how knock 24v dc supply to 12v?

Discussion in 'The Projects Forum' started by fawcetteng, Jan 11, 2012.

  1. fawcetteng

    Thread Starter New Member

    Dec 11, 2010
    if i have a 24v dc power supply such as linked below, what is the best way to "add" a 12v capability? As in i'd like to throw a switch in the output lines to select either 24 or 12 volts, by having the output when switched go through a voltage regulator or divider. Or can I hack into the power supply and put a second resistor in somewhere that will change the output? Keep in mind i want to do this for up to the 6amp capability. It is my understanding that a plain old resistor based voltage divider will vary the ouput voltage as my load varies.
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    A resistive dropper will not be practicable for a variable load, and in any case dropping 24V to 12V at up to 6A resistively would mean burning up to 72W.

    A linear regulator could give a constant output, but would still waste up to 72W. In principle, a switching regulator could do better, but I seriously doubt that it would be practicable for you to make one.

    You would be much better advised to obtain a power supply (or a pair of power supplies) that meets your requirements without modification.

    Alternatively, adapters are made that allow truck drivers (lorry drivers) to use things like 12V car radios on vehicles with 24V electrical systems. Perhaps something of that kind would fit your requirements.
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
    I don't think you want to do this but I mention it anyway.
    It should be possible to open the power supply, find the feedback loop and modify it in order to change the output voltage.

    I did it before for small switching power supplies.
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    Yes, but this might result in problems such as poorer efficiency leading to overheating. In any case, it will invalidate any warranty unless the modification is sanctioned by the manufacturer, which seems unlikely.

    Somebody with plenty of experience might contemplating doing such a thing. I think though that if they have to ask confirmation whether a resistive dropper would be unsuitable, it is unlikely that they have the knowledge to do this safely.
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    There is usually a resistive divider (two resistors) that sets the output voktage. Put a resistor in parallel with the top one and the output voltage drops.