separate circuits that can be simultaneously controlled by the inductor determines the pole count.
Throw or Way
Throw refers to how many circuits, or fixed contacts, a movable contact (i.e. armature) can switch between. Thus, a simple way to check the number of a throws is to count the number of fixed contacts for that armature. The term “way” is sometimes used instead of throw.
Common Pole and Throw Counts for Electrical Switches:
SPST - Single Pole, Single Throw
SPDT - Single Pole, Double Throw
DPST - Double Pole, Single Throw
DPDT - A Double Pole, Double Throw
Momentary Switches: Normally-closed and Normally-open Contacts
For many applications, switches are made so that they stay in a “normal” position until actuated with the contacts being either “normally-closed” (N.O.) or “normally-open” (N.O.). An electrical switch that returns to its “normal” position once the actuating force is removed is called a momentary action switch or simply a momentary switch.
For example, traditional keyboards have push button switches with normally-open contacts (e.g. alphabet keys) where the keys function only when pressed (actuated) by a finger, and a spring returns the keys to their normally-open position once they are released.
Types of Electrical Switches
Switches control the flow of current in circuits, which is why they can also be called circuit control devices. Though the basic purpose of all switches is the same, different switch designs provide further controls over the conditions of switch actuation and allow or impede current flow if certain conditions are met. Thus, we can distinguish various types of switches by looking at the conditions themselves (e.g. pressure and temperature).
Manual or Hand Switch
These switches are designed for use with human operators/controllers. Examples of manually actuated switches include push button and toggle switches. The diagram of a single-pole single-throw (SPST) switch can also be used to represent a toggle switch. Another type of manual switch is the foot switch that is actuated by stepping on a button or pedal and has the advantage of freeing up the hands of a human operator for other uses.
These kinds of switches are typically used for remote detection and sensing applications. Manufacturing facilities depend on numerous automatic switches that function as the “eyes and ears” of machine equipment.
The actuation methods for several types of automatic switches are described below, but this list is by no means exhaustive.
Also known as a micro switch, the key feature of these switches is that the contacts change state at specific points along the actuator’s range of motion. These “operating” and “release” points allow the movement of the contacts to be largely independent of the movement of the actuator.
In a limit switch, physical contact between a target object and the actuator opens or closes a set of electrical contacts. Limit switches function as a sensor that detects the position of an object (e.g. sliding doors, items on a conveyor belt, etc.).
As the name suggests, photoelectric sensors use the emission and detection of light, specifically LED lights, to control the flow of current in larger circuits. Photoelectric sensors are growing in popularity and pushing the bounds of sensor switching technology.
Actuated once a specific temperature is reached. Thermal overload relays, used for motor protection, are examples of temperature actuated switches.
Low or high pressure actuation. These kind of switches are commonly used in circuits powering HVAC systems.
Rotary Speed Switch
Actuated if a rotor is spinning too fast or too slow. These switches are used to monitor and protect engines and motors.
These switches are connected in such a way so that they actuate once the liquid in some tank, container, pool, etc. reaches a certain level. Float switches can vary in complexity but their name comes from the simplest design, which is a float attached to a lever that actuates a switch (typically a micro-switch).
Unlike limit switches, proximity sensors detect the position of an object without requiring physical contact with the switch actuator. The two types of proximity sensors are inductive (for detecting metals) and capacitive (can detect metals and non-metals) sensors.
Differences between limit and proximity sensor switches
Limit switches typically cost less and are more accurate, but have shorter life spans and lower maximum switching frequencies (i.e. number of ON/OFF cycles per second) compared to proximity sensors.
Only the physical condition of a limit switch will affect its operation. In contrast, though physical wear and tear is not really an issue, a proximity sensor is vulnerable to anything that can affect inductance or capacitance (e.g. magnetic fields from nearby electrical wiring, dust or metal chips accumulating on the sensor, and ambient temperature).
How to Select an Electrical Switch
The content we provide is meant to inform you and help support the proper selection and use of switches. As always, we recommend you consult a licensed and competent electrician to help you with the sizing and selection of parts for your particular application.
There is no hard and fast rule for selecting the best switch for a given application. There may be several switches that are suitable for the same application, and each will have its own specific advantages and disadvantages depending on the design.
AC or DC
First, determine whether you are switching AC or DC because your power calculations will vary widely depending on whether you are switching alternating or direct current.
Voltage and Current
Voltage ratings are meant to prevent damage from electric arcs as contacts open and close. Current ratings are meant to prevent contacts from overheating. Make sure your switches can handle the voltages and currents of the circuit they are switching.
Number of poles and throws
For electromechanical switches, determine how many movable contacts you need to figure out the pole count.
Determine how many fixed contacts you want a movable contact to switch between to figure out how many throws you need for a pole.
Number of N.O. and N.C. contacts
Actuating a switch with N.O. contacts means you will be momentarily closing circuits (allowing current to flow), while a switch with N.C. contacts means you will be momentarily opening circuits (stopping the current).
Intended Method of Actuation
How do you want to operate the switch? Under what conditions do you want your switch to actuate?
If you need to monitor the movement of a machine part or count products on an assembly line, then you may want to get a limit switch. Additionally, you may need a proximity sensor switch if you want a sensor that does not touch the object(s) being detected. If you are looking for a switch to start a motor, then you will need both a motor contactor with start and stop buttons and a thermal overload relay, which is a temperature switch.
Remember that to actuate a switch means either opening or closing a circuit, and does not necessarily mean you are turning equipment ON.
How many times do you need a circuit to change states, from open to closed or closed to open, per minute?
Look for a maximum switching frequency rating that matches the requirements of your application. For example, a sensor switch in a manufacturing facility may need to open and close the control circuit for a relay or some other monitoring device hundreds of times every minute as it detects all the passing objects on an assembly line.
Minimum Number of Switching Operations
Also called the electrical or contact service life, this specification refers to the minimum number of total ON/OFF operations the contacts can be expected to reliably perform.
This refers to how fast a circuit can change states. In electromechanical switches where contacts physical touch and separate in order to complete or break a circuit, arcing will eventually wear out the contact surface. Having fast switching speeds helps in suppress and reduce the damage from arcs.
Enclosure Protection Rating (NEMA or IP)
Enclosures provide circuit components, switches in this case, a much needed extra layer of protection against the surrounding environment (e.g. dust, moisture, chemicals, fumes, etc.). Depending on your application it may be perfectly fine to use a cheaper enclosure that leaves your switches and contacts slightly more exposed.
For applications in hazardous environments, you may require enclosures that can contain small explosions, can handle being submerged in water, or being exposed to high pressure water jets.
Different alloys and materials will conduct heat and electricity more or less easily and have varying degrees of resistance to oxidation and erosion of contact surfaces. For example, most electrical contacts are made using silver as one of the materials because, out of all the metals, silver is the best conductor of heat and electricity. Other materials commonly found in the alloys used to make contacts include copper, gold, tungsten, and graphite.