Double Coil Syndrome In Plc Ladder logic and how to cure it

Double coil syndrome though sound like a human disease but it not . This disease is found in PLC programming.

Let us investigate what is this syndrome and how to eradicate the cause of it.

Before that let us discuss a logic in ladder Can we use same output coil for two more

than once in our ladder program? Yeah you herder right we can use the same output coil for more than once in our ladder program lets see how

Let us take an example. Suppose we have taken two toggles switches no- x01 and x02. If either x01 turns on or x02 turns on, the output coil named Q06 turn on which will turn on the motor. 

Let’s create a ladder program on this logic

x01             Q06

--| |-----------( )---

x02              Q06

--| |-----------( )---

So, If either toggle switch x01 or x02 turn on, then output Q06 should turn on which should start the motor. But after compiling/running the program you will see that it won't generate output as per our demand .Because of Double Coil Syndrome. Here as Output is taken twice hence it is called Double. If we will take it thrice then it will be called Triple Coil Syndrome.

Now let's analyze the logic

 by running the program we will see that by turning on the toggle switch x01  motor won't run. But when we turn on the toggle switch x02 then motor will run. as motor is running with toggle switch x02 that means motor is good .Now further checking revels that PLC card is also good and in working condition. So we have to get back to our program check it from top to bottom

x01                 Q06

--| a |-----------(c )---

x02                 Q06

--| b |-----------( d )---

Now take two cases

Case 1

lets take all switches that is a&b are turn off so output c&d will also be in off condition.

1- when we turn on the switch x01 so a is now on.

2- Since a is ON ,c(Q06) Will turn on.

3- as b is physically off ,so d(Q06) will also be off.

so when PLC will scroll down it will take the last command as its final command so output Q06 will stay off though we have turned on the x01.

Case 2

Let’s take all switches that is a&b are turn off so output c&d will also be in off condition.

now

1--a is off so c (Q06) is off.

2--b is on so d (Q06) is will turn on.

We can see here PLC will take the last command as its final command so output Q06 will stay on irrespective of the toggle switch x01.

This is what we call Double Coil Syndrome

Now let's cure the Disease

Just add an OR ladder to the logic.

As we want the motor to be on when x01 OR x02 turns on. Here's the situation.

|       x01             Q06

|------| |----|-------( )---|

|                |

|       x02    |

|------| |----|

|

Now, If switch x01 OR x02 turn on then Q06 will turn on .Double Coil Syndrome has been cured.