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A Basic Guide to Understanding Electrical Schematics and Relay Logic

By Daniel Wade, Copyright 2014 - All rights Reserved
In this tutorial, I'll show you how to read a basic ladder diagram schematic and how to grasp the logic that goes behind it. This same type of logic is applied to both PLC and Motor control circuits as well.

Before we begin, you may want to start with the basics of how a relay works. Having said that, it's important to have an understanding of the layout of a basic circuit - what the symbols are and how they're represented. To keep it simple, we'll introduce you to only the 6 electrical schematic symbols as shown below.



Referring to the symbols above, you'll notice that we've colored some of the symbols. This will help in making your comprehension of the logic a little easier. Picture the red ones as a red light - STOP - no current can flow past it in the state as shown. Current will flow through the green ones - GO - as they are drawn. It's important for you to remember  that electrical schematics are always drawn in the de-energized state - in other words, as if no outside action has occurred. The six above symbols represent some of the most common controls that will be found in most drawings. So let's list and describe them.

1. Normally Open Pushbutton: As indicated by the red color, no current can flow past this device (control) unless the pushbutton is pressed. Once pressed, current can continue further down the circuit. So long as it is pressed, that is. Once released, it will go back to the open state and current will not flow.

2. Normally Closed Pushbutton: The green indicates that current usually flows through this device without having to press it. Pressing it will stop the flow- however, once we release it, current will again flow.

3. Pilot Light: We use these devices as indictors for the machine operator. For example, a red light may indicate a warning of some sort. A green light may signal that everything's ok.

4. Relay Coil: This same symbol may be used to represent the coil for any of the following:

        Control Relay
        Contactor
        Motor Starter
        Timer

The labeling inside is usually an indication of what type of control it represents. More on nomenclature later.

5. Normally Open Relay Contact: As the color suggest, no current flows until the relay has been energized. Once the relay is de-energized. no more current will flow. This same symbol applies to the previously given list above.

6. Normally Closed Relay Contact: When the relay is de-energized, current will flow and vise-versa. This same symbol applies to the previously given list above as well.

About the Symbol Nomenclature (Labeling): Usually we precede the label with a description of the device followed by a number representing its distinction from similarly labeled controls. For example, PB1 would represent a pushbutton. We distinguish which type, whether NO or NC, by the symbol itself. If we were to add another pushbutton, it's label might be PB2 followed by PB3 and so on.

Regarding control relays, we would use a similar method - CR1 as an abbreviation for Control Relay 1 for instance, and CR1.1 and CR1.2 as subsequent names for contacts belonging to CR1. This is just an example - you could name each of those contacts as simply CR1 but such naming conventions can prove problematic with large scale drawings; esp. ones that contain a lot of contacts.

In the image above (and the ones that follow), we've created a very basic circuit to demonstrate how relay logic works. We've drawn the above circuit exactly how it should appear - in the de-energized state. We've also colored CR1, CR1.1 and CR1.2 in dark green to symbolize that they are representative as one single control that has three components. That being, the relay coil and its two contacts - one normally open and one normally closed. Note: You won't see this colorization in standard drawings. It's only been done here to illustrate that they're all related.

The two vertical lines represent our voltage source - L1 is usually high and L2 is usually low. These vertical lines are intercepted by horizontal lines which we call rungs - hence the term, ladder logic. We number the rungs in numerical order from top to bottom. So the rung containing PB1 would be our first rung or rung 1.

Load devices (those that consume power) are placed on the right hand side of the drawing. Control devices are usually placed to the left of the load devices. These are also called pilot devices as they direct (or steer) the energy (voltage) to the loads. In this case, CR1, PL1 and PL2 are current consuming devices or loads. PB1, CR1.1 and CR1.2 are pilot or control components.

 

In reference to the illustration below, I've taken the liberty to highlight the high voltage potential in red and the low voltage potential (or neutral) in blue. We'll assume that the voltage difference between L1 and L2 is 120 VAC.

Notice, that if PB1 is not pressed, CR1 will remain un-energized because of the normally open PB1. Try to picture contacts as bridges - if they're open, it's like the drawbridge is up (blocking the voltage) and if they're closed, it's like the drawbridge is down (allowing voltage to pass).

You'll see that the same condition is true on the second rung. CR1.1 is also normally open - blocking the voltage and preventing PL1 from energizing.

However... notice on the third rung that CR1.2 is normally closed thus allowing the 120 volt potential to reach PL2, enabling the current to flow through PL2, energizing it. And in this case, our light is green (G).

It's worth noting that if you were to take a voltmeter and place one probe on red and one probe on blue, you would read 120 volts. What do you think you would read if you placed both probes on red or both probes on blue? In either scenario, is there a potential difference?

In our last illustration below, we're simulating the pushbutton (PB1) being pressed. In so doing, we're allowing voltage to pass through PB1 and reach our relay coil (CR1) allowing it to energize. Now look what happens to CR1.1 and CR1.2. Their conditions both change. CR1.1, which was open, has now closed - allowing voltage to pass on to PL1 - energizing it. CR1.2 is now open - de-energizing PL2. Notice how our voltage potentials in the circuit have changed as well. So long as we continue to press down on the pushbutton, this condition will remain as shown. Once we let go, it will revert to the above condition.

Well, that's a crash course on relay logic (ladder logic). I hope this has been helpful.

All illustrative drawings in this tutorial were created using EZ Schematics. A description of the program can be found here. It's absolutely free to try for 21 days.

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