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Can I share an AC neutral w/ two different voltage requirements?

  1. May 16, 2012 #1
    I have a situation on a dashboard where the dashboard itself (metal) is the “ground” and two different items require a different voltage. One item requires 5 volts and the other requires 25-28 volts. Each of these items has a single lead. How can I attach two different ac power supplies when there is a shared neutral/ground?
  2. jcsd
  3. May 16, 2012 #2


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    AC power supplies? Dashboard? I guess I would like more info.
  4. May 16, 2012 #3


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    you have to somehow ground the return to the power supplies.

    Connect each source to its relevant load, and it will only draw as much current as required, the other supply won't effect the opposite load, since current isn't distributed between supplies?

    It's like measuring voltage from one battery's positive terminal to the negative of another, no potential.
    Last edited: May 16, 2012
  5. May 18, 2012 #4
    Thanks for the responses. Sorry so long to get back; the paycheck is getting in way of my project…

    Let’s see if I can explain this better. I do need Alternating Current. It’s an avionic dashboard. The dash is aluminum and serves as a common neutral. Many of the items (loads) requiring AC power share this neutral by design of their attachment to the dashboard. Each of these loads have a single wire lead for the “hot”. Some loads require 5 vac and others require ~25 vac.

    If it were a dc power requirement, I know I could use two different power supplies using the dashboard as a shared ground (-). I am not sure how to do this with ac power. Currently I have one transformer that has three secondary leads (1 yellow, 2 black). Using a yellow and one black gives me 12.5 vac. Using only the two black leads gives me 25 vac. This 25 vac variation works great for powering the dashboard items requiring ~25 vac. I connect one transformer black lead to the dashboard as a neutral, and the other black lead directly to the wire lead on the load.

    Even if I had a separate power source of 5vac, how would I wire these two different voltages using the shared neutral of the dashboard?

    Did that clear the water, or just add dirt…
  6. May 18, 2012 #5


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    Are you building a airworthy home built aircraft? You haven't said at what voltage and frequency the transformer primaries are.
  7. May 18, 2012 #6
    I should have mentioned it is NOT going in an aircraft, just a conversation piece. I only have one transformer at this time. It's an el-cheapo from radio shack. The primary is household 120 vac/60hz. I'm open for recomendations for power sources. I would prefer small enough to attach to the back of the dashboard.
  8. May 18, 2012 #7


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    What are the loads going to be? If you are going to fire up some avionics that require a.c., the supply frequency was probably not 60Hz.


    Instead of multiple transformers, each grounding to the panel, you may be able to find a single primary transformer with multiple secondaries (including center taps) to run all your devices. But you'll still need to common the secondaries and/or the center taps (depending on how you intend to make use of their voltages).


    You'll have to do some checking as to the your loads maximum powers.
  9. May 19, 2012 #8
    I have built systems similar in concept to the one you are proposing. mine are for industrial automation.

    We typically build machines with several three phase transformers and single phase transformers using the same connection to ground, and they are fine. even in buildings, there can be dozens of transformers all grounded to the steel (chassis ;) of the building itself. Each of these transformers interconnected to the same ground tie their respective neutrals to this point.

    Barring being personally able to see what you are building, here is what i tend to think. just be careful, and don't proceed if you aren't sure what to do.

    Any number of transformers can share the dashboard chassis, just so long as only one side of the transformer is connected as the neutral to it. that would be one side of a two-lead secondary or the middle lead of a dual voltage secondary. We use a lot of 12/24 volt secondaries. 24 v between the two black terminals, and 12 volts between each one and the centertap, with the two 12 v outputs opposite in polarity with each other.

    what is important is to remember that in vehicles the chassis isn't a true ground, it's just an extension of hte return terminal of whatever power supply is in it. at 12 or 24 volts, it isn't a big deal, but at higher voltages you would HAVE to ground the chassis to avoid a potential life threatening shock hazard.

    the chassis used this way is a shared return terminal to all the power supplies sharing it, and shouldn't interfere with each other.
  10. May 19, 2012 #9


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    This is an important point. Even though the transformer's secondary voltages may not be a big deal, the primary voltage is.

    One way to be safe is to use a Underwriters Laboratories approved AC adapters aka "wall warts".

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