plc wiring

I have Melsex FX 32 MR mitsubishi PLC. I wired it PNP(+ input) This is my first PLC job. According to some liturature I read about reed switches they suggest adding a resistor to protect the reed switch.

Taken from reed manufacturer(COTO),"For inductive, capacitive or lamp loads, inrush current or reverse voltage can affect the life of a reed switch. For maximum lifetime, contact protection is advised."

So if the manufacturer of the reed switch wants a resistor to protect its contacts are there resistors built into the PLC to protect its contacts and if not can I replace the contacts on the PLC if they go bad or do I have to buy a new PLC. And if I do need to add a resistor to protect the PLC does it go between the reed sensor and x(+ input) or between the reed sensor and 24v. Right now I have is a 1/4A, 250V, fuse between all sensors and 24V.

Niles

While reed switches are not good for directly operating loads such as lamps & solenoids, they are just fine for use as inputs to a PLC (DC is best).

I have had extensive experience with the Mitsubishi PLC in question (I was an applications engineer for their product line) and you should not need any type of resistor or snubber in the input circuit. The inputs are very low current consumption and the current draw is what causes the contacts to weld.

Enjoy

Don't take this as gospel! but working as a maintenance engineer for the past 12 years or so i have delt with alot of mits plc's..from the very first fx to the A now the Q have come across all sorts of weired and strange ways people interprit how to wire plc's up but never have i come accross any one wireing resistors to an input!! I have had inputs and outputs burn out for all manors of reasons mainly on plc that are fitted to machines working in the food industry in a very harsh wet inviroment. Nine times out of ten could put down to water damage to the plc its self or external I/O Yes it is possiable to repair plc's if you have a little electronic knowledge but can be time comsumeing always pays to have a spare one to hand! But dont get me wrong installed in a good panel and I/O installed correctly fused outputs etc... you can usely shut the door once comissioned and forget about it so to speak!!!!!!


if you aint got it spec for 32mrhttp://194.130.244.129/sourceandselect/Resources/sourceandselect/0000002001/2150.PDF
hope that rabel makes some sence

Niles,

From my knowledge of the input circuitry on FX PLC's, and reed switches (and what causes them to fail), I agree completely with WireGuy1950's answer.

Nigel