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AC/DC Power supply with no neutral ???

Discussion in 'Power Electronics' started by 24Volts, Jun 21, 2013.

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  1. 24Volts


    Mar 21, 2010

    I am working on a home automation device where I need low powered electronics within the electrical box of a household switch. Typically we have the line and the neutral wires available in the box. Therefore a small AC/DC power supply would use the existing 120VAC line as the input to a 5VDC power source as shown in #1 of my attachment. The power supply I would use is the following: ACDC MPM02V.html

    The only problem is that sometimes in my switch box I don't have the line and the neutral wires!! In this case I only have the line and a return wire which will connect to the light. However, in this case, I still need my power supply in the switch box to provide the +5VDC as shown in diagram #2 of my attachment!

    I have heard that such issues are possible by using the line wire only. My question is:

    - Can someone explain to me how this can be done?
    - Does someone know of any off-the shelf power supplies that would handle this connection!

    Thanks all for your help

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Scheme #2 will usually not work.
    The converter is in series with the lamp and therefore limits the lamp current. Conversely, the lamp is in series with the converter. Thus the lamp will not light and/or the converter will not produce enough output.

    Only scheme #1 is useable.

    I don't know of any power supply that could be used in this circuit because all supplies face the same problem: the lamp in series will limit the energy available to the converter.
    For very low power requirements one could construct such a converter using a low-resistance shunt and converting (and isolating!) the voltage drop across the shunt. For moderate voltage drops this will not visibly influence the brightness of the lamp.
    However, since besides the lamp the switch is in series, too, the converter will produce output only when the lamp is turned on.

    Im afraid you need to insert a neutral wire into the boxes that don't have one.

    One note in addition: Your question suggests that you are not very experienced with electricity. I therefore seriously warn you not to tinker with the instalation. It bears danger of live for you and possibly others. Get expert help from someone near you.
  3. 24Volts


    Mar 21, 2010
    Hi Harald Kapp,

    Yes, this is exactly what I would like to do is perhaps a shunt resistor (High ohms) across the switch just so to power my power supply?

    Bringing a neutral in the box is not always feasible. So I am snooping around for some alternate solution.

    I was just wondering if there would be any article that can show how to accomplish this.

    Thank you so much for your reply!
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