• SkillCat Team

Wiring Diagrams

Updated: Apr 22

Wiring Diagrams: Chapter 3


Schematic Diagrams


In this module, we will introduce you to the schematic diagram. We will cover: The diagram, The notes, The legend, and How to read a schematic diagram. Skip to quiz!


Overview


A schematic diagram is a blueprint of the electrical components within a system. Schematic diagrams include a diagram, notes, and a legend. See the picture to the right for an example of schematic diagram.


Schematic diagrams can be used to:

  • See components in a circuit, and

  • See how components are wired

A schematic diagram shows the wiring of each component in the circuit. Schematic diagrams are very useful when you are troubleshooting or wiring a circuit.


The best place to start when reading a schematic diagram is with the legend. The legend will explain what some symbols on the schematic diagram mean.


For example, look at the schematic diagram to the right. You can see that there are abbreviations on the diagram like CONT, CAP, and CH. Without reading the legend, you cannot understand the abbreviations and the components of the circuit.


After you read the legend, it is important to read the notes section of the diagram. The notes section can have important information about the circuit.


For example, the notes section may have a note about the crankcase heater (CH) on a schematic diagram. The notes may say something like “CH not used on all diagrams”. If you did not read the notes, you may have an incorrect understanding of the circuit. The CH may not be present.


The next step is to look at the diagram on the schematic. Each symbol on the diagram represents a component within the circuit. The lines running between each component represent a wire.


Voltages


There are two sides of every circuit:

  • Line side

  • Control side

Components on the line side of the circuit will receive a higher voltage. For example, 230V. Components on the control side of the circuit will receive a lower voltage. For example, 24V.

L1 and L2 power the line side of the circuit. L1 and L2 represent the two poles on an AC power supply. The picture to the right shows L1 and L2 on a schematic diagram. Note that the line voltage is labeled above L1 and L2.


The line side of the circuit will have bolder wiring connections than the control side. The line side will also have L1 and L2 nearby. The picture to the right shows the line side of the circuit. Note that the wiring lines are bolder than the bottom half of the diagram.


The control side of the circuit can be powered by an external power supply. The control voltage will usually be a lower value than the line voltage, for example, 24V.


The control side of the circuit will have thinner wiring lines than the line side of the circuit. Note that the wiring lines are thinner than the top part of the diagram.


How to Read a Schematic Diagram


Recall that the lines between components on a schematic diagram represent wires.

There are two main types of wiring on a circuit:

  • Field wiring

  • Factory wiring

Recall that factory wiring is done by the manufacturer. Field wiring is done by the technician. Factory wiring is represented by a solid line on the diagram. Field wiring is represented by a dashed line on the diagram.


Most schematic diagrams will also tell you the color of wires. On each line in the diagram, you will see a symbol. For example, BLK or BLU. BLK stands for black wiring and BLU stands for blue wiring. Some legends will define the color symbols.


Recall that a schematic diagram represents components with symbols. On a diagram, some symbols will have a black border around them. You can see an example in the picture to the right.


A black border around a group of symbols indicates the internals of a single component. For example, the image to the right shows a contactor on a schematic diagram. You can see that the contactor is labeled CONT, and it has a black box around several internal components.


The diagram shows the internal components so that you can determine the type of contactor. In the picture, you can see that this is a single pole contactor since there is only one contactor switch. There is one contactor switch between 11 and 12, a wire running beneath the switch, and a coil underneath the wire.


A schematic diagram can also be used to determine how components are wired to each other. For example, view the image to the right. You can see that this image shows a dual run capacitor, start capacitor, start relay, and a start thermistor.


Start by looking at the dual run capacitor on the left hand side of the image. You can see that a blue (BLU) wire runs from the HERM terminal on the compressor to the start thermistor. The wire is highlighted in blue in the image.


Look at the dual run capacitor again. You can also see that a yellow wire runs from the common terminal on the dual run capacitor to the start capacitor. The wire is highlighted in yellow in the image.


In this module, you learned how to read a schematic diagram. Always read the notes and legend before looking at the diagram. Schematic diagrams display the components and wiring within a circuit.



Ladder Diagrams


In this module, we will introduce you to the ladder diagram. We will cover: The diagram, The legend, and How to read a ladder diagram. Skip to quiz!


Overview


A ladder diagram is a blueprint of the electrical components within a system. Ladder diagrams will have a diagram and a legend. See the picture to the right for an example of a ladder diagram.


Ladder diagrams specialize in displaying the logic of a circuit. Recall that circuit logic explains what happens to the circuit when a component switches its position. For example, circuit logic would explain which components gain or lose power when a switch opens.


There are two parts to a ladder diagram:

  • The diagram, and

  • The legend

The legend is the best place to start when reading a ladder diagram. The legend will explain the meaning of certain symbols on the ladder diagram.


For example, look at the legend to the right. You can see that the legend explains abbreviations like CC, EFR, and HR. Without reading the legend, you cannot understand the abbreviations on the diagram.


Recall that each symbol on a diagram represents a component within the circuit. Ladder diagrams place each component in the circuit between two vertical power lines called L1 and L2. Each symbol is on a horizontal line between L1 and L2.


