JohdotaThe same self-holding as with relays, but the holding is done in the program. The start and stop buttons are wired to the PLC inputs and the contactor coil to a relay output.
Stop button as an NC contact
The stop button is wired to its input as an NC contact. The input is on at rest, and the program uses an NO contact of that input. If a wire comes loose, the input loses its voltage and the machine stops, just as in relay control.
Program
The rung has I1 with Q1's own contact in parallel. After them come I2 and coil Q1. Because I2 is in series with the whole branch, stop has priority even if both buttons are pressed at the same time.
Relay output
A relay output is the NO contact of a relay with its own terminals 1 and 2. Connect +24 V to terminal 1, and run a wire from terminal 2 to A1 of the contactor coil. A2 is connected to 0 V.
Common mistakes
- The stop button is wired as an NO contact and the program uses an NC contact. The machine works, but a loose wire does not stop it.
- The stop button is wired as an NC contact, but the program uses an NC contact of I2. Then the machine does not start at all.