How to Convert a Basic Wiring Diagram to a PLC Program

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Welcome back and thank you for
checking out this latest video brought to you by RealPars. Today you will learn how to
convert a basic wiring diagram to a ladder logic PLC program. This can be a very
handy skill to learn, especially if you are converting
a machine to PLC control. Upgrading a machine to PLC control
may seem like a daunting task. However, if you take your
time and learn the basics, it can be an easily
achievable task. Today I will walk you through
programming a very basic “stop-start” circuit
for an electric motor.

This should help you get a
handle on some of the basics. before we get started on today's video if you love our videos, be sure to click the
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notifications of new RealPars videos. This way you never
miss another one! First, let’s look at our basic
start and stop diagram. I like to start on the
left and follow the line. Each line represents a
wire that is connected. L1 is our 120v hot wire. It will run straight to the normally
closed Stop button labeled PB1. From the other terminal on PB1, it runs to a normally open PB2 which is our Start button.

From the other terminal on PB2, a wire connects to the A1 terminal
on the Motor Starter Coil. The A2 terminal runs to the normally
closed overload auxiliary contact. The next wire runs from the other
overload terminal to neutral. The next line down shows how a
motor starter stays energized. One wire needs to be connected
to the input terminal of the start push button. It will run to one terminal of the auxiliary contact
set on the motor starter. This ensures power is on
the auxiliary contact. The second terminal of
the auxiliary contact is connected to the output
side of the start push button. When the start button is pushed, the circuit is complete and the
motor starter will energize. This closes the auxiliary contact and allows the operator to
release the start button. The motor starter will
now stay energized until the stop button is
pressed and the circuit opens. Now that you are familiar
with the wiring diagram, let's program it in
ladder logic now. When you are finished, your program will look very
similar to our wiring diagram. Programming software is all
different but similar.

Most have a toolbox somewhere that you
will drag your contacts and coils from. Just like on the diagram, we
start with the stop pushbutton. It will be represented
with an examine OFF bit. These look like a
normally closed contact. Drag it to the first ladder rung and the program will place
it to the far left. This will be a stop push button. Next, drag an examine ON bit and drop it to the right
of the first bit. This looks like
normally open contact and will be the
start push button. From there you need to drag an output
coil to the right of the other bits. It looks like a set
of parenthesis. This is the motor starter coil. Most PLC programs will not allow an
input to the right of the output bit. So the overload contact
from the print can be left as a physical
protection device. The last thing to add is the auxiliary holding contact. To do this, we need to add a branch
around the start button examine ON bit.

Then, drag another examine
ON bit to that branch. This will be the auxiliary
holding contact. Your program should now look
like the wiring diagram. Let’s keep in mind that we will
still need to address each bit that we have added
to the program to make the program
work physically. This simply means that we
have to match the program to the physical location on the input
and output locations of the PLC where the push buttons and
motor starter were wired. This will be covered in
another video very soon. Let’s look back at
what we have learned. To program a simple motor
start-stop circuit, we need to read the electrical
diagram from left to right and use our toolbox on the
programming software.

You will be using the most
common bits in the examine ON, which will represent a
normally open contact, examine OFF, which will represent
a normally closed contact, and output coil or
output energize. Again, the language is sometimes
different between manufacturers, but the symbols pretty
well look the same. You will also need to use
a branch around one bit. This is basically a parallel
circuit called OR logic. The start push button OR the auxiliary contact
will energize the circuit. I hope this video has been helpful in
the transition from wiring diagrams to plc programming. This was a very basic program
that can run a motor. As always, be sure to check
back for a follow-up video and more RealPars videos. Want to learn PLC programming
in an easy to understand format and take your career
to the next level? Head on over to realpars.com

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