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Can I use two AC TRs with ONE neutral at sec. and prim.?

  1. Mar 1, 2016 #1
    The situation:
    I have one BMS (Building Management System) controller supplied from TR1 (220Vac to 24Vac transformer) and provides an analog control signal (0-10VDC) with the (-) of that DC being the same as the Neutral of the controller (from TR1) and should control a valve actuator.

    The controlled valve actuator uses 24Vac supplied control circuit....supplied from TR2 (same as TR1 and primary winding are supplied from the same source "220Vac outlet").
    The actuator control circuit has terminals:
    1- G "24Vac"
    2- G0 "Neutral and DC reference point"
    3- X "control signal...DC 0-10V"

    Now, the problem I'm afraid to face:
    The actuator should have the control signal (0-10VDC) with a reference (-) that should be at the same time the 24Vac source neutral....Should I SHORT CIRCUIT TR1 and TR2 secondary winding neutrals or do what??

    Note: Unfortunately, I cannot use one transformer for both the controller and actuator.

    Thanks
     
  2. jcsd
  3. Mar 1, 2016 #2

    Svein

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    You can. I am a pessimist, and would insert a 1Ω resistor between them.
     
  4. Mar 1, 2016 #3
    As long as the "positive" terminals of the transformers are not connected two (to avoid paralelling the transformers) there should be no problem with the connection you are proposing. Indeed, if you are using a TN scheme, the neutral points should be common and at ground potential
     
  5. Mar 1, 2016 #4
    What power rating should that resistor have?
     
  6. Mar 1, 2016 #5
    Thanks
     
  7. Mar 1, 2016 #6
    The resistor is not advisable. As I explained before, the neutrals must be solidly connected. If you want to protect the circuits (you should), you can used Miniature Circuit Breakers in each phase leg (not in the neutral in this particular case).
     
  8. Mar 1, 2016 #7

    Svein

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    0.25W is more than enough. The only current that should pass through the resistor is the current due to the control signal. The actuator should be connected directly to its transformer. The purpose of the 1Ω resistor is to break up any ground loops.
     
  9. Mar 1, 2016 #8
    The transformers neutral must be solidly interconnected, without the resistor . Please check neutral earthing schemes. The control voltages are 0-10 V and fed from the controllers. As for the control signal: most probably the internal circuitry of the controllers isolate the 0-10 Vdc from the 24 Vac so no common ground will be expected. If you place a resistor in the control loop, you are disturbing the voltage signal (it´s not a 4-20 mA loop).
     
  10. Mar 1, 2016 #9

    jim hardy

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    there's an unanswered question...

    How is that DC from TR1 derived?

    EDIT i cut and pasted pictures below without checking closely enough
    that author drew his diode bridges rotated 1/4 turn from correct orientation
    rotate them 90 degrees CCW before using
    see dave's post below
    thanks Dave
    (chagrin icon) :eek:


    diode-bridge-single-supply.png

    or

    full-wave-rectifier-lc-choke-filter.jpg
    or

    diode-bridge-split-supply.png

    In either of the latter two your TR1 transformer's centertap is already circuit common
    so you also could tie the TR2 transformer to circuit common .
    If the first, don't connect TR1's winding to TR2's winding.
     
    Last edited: Mar 1, 2016
  11. Mar 1, 2016 #10

    davenn

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    Those bridge diodes are incorrectly connected
    whoever drew them didn't see their mistake or don't understand

    take the top circuit

    diode-bridge-single-supply.png

    voltage from the 1/2 cycle coming out the top of the transformer winding is going both to ground and the V+ output
    don't think that will work too well ... the same error is perpetrated in bottom drawing as well

    correctly drawn example ....
    upload_2016-3-2_9-46-8.png


    Did I miss something in the discussion, @jim hardy ?
    Did you show those incorrectly on purpose to make a point ?


    Dave
     
  12. Mar 1, 2016 #11

    jim hardy

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  13. Mar 1, 2016 #12

    davenn

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    no probs ... just wanted to make sure you weren't trying to teach the OP something and see if he could pick the problem LOL


    D
     
  14. Mar 2, 2016 #13

    Svein

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    A 1Ω resistor? Even if your activation signal is as greedy as old-fashioned TTL (1.6mA), the voltage drop will be 1.6mV!
     
  15. Mar 2, 2016 #14
    This is a matter of engineering, not laboratory tampering with circuits.
     
  16. Mar 2, 2016 #15
    At any rate, this circuit belongs to a "power" rectifier, but I believe that for the 0...10 Vdc the control circuitry isolates the variable 0...10 Vdc from the 24 Vac input
     
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