Learning about Relay Logic; What’s a Relay?

In my last post, I talked about the Jeopardy game I had built that used relay logic and gave you a copy of my wiring diagrams in case you were interested in learning about relay logic by building your own version of the game.  In this post, I thought I would talk a bit about what a relay is since I am asked that question on occasion.  That will also set the stage for a discussion of Boolean algebra, which is the algebra of 1s and 0s and is the basis of doing logic with relays (excitement, excitement, excitement).

Relay logic has been with us for a long time, as you may have garnered from a post I did a while back titled Control Technology; a Glimpse Backwards and Some Thoughts on the Future where I discussed a legacy control system I had run into.

Old panel 01

What you are looking at represented the state of the art in the 1970s when I first started working in this business and is a far cry from the current approach to the control system implementation.   The control panel in the picture above originally sat behind where the blue chair is in the picture below.  All of its functionality, and then some, was replaced by a PC that is not visible in the picture below due to the camera angle, other than the display and keypad, which you can see sitting there on the desk.


If you looked inside the old panel, you would have seen what appears below;  hundreds of discrete electric and pneumatic control elements that were wired piped together so that they performed the desired control functions, some of which were analog (meaning the output of the devices was continuously variable) and some of which were digital (meaning the output of the device was in one of two states, like on or off, or open or closed).

Old panel 11 Sharpen

The yellow plastic cubes sitting on a gray base towards the bottom of the picture are electro-mechanical relays.  If you are wondering what the rest of the stuff is, I talk about it more in the post I mentioned previously.  For this post, I want to focus on the relays.

I have always, for what ever reason, kind of liked relays.  I didn’t know it at the time, but as a kid, I was making them out of nails with wire wrapped around them for an armature that moved pieces of tin can that connected other nails with wires on them to control my toy trains.  (Its probably a miracle that I did not electrocute myself).  In any case, I sort of have this weird natural curiosity and interest in them, so fair warning for what is to follow.

The picture below is a close-up of what one of the little cube relays in the picture above typically looks like.  The one after it labels all of the parts.  Incidentally, a common term used to refer to these relays is “ice cube relay” because in size and shape, its similar to an ice cube.

Slide Show 12162011 113827 AM

Slide Show 12162011 113330 AM

As you can see, its really a collection of levers and wires and the  “electro-mechanical relay” term I used previously is appropriate.  And as you might suspect, it depends on both electrical and mechanical principles to operate.

Fundamentally, a relay is simply an electrical switch that is operated by another  electric circuit instead of a lever that someone moves manually.  Thus, they provide a way to automate the operation of equipment using circuits that are electrically isolated from each other.  We use them a lot in building systems for that reason.  The isolation feature is important because it allows a low power, low voltage control signal to operate a high voltage, high amperage piece of machinery.  this makes things safer and keeps costs because the low voltage wires can be smaller and used for the long runs to the high voltage/amperage equipment point of use.  That is their other big application in building systems with prime examples being the interface a DDC control system output and a motor starter or lighting contactor. 

And, the motor starter or lighting contactor themselves are also relays, allowing high amperage, high voltage loads like 1,000 ton chillers or a 277 volt lighting distribution panel to be controlled by a 120 vac control circuit that is in turn, controlled by a 24 vdc DDC control system output.  So, ultimately, a  little tiny transistor or op-amp ends up controlling a 4 kV chiller motor.

The relay in the picture is a control relay like you apply as the interface between a DDC control system and a starter (or in a Jeopardy game).  Its not intended to handle heavy currents like three phase motor or lighting loads,  But having said that, the operating principle is fairly straight-forward an very similar to that associated with a motor starter or a lighting contactor:

  1. The armature is connected to a moving contact and swivels on a pivot, just like the see-saw you used to play on when you were a kid.  The contact is called the “common” contact since it will be in the circuit no matter what the state of the relay is (On or Off).
  2. When the relay is “off” (the coil has no power applied, which is termed “de-energized), a spring on the left side of the armature pulls that side down, causing the armature to rotate counter-clockwise. which moves the contact it is attached to up until it hits the fixed contact above it, which is termed the “normally closed” contact.  This completes a circuit through the common contact to the normally closed contact.  So wires that were hooked to the blades associated with these two contacts could be connected to light a light or sound a buzzer or start a motor if the relay was off.
  3. When the relay is “on” (the coil has power applied, in other words, it’s energized) a magnetic field is created.  This attracts the right side of the armature towards the coil and the armature rotates clockwise on the pivot until the common contact hits the fixed contact located below it, which is termed the “normally open” contact.  This opens up the circuit from common to normally closed and completes the circuit from common to normally open.  And, as was the case for the normally closed contact, wires could be attached to the blades associated with these contacts to perform useful functions.

