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Adding DC voltages in Series or Paralell

  1. Oct 12, 2012 #1
    Hello all,

    I am trying to get my head straight about wiring multiple DC voltages together. I should know the answer to this question, but I thought I'd ask someone else.

    If I have two DC power sources....Source A is 12V 1A and Source B is 5.3V 980mA.....and I want to combine A+B to get the most power out of the two sources... How should I combine them? In Series would I need to place diodes on the lower voltage source so that the higher voltage source does not short the lower? And in parallel I know the voltage would not increase but would the current? Is there even a way to add the sources if they are not equal?
  2. jcsd
  3. Oct 12, 2012 #2


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    Staff: Mentor

    It is not practical to parallel voltage sources of different values.

    You can put them in series to get a total of 17.3V, but you will be limited by the max current of the 5.3V supply (980mA).

    If you want to combine the powers to get as much power out as possible, you can buck the 12V supply down to 5.3V, and then Schottky diode OR the two power sources together.
  4. Oct 15, 2012 #3
    Thanks for the info berkeman, Lets say I have two voltage sources 9V 1.2A and 12V 1.1 and I want to add these together in series to provide a regulated 5V 900Ma. Would it be best to add the sources and then regulate to 5V? or regulate both to 5V seperately and then add together as equal sources....such as (2.5V) X 2 @ 450mA?
  5. Oct 15, 2012 #4
    I suggest you look again to check your posted figures.

    Either of your supplies can be made to provide a regulated 5 volts at 0.9 amps on their own, so why would you want to combine them?

    I am not sure if you understand about power supplies.
    The specification is for a particular voltage. The idea is that this does not vary.
    The current is determined by the load, not by the power supply.

    So a 9 volt supply at 1200 mA will (try to) provide 9 volts at any current drawn by the load from zero to 1200mA.
  6. Oct 15, 2012 #5
    True, I understand that, but these power supplies are just examples. In my case I want to combine them because I have two renewable energy sources and I have a known load value that will draw lower than the rating of each power source individually . The sources are subject to fluctuation as a result of heat or pressure change, therefore I want to provide enough voltage above the minimum requirement of the voltage regulator. For example a 5V regulator needs 8V to 30V to operate. I want to provide a bit more than 8V so that if my two sources fluctuate, my capacitor before regulation will hopefully not drain enough to drop below 8V while powering my load.

    So I guess what im trying to confirm is...that if two sources are connected in series...the output is limited by the source with the lower amperage rating? and the load should not exceed that....but the more powerful source will not affect the lower power source?

    As (PowerHigh) + (PowerLow) = (Power combined) and R should not exceed amperage rating of (PowerLoW) and that (PowerHigh) will not affect or short out (PowerLow) Would this be correct?
  7. Oct 15, 2012 #6
    Your 5 volt regulatior needs a minimum of 3 volts above the regulated voltage. Anyhting beyond that is merely wasteful bad practice.
    Thre are versions that require less than 1 volts above the regulated.
    This is known as the dropout voltage.

    The 30 volts is the absolute maximum that can be applied without destroying the regulator.

    All that will happen will be that you will generate a good deal of heat.

    9v + 12 v = 21 v.

    So at 0.9 amps draw this will dissipate 0.9 (21-5) = nearly 15 watts.
    This will require significant heatsinking to dissipate.
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