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  1. #1
    Join Date
    Jan 2003


    We all know that a regular control (like a Hight limit switch or a fusible link) is located on the hot side (not common) of a line-voltage circuit. However, I noticed that certain DDC output (24 vac), the control relay is on the common side (embedded in the board). Why is that?

    Whice takes me to my next question. Take a generic VAV actuator like a Siemen GDE131. The drawing shows 24vac on the common wire.

    When I apply 24vac to common and (let's say) drive open. (Drive close is not hook-up) I would read 24vac between drive close and common. I'm thinking this has something to do with the control relay being on the common side.

  2. #2

    Talking Source/Sink

    In an effort to save pennies, the electronic design
    whiz's utilize negative logic to operate some DDC
    switching mechanisms. Over the product line those
    pennies grow to big dollar signs.

    Here is a brief explaination:

    Sourcing: When the 24V HIGH LEG is sw'ing the load
    to it's common neutral (24G)
    Sinking: When the circuit board uses a triac on the
    common "Output". The HOT 24H is the common
    and the load wires come to the Sw'd common.
    So simple...!

  3. #3
    Join Date
    Jan 2003
    Thanks for replying HVAC-Controls. But I'm still confuse. Can you suggest any reading about HVAC electronics and Triac.

  4. #4
    Join Date
    Nov 2003
    Try this link to belimo:
    It touches on sinking/sourcing, triacs, and all kinds of vav and damper wiring and hook ups

    Check page 11 and 15

    I have a hard copy of the above pdf called "Product Documentation, Damper Actuators and Accessories" and it has a lot of good info. It also covers Power supplies, Basic electricity, Formulas, Control signals, Wiring Mistakes, Multiple Actuators, Johnson, Honeywell, Seimens wiring, etc. Ask your Belimo dealer to get you one.

    [Edited by powerhead on 10-04-2005 at 05:06 PM]

  5. #5
    Join Date
    Apr 2005
    With the contact of the relay tied into the hot leg it's called switching the hot or sometimes called hi. Put the contact of the relay on the common leg and it's called switching low. Everyone does it differently, switching the common will work but there are problems with it sometimes. I try to syat away from it and I never have a problem switching high.
    I Bleed Johnson Blue

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