PLC Programming / Hardware issue

Discussion in 'General Electronics Chat' started by MemorX, Nov 1, 2010.

  1. MemorX

    Thread Starter New Member

    Nov 1, 2010

    I am having an issue with PLC programming / hardware.

    Here is the setup:
    I am supposed to program a PLC to control three outputs on an power supply (UPS). The outputs are controlled by three signals, where leaving them open keeps the outputs on.

    The software runs a check, a timer and a couple of other things to check that the system is ready to shut down when the main power is lost, and ultimately shuts all three outputs off.

    The issue is that the PLC itself is powered by the power supply, and when it shuts of the last output, the system resets, and starts all over again.

    This is kind of obvious, as the PLC opens the contact when it looses power (and so turning the outputs on again).

    I did not design the hardware of the system, and I do not have access to another power source.

    But my question is, is this even solvable without one?
    I have been thinking, and I cant think of any way this would work properly without having power to the control system (the PLC) all the time (the way the setup is now).

    The power supply itself is the only thing that has access to its internal battery all the time, so I am guessing that the only other way is by modifying it to work a little differently.

    Any ideas would be greatly appreciated.
    Last edited: Nov 1, 2010
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Is it the internal battery that is powering the circuit as it 'reboots'? or is it sensing something else is present and powering back up because of that?

    more info is needed here.
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    How do you purpose to start the power sequence if the PLC is not initially powered?

    Most systems I've worked with are continously on, or manually shut off.

    Programatically turning off it's own power may not be a safe approach, however, you could possibly leave the PLC powered, but remove power from the field devices.
  4. DonQ

    Active Member

    May 6, 2009
    Reverse the logic of the relay, and provide an override for initial power up. Once the PLC is powered up, it can supply power to keep the relay on. Then, when the PLC opens the relay, it powers down, and the relay stays open.

    The other option is to have the PLC only control power to the external devices. This is like an E-stop chain. All the E-stops are in series and the PLC also has a contact in the series. When any of the E-stops are open, or the PLC opens the chain, all the external devices go to a power-down state. At that point, who cares if the PLC keeps running. If you worry about the PLC repowering the chain (you should worry about this), put a latching relay in the chain, with a manual override, in much the same way as the first solution.