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how to determine the right voltage for led lamp?

Discussion in 'Troubleshooting and Repair' started by pharaon, Jun 3, 2019.

  1. pharaon

    pharaon

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    3
    Oct 28, 2014
    how can i determine the right voltage that could light up some leds in old lamp because it's own adapter is missing
    is it related to the number of the leds in it or what
    it's a rounded lamp and the leds like a string around it
    when i test it on the diode mode with my multimeter i find out that each 4 leds light up together as they are connected in parallel ..so to light the whole string how much voltage do i need
     
    Last edited: Jun 3, 2019
  2. Alec_t

    Alec_t

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    Jul 7, 2015
    Yes, the number is important.
    LEDs should be current-driven. You need to know the maximum allowable current draw of each LED (at least approximately) as well as the series/parallel configuration of all the LEDs and the number of LEDs.
     
  3. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi, first of all, how many groups of four leds are in
    Parallel ?. What kind of LEDs are they?.
    LEDs are current driven devices. They have a ‘turn on’ voltage but that depends on what kind of LED they are. For example, if four 2 volt leds are in series, that would be 8 volts. They might require 25 milliamps total.
    If in parallel, it would be 2 volts and 100 milliamps.
    So a correct answer cannot be given without knowing what LEDs you have.
    Has the lamp got a label on it with any information?.
    The connecter where the adapter goes, has that got any information on?.
    Failing that, you’ll need to take it apart and post some pictures of both sides of the LED board.

    Martin
     
  4. Martaine2005

    Martaine2005

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    May 12, 2015
    @Alec_t beat me to it. Have you WWW. Searched for your lamp?. Google images is great.

    Martin
     
  5. pharaon

    pharaon

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    Oct 28, 2014
    how can i find out

    [​IMG]

    [​IMG]

    and i count it's all 90 leds
     
  6. Bluejets

    Bluejets

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    706
    Oct 5, 2014
    From the label, it looks to me like whatever that LED strip came out of already has a mains driven unit inside it.

    Not a monitor by any chance..??
     
  7. pharaon

    pharaon

    239
    3
    Oct 28, 2014
    that wire should be connected to the adapter that give it the power it needs to
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    The question i submitted was to find where the LED strip originated as it might give some insight as to the control required.

    Your photo suggests it came from some piece of equipment.
     
  9. pharaon

    pharaon

    239
    3
    Oct 28, 2014
    it's a lamp and there's no other data available , only 18 watt on the back of the lamp

    so based on the number and the watt can't i determine the required voltage to lighten it up
     
    Last edited: Jun 4, 2019
  10. Alec_t

    Alec_t

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    Jul 7, 2015
    Assuming all the LEDs are the same, then 18W/90 = 0.2W per LED. If we now assume the LED Vf is 3V, then the implied current per LED is 200mW/3V = ~66mA. So a group of 4 LEDs in parallel would draw 264mA max. There could be 90/4 = 22 such groups wired in series (can you check?).
    The wide input voltage range of the lamp indicates the power supply was a SwitchMode type. My guess is that it was configured as a 250mA constant-current supply, with an output voltage rating of just over 3V x 22 = 66V.
     
  11. pharaon

    pharaon

    239
    3
    Oct 28, 2014
    actually it's 90/5 =18 group i counted wrong sorry

    how can i check? they all share the positive line but the negative is every 5 leds
    [​IMG]
    is that enough to light up 90 leds?
     
  12. Bluejets

    Bluejets

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    Oct 5, 2014
    In the original photo it shows 2835 which is, as Alec_t says, 0.2W per LED.

    Alec_t has already shown how to calculate the voltage and current requirements for the supply.

    If you changed the number per group, recalculate with the new number and you have it.

    Just be aware there are various types of lED strip and it appears you are doing a refit with new strip...?

    Best of luck getting a driver for the original at such a comparitively high voltage as they are from my experience, at a cost in excess of the complete piece of gear. In this case some light or other.

    There are also out there, depending on application, constant voltage LED drivers.

    We use them often for downlight installs.

    Guide here........

    https://www.power-supplies-australia.com.au/blog/constant-voltage-or-constant-current-led-driver
     
    Last edited: Jun 5, 2019
  13. Alec_t

    Alec_t

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    Jul 7, 2015
    o_O That rules out a series connection of groups each having 5-in-parallel. The only reason for wiring them that way, that I can think of, is if the groups are multiplexed, or selectively switched. Seems unlikely.

    Edit: It's hard to tell from the post #10 picture, but it looks like the 5 LEDs of a group could be in series, with the 18 groups in parallel across the +ve and -ve rails. If that were the case, then the power supply would be something like 5 x 3V = 15V with a current rating of 18 x 60mA = 1.08A, which seems plausible and probably obtainable.
    Are you absolutely certain the 5 LEDs of a group are electrically in parallel?
     
    Last edited: Jun 5, 2019
  14. BobK

    BobK

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    Jan 5, 2010
    There do not appear to be any resistors on the strip, which means, whatever configuration they are using, it is not likely designed for a constant voltage supply. I am going to take a wild guess and say they have paralleled 5 strips of 18, which would make the voltage somewhere in the range of 50-60V and driven it with a constant current driver at around 300mA. That sounds about right as it would have about 60mA on each LED, and those look like medium power LEDs like that.

    If you can determine the actual connection scheme and verify that there are no resistors, or tell us if there are, it would help.

    Bob
     
  15. Bluejets

    Bluejets

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    Oct 5, 2014
    Bob, I have a funny feeling the Op is using a different LED strip in the 2 photos supplied.
    Bottom photo is what appears to be the protective paper stripped back from the "sticky surface" and I assume it to be a new strip.
    Whereas the top photo is all gunky and grimy like it's been around for a while in the garden.
     
  16. BobK

    BobK

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    Jan 5, 2010
    Hmm. That would be unfortunate.

    Now that I look at the back side of the strip, it looks like 5 in series.

    That wold argue for a lower vottage of 15 or 16V.

    Bob
     
  17. pharaon

    pharaon

    239
    3
    Oct 28, 2014
    so which one is the right one?

    here, this pic for me using 3.5 v with 0.2 ampere on one led and you can see the 5 is light up together
    [​IMG]

    also i made some other picture for the led stripe
    the little dots shows where the led is connected from the other side
    [​IMG]
    [​IMG]
     
  18. Alec_t

    Alec_t

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    Jul 7, 2015
    Does each dot have 2 conductive tracks going to/from it? They don't show up in the picture.
     
  19. Bluejets

    Bluejets

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    Oct 5, 2014
    ahhh....no we can't. Did you even look at the photo you took.
    It is just a bright overwhelming flash where zero detail is visible.

    Blowed if I can see any "white dots" except for the previous flash that made my eyes kinda wonky. :eek:o_O

    Here's an idea....why don't you simply post a link to where you bought the "obviously new" LED strip.
     
  20. BobK

    BobK

    7,626
    1,654
    Jan 5, 2010
    I don't see how you can light up just 5 of the LEDs on that strip. It looks like there are conductors top and bottom, which would be power and ground, and then 5 LEDs in series between them. If you power any of it, it should be powering all of it.

    In any case, it cannot be driven by a voltage source since there appears to be no current limiting (normally resistors) on the strip.

    Bob
     
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