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LED project power supplies in parallel?

  1. Aug 16, 2011 #1
    So I am going to be building an LED light for an aquarium.

    What I am doing is...

    10- 10k 3w LED 3.4-3.6 fv 700ma

    10- 455nm 3w 3.4-3.6 fv 700ma

    I have ordered 6 10w 9-12v 900ma power supplies.

    I was planning on doing 9 of each blue, and white on 3 circuits each, with a 2.7ohm resistor in series. (3 leds + resistor)

    What I got thinking about, is it possible to combine 2 of the PS in parallel so I can run all 10 instead of 9?

    Maybe with a diode to keep from back feeding
  2. jcsd
  3. Aug 16, 2011 #2


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    Yes you can try a diode-Or of two or more DC power supplies.
  4. Aug 17, 2011 #3


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    Putting power supplies in parallel probably won't work, but putting them in series may help.

    So, if you put two 12 V supplies in series to get 24 volts, you would then allocate 25 % of the 24 volts (ie 6 volts) to a resistor and the rest to the LEDs.

    Your LEDs have about 3.5 volts across them, so you could have 5 in a string to use 17.5 volts and then the remaining 6.5 volts for a resistor. (6.5 Volts at 500 mA would mean you need a 13 ohm resistor)

    You will notice that this isn't even as many LEDs as you have now.

    The problem is that if you put in more LEDs and have less voltage across the resistor, then small variations in the supply voltage have a large effect on the LED current.

    For example, if you have 3 LEDs with 3.5 volts across them and a 12 volt supply, and a 2.7 ohm resistor, there will be (12 - 10.5 volts / 2.7 ohms) or 555 mA flowing.
    If the supply voltage went up to 13 volts, there would be (13 - 10.5 volts / 2.7 ohms) or 925 mA flowing.
    The LED voltage would rise slightly, but this effect can make the LED brightness vary a lot or even destroy LEDs.
    It can be reduced by allocating more of the supply voltage to the resistor and less to the LEDs.

    There are dedicated modules for driving high powered LED arrays like these.
    For example, this one:
    I can't vouch for the quality, but it appears that each driver would provide about 600 mA to one LED but at greater efficiency than using resistors to limit the current.
    There are 10 drivers in this package for $10.
    It uses 12 Volts AC to drive the modules.

    Unfortunately, the efficiency is not given, but it may be something like 80%.
    So, a 3.5 volt LED drawing 600 mA would be using 2.1 watts.
    This would take 2.1 watts * 100 / 80 or 2.625 watts from a 12 volt supply or 218 mA.

    So, you would gain a little in efficiency, but the current through the LEDs would be regulated and so the brightness would not be highly dependent on the supply voltage.
    Last edited by a moderator: Apr 26, 2017
  5. Aug 18, 2011 #4
    Thank you for your help!

    As another option, could I put a 12v in to 5v out Voltage Regulator behind each power supply and then run the LEDs in parallel with a 2.2ohm resistor?

    Am I understanding correctly that the regulator would give me a constant 5vdc even if the PS fluctuated?

  6. Aug 18, 2011 #5
    I am assuming you were planning on putting the LED's in series from the source, why not place them in parallel to each other, and provide each with a current limiting resistor? thus negating the issue of varying current levels.
  7. Aug 18, 2011 #6


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    These options would work, but they are wasteful of power and components.

    A linear regulator producing 5 volts from 12 volts wastes 58% of the power supplied to it.
    Giving each LED its own resistor wastes even more power and uses a lot of expensive high powered resistors.

    This power loss would not matter with low powered LEDs, but these are high powered and expensive and also quite capable of getting destroyed if they are not handled carefully.

    I would look into dedicated LED drivers. These give constant current output and good efficiency. If the ones on EBay are proved to be OK then they are not a lot more expensive than resistors and give a lot of benefits.
  8. Aug 18, 2011 #7
    I will have to look into these, havn't come across them before, good to know
  9. Aug 19, 2011 #8
    So would something like this be the same or very wasteful as well?

    Last edited by a moderator: Apr 26, 2017
  10. Aug 19, 2011 #9
    Last edited by a moderator: Apr 26, 2017
  11. Aug 19, 2011 #10


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    So would something like this be the same or very wasteful as well?

    AC 85-265V 1x3W Power Constant Current LED Driver

    These run directly off the mains. I think this adds an unjustifiable element of danger to the project unless you are very sure of what you are doing.

    I prefer the idea of getting 12 volts from a transformer and then just distributing this to the LED drivers.
  12. Aug 19, 2011 #11


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    So my problem is I have already purchased some 9-12vdc power supplies

    So I am trying to use what I have spent money on, if there is no good way to do that I will have to abandon that.

    I did find 3w Drivers that seem like they will work with the 12vdc

    What do you think about this?

    Use my 120vac - 12vdc with 3 drivers in parallel.


    These are not just power supplies. They have built in LED drivers that drive at 900 mA.
    As I read it, they will try to destroy your LEDs.
  13. Aug 19, 2011 #12
    Ok, sorry I don't know much about this.
    Didn't realize it was quite so involved.

    If you have time would you mind a link to a transformer you have in mind? I just seem to be getting confused now.

  14. Aug 19, 2011 #13
    Last edited by a moderator: Apr 26, 2017
  15. Aug 19, 2011 #14


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    That power supply looks OK.

    They don't say if the input and output are isolated from each other. Maybe it would be worth asking the seller to check this, although the device is so cheap that you could just get one and take the risk.

    If it is isolated, you should not be able to measure any DC connection between primary and secondary with a multimeter.

    If it isn't, you can still use it, but you would have to be very careful with safety.
    For example, wire connections should only be made inside plastic boxes. This is probably not a bad idea anyway, especially near water.

    Note that it requires a minimum load of 20 watts, so you would not be able to just run one LED to try it out.

    I was hoping to find a conventional transformer giving 12 volts at about 5 amps, but these seem to be rare, expensive and heavy.
  16. Aug 20, 2011 #15
    Thank you for all your help!
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