The very first programmable logic controllers were designed to replace relays.  PLCs have changed a lot since then, but as an artifact of time some of the terms used in PLC programming still relate to relays, namely a "coil" and "contact".  In typical computer programming these would be strange terms but they make perfect sense in an industrial automation sense seeing that the relay has (and still does) form a vital role in controls.  Following is an attempt to dissect a relay and understand how it works.  All though simple in nature I hope it will be appreciated how important it is to understand this most basic of components.

What is a Relay?

A relay can be thought of as a basic switch that relies on an electrical signal to move the contacts.  For example, instead of your hand flipping a switch to turn on a light you can have the relay perform the same action when powered by an electrical signal.  The figures below represent two ways to show a relay with coil and contact(s) in an electrical schematic.

In the real world a relay can take on many forms.  The image below is of a common general purpose relay. 

The Basic Physics: Electromagnetism

The basic function of a relay begins with the physics of electromagnetism.  The knowledge of magnets and electricity extends way back in time but it wasn't until the 1800's that any connection was made between the two.  In 1820 a Danish physicist named Hans Christian Ørsted was the first (quite accidentally) to notice that an electrical current produced a magnetic field (in this case it deflected a compass placed near his battery).  This discovery was extended and formulated by Ampere, Faraday and Maxwell.  Joseph Henry invented the first relay in 1835.

In the simplest of terms this means that any wire that has an electrical current passing through it will also have a magnetic field.  The higher the current flow the stronger the magnetic field.

The Coil 

One way to increase the magnetic field is to put more then one wire together (thus increasing the total current in that particular area).  In this case the magnetic fields add together to form a stronger magnet.   The neat trick in relays is that it doesn't matter when doubling up the wired if the current is in the same wire.  Thus the magnetic fields in a coil of wire will add up to form a more powerful magnet. 

Now you can see why the term "coil" is used in a relay.  It is the essential first part of a relay that when electricity is applied to the coil it makes a magnet which we can turn on or off.  It's the ultimate control we all desire.

Coil Concerns 

The coil is rated by voltage, either DC or AC, and the current it will draw.  It's important to pay attention to that as some coils being driven by a PLC may draw more current then the PLC can deliver and thus blow a fuse or cause damage.

Another area of concern is providing a measure of protection against the coils magnetic field causing electrical spikes in the circuit or wiring.  Because of the connection between magnets and electricity when the coil is de-energized it still has magnetism seeking to create electricity.  This residual magnetism will create electricity where ever it can and as fast as it can.  You can see this effect by wiring an incandescent light in parallel with the coil.  When you turn it off you'll see the light bulb become very bright for a split second.  The trick to protect light bulbs and other components from coils is to put a simple 1 Amp diode across the coil.

Now this may look weird but in normal circumstances the diode does not conduct and the relay works as expected.  When the power is removed it provides the shortest path (going the other way along the red arrow) for the energy to travel.  Thus the rest of the system is spared.  That's a pretty neat trick.

The Contact

The other important half of the relay is what is called the contact(s).  Sometimes they are referred to as "poles" and there can be one, two or three of these on one relay (any more then this is not typical).  It is made up of a flexible piece of conductive metal fixed on one end to a larger piece of metal (the armature) while the other end is allowed to float between to different electrical connections.  The armature is hinged and forced to one side by a spring.  When the coil is energized the magnetic force overcomes the spring tension and pulls the armature to the other side creating a different path for the current to flow.

I pulled the hood off of my vintage 1967 Potter and Brumfield relay to get a closer look at what we're talking about.

For the kids in the audience there's even a movie showing this relay in action.  Those clicking sounds are music to a control engineers ears.  The man behind the curtain is me turning the relay on and off. 

The Relay in Ladder Logic

The earliest and still most common type of PLC programming uses ladder logic which mimics the relay logic diagrams made so long ago.  The beauty in the world of software is that an internal relay can now have as many contacts as the memory can hold.  It's not limited to the physical size of the relay. 

The coil is typically represented by a () or circle while contacts are ][ or || symbols.  The left hand vertical line represents the positive power supply and on the right is the electrical ground or return.  Thus this represents an electrical schematic.  When the coil is energized then the correspondingly named coils close letting current flow from left to right.  A slash through the contact symbol represents a contact that is normally closed when the coil is de-energized.  So a normally closed contact will open when the coil is energized.

Other Applications

So if PLCs are such good relay replacers then why are still used so much?  Good question.  While PLCs replaced the logic ability of relays there are still a lot of applications where a relay is needed.

• High powered circuits will need relays as the PLC contacts would not be able to handle such high currents or voltages.  In this case a "contactor" (basically a heavy duty relay) would be needed.
• Motor starting has high inrush currents that are best handled by relays or contactors specially designed for that application.
• On the other hand, there are simple applications that don't need the power and flexibility of a PLC.  For instance, a dedicated circuit board that provides some alarm contacts.