Ladder diagram?

I just started reading about ladder diagrams. Couple of things I don't understand about the Stop/Start/Starter overloads/Motor starter ladder rung.

Do all of the conditions have to be true along the rung of the ladder for the output to be set? Anded?

I understand why a stop is a normally closed switch for safety but why is it a normally open on ladder diagram?

Stop is a closed switch and the Start is a open switch but both are normally open on the ladder diagram.

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Because if you programmed it normally closed it would be open in the program. :)

Is this the circuit you have?

Start Stop

-------[ ]----|--------[ ]------------------(out) out |

-------[ ]----|-

Stop, being wired NC in the real world, is ON in the program. Start is OFF in the program because the switch is wired NO. The OUT contact is OFF. When you press the START button, START is anded with STOP... and OUT comes on. Notice the OUT contact or'ed with START. When you release START button the OUT contact keeps the circuit on until STOP is pressed (goes "off" in the program because now your NC switch is open)

I had this same thought when I started in PLC's after years of relay logic. Just look at it from the PLC input card side of things. With the NC switch, the STOP input point is normally "ON".

Good luck in your new maddening career.

Reply to
Steve Cothran

The STOP button in a motor-control switch box is normally closed, too. It should be wired in series with the motor relay coil.

Indeed! from me too.


Reply to
Jerry Avins

Huh shouldn't :: Start Stop

be Start Stop

or have I been missing things all these years?


Reply to

It would be if your stop switch was wired normally open out in the field, or if this was electro-mech instead of a plc.

Reply to
Steve Cothran

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