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Noob question: Multiple LED modules in parallel or in Series?

Discussion in 'LEDs and Optoelectronics' started by Novogenia, Feb 23, 2016.

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  1. Novogenia


    Nov 26, 2015

    I want to build my own aquarium lighting system with multiple LED modules.

    Each module contains 12 LEDs and is meant to run at 34V and 350mA.

    I have a QJE Lab-Power supply that lets me define the voltage and Amperes. I have my first module and attach it... set the voltage to 33V (the highest it can go) and the LEDs will shine. Changing the mA to a higher setting has no impact. Shorting one of the LEDs on the module makes it a lot brighter.. so more Volts seem to cause brighter light.

    Now.. if I get additional mudules and I want to connect them to the same power supply... how do I do this? If I go in series, the voltage will go down if I am correc,t making the light dimmer.

    If I go in Parallel, all get the same voltage.. right?

    What will the limiting factor be of how many modules the power supply can run? The Amps it can supply? Will the amps go down if I connect more modules?

    How many modules do you think I can run in parallel?

    Thanks for the help
  2. Gryd3


    Jun 25, 2014
    You can't actually control Voltage AND Amps at the same time. You can control only one, and limit the other... when the limit is reached, the one you 'control' will actually turn itself down to compensate for reaching/exceeding the limit.

    The modules sound like they are wired internally as a string of 12 LEDs with a current limiting resistor. Are they Red?

    Anyway... If your Power supply can't reach the 34V required, then the current (which is the important part) will not reach the designed amount and the LEDs will run dimmer. My shorting one of the LEDs, you are reducing the voltage required by between 2V and 3.5V, so your new required voltage would be between 30.5V-32V which your Power Supply CAN do properly.
    If you short that LED, and then play with the current adjustment knob, then you will see the brightness change, and when the brightness begins to dim, you will find that the voltage being output is now being limited.

    Now to answer your actual question:
    - If the LED modules have a current limiting resistor inside (they should!) then you can connect all the modules together in Parallel. This will add up their Current, and the Voltage will remain the same. So 5x modules would be 34V and 1750mA
    - If the LED modules do NOT have a current limiting resistor, then I hope they were cheap. You get what you pay for. You will need a slightly higher voltage, and add your own resistor in series on EACH module, then connect all of the modified modules in Parallel.
  3. AnalogKid


    Jun 10, 2015
    Most of your guesses are correct. What is the maximum output current of the supply - and its complete model number?

  4. Novogenia


    Nov 26, 2015
    Wow.. great reply.. thx a lot for the clarification. So I will short one LED and then move within the capabilities of my power supply. This will also be a suitable technique if I add other modules of different design and voltage requirements.. right? Just short out so many that all require the same voltage.

    Concerning the current limiting resistor... hard to say. Here is what I have so far:

    Doesnt look like it form the electrical circuit drawing.

    Any recommendations what resistor I could use for the parallel setup?
  5. Novogenia


    Nov 26, 2015
    thx for the help.

    The modules I want to use are:


    The power supply is a:

    QJE - QJ3005E III

    its a german production I think.. so I will translate the parameters:
    2 regulatable outputs with 0 to 30V and 0 to 5A
    one static output with 5V and 3A

    Would be interesting how many modules I could run with this power supply.
  6. AnalogKid


    Jun 10, 2015
  7. Gryd3


    Jun 25, 2014
    The 8x LED modules do not have a current limiting resistor. This is given away by selling it as a 24V source when each LED requires 3V. The first link did not work for me, but from the sounds of things, there may not be a resistor in there either.

    Anyway, there are two ways to drive those LEDs... Go buy a small 'Constant Current' supply for each LED module. Current = 350mA, and the output voltage does not need to be any higher than 36V-40V. This is the ideal method, and the input of each power supply can be wired in parallel.
    **Additionally, you can find a Constant Current LED driver that operates at a much higher voltage, and string all the modules together in series! Output will need to be 350mA, and the voltage is higher than the the SUM of all modules you want to run.
    The alternative method is adding a resistor to each module and connecting them all to a 'Constant Voltage' supply. The current will need to be the sum of all the modules used... ie, 3 or 4 modules would draw 950mA or 1400mA. The voltage here would be set to a little higher than the largest module and a resistor is required to limit the current and to take up the difference in voltage.

