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Parallel Power Sources

  1. Feb 27, 2016 #1
    Hi, I am trying to make one of my project plug into the wall. It needs 150 Watts, at 12 volts. I am going to use laptop chargers to power it, but I only have 2, 75 Watt chargers.

    Both of the chargers are 12 volt chargers, and I was wondering, could I use these two chargers in parallel to achieve my 150 watts?

    I do not have money to buy another laptop charger, and was wondering, if i could have them in parallel, how would I go about doing that?
     
  2. jcsd
  3. Feb 27, 2016 #2

    davenn

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    that is 2.75W is it ?

    so do the math, does 2.75 + 2.75 = 150?

    150W @ 12V is quite substantial = 12.5A and if that is what the load needs, then you are going to require somewhat more
    than that to have some headroom ie. around 175 - 200W ... now you are up to around 15 Amps

    best option would be to use a PSU out of a desktop PC get one that can produce around 15Amps on the +12V rail


    Dave
     
  4. Feb 27, 2016 #3
    It is 2 comma 75, sorry i did not make that clear. I have two separate 75 watt laptop chargers. I do not have money to get a desktop psu, nor can I scrap one from one of my computers. I need to know if i can use the two i have in parallel to get the 150 watts
     
  5. Feb 28, 2016 #4

    davenn

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    ok, yes I saw they were commas, in dome areas of the world a comma is used as a decimal point so 2,75 = 2.75

    for future reference, if you are describing 2 of a thing as you were trying to do either state eg ...
    I have two by 75W PSU's or I have 2 x 75W PSU's .... saves much confusion :wink::smile:

    in a simply world, yes, you could just parallel the two supplies, this would give you twice the capability at the same voltage

    but the world isn't simple or perfect, nor are power supplies and if the supplies are not exactly identical in their outputs
    (highly unlikely that they would be) ... then one will be likely to supplying more of the current than the other
    Unevenly shared load means one supply works harder and is likely to fail quicker


    You also didn't comment on my comments re max power needed by the load and the need to have headroom above that amount

    you unclearly hinted that you need 150W, I said if that is the case, then you need a power supply capable of more power out
    so again ... what is the maximum load ? is it 150W or a lot less ?

    Dave
     
  6. Feb 28, 2016 #5
    The maximium load is indeed 150 Watts, and for the future, I will be using "two" instead of two :wink:. They are the exact same laptop charger, I just so happen to have two of them. So it can work? how quickley would the first one die? would it be a faster death simply because their are two of them, or would it be because of its current draw? Thank you, your help has been really appreciated !
     
  7. Feb 28, 2016 #6

    davenn

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    OK so that doesn't give you any headroom and the PSU's would be working to the max ... this will be very bad for them
    they will fail much quicker

    I don't know what that means ???

    yes same make and model but not necessarily identical outputs ... see comments in previous post

    it will work for a while till they overheat and fail, could be hours, days or a few weeks

    unknown ... too many unknown factors


    as I stated way back earlier in the thread ..... you need a single, hi power PSU at least 175W capability
    to ensure reasonable reliability


    Dave
     
  8. Feb 28, 2016 #7
    Ok, thank you, and what i meant by two, is based on the mix up, where i had written 2, 75 instead of two, 75.

    What would happen, if i were to only drain 140 watts, so that none of the power sources are working on 100% output.

    Also, Thank you for the help!
     
  9. Feb 28, 2016 #8

    berkeman

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    Did you say what your project is? Sorry if I missed it.

    You may be able to diode OR the two chargers, using two high current Schottky diodes. That might give you an overall 11V at 150W maximum load. How sensitive is your project to the input voltage?
     
  10. Feb 28, 2016 #9
    how exactly would I go about doing that?
     
  11. Feb 28, 2016 #10

    berkeman

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    Use Google Images, and search on Diode OR Power Supplies... :smile:
     
  12. Feb 28, 2016 #11
    So this is the sort of thing you are talking about?
    wU42P.png

    and this would work, and solve all issues?
     
  13. Feb 28, 2016 #12

    berkeman

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    That's what I was referring to.

    Would it "solve all issues"? Not necessarily. As I alluded to, you lose voltage across the diodes, so you will likely be down around 11Vdc at the max 150W load. Also, that's a lot of current for a diode, so you will need to be sure to use Schottky power diodes that can handle the current (and they may need head sinking). 150W/12V = 12.5A, so you would need to find some 15A or bigger Schottky diodes and heat sink them properly. The BAT54 shown in that image is tiny, for example, and only rated at something like 100mA.
     
  14. Feb 28, 2016 #13
    Thanks, a ton, You all have been extremely helpful. Thank you so much!:eek:
     
  15. Feb 28, 2016 #14

    Merlin3189

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    I notice you have not said what you want to use them for. I'm still a bit sceptical about paralleling them. It would help to know how they were to be used.
     
  16. Feb 28, 2016 #15
  17. Feb 28, 2016 #16

    berkeman

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    I've closed that thread due to safety concerns. We do not think you should be trying to design a Lithium Ion battery charger. There is too high a risk of fire if not done very well. Save that for a project that you do in a year, when you have much more experience. Just buy a charger for that now.
     
  18. Feb 28, 2016 #17

    davenn

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    That thread was closed for a reason
    That being, you wouldn't listen to good advice on how LiPO's need to be charged safely

    It doesn't need to be discussed further in this thread


    Dave
     
  19. Feb 28, 2016 #18

    Merlin3189

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    So would it be possible to power this as two half matrices?
    Open collecter drivers should not care where the power comes from.
     
  20. Feb 28, 2016 #19
    sorry, i didnt realize at the time that it was closed
     
  21. Feb 28, 2016 #20
    Why can I still post here? (#$%^& hackers)

    In general it's not possible to diode power supplies like that. This is because some semiconductors have a positive temperature coefficient and some have a negative. With the positive (I think) one draws slightly more power, gets hotter, then draws more still until it dies. The negative gets hotter, draws less power, the other picks up the slack and both work fine.

    So it's at best a 50-50 shot if it works (BJT vs. CMOS, BTW). So there is some risk to equipment and conceivably a risk of fire.

    Also you shouldn't push your power supplies beyond about 50% of nominal for projects like this. Power supplies are rated for nominal maximum with specific equipment. Guessing is a good way to blow things up.
     
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