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All Things Rain Bird

Discussion in 'Irrigation' started by Ron Wolfarth, Aug 11, 2014.

  1. SoCalLandscapeMgmt

    SoCalLandscapeMgmt LawnSite Bronze Member
    Posts: 1,699

    Not to sound like a jerk.... But why would you spend the money to install a 2-wire system and then cheap out by installing DV valves? Especially when the PGA is only a couple of bucks more.
     
  2. Ron Wolfarth

    Ron Wolfarth LawnSite Senior Member
    Posts: 509

    The DV Valve does not have a core. Without that, it will not operate reliably with 2 wire. A design change is a whole new solenoid. This is what we now have in the HV valve. We are testing to determine it's reliability with 2 wire now. It is a lower priority for us so it will take some time to complete.
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  3. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,432

    Any technical explanation for why a solenoid must have a core for 2-wire compatibility?
     
  4. Ron Wolfarth

    Ron Wolfarth LawnSite Senior Member
    Posts: 509

    Two-wire systems are a very low power system. They use an initial high power pulse to activate a solenoid (pull the plunger up toward the coil) and then the power drops very significantly. That power pulse is then used to turn on the next solenoid, and so on.

    In a solenoid with a core (metal cylinder that is ferrous and sits in the middle of the coil), the low power supplied by the two-wire system is enough to generate enough of a magnetic field to hold the plunger against the core. It is not enough to pull it up in the first place. The core becomes polarized, becoming magnitized. The plunger can easily 'hang on' to the core even at the low power level supplied by two-wire systems because of this magnetized metal to metal contact.

    On top of this, two-wire systems are sometimes used with wire runs that are extremely long, miles sometimes. This extra wire length generates more electrical resistance and the lower power supplied by two-wire systems is even lower at the end of long runs. (This is one reason why many times designers try to loop the wire path much like looping a main line.)

    The DV design has no core, so the higher power is required to pull the plunger up and keep the plunger in the up position. There is not a metal magnitized part (core) for the plunger to more easily stick to. When the power in a two-wire system drops, the plunger often drops also. This does not happen every single time, but very often; certainly enough that it is not reliable and cannot be used in two-wire systems. Sometimes a contractor will try using DV because they don't know if it will work. It might work the first few times and they think they are good. Then,.......call back!
     
  5. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,432

    Is the power problem with a DV solenoid addressable by carefully specifying the wire and supply power? My simplistic view of a 2-wire decoder was as its being a remote-operated solid-state switch. That it makes a difference whether the switching triac is in a field decoder, as opposed to its being in a conventional controller, is a bit surprising.
     
  6. Ron Wolfarth

    Ron Wolfarth LawnSite Senior Member
    Posts: 509

    In short, no, it cannot be addressed with a larger wire path and transformer.

    It isn't just that in a two-wire system there are many valves operating off a single two wire path as opposed to in a conventional system where you have one or two solenoids on a single wire path. The two-wire system has to deliver power to supply enough power to run many solenoids on that path. Hence, the strategy that is used is that inital high power pulse to pull up the plunger and then a drop in power applied to that same solenoid to keep the plunger up. The triac in a conventional system likely is very similar to a triac in a decoder and it is okay to think of it as you do, but it goes beyond that. The decoder (or controller, not sure which) also varies that power so that the inrush is high (probably close to a conventional system inrush) and then the holding current is very low (much lower than a conventional system.) It is kept intentionally very low so the power on the two-wire path can then be used to power the inrush requirement to open the next valve.

    So, a two-wire system is more complicated than just putting the electronic switch (triac) out near the valve. The high/low power strategy would still be applied by the controller/decoder and the larger wire path/transformer would be wasted if it were to be supplied by the installer.

    Said yet another way, even if you provided an adequate wire path to handle all that power, the controller/decoder would still drop the power after the initial inrush pulse because it is designed to do that. Even with the big wire path, the holding power would not be high enough after the initial inrush pulse to hold up the plunger.
     
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,432

    I could picture a more advanced decoder that would store its own charge of power, using it to actuate the solenoid, before switching to a sustain mode.
     
  8. Ron Wolfarth

    Ron Wolfarth LawnSite Senior Member
    Posts: 509

    I would not rule out that this is what is already happening in some decoders today by means of a capacitor.

    I am already beyond my technical expertise, so take what I say on this with a grain of salt.
     
  9. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,432

    I pictured the storage capacitor, but of course it stores DC, and the solenoids are AC.
     
  10. Ron Wolfarth

    Ron Wolfarth LawnSite Senior Member
    Posts: 509

    Good catch. Good thing I warned you about me being beyond my expertise in electrical things. I wonder if there is a way to store the energy in a capacitor (DC) and discharge it in AC through some simple inverter?
     

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