The horizontal lines are called rungs. Each rung represents one circuit within the electrical system. Recall that electrical systems have multiple circuits. Since systems have multiple circuits, the ladder diagram will have multiple rungs.


Lines between symbols represent the flow of current in the circuit. Lines between components do not represent the exact wiring you will find in the field.


Voltages


There are two sides to every circuit:

  • Line side

  • Control side

Components on the line side of the circuit will receive a higher voltage, for example, 230V. Components on the control side of the circuit will receive a lower voltage, for example, 24V.


Components that receive less voltage, like a 24V coil, will be placed on the control side of the diagram. Components that receive higher voltage, like a 240V compressor, will be placed on the line side of the diagram.

A ladder diagram will have all components on the line side of the circuit between two vertical lines labeled L1 and L2. L1 and L2 represent the two poles on an AC power supply. L1 and L2 provide power to the components on the line side of the circuit.


The control side of the circuit will not have lines for L1 and L2. The control side of the circuit will be below the line side on a diagram. A transformer usually marks the separation between the line and the control side. The picture to the right shows the control side of a ladder diagram.


It is important to note that the two sides of a circuit are not separate systems. The line side and control side refer to components that receive power from different sources.


A component can receive power from both the line and control voltage. This is common with contactors or switches. The 24V control voltage can energize the coil within a contactor. The switch within the contactor may reside on the line voltage at 240V.


For example, in the diagram to the right, the compressor contactor (CC) has components on the line and control side. You can see that there are contactor switches labeled CC on the line side, and coils labeled CC on the control side. Both symbols are a part of the same CC but receive different voltages.


How to Read a Ladder Diagram


Recall that there are three parts to the line side of the ladder diagram:

  • L1,

  • Rungs, and

  • L2

The current flows into the line side of the circuit through L1.


Once L1 is energized, the current flows across each rung of the ladder diagram. This assumes that the rung is a closed connection. If the rung has an open switch, the current will not flow across the rung.


Once the current crosses the rung, it arrives at L2. The current flows back to the top of L2, and then the process begins again. This process happens almost instantaneously. It is broken into steps here to make it easier to understand.


Recall that symbols on the same rung of the ladder diagram are part of the same circuit. For example, look at the ladder diagram in the image to the right. You can see that the compressor and compressor contactor are on the same rung of the line side of the diagram.

Since the components are on the same rung, it becomes easier to understand the circuit logic. If the contactor on the rung is open, then the compressor will not receive power. The open contactor only prevents current from flowing through this rung.


Recall that components in a circuit can be wired in series or parallel. Ladder diagrams make it easier to see if components are in parallel or series.


For example, you can see that the compressor contactor (CC) is wired in series with the compressor. Between L1, CC, and the compressor, there is only one path for current to follow. One path between components means that the components are wired in series.


The ladder diagram to the right also has components in parallel. You can see that the EFR and HR are in parallel with each other.


You can tell components are in parallel if there is more than one path for current to flow through a rung. In the diagram, the current could go from L1 through the EFR and then through the evaporator fan. The current could also go from L1 through the HR and then through the evaporator fan.


Since current can take two paths from L1 to the evaporator fan, the EFR and HR contactors are in parallel.


In this module, you learned how to read a ladder diagram. Always read the legend before looking at the diagram. Remember that a ladder diagram places the components between L1 and L2.



Question #1: What is included in a schematic diagram:

  1. A diagram

  2. Notes

  3. A legend

  4. All of the above

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Answer: All of the above

A schematic diagram will include a diagram, a legend, and notes on the circuit.


Question #2: A schematic diagram displays the wiring between components.

  1. True

  2. False

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Answer: True

A schematic diagram will show you the components in a circuit and how they are wired.


Question #3: What do L1 and L2 represent on a schematic diagram?

  1. Line voltage

  2. Control voltage

  3. Both

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Answer: Line voltage

L1 and L2 represent the line voltage side of the circuit.


Question #4: A black border around multiple symbols indicates that the symbols are a part of a larger component.

  1. True

  2. False

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Answer: True

A black border around multiple components on the diagram indicates that they are part of the same component.


Question #5: Horizontal lines between L1 and L2 on a ladder diagram are called:

  1. Rungs

  2. L1

  3. L2

  4. Symbols

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Answer: Rungs

The horizontal lines running between L1 and L2 are called rungs.


Question #6: Symbols between L1 and L2 are on which side of the circuit:

  1. Line side

  2. Control side

  3. Free side

  4. None of the above

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Answer: Line side

Components between L1 and L2 on a ladder diagram are on the line side of the circuit.


Question #7: In a ladder diagram, current flows from L1, across the rungs, and then through L2.

  1. True

  2. False

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Answer: True

True, in a ladder diagram current flows from L1 to the rungs and then from the rungs to L2. This process is instantaneous.


Question #8: Two components in parallel have:

  1. More than one path for current to flow

  2. One path for current to flow

  3. No paths for current to flow

  4. All of the above

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Answer: More than one path for current to flow

Components in parallel will have two paths for the current to flow into the next component in the circuit.


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