Ladder diagram
Ladder diagram
Ladder diagrams are ladder-shaped electrical diagrams often for logic system controls and for circuits like control circuits.
Embedded Files
[4] “L1” and “L2” refer to 2 poles of a 120 VAC supply unless otherwise noted.
L1 is “hot” conductor and L2 is grounded/neutral conductor. These are unrelated to inductors.
The true transformer supplying power to this circuit is omitted for simplicity.
omit:
In truth, the circuit looks like this:
Workplace requirements of drawings
Workplace requirements of drawings
A must match a lab's one--control power at 120 V from power circuits.
For a given circuit, voltage must match a coil and all control devices are rated at least used voltage.
We often need 2 drawings: Power and control--what's often done in industry, an approach facilitating trouble shooting.
Rules
Rules
[3]
Highest voltages are often at the top of a drawing.
Line and neutral
Line and neutral
A ladder diagram's line is its left vertical rail of the positive power supply (L2).
Its neutral is its right vertical rail of the negative power supply (N or L2)--The "hot" and "ground" wires in a normal electrical circuit; both rails together are called "power rails."
The first rung tells control power source, in this case a transformer--could be L1 and N.
Rungs 2, 3 and so on have the control logic.
The rungs’ order often tell the event's sequence.
Reference numbers
Reference numbers
Rungs are horizontal lines of ladder diagrams.
[1] E.g. This diagram has 4 rungs (horizontal lines.
[1]
Wiring to ladder diagram conversion
Wiring to ladder diagram conversion
A ladder diagram's line (hot wire) is often on the left side and its neutral is on its right.
Connection are at the nodes.
If there's no node, wires are crossing
See also: Lab 9 - Interpreting Wiring Diagrams
[3] Here, C has both Light (L) and Neutral (N), so it touches both L and N on the ladder diagram
Since C blocks L for everyone else.
A and B have only N and IW--
D has N on both side--Has 2 wires connecting to N.
Drawing conventions
Drawing conventions
These 2 wires connect where they cross.
These 2 wires don’t connect where they cross.
The meter reads “OL”, meaning infinite resistance in ohms.
These 2 wires don’t connect where they cross--an obsolete notation, as it’s hard to draw and follow with many wires.
Symbols
Symbols
[5], [8] Basic ladder diagram symbols used are often (in college):
Coil labelling
Coil labelling
[2] North American standard symbol of a coil is a circle.
“M” is for general circuits applied anywhere.
“F” = coil controls a fan
A number means multiple fans are used.
Control relays are small relays for interlock and other control functions.
“CR” = control relay
“P” = a coil controlling a pump
Note: Label to avoid confusion.
Contactor coil
A contactor coil/contactor coil is the electromagnetic core in a contactor, an electrically controlled switch, creating a magnetic field to close/open the contactor's contacts--it's the actuator switching the circuit on/off.
In ladder diagrams, they're often drawn as M coils controlling a motor, often in industrial control circuits to energize motor starters/contactors, which then switches power to the motor.
Contact labelling
Contact labelling
Contacts are labelled according to the coil controlling them and may be drawn next to the coil/anywhere.
The label tells how they're controlled.
What controls a contact labelled “P12A”?
Numering
Numering
As control drawing become complex, do careful documentation for correct installation, which involves numbering points on drawing in a consistently.
In a usual ladder diagram to be familiar with in Algonquin College, this other stop button's side is a different part of the circuit.
Label it as '2'.
And the rest of the circuit.
Any part labelled “2” is equivalent. Here are 3 ways to wire a circuit that all work.
This is a terminal block of a push button in Algonquin College labs' motor control centers.
The number tells the slot as we build the drawings.
The terminals are inside the push buttons of this motor control center.
E.g. To draw the 1st circuit, we must also use the contactor panel to access the overload and coil.
[1]
A control circuit is a part of a circuit where a panel switches. Limit switches and sensors connect to control a larger machine, using timers, small relays, sensing equipment, overload, interlock contacts and switches. They're often low current input and control devices, often in control cubicles. Control switches can be remote, giving the switching and control of the power circuit.
Fig 1. is the basic power circuit for many applications.
The “M” contacts control the motor.
Other devices protect the motor from overloads.
Fig 1. Power for controls is often tapped off ahead of M contacts, at ~208, 240, 480 or 600 V.
to tap off: to draw power/make a connection to a main electrical line/source for purposes like powering an equipment piece or circuit
We often use a control transformer, often either 24 or 120 V, with other voltages.
Often a control circuit is grounded and has a fuse.
2 common styles of power drawings exist for 3 phase motors--can be extended/modified.
Control devices
Control devices
A thermostat is often represented as a normally open (NO) or normally closed (NC) contact responding to temperature changes.
Auxiliary contacts are additional switches, often for devices like contactors, relays, and circuit breakers, that mirror the state of the main contacts in a separate, lower-current circuit. They provide remote status information and can be used for control, indication, or interlocking.
In ladder diagrams, M contacts linked to an M coil or contactor (KM). These contacts are controlled by the M coil and are used to:
See also
See also
References
References
[3] 6 Rules for PLC Ladder Diagram Programming | Explained with Diagram - DipsLab
[5] Electrical ladder diagram symbols - (Algonquin College) Canva
[7] ELE8937 All Sections Electromagnetic Control: Lab essentials - (Algonquin College) Canva
[9] Decoding Switch and Push Button Symbols in Electrical Diagrams - Igoto
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Quizzes
Quizzes
[Q1] Flashcards
[Q2] Exercises
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