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Thread: resistor question(noob needs help)!

  1. #1
    Join Date
    Jul 2012

    Default resistor question(noob needs help)!

    Hello again forum users, it has been a while since I have been on here. I had a quick and hopefully easy question for some of you LED techs out there. I am planing on making my own LED grow lamp for personal use. It is going to be a simple design using 3w Blue and Red LEDs which I will supplement with florescent lighting until I can make a lamp with a larger verity of colors. Anyway, I digress. The issue you I am running into is how should I go about wiring them to resistors. I want to do this in the most efficient way possible. Here are some specs to help out.

    3w 440nm BLUE LED w/heatsink fv 3.6v fc 700mA, I will be using 67 of these
    3w 660nm RED LED w/heatsink fv 2.4v fc 700mA, I will be using 33 of there, for a total of 100 LEDs.

    The simple LED array tool I used tells me to wire the blues as such 22 sets of 3 blue including a 1.8ohm 1 watt resistor with a remainder of 1 LED with a 12ohm 6watt resistor. The reds are wired as such 6 sets of 5 red with a 1ohm 1/2 watt resistor with a remainder of 3 leds on a string using a 7.5ohm 4watt resistor.

    My question is, is there any way I could string the Blue and Red leds together to make a more efficient system and use less resistors. Any help would be appreciated, thank you LOKE!

    OOOPS!!! I forgot to add that I will be using a 350watt CPU power supply with the 12v rail.

    So while I was waiting for a reply I was doing some reading. On a couple of forum posts on this site and others I was seeing an argument. The argument being that if you use up all of the voltage IE if i were to use 2 BLUE leds and 2 RED LEDs that would = 12v ie I would not need a resistor. Some people were saying that would not be the case and just the way that LEDs worked I would atleast want a 1ohm 1watt resistor on the string just to have it there. So the question is do I need to place a resistor on a 12v string of LEDs on a 12v line and if so what would be the correct resistor.

    Also IF I did my math correct this is the most efficient way to string them 16x 2blue2red +resistor(?needed?) with 11 strings of 3 blue with a 1.8ohm 1w resistor and then 1 string of 2blue 1 red with a resistor(which resistor should I use?). Please correct me if I am wrong!!!

    Again any help is greatly appreciated!
    Last edited by Drdallon; 11-02-2013 at 08:37 PM.

  2. #2
    Join Date
    Jan 2004
    Pleasanton (Bay Area), CA, USA

    Default Re: resistor question(noob needs help)!

    You are on the right general track with your project for a resistor based setup. As you can see, some combination of LEDs and resistance in a string is needed to get you to the supply voltage. You can mix and match the LEDs and resistance in series as needed.

    In a perfect world, all of this can be calculated exactly, but in the real world, some variables exist:
    - 1 ohm resistors are not really 1 ohm, especially once they warm up
    - Parallel vs series changes the effective resistance
    - The 12 VDC supply will not always be 12 volts
    - The LED Vf is not a constant, rather, it depends on current, and the individual LED itself
    - LEDs sometimes fail. Some brands fail open, while others are designed to fail close (such as Philps)

    What this means, is that it is a bit trial and error. The nice thing, is that LEDs are pretty cheap compared to in the past, so when (not if) you blow a few, it isn't that bad. Buy some sunglasses.

    In theory, if you had 36 VDC, then fewer strings are needed, so fewer resistors, and the system as a whole would be more efficient, but 12 VDC is common.

    When you work with "resistance to manage the current", then it helps to just accept that you cannot run the LEDs as close to their design spec as you could with a true “current driver”, perhaps assume 75%. This gives them a little room to spare as things vary.

    I have had good luck using strings with a 1 or 2 ohm resistor in each string, but I didn't attempt to assemble as many strings in parallