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Timed Coils not keeping time

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  • Timed Coils not keeping time

    We are using the PLC to receive an input from an HMI to fire a timed coil for a door opening/closing. We have it programmed so each timed coil is specific to an individual input and have the time we desire correctly programmed (in milliseconds) for each coil however, twice now we have seen a door revert to a near instantaneous firing of the output instead of firing for our programmed 10 seconds.

    I called tech support and went through their troubleshooting ideas and nothing came back on the hardware side. Even with the door completely disconnected the output fires only for a moment and then stops so it seems like a programming issue. We have 8 doors on this PLC programmed the exact same method, an individual input triggering a timed coil for 10 seconds, and none of the others seem to have an issue. The first time we were able to "fix" the problem by landing the door on a different output and programming the PLC to fire it off of that.

    Any ideas to how I should be programming this to fix this? Or does anyone have an example of how to do this more properly? Thank you.

  • #2
    Zip and attach the program indicating the problem output.



    • #3
      We had about the same problem on our last project with the timed coils (first time to try them). Some times they work right and some times they don't. We use the Pro 3000, had to re-program the whole thing and use standard timers. About 150 doors in a jail.


      • #4
        you may be trying to energize a coil straight from the plc output. if that coil is not secured for backfire, it will backfire on the plc output.

        you need to check a couple of things:
        1. the current draw of the coil is less than output max draw- protect the plc output against overload
        2. the coil is internally secured against backfire. when dc output powers down, the coil will develop a forward spike which could damage the output.

        when connecting a relay to the output, these are the first checks to make sure the plc output won't be fried.

        fact that you managed to get a coil running by relocating it to a different output seems to confirm that the plc outputs tend to fail with these specific coils.


        • #5
          Double check that you are not using that output anywhere else in the program. The last use of the output in the scan cycle wins so somewhere else in the program might be driving the bit low


          • #6
            We swapped the outputs the timers are affecting and everything seems to be working now.

            We went back and without anything wired to the output board tried the timer and they still are not operating as expected.

            To add to this the timers on all of the outputs are all variable by 1.5-3 seconds. Any input on why the timers are acting up? I have confirmed that the outputs are not being called anywhere else in the programming


            • #7
              cant help what we cant see