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Triac weird behavior on Building Automation card

  • t
    I have my building automation card on a water well controller where one of the triacs controls a relay that controls the pump relay. Control power to the relay is 24VAC, I have had this happen twice now where the RPI has done something weird and the triac goes closed and stays there. The first time it ocurred, I turned off the RPI on accident. It happened again overnight last night, the power may have blinked causing the RPI to restart ( I am ordering the UPS for it now) . Hopefully it did not kill the pump. I have a second set of contacts on the pressure switch and am going to run the relay control power through there as a failsafe.

    Do you have any ideas ?

    thanks!
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  • alexburcea
    Hi,

    The Building automation card has no power backup, so if the power glitches, all the outputs return to the default state (off for triacs and 0V for 0-10V outputs). It is recommended that the software on the Raspberry to detect power losses and restore the outputs to the appropriate state.

    Alex.
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  • t
    I'm going to do more investigation on this today. I think it's doing this in the time when the raspberry pi is not in a operational state, once the RPI boots up and open PLC is up and running it goes back to normal. I am using open PLC.
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  • t
    I tried to get it to repeat the problem today and could not get it to repeat. Unplugged the power suppy that feeds the RPI and the Sequent boards, waited 30 seconds and plugged back in. System restarted just like it should. Tried pushing the power button on the RPI, tried holding it down to see if I could make it angry. I tried all the above while it was in the middle of a run cycle, it just shut down and restarted once the RPI booted back up and OPLC started back up. It is weird that this has happened twice now where it freaks out and makes all the triac outputs true. I am powered from an official RPI 5 power supply. I could not get any other power supply to work without getting a low power alert on the RPI. The first time this happened I remember opening the cabinet and seeing all the output LED indicators on. The last time it did it overnight, I am just glad it did not kill the pump or break something. The 100PSI gauge was wrapped all the way around to the back side of the needle at 0.
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  • t
    It finally repeated and figured it out.

    My circuit went from the control transformer 24vac to Triac1 first terminal, then from the second terminal to the relay, then to the other side of the transformer. I guess the traic was still partially on and flowing enough current to keep the control relay pulled in. There was about 8vac drop across the triac during this. I swapped the wires and it acted like it should. Then I went and looked at the schematic for the board which confirmed that I had it wired incorrectly. Well hope this will help someone else in the future!
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  • alexburcea
    Thanks for reporting this, and sorry I did not figure it out.
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  • t
    No worries! Its not like you have a truckload of other questions coming in all day! It was so weird when it would randomly do it, and only seemed to occur after a loss of power and restoration. I put the pressure switch contact in the circuit with the relay to eliminate the possibility of the over pressurization, along with adding an alarm to monitor for pump current when there was not supposed to be pump current. I had a different failure mode which led me to this, the system went into pump protection as we drew too hard on the well, and the pressure switch contact was closed but the triac did not clear. I found it very odd that it would only randomly do this, it did it twice in 6 months.
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  • t
    Well my Triacs are back to misbehaving again. The triac will close, when it is commanded to, then it will not stay closed. The relay that it is controlling will drop out, then a few seconds later the triac will close, relay closes....cycle continues. The indicator light for the triac stays lit, Open PLC shows the output is energized through the whole time frame.

    The relay is an Omron panel mounted 25A 240V double pole relay, 24VAC coil
    https://www.mouser.com/ProductDetail/Omron-Electronics/G7L-2A-TUB-CB-AC24?qs=GLD16PGLtAGcCk5h0498lQ==
    It states the coil current is 71mA @ 24VAC. IT had been running for nearly a year with no issues. I would think the 71 mA is enough to keep it closed. I'm going to add a dummy load on it tomorrow and see if it stabilizes with that.
    Control voltage is a bit high at 28VAC. That is 17% over stated operating voltage for the triac. Could the overvoltage be upsetting the triac?
    .
    The electric company has done something recently, I think they made the change Saturday morning/mid day, which this issue showed up Saturday mid day. Our line voltage is higher and closer to spec now. They either changed regulator settings, closed in a capacitor bank, swapped some load around....we were 115-120vac, now were 120-124vac. Control voltage was 26vac when i put it in about a year ago. Pump run current has dropped about 0.5amps which caught my attention, I need to add a voltage transducer to this so I can voltage compensate the pump current.

    You can connect the relay directly to the control voltage bus, and it will stay closed as long as it is powered. This just started doing this yesterday out of the blue. I have swapped triacs, and it will do it with the same behavior on another triac. This is a Building automation card, with a UPS, and a fan. Powered by 20V AC 65W power supply. The card is in a cabinet outside that is shaded so it will see full simmer ambient temps in Arkansas. Everything else is working as expected still. I ordered an 8 relay card so I can run the line voltage relay from it. I have been thinking it is a heat issue, or power supply issue. I have not been getting the low power supply warning on the RPI recently.

    For now I have the relay controlled from the pressure switch so I still have indication if I run out of water, just takes manual action to shut it down.

    Thanks!
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  • t
    My power supply connector on the board was not snapped in all the way for the 24VAC.
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