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Detecting End-of-cycle for 24VAC Zone valve

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SlashDev

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I am using a White & Rodgers 1311 zone valve for a snowmelt system (spec here):

White%2520Rodgers%2520-1311-104%2520schematic.jpg


This device is typically controlled by a SPDT relay that connects input pins 4 & 5 to open the valve (thermostat instead shown above -- not used in my application). 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:
State        Pin 3   4     5     6
---------------------------------------
OPENING      float  24V   24V  float
OPENED        24V   24V   24V  float
CLOSING      float  float 24V   24V
CLOSED       float   0V   24V   24V

(voltages measured WRT pin 1.) 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

[Edited to add schematic]
 
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Doesn't 2 (common), 6 (open)and 4 (closed) make up a state switch?
I guess you're going to have to spell it out for me... Pins 5 and 2 are always 24V, so they don't help identify the CLOSED state of the valve.

floating 6 indicates OPENING or OPENED, and floating 4 indicates CLOSING or CLOSED.

By "state switch" do you mean "state transition"? Sorry, I'm not following...

Thanks,
/dev
 
I see, you don't have access to the common terminal of the motor, but all of the states (open) moving and closed are represented internally. Can you look at the voltage across pins 4 and 5 of the anticipator in call for heat. satisfied and motor valve open, closed and moving?

Why not take a look at measuring current to the valve and "not call for heat"?

e.g.

**broken link removed**
 
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Can you look at the voltage across pins 4 and 5 of the anticipator in call for heat. satisfied and motor valve open, closed and moving?

The schematic is an excerpt from the spec, and shows a thermostat instead of the SPDT relay that I will be using (OP edited to clarify). That relay will be controlled by a microcontroller or a switch.

When the microcontroller or switch calls for heat, the relay will connect 4 to 5. Pin 5 is just connected to pin 2, the hot side of the 24V transformer, so the voltage will be 0VAC across 4 and 5 (i.e. 24V from pins 4/5 to pin 1). Pin 6 will be floating until the contacts rotate forward and connect pin 6 to the motor. The other side of the motor is tied to neutral, so I think the voltage across pin 6 and pin 1 (neutral) will then become 0V, which is also when the valve is fully opened. Simultaneously, pin 3 is floating until the contacts connect it to pin 2, which is 24V (across pin 1 an 2/3). The pump is powered by pin 3.

When satisfied, the relay will connect 6 to 5. The voltage across 6 and 5 will be 0VAC (i.e., 24V from pins 5/6 to pin 1). However, pin 4 will be floating until the contacts rotate back and connect pin 4 to the motor. The other side of the motor is tied to neutral, so I think the voltage across pin 4 and pin 1 (neutral) will then become 0V, which is also when the valve is fully closed.

Pin 4 appears to be the key. It is the only thing that changes when the valve is fully closed and power can be removed.

Thanks,
/dev
 
Why not take a look at measuring current to the valve and "not call for heat"?

Ah, so I could measure the current from pin 5 to pins 4/6, and when it drops below a threshold, I know the motor isn't turning any more? That sounds like a shunt resistor that may drive a FET? What would you suggest?

Thanks,
/dev
 
These https://www.mouser.com/ProductDetail/Honeywell/CSDC1BA/ are not that expensive. Datasheets: https://download.siliconexpert.com/pdfs/2004/0802/pemco_b/1/hnw/qd/c20057.pdf

You can generally increase sensitivity by adding more turns. Those are digital, so you get an isolated digital output. These https://www.pololu.com/product/1185 are always a good choice, but it might be more trouble than it's worth. You get isolation though.

The valve probably doesn''t draw much current. Need that to really go any further with current sensing.

So, I'm saying if there is no call for heat and the valve/motor is not dwarfing current, then it has to be closed. You are still using "moving" and the "commanded direction" to determine the state. You just have to wait until the motor stops to figure out if it's fully closed or fully open.

With a purely mechanical system, it's probably not worth the effort for energy savings because energy consumption should be zero when at either extreme.
 
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