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  1. #1
    Join Date
    Jul 2008
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    414

    Triacs in modulating controls

    Im having trouble understanding how modulating proportinal controls that utilize a 24V AC signal to open and close work exactly. Case in point J/C VG 7281GS + 8020G zone valve has 3 control wires up, down and common- there is 24V present at all times to both the up and down to common. Ive been told this is do to the use of a triac which allows voltage but no current Im having trouble wrapping my head around that so if anyone could explain it to a simple mind like myself that would be much appreciated.

  2. #2
    Join Date
    May 2008
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    Jax Fl.
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    1,943
    A Triac [AC transistor switch] will show continuious output voltage as long as there is no load on the output. Your meter reads voltage since it does not load down the circuit. If you read the output with the actuator motor connected, the continuious voltage is not there.
    Clear as mud?

  3. #3
    Join Date
    Jul 2008
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    414
    Im pretty sure I checked voltage at the atuator with it commanded all the way open and closed at one point with a known good actuator and found there to be 24v to com at both up and down. Thats what peaked my curiousity.

  4. #4
    Join Date
    Oct 2003
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    Michigan
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    706
    Quote Originally Posted by dave1234 View Post
    Im pretty sure I checked voltage at the actuator with it commanded all the way open and closed at one point with a known good actuator and found there to be 24v to com at both up and down. Thats what peaked my curiosity.
    Power feeds to COM through both motor windings and the Triac shorts to ground to complete the circuit.
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  5. #5
    Join Date
    Aug 2007
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    Western Colorado
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    497
    Put the coils of two relays on the outputs (Open & Close), then command the outputs and watch your relay energize. With no command you will still see the 24v with your meter, but no relay closure. I use a meter that "dampens ghost voltages" to read circuits controlled by Triacs. I use the Fluke 16 with Lo-Z.
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  6. #6
    Join Date
    May 2008
    Posts
    290
    I have a couple of relay's with tails on them in my back for testing triac outputs.

    Dazza

  7. #7
    Join Date
    Aug 2007
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    Western Colorado
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    Quote Originally Posted by azzad View Post
    I have a couple of relay's with tails on them in my back for testing triac outputs.

    Dazza
    That works too!
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  8. #8
    Join Date
    Mar 2009
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    Several Miles from Sane
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    Quote Originally Posted by azzad View Post
    I have a couple of relay's with tails on them in my back for testing triac outputs.

    Dazza
    As said, Triacs "Leak voltage".

    I have relays (RIBU1C/V-100) with pigtails on them just for such an occasion, same as azzad.

    yllekicc (not as cool as "Dazza" but hey, I had to do something. Almost looks like a disease.)
    If sense were so common everyone would have it !

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  9. #9
    Join Date
    May 2008
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    290
    Oops, I guess it goes with my 2 heads

  10. #10
    Join Date
    Mar 2009
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    Sorry azzad,
    What I ment was...

    yllekicc is (not as cool as "Dazza" but hey, I had to do something. "yllekicc" Almost looks like a disease.)
    If sense were so common everyone would have it !

    All opinions expressed are my own. Any advice provided is based on personal experience, generally accepted fact or publicly available information. As such, it is worth exactly what you paid for it, not a penny more not a penny less !!

  11. #11
    Join Date
    May 2008
    Posts
    290
    Its all good; I knew what u meant.

  12. #12
    Join Date
    Jun 2006
    Location
    Athens Greece
    Posts
    108
    If you use a multimeter you will actually see that when triac (output) is off the voltage is slightly different than when output is on (if I can recall right about 21.5 V in of where 24V in on).
    But this is always relied on triac leackage as mentioned.

    A relay or in my case a 24VAC lamp with clamps is better to be used.

  13. #13
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
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    4,151
    FYI - there are voltage sourcing (power switched) and voltage sinking (common switched) triacs. Some triacs have you wire both sides of the load back to the triac points and some have you wire one side to the triac point and the other to the common side. Hook it up wrong and you'll burn out the triac. By know means do I feel like an expert on triacs, but working with both voltage sourcing and sinking I've burnt up more than one triac, due to voltage sourcing vs. sinking and not looking closely enough at the wiring diagram.
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