PLC OUTPUT WIRING

[MIKE31 0]

HI which method is preferable: 1- connect the devices (solenoid valves, contactor, lights, ........) direct to plc output using a fuse in series with it. 2- connect a relay directly to plc output and then activate the output devices by this relay . I have seen both methods mike

[Shiner 2]

There are some things we directly connect to plc outputs eg: indicating lights on enclosure doors, but as a general practice anything with a field or a decent current draw gets an interposing relay. The main reason is maintenance cost. One of these is that when a coil field breaks down during deenergizing it puts a spike on the output, which can lead to failure. Then you are faced with replacing the output card, or if it is an integrated unit, the whole PLC. Much cheaper and easier to replace the relay.

[drforsythe 15]

I agree with Shiner. I do the same thing in my designs. You need to look at the type of load you have and the amount of times it will turn on/off. Inductive loads which turn on and off several times per hour are good candidates for using a relay to act as a cheap buffer between the plc and load.

[BobB 58]

I only use transistor outputs and use external relays for field connections. The relays also have to be fitted with flywheel diodes or I will not use them - they protect the transistor output. I also wire indicating ligfhts directly to the transistor cards. I only use IDEC (IZUMI) as these have the best diode protection built in - have never had a failure. The IDEC lights also have BA9S LED bulbs - no need to open the board if you do ever need to replace a bulb. I dislike immensely the stupid European lights form Schneider, Sprechur + Schuh (Alan Bradley) and others. The darn things are not as reliable as the IDEC ones by a long shot. It is much easier to replace an external relay in the middle of the night than replacing a PLC or PLC card that has relays.

[MIKE31 0]

Thanks for the reply and information .mike