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Help building LED power supply

Discussion in 'Power Electronics' started by Rolltop, Dec 7, 2016.

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  1. Rolltop

    Rolltop

    5
    0
    Dec 7, 2016
    I am trying to power 10 strings of small LED lights. Each string came with it's own solar power battery power supply. These are 1.2 volt AA save batteries.
    What I want to do is wire all the 100 strings together in parallel and supply then with a wall powered AC to DC power supply.
    My question is on how to do this as I do not have specs on the LED except that each 20' strand runs off a AA 1.2 Volt battery pack. The Battery pack has some circuit in it to switch between blink and study, charge with sun light on when dark.
    I do not need all of this just a constant on when plugged in.
    Here is a drawing of what I am trying to do.
    Any solutions?
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,500
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    Jan 21, 2010
    It's likely that those strings of LEDs are in parallel. This is a really bad way to connect LEDs, but is not uncommon in battery powered LED christmas lights. It also relies to some extent on the internal impedance of the batteries to limit current.

    Connecting a large number of these in parallel only makes things worse.

    The best option (and it's not necessarily a good option) is to place several of these strings in series with a "current limiting" resistor, and then parallel them all up.

    The first thing you could do is to measure the voltage across, and the current through a single string when powered by the normal power source. Let's say it's 3.2V and 120mA.

    Connecting three of these strings in series would require 9.6V at 120mA. If connected to a 12V power supply, a (12-9.6)/0.12 = 20 ohm (use 22 ohms) resistor would be suitable. If you have 100 strings, then you can connect 99 of them in 33 strings to a 12V power supply capable of 4A or more (I'd recommend at least a 5A power supply).

    Remember that my recommendations are based on assumptions and totally "pulled out of my a... um hat" figures that you need to measure. Do you have a multimeter? Do you know how to measure current?
     
  3. Rolltop

    Rolltop

    5
    0
    Dec 7, 2016
    Hi Steve thanks for getting back so soon.
    I looked under magnification and looks like the bulbs in each string are in parallel.
    I do have a multi meter but am un sure how to measure current.
    Do I do this with the lights on and where they connect to the power supply?
    Also which setting on the meter?

    I have only 10 strings to put together so I would connect 3 sets of 3 in series and 1 string together with them in barrel?

    Thanks for the help, I am pretty much a beginner at this.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,500
    2,840
    Jan 21, 2010
    To measure voltage, simply set your meter to a voltage range (it may autorange, or you might have to set it to (say) 20V.

    typically the black meter lead goes into the common connection and never moves. The red lead will connect to the point often labelled as V/Ω, or V/Ω/mA.

    Place one lead on each wire leading to the LEDs.

    To measure current you need to select a current range. choose one that is at least 250mA. You *may* nead to move the red lead to another socket to measure current. This will almost certainly be true if you pick a large current range (like 10A).

    To measure current you need to breal one wire leading to the LEDs and place one probe of the meter on each pf the separated ends of the wire (placing the meter in series).

    Be very careful making current measurements because you can damage your meter if you do it wrong. There is probably ness chance of doing this with battery powered equipment, but you can certainly blow a fuse in your meter. Depending on the meter the fuse can be *very* expensive.
     
  5. Rolltop

    Rolltop

    5
    0
    Dec 7, 2016
    I measured the voltage out put of the supply at 1.2 V DC .
    So I will break one of the wires between the led string and the power supply and then measure the current in mA between the the supply and string.
    my meter has 10 and 250 mA setting.
     
  6. Rolltop

    Rolltop

    5
    0
    Dec 7, 2016
    On the 10 mA setting it reads 9
     
  7. Rolltop

    Rolltop

    5
    0
    Dec 7, 2016
    the voltage across the leads when plugged in and running is 1.24 V
    the mA measured as you directed reads 9 with lights burning
     
  8. Audioguru

    Audioguru

    3,440
    738
    Sep 24, 2016
    The LEDs are probably driven from a voltage stepup circuit that limits the current. You cannot measure the LED voltage or current because it is pulsing on and off at a high frequency. 1.24V continuously is for an infrared LED that is invisible, 1.8V is for a red LED and 3.2V is for white, blue and bright green LEDs.

    Each string of LEDs must be in series with its own current limiting resistor then all the strings with resistors can be powered from an AC-DC power supply.

    10 LEDs would use a current of from 30mA (3mA each) to 200mA (20mA each).
     
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