How to detect a 24VAC Zone Valve has closed?

Discussion in 'The Projects Forum' started by SlashDev, Mar 21, 2015.

  1. SlashDev

    Thread Starter New Member

    Apr 28, 2013
    7
    0
    I am using a White & Rodgers 1311 zone valve for a snowmelt system (spec here):

    [​IMG]

    This device is typically controlled by a SPDT relay that connects input pins 4 & 5 to open the valve (thermostat shown above). Connecting pins 5 & 6 will close the valve. Pin 5 is internally connected to pin 2 (24VAC Line or "hot" side). It takes about 45 seconds to change from open to closed, or vice versa. The "holding contact" makes sure the motor completes the open/close cycle, even if the request is cancelled.

    When the open cycle is completed, this device connects pin 3 to 24VAC (Pin 2). (Pin 1 can be considered the neutral side of the 24VAC.) Because the pump should not run until the valve is fully open, the pump is usually connected to pins 2 and 3 (or to a relay attached to those pins). Pin 3 is disconnected immediately upon starting the close cycle, which would stop the pump. However, there is no such signal that indicates that the close cycle has completed. There is no Side B CLOSED POSITION.

    Why do I care when the valve is closed? It turns out that this valve is the only 24V part of the system so far. It will not be used very frequently, so I would like to power down this section after the valve has fully closed. I have a microcontroller for this and many other parts of the system, but there is also a manual control panel for testing, and (gasp) a fall-back if there are bugs in the firmware or hardware failures. From the manual control panel, one switch should be able to start circulating (24V power on, open valve, pump starts when open) and stop circulating (close valve, stop pump, 24V power off when completely closed).

    I can see how to latch the 24V power on with a relay -- that will allow the valve to have power while it's closing. I'm having trouble with knowing when it can be turned off.

    From the spec, it appears that pin 4 gets connected to the motor when the cycle is complete, in preparation for opening it again. The other side of the motor is connected to neutral (pin 1). I think there are 4 possible states:
    Code (Text):
    1. State        Pin 3   4     5     6
    2. ---------------------------------------
    3. OPENING      float  24V   24V  float
    4. OPENED        24V   24V   24V  float
    5. CLOSING      float  float 24V   24V
    6. CLOSED       float   0V   24V   24V
    It looks like using a 10K pull-up between pins 4 and 5 would make the 0V unambiguous. But that couldn't drive a relay. A FET would work, I think. Along with some diodes for half-wave rectification, smoothing caps and resistors, I think I could control a relay to break the latched power. Yes, the microcontroller could easily support 3 outputs, but I need idiot-proof manual control ("Throw the switch. NOT THAT SWITCH!" ;) ), and microcontroller-proof sequencing.

    Suggestions?

    Thanks,
    /dev
     
    Last edited: Mar 21, 2015
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Can you install a microswitch or optical interrupter external to the valve in order to detect that the valve is in position?
     
  3. SlashDev

    Thread Starter New Member

    Apr 28, 2013
    7
    0
    There is a manual wheel that can be turned, but it's not very exposed:
    [​IMG]
    There is some play in that wheel, so I think I'd be concerned about registration of a hole or notch being spot on. I think I'd rather not modify the valve, as that would guarantee it would be the first thing I have to replace. :D

    Thanks!
    /dev
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    can you not connect a bridge rectifier on pins 2, 4 then when the motor is closed, you would read 24v dc through the motor coil, and use this to fire an opto coupler or small relay to connect to your micro.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Interesting. I have never done this sort of modification before but I would try painting a contrasting spot on the wheel, either white or black, depending on how reflective that wheel is.

    Then I would use a reflective optical sensor to detect the spot:

    upload_2015-3-21_11-36-40.jpeg


    Some electronics assembly required.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    3M make a special reflective tape for this purpose.
    Max.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    I'm having difficulty interpreting the thermostat symbol.
    What is the purpose of the rheostat inside the thermostat? It seems to be placed in series with the motor when there is a call for heat?

    Edit:
    I agree with Dodgydave (post #4). The 24VAC between pins 4 and 2/5 is your indication that the valve is closed.
     
    Last edited: Mar 21, 2015
    Dodgydave likes this.
  8. SlashDev

    Thread Starter New Member

    Apr 28, 2013
    7
    0
    Well, of course I can!

    [​IMG]
    And thanks for asking! ;) Is this what you're suggesting?

    Setting R5 to 1e7 floats pin 4, and then setting R2 to 1 connects pin 4 to 24VAC. The opto is driven only when R5 is the (low?) motor resistance.

    However, I just tried to measure the motor resistance, and I'm getting nothin' (open-circuit), for all positions of the valve. Is that a silly thing to do to an AC motor? The rating on the motor is 0.4A, so the 10mA for the opto shouldn't actually make it move. But now I'm wondering if the stopped motor will even allow the 10mA through?


    Cheers,
    /dev
     
  9. SlashDev

    Thread Starter New Member

    Apr 28, 2013
    7
    0
    I do not pretend to understand those scribbles. o_O I'll be calling for heat with a SPDT relay: Common to 5, NC to 6, and NO to 4.

    Thanks!
    /dev
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    don't know why you just don't measure the voltage across pins 2,4 and see if you get 24 v ac ,then see if it drives an opto.
     
  11. SlashDev

    Thread Starter New Member

    Apr 28, 2013
    7
    0
    Haha, because I don't know what I'm doing. And it's not installed yet. And I just realized you mean an AC-input opto, duh.

    To summarize: "Why not this?"
    [​IMG]
    Why not, indeed?

    Thanks!
    /dev
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    yes that would work ,with a series resistor
     
Loading...