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Thread: I need help with LED strings

  1. #1
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    Default I need help with LED strings

    OK I know enough about electronics to build most things but I am having a world of trouble with LED strings and their operation. Here is the deal. On my gutters I want to put a run of LED lights that contains 6 strings (Green, Blue, Amber, Purple, Yellow and Red) they are set up each as a separate run with a common return (the common is the positive side which is common on the board. Each string is in series and I did the calculations for the resistor for a 48volt supply. I am using two of the DC ssr from Labrat (8 total channels with 2 unused). My problem is that each string runs fine by its self but if two string go on at the same time they drop out and just glow. What am I doing wrong? My supply has 8 amps so there should be enough to drive it all. I am stumped and could use some help.

  2. #2
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    Default Re: I need help with LED strings

    Does each string have is own resistor, and what voltages are the strings? What is the voltage on the positive side wher we iit connects to the strings? I assume the positive goes to all the strings and doesn't go through the string before choosing to the nest string?

  3. #3
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    Default Re: I need help with LED strings

    Each color channel is on its own circuit of the DC SSRs and is 48v. I am using D5NK60 MOSFETs which are rated a 650v and 5a. There are 14 LEDs in each color string. I used an online resistor calculator for the proper value for the resistors. They are as follows Blue 3volt - 330 Ohms, Green2.2volt - 1000 Ohms, Red 1.8volt - 1200, Amber 2volt - 1000 Ohms, Yellow 2volt - 1000 Ohms, Purple 3.3volt - 100Ohms. Also the resistors 1/2 watt and they run cool with none of them getting hot.

  4. #4
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    Default Re: I need help with LED strings

    How much current to each of the strings draw? What gauge wire are you using and length? I'm wondering if maybe your common has to much voltage drop.
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  5. #5
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    Default Re: I need help with LED strings

    I am not sure of the total current for each string but it can't be more than .1 amp. I am using 22 AWG wire for the common and 24 for the strings.

  6. #6
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    Default Re: I need help with LED strings

    What controllers and/or SSR are you using? Those MOSFETs require more than 5V gate drive in order to work well, they are not logic-level parts.

    Never mind about the controller/ssr question - I see the answer now.
    Phil

  7. #7
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    Default Re: I need help with LED strings

    I think that using a higher-voltage regulator on the board (perhaps 10V or 12V) in place of the LM7805/LM78L05 would help a lot. Perhaps Ernie or someone else might have something to say on this.
    Phil

  8. #8
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    Default Re: I need help with LED strings

    P. Short,I did preliminary test using a 12v regulator and I had success! I checked the specs on the MOSFETs and they need about 4.5v the 12v gives all of them what they need. I will do a full test in the next few days and let you know. For now I am pumped!

  9. #9
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    Default Re: I need help with LED strings

    without knowing the current through the string, there is no way you could calculate a resistsnce value.


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  10. #10
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    Default Re: I need help with LED strings

    MAJOR success! The lights are now functioning great and the load is no problem with several strings hooked up. Now I just need to get them up. I'm in the Minneapolis area and we got 14 in of snow and cold weather to go with it. My roof is a little too icy now to get up there. They say that some warmer weather is coming but I will believe it when I see it. For now I'm getting things ready.
    Thank you so much P. Short for your assistance. Martin, I have found a website which is great in finding the proper resistor value for LED strings. The site is http://led.linear1.org/led.wiz

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