Relays and contactors have two types of contacts each suited for different applications.
Normally open contact in plc.
In other words when voltage is applied to the relay contactor terminals this contact closes.
A relay contact in the ladder diagram can be normally open or normally closed and there is no limit to the number of contacts a relay can have.
Normally open no contacts allow current when the relay or contactor is energized.
Many beginners will get caught trying to make the ladder logic match the input types.
Normally open and normally closed electrical contacts make up electrical switches relays circuit breakers and most any other electrical component that switches something on off or can be switched on off.
Normally open asks if a bit is 1.
How could a circuit like the one in the example with just 2 inputs fail.
Notice that both of the input push buttons are normally open but the ladder logic inside the plc has one normally open contact and one normally closed contact.
If a contact is tied to an input then when the power at the input changes it s contact value in the ladder logic will change.
The normally closed nc contact is in plc programming referred to as an examine if open instruction xio.
Difference between normally closed and normally open inputs.
In order for the plc program to work properly with this new switch wiring the virtual contact for in switch stop must be changed from a normally closed nc to a normally open no.
Normally open is a contact that does not flow.
Normally closed nc contacts allow current through when the relay or contactor is not energized.
The easiest way to explain them is in the example of a relay.
But for stop functions it can be bad.
Normally open contacts would represent relay contacts that would perform a connection upon receipt of a signal.
Plc training electrical contacts normally open and normally closed contacts.
Contacts and coils are often associated with the physical inputs outputs on the plc.
Just like the input actuators you have both normally open and normally closed contacts available in plc programming.
As before the in switch stop virtual contact is in the closed state when no one presses the stop switch enabling the motor to start any time the.
The ladder logic symbols look almost like the electrical symbols for normally open and normally closed contacts and the function of these are the.
Concepts normally open and normally closed contact ought to be clarified and explained in detail in the example of a plc controller input and output.
Using normally open contacts as plc inputs is good.
Often they are referred to as examine if closed xic and examine if open xio.
To illustrate that with the normally open contact you can think of the two sides of the contact as an input and an output.
Examine if closed is an instruction that examines a bit for the off condition.
A plc output is really just an internal relay with a physical connection that can supply electrical power to control an external load.
Do not think that the ladder logic in the plc needs to match the inputs or outputs.
