Let’s dive deep into the switches vs. relays comparison to see how these two devices operate and differ.
Switching is critical in electronic circuits, and switches and relays are the primary components that provide this function.
Both are electromechanical devices, but switches require manual or mechanical operation to open or close the circuit.
On the other hand, relays require optical or electromechanical signals for switching. Read on to learn more about the comparison of switches vs. relays.
Table of Contents
- What Is a Switch?
- What Is a Relay?
- Switches vs. Relays: Poles and Throws
- Switches vs. Relays: Switching Speed
- Switches vs. Relays: Control
- Wrap Up
What Is a Switch?
A switch is an electrical device that links, breaks, or diverts electrical current from one electrical line to another. The defining factor of this component is it requires manual (human operation) or a moving object to control circuits.
For instance, you might have to press the switch manually to operate it or use a door/lid to activate/deactivate the component.
Some even have sensors to detect flow, temperature, pressure, etc., to activate or deactivate the switch.
Another defining factor is switches make direct contact with the wires in the transmission line.
Types of Switches
Switches generally fall into two categories: hand and process.
Hand Switches
Hand switches require a human touch, and they include the following.
- Toggle switches: actuated by an angled lever that is in one or two positions
- Pushbutton switch: actuated by a button that alternately latches on and off
- Selector switch: contain lever or rotary knob actuation
- Joystick switches: feature lever actuation that moves in more than one axis
- Limit switches: resemble toggle switches, but the levers have tiny roller bearings to minimize wear
- Proximity switches: feature permanent magnet levers in sealed enclosures that switch when close to metal objects (metal detector). Some can have photocell actuators (optical switches).
A car battery disconnect switch
Process Switches
These switches are ideal for industrial control, where they monitor physical quantities to sound alarms or give warnings if some parameters exceed preset values. They include:
- Speed switches: these devices sense rotary speeds using non-contact detection or centrifugal weight mechanisms
- Pressure switches: actuated by liquid or gas pressure applied to a diaphragm or piston
Pressure switches linked to a pipeline
- Liquid level switches: uses a floating object to switch when the fluid level in the tank rises past a certain point
- Temperature switches/thermostats: contain bimetallic strips that expand at different rates when heated. This expansion variation causes bending, which actuates the switch.
- Liquid flow switches: these devices detect fluid flow in pipes using tiny vanes or paddles to actuate the circuit connection if the rate exceeds a preset value.
What Is a Relay?
Relays are more complex than switches because they rely on electrical signals to actuate the circuit.
These signals power electromagnets that connect or disconnect other circuits.
Usually, the control voltage in the control circuit runs at lower levels than the voltage in the load contact points.
So these switches provide safety because they eliminate direct contact with the high-load current lines.
A simple relay load and control switch circuit diagram
For instance, transferring the high voltage and current to the dashboard switch when turning your vehicle’s headlights on is risky.
So cars have relays that allow you to control a low voltage circuit from the knob switch, which activates/deactivates a relay to turn the headlights on or off.
Types of Relays
Relays fall into two categories.
Electromechanical Relays
The traditional electromechanical relays operate on the principle of electromagnetism to open and close the circuit.
A simple electromechanical relay has one input and two output contact points: NC (normally closed) and NO (normally open).
An electromagnetic relay
The electromagnet consists of a copper relay coil and an iron core. When the relay is off (de-energized), a spring keeps the input connected to the normally closed position.
Applying a DC electric current to the electromagnet attracts the armature, pulling the movable contact to connect the input to the normally open output terminal.
This connection closes the circuit, allowing the load current to reach the device.
A car electromagnetic relay switch
The electrical relays that fall into this category include:
- Reed relay: feature relay coils wound around two hermetically sealed reed switches in a glass tube filled with an inert gas
- Latching relays: also known as impulse, stay, or keep relays, these devices maintain their state after actuation and are ideal for low power consumption applications
- Buchholz relays: actuated or gas relays for detecting minor internal faults, usually in power transformers
- Differential relay: actuate if the phase difference between two or more electrical quantities exceeds predetermined values.
- Polarized relay: this DC electromagnetic relay relies on coil current polarity changes to alter the magnetic forces to attract or repel the armature instead of a spring.
Solid State Relays (SSR)
Solid-state relays don’t have moving parts. So instead of using mechanical armature movement, these relays feature solid-state switching devices like thyristors, MOSFETs, bipolar junction transistors, TRIACs, and IGBTs to handle the process.
Solid-state relay modules
These components only need control signals for activation to switch the electrical load circuit.
And they usually have optocouplers to isolate the control and controlled circuits, plus sensors to respond to the control signal to switch power to the load.
The lack of moving parts makes this relay more durable than the electromagnetic type because there is no hot electrical arc across relay contacts caused by high inrush currents.
Although you can reduce contact wear using snubber circuits in electromechanical relays, SSRs are still more reliable.
Also, they have faster switching speeds and are less noisy. And unlike their electromechanical counterparts, some solid-state relays can operate using AC and DC.
They generally fall into these categories.
Solid state relays
- Transformer-coupled SSR: contains a DC-to-AC converter that provides low alternating current to the primary transformer section for amplification to run the TRIAC and triggering circuit. The isolation degree between the input and output depends on the transformer design.
- Photo-coupled SSR: features a photosensitive semiconductor that switches based on input signals from a light-emitting diode, which receives the control signal. If the LED turns on, the semiconductor becomes conductive. This operating mechanism gives higher isolation than the transformer type.
Switches vs. Relays: Poles and Throws
Both devices operate on the mechanism of poles and throws. And they fall into these four categories.
Single Pole Single Throw (SPST)
This switch or relay has two contacts and can only control one circuit because it has a single input and output.
The device is ideal for applications requiring on and off states, like lighting.
A single pole single throw switch/relay diagram
Single Pole Double Throw (SPDT)
Also known as changeover switches/relays, these devices have a single input and two outputs, meaning they can control two circuits.
And they can have more than two throws to suit the application requirements, but the throw (input) is only one.
A single pole double throw connection diagram
Double Pole Single Throw (DPST)
This device has two poles (inputs), each with a single output. So it has four terminals and operates like two identical SPST switches.
Turning the switch or relay on/off regulates two discrete circuits.
For instance, you can connect two biased wires to the load via this device, then leave a direct connection for the unbiased cable.
A double pole single throw connection diagram
Double Pole Double Throw (DPDT)
A DPDT device has two input terminals and two throws for each pole.
So it has six terminals, and each input can control two circuits, meaning you can regulate up to four.
This switch or relay functions like two separate SPDT devices because it can handle two loads concurrently.
A double pole double throw connection diagram
DPDT switches/relays are ideal for motor direction control, where they regulate the polarity or phase along the electrical lines by throwing the signal to either output terminal.
Switches vs. Relays: Switching Speed
Relays operate faster than switches because they don’t require a person or physical object to handle the switching.
And within the relay category, solid-state relays operate faster than their electromechanical counterparts because they don’t have moving parts.
A four-channel solid-state relay
Switches vs. Relays: Control
Switches have direct contact with the load circuit terminals or contacts. So they are not ideal for controlling high-voltage or high-current circuits.
But relays are like remote control switches because their control terminals don’t contact the electrical load directly.
So you can use them to regulate circuits or devices with high current ratings like motor starters and headlights.
However, relays have the maximum current they can handle. So you cannot connect any relay to a high-power circuit. Check their current rating first.
Wrap Up
Switches and relays are critical for automotive electrical circuits because cars have high and low current/voltage circuits.
And it is essential to understand their types and operating mechanisms to avoid making mistakes when installing them in your car.
We hope this has been an insightful article. Don’t forget to share your thoughts in the comments below.