The blades sticking out of the relay are designed so that you can solder wires to them in many cases.  But in most applications, they will also plug into a base that allows wires to be terminated under screws.  You can see these bases in the picture at the beginning of the post;  they are the gray squares with all of the wires hooked to them that are under the yellow relay cubes. 

This arrangement allows a failed relay to be quickly replaced with out having to re-wire anything.  In addition to saving time, this arrangement ensures the persistence of the logic created by the relay circuit because you never lift a wire to change a relay.  You just unplug it and plug in a new one.

Note that this “plug in” feature is something generally found with low voltage control relays vs. motor starters and lighting contactors.  But, in a motor control center, the entire enclosure containing the motor starter is often designed to be removable and simply plugs into the buss bars inside the motor control center, so again, the concept is similar.

The relay in the pictures also has a couple of handy features that are options for most manufacturers.  One is the LED that lights up when the coil is energized.   This is a good news/bad news thing.  The good news is that it tells you there is power to the coil.  But, the bad news is that it doesn’t really tell you if the coil actually moved the armature.

That is the benefit of the orange colored plastic lever.  If you look closely, you will notice that it is connected to the armature.  It turns out that the connection is arranged in a way that causes a little window on the top of the relay to turn orange if the armature has moved and keep it clear if the armature has not moved, as illustrated in this picture.

Slide Show 12162011 123148 PM

The blue lever allows you to manually move the contacts with out energizing the coil, a handy feature for verifying your wiring and trouble shooting problems.  Its also handy in an emergency when the control system is down and you want to force the output that it would have controlled had it been up and running to do something.

The terms “normally closed” and “normally open” are used because this is the state the relay will be in when no power is applied to the coil;  thus, the circuit through the “normally closed” contact would allow current to flow through that path as shown below.

Slide Show 12162011 123204 PM

Meanwhile, no current could flow through the “normally open” contact since it is not touching the common contact.

As you can surmise, when the relay is energized, the contacts switch and the normally closed contact opens and the normally open contact closes and current goes the other way as illustrated by the red line in the figure below, when contrasted with the yellow line in the figure above.

Slide 12162011 52943 PM

If you think about that for a minute, you might (correctly) conclude that in a way, the relay can “think”.  If it has power it reaches one conclusion represented by the normally closed contact being open and the normally open contact being closed.  If its not powered, then it reaches a different conclusion represented by the contacts being in their “normal” state.  Its this simple decision making capability that is the basis of relay logic, another major application for relays in buildings, especially in legacy control systems.

Its also important to note that the coil can control more than one set of contacts.  For instance, in a three phase motor starter or a three phase lighting contactor, the coil controls a set of contacts for each phase.  And in the control logic world, as you will notice if you study the wiring diagrams in my previous post, one coil controls three or four different contacts, meaning it can influence three of four different streams of relay “thought”.

Each set of contacts that can serve an independent circuit is called a pole.  So, in the drawings in the previous post for the Jeopardy game, the relays are three pole and four pole relays.

And while most control relays have a contact that can have two states as shown in the pictures above and in the wiring diagrams for the Jeopardy game, it is also possible to have a contact that is only “normally open” or only “normally closed”. Motor starters and lighting contactors are common examples, containing three normally open contacts (one for each phase) that close when the contactor coil is energized and open, but do not (typically) complete a different circuit, when the contactor coil is de-energized.

A set of contacts that completes one circuit when it is energized and a different circuit when it is de-energized, like in the pictures above and in the Jeopardy game is called a “double throw” contact.  A contact that completes or opens up only one circuit when it is energized and does not directly impact a second circuit when it is de-energized is called a “single throw” contact.

Relay engineers have combined these two terms and will say, for instance, that a relay is a four pole double throw relay, meaning it is a relay that can control four independent circuits and that it can connect each circuit in a different way depending on if the relay is energized or de-energized.  The relays in the Jeopardy game wiring diagram are three pole double throw and four pole double throw relays. In contrast contacts like those in motor starters and lighting contactors are usually what is termed “single throw”.

And, being engineers, relay engineers have created a set of acronyms to describe all of this.  A Double Pole Double Throw relay is abbreviated as DPDT.  A 4 Pole Single Throw relay would be called a 4PST relay.

If you really are interested in this (and who wouldn’t be), then you may want to poke around and see if you can find a copy of the Engineer’s Relay Handbook.  I, as you may have guessed (perhaps with some alarm) happen to have a copy and I can tell you that it contains everything you want to know and more about relays.  

In the course of developing this post, I looked around to see how easy it was to find a current copy and it seems to be not that easy.  My guess is its not published anymore so you will probably only find one at a used book store or a garage sale.

The good news is that the most relevant chapters seem to have been posted on line as HTML files on the Easterline Power Systems web site.  This includes the chapters on terminology and on principles of operation.  The files can be downloaded to your hard drive for off-line reference.

So there you have it;  probably more than you care to know about relays.  The next post will look at how we can use them to “think” just like I did in the Jeopardy game.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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