I have a normal contact switch (like a regular light switch to turn on and off a light bulb), but this one is for a power supply. I want to replace this switch by putting the two wires to two I/O ports on my PLC. How would I make the two wires connect (make contact) with just a changing an internal ladder logic NC/NO?

Basically, I don't understand how an internal relay energize a normally close contact through a PLC logic to physicaly make contact to the wires to turn on/off. I have a PLC that does analog and digital in/out. Is it possible to make two I/O ports make physical contact? Maybe a stupid question, but am I missing something??? Thanks for your advice.

 

You need to research your PLC output type. Usually the digital IO is either a opto coupler transistor (NPN or PNP, i.e sink or source) or a relay. Usually they both have a common. PNP common will usually be IO positive voltage(usually 12-24V), and NPN common will be IO ground (0V). For relays common can be anything as long as you observe the rated voltage and current of the contacts and connectors. You should be able to find example circuits in the PLC manual.

Unless your power supply that you are turning on is very small I would interpose a relay between it and the PLC. If nothing else they are usually cheaper to replace if you blow it up. If you are switching the relay on and off many many times you might consider using a solid state relay. You should be able to find simple relay circuits on a web search.

One last note, many power supply inputs (and servo drives too...) have very high inrush currents due to the charging of all of those electrolytic filtering capacitors. Usually you have to upsize your relay substantially to meet inrush, but you can also devise a soft start circuit to limit the current for a brief number of milliseconds upon initial turn-on. You probably don't need this, but if you find your relay is welding closed you may want to upsize the relay or do a soft start.

KEJR

 

Hi,

i read your message it generate lot of questions. what type of plc are you used?

what type of logic you want?

i mean you want to a fail safe control. if you send me your piece of logic by image i explain you well.

Hamad

 

Use a relay to interface the PLC to the power supply. That will isolate the PLC from the PS, and take care of any voltage or ground incompatibilities.