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IO-DI8-RO8, Relay outputs "hangs"


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I have a problem with the output on the IO-DI8-RO8 module. it seems like the output "hangs".

Let say that the plc software is working perfecly,

Connection is as follow:

PLC -> EX-A2X -> IO-DI8-RO8

inputs and outputs wierd correctly, all working.

Concerning the 1 output that hangs, the output control a contactor that runs on 24 + dc. contactor is used to make a small engine switch rotation.

it seems that the plc is giving the signal to the IO module ( can see the light turn on the led) but the contactor is not responsing on the signal.

either the output "hangs" or the contactor not responsing on the signal.

Can this problem be bc getting spikes on the output everytime the contactor change the rotation of the engine ?



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  • MVP 2014

It sounds like you need a flywheel diode on the contactor coil. This is a diode across the coil terminals, that is reverse-biased when the coil is powered, but allows current to flow when the PLC output relay turns off. Basic power diodes like IN4004 would usually be adequate for this.

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Thanks for the answear,

Have a question about the diode and the contactor i use.


TeSys® Model U – the

ultimate starter-controller

This is contactor i use, LU2B (reversing) this have A1,A2 and A3 (A1 is negative and A2 and A3 is positive).

So i need 2 diode for this ? and connect between A1 & A2 and A1 & A3 ?

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  • MVP 2014

Firstly, from the diagrams I downloaded, A2 is the negative, A1 and A3 are positive. I would double-check that first.

Otherwise, the principle is correct, you would need a diode from each of the positive control terminals to the negative terminal.

It seems odd that these don't have it built-in flywheel diodes, but the datasheet does not mention it at all.

Also make sure you have "break before make" with your relays. If you turn one output off and the other on in the same PLC scan, there is the chance that both relays will be on for a fraction of a second. I have seen this happen. My solution was to insert a small delay between turning off one relay and turning on the other relay (say 100ms).

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Yes A2 is negativ and A1 and A3 is positive, i was wrong about that because i did not have that contactor in front of me when i wrote it I just assumed.

Before the turning of the engine (signal from A3 to A1 the engine would have been stopped by sensor so it wil have a "break" before change of signal.

Engine is used to rotate a arm with a pipe, i have basicly 2 types of these.

1. big pipes and 3 pipes on one arm

2. small pipes and 24 - 32 pipes on one arm.

this problem only happends on the small pipes with 24-32 pipes on it. why is this so? Is this because this engine wil have more movement (contactor would be used more) ?

thanks for all the answear, i realy hope this wil help on my problem. i will certainly try it out .

If this works out i wil reply here

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Pretty much all of the manufacturers of motor contactors do not include diodes (and a majority of DC relays are also without diodes). They generally do this so that that polarity does not matter. If they include the diode and a user connects positive and negative backwards bad things happen and and there is a huge quantity of "waranty" claims. If they leave it out and polarity is reversed everything will work fine with the customer being none the wiser. They are expecting those people who know what they are doing to just add the 3 cent diode. And if they are smart enough to add the diode, then they are probably smart enough to get the diode band going in the right direction to the positive side as well.

It is a pain for those of us that would typically add diodes to everything, but thats just the way the world seems to be put together for now.

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