    If your supply is set at 34V, and your LED module has been modified to require 32V, then you need a resistor to drop 2V @ 350mA.
    The formula is as follows:
    Voltage = Current * Resistance
    So you will need a 2V / 350mA = 6Ω resistor.
    But remember I said 350mA is a little high for it... It will need to be able to handle 0.70Watts, so you will need to buy 6Ω resistors rated at least 1 Watt for this particular example.
    Power = Voltage * Current
    So if you need to take up a different amount of voltage, the 0.7W may go up or down. You have the math here, you can look at examples.
    Last edited: Feb 24, 2016
    BobK likes this.
  8. Novogenia


    Nov 26, 2015
    Great reply.. thanks so much!

    One question: The Constant current Supplies.. they just limit the mA and let the same voltage go through, that goes into it.. right?

    I have the following dilemma:

    I have:

    2 modules with 38V and 350mA
    3 Modules with 36V and unknown Ampage:

    • Power: 100W
    • Model Name: CREE XLamp XP-G2
    • Emitted Color: White
    • Color Temperature: 6000K-6500K
    • DC Forward Voltage (VF): 30V-36Vdc
    • DC Forward Current (IF): 3A <<<<<<<<<< is it 3 Amps? Really?
    • Maximum Pulse Voltage: 38Vdc
    • Maximum Pulse Current: 3200mA
    • LED Viewing Angle: 115 degree
    • Intensity Luminous: 10000LM
    • Size: (L)55 x (W)40 x (H)2mm

    When I put them in Parallel on my power supply, the obvious happens, the 3 modules with the lower voltage glow before the higher voltage modules glow.

    Now trying to follow your advice, I now have to do the following (I think):

    I need to get 5 constant current supplies:

    2x 350mA and about 38V

    3x 3000mA and 36V

    Planning to attach all to the same power supply, I need to add resistors before??? the 3 modules to reduce their Voltage by 2v.

    Using your formula, that should be resistors like this:

    2v/3A = 0.66 Ohm Resistor surviving 100 Watts.

    Is this correct?

    Would this resistor be suitable?

    Thx so much for helping me learn this stuff.
  9. Novogenia


    Nov 26, 2015
    ok, another question, trying to curcumvent having to use my power supply which will reach the limit in wattage (I think)... I came across this:

    If I understood correctly, I can attach that directlyto the mains (230V here) and on the output I get enough juice to run one module. Is that correct? Only question: It says it outputs between 9-48V. How will this be regulated? Is there a regulation crontroller on this thing?
  10. Gryd3


    Jun 25, 2014
    A constant current supply, (Often LED drivers) will keep the output current the same. They adjust output voltage within an advertised range to keep that current the same.

    100W White led at 30-36V is most definitely 3A... To get power, you multiply voltage and amperage.
    3A x average(30-36V) = ~100W

    If you are connecting the 'input' of the LED drivers in parallel, you do NOT need resistors or anything else. Just the power supply, LED drivers and LEDs are required. Make a note of the 'input voltage' on the LED drivers and make sure the power supply provides that. Nice and easy. The power supply will have to be able to supply enough amps though... So add the amps for each LED together and tack on at least 20% more to know your requirement..
    (ie... 3x 100W LEDs will draw 9Amps. Try to get a 12A or more supply.)

    You are absolutely correct. Right from mains to the LED.
    I mentioned above that a constant current supply is more concerned with keeping the current at the set value. (350mA in this case)
    It will vary the output voltage as it sees fit to make sure 350mA is going out.
    Don't worry though... the specs given on LEDs state the working 'current' that MUST be met. The voltage is a 'typical' voltage range that can be expected to meet that 'current' rating.
    So no... it does not regulate voltage at all, but that's the point. The important thing here is that the current is a fixed value and will not change.
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