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Thread: Discrete/Dry contact in Allen Bradley PLC Programs

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    Senior Member maitrey27's Avatar
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    Discrete/Dry contact in Allen Bradley PLC Programs

    Hello Everyone,

    I am working on a project where I have to create discrete contact to let the PCS system know that "X" valve is open. I have this X valve at output pin O.1/ on Micro820 controller. The PCS system has simple LED which will illuminate when O.1 is active. I was thinking of using relay to achieve this but confused over NO/NC/C and A2/A1 connections on relay. Relay p/n: PLC-BSC-24DC/21

    Please share your thoughts guys. Any help would be highly appreciated.

    Thank you in advance.

    Best Regards,
    Maitrey

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    Administrator Shawn Tierney's Avatar
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    Good morning @maitrey27,

    Do you have a link to the relay's wiring diagram?

    Shawn
    Instructor at www.TheAutomationSchool.com
    Looking for affordable automation training? If you are, check out my courses at TheAutomationSchool.com!

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    Senior Member maitrey27's Avatar
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    Hello Shawn,

    Please see attached,
    relay wiring.PNG

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    Administrator Shawn Tierney's Avatar
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    Good morning @maitrey27,

    I can't be sure without the full diagram, but it looks to me like 11 to 12 is normally closed, and 11 to 14 is normally open. So energizing the coil from the PLC output would open 11-12, and close 11-14.

    Is that what you're reading?

    Shawn
    Instructor at www.TheAutomationSchool.com
    Looking for affordable automation training? If you are, check out my courses at TheAutomationSchool.com!

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    Senior Member maitrey27's Avatar
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    Hi Shawn,

    Hope all is well.

    Yes, you are right about the connections on relay part. Actually, I want to understand the fundamentals of using relay w/ PLC in panel. Please see below, this is exactly what I am trying to understand.

    1. Illuminate an LED when O-01 is active. (Assuming LED only requiers 24 VDC.)
    2. Perform remote start/stop operation through a relay. Energize relay to activate PLC input (let's say I-01) to start remote start/stop operation. My question is, I have seen systems installing relays in their panel to satisfy this operation. But, not sure how actually they do it? Do they another PLC connected to their end, etc.,?

    Thanks Shawn,

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    Administrator Shawn Tierney's Avatar
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    Good morning @maitrey27,

    I'm not sure I understand you correctly, but I'll give it a shot:

    If you need to turn on an LED, just wire it to your PLC output... most PLC transistor outputs can provide enough current to power the typical LED Pilot Light. This is how my trainers are setup at https://TheAutomationSchool.com. Checkout the sample lesson for my Compact Basics course for a closer look.

    If you need to start a high current load like a Motor, then of course you'll need an "interposing" relay / contactor/ motor starter between the PLC output and high current load.

    You can also wire a button in to a relay's coil, and then wire the relay's contacts to the PLC input, but typically Push Buttons are wired directly into PLC inputs since the PLC Input is a very low current load that most push button contacts can handle without issue.

    Hope this helps,

    Shawn Tierney
    Instructor at www.TheAutomationSchool.com
    Looking for affordable automation training? If you are, check out my courses at TheAutomationSchool.com!

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    Senior Member maitrey27's Avatar
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    Hi Shawn,

    Thanks for a quick response on this. I will check your videos and figure this out. Appreciate your time.

    Have a nice weekend,
    Maitrey

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    Senior Member maitrey27's Avatar
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    Shawn,

    I was able to simulate this for both Discrete output and input scenario. Funny part is that I was making connections on relay socket without an actual relay mounted on the top (No wonder why it didn't work), which was quite embarrassing! Thanks,

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    Administrator Shawn Tierney's Avatar
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    Good morning @maitrey27,

    You're very welcome and congrats on getting it working!

    Sincerely,

    Shawn Tierney,
    Instructor at www.TheAutomationSchool.com
    Looking for affordable automation training? If you are, check out my courses at TheAutomationSchool.com!

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