What is an electromechanical relay? A relay is a switch that is electrically operated rather than manually operated. It allows a low power circuit to control a higher power circuit – a relay amplifies signals. The first relays were purely mechanical but more recent ones use electromagnetism and are called electromechanical relays. These paved the way for electronics and computing as we know it today.
A simple electromechanical relay contains:
- An electromagnet made of a coil of wire wrapped around an iron core
- A movable iron armature that becomes magnetized when the electromagnet is powered
- Spring loaded electrical contacts that are opened or closed when the armature moves
Relays as Electrically Controlled Switches
When voltage is applied across the electromagnet coil, it generates a magnetic field that magnetizes the armature. This pulls the armature toward the electromagnet, against the force of the springs. As it moves it causes the electrical contacts to touch, closing the output circuit. When power is removed the springs snap the armature and contacts back to the open position.
So in summary, relays allow a low power controlling circuit to switch a higher power controlled circuit on or off – like a remote control switch! Next we‘ll look at the history of how this deceptively simple device transformed technology…
From Lab Curiosity to Global Telegraph Network
Henry & Sturgeon Lay the Foundations
The electromechanical relay story starts with two 19th century scientists – Joseph Henry in America and William Sturgeon in Britain…
We‘ll continue tracing the timeline of key milestones in the development of relays and their applications next. But first, let‘s take a closer look at what exactly goes on inside one of these devices…
Inside the Electromechanical Relay
The telephone relay became one of the most widely used configurations of the electromechanical relay. Let‘s examine its internal structure and operation using this annotated cross-section diagram:
- Electromagnet coil
- Iron core that magnetizes when coil energized
- Movable iron armature that becomes magnetized
- Insulating pin on armature holds contacts
- Electrical contact springs normally open
- Contact springs close when armature pulled
Step by Step Principles
Now let‘s walk through the sequence of events that occurs when the relay is activated:
- No power is applied – contact springs hold circuit open
- Power applied to electromagnetic coil
- Iron core magnetized, magnetic field attracts armature
- Armature rotates on pivot, insulating pin pushes contacts closed
- Contacts connect two separate circuits together
- Power cut from coil, magnetic field collapses
- Spring tension pulls back armature, breaking contact
So in summary, the relay converts an electrical control signal into mechanical motion that opens or closes a switch in a separate circuit…
Drivers for Improvement – Scaling Up Switching Capacity
As relays were increasingly used in telephone exchanges, the demand grew for more reliable, faster, and higher capacity switching systems. Engineers incrementally improved relay designs over decades to keep pace.
Telephone Relays in Numbers
| Year | Relays Produced | Failure Rate | Speed |
|---|---|---|---|
| 1925 | 1 million | 1 per 33,000 ops | 20 per min |
| 1955 | 6 million | 1 per 1.6 million ops | 60 per min |
Source: AT&T Bell Labs
Fully electronic systems started replacing electromechanical relays from the 1960‘s onwards. But relays remained a core technology enabling global communications for over a century!
Now let‘s look at some other pioneering applications of relays in scientific and computing devices in the early 20th century…
Revolutionizing Computing
Flexibility Unlocks Innovation
Unlike mechanical calculators, relays offered flexibility – by simply rearranging wires, electrical pathways could change a circuit‘s logic and behavior. This caught the attention of an innovative group of engineers who saw the possibility for programmable, automated computation.
Several breakthrough devices built on relay networks include:
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Zuse Z3 Computer – Used 2600 relays to become the world‘s first fully programmable, Turing complete calculator in 1941. Built by German engineer Konrad Zuse.
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Harvard Mark I – IBM electromechanical computer built in 1944 with dials, switches, gears and about 3 million parts. Programmed using punched tape and wire connections.
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Bell Labs Model I – George Stibitz demonstrated circuitry in 1937 for complex number calculations. This led to Model I in 1940, using ~500 relays performing arithmetic guided by telephone relays.
Legacy of Flexibility
While electronic circuits ultimately outperformed them, the flexibility of relays was key in demonstrating the possibilities of complex programmable computing machines. Modern computers owe a debt to the electromechanical relay!
Present & Future – Niche Applications Remain
So while relays have been superseded by solid state electronics in most mainstream applications, they remain preferable in some niche cases:
Benefits
- Extreme ruggedness and resistance to harsh environments like heat, radiation, shock
- Ability to directly control high power loads like motors or heaters safely
- No semiconductor leakage unlike transistors – can stay idle indefinitely
This makes them well suited for critical infrastructure roles where resilience is vital, like:
- Railway trackside signaling equipment
- Grid power distribution automation
- Industrial motor or process control
- Spacecraft systems
So while electromechanical relays are a 19th century technology, their flexibility spawned innovations that led to computers and automation as we know it. Next time you tap on your smartphone, spare a thought for the unassuming relay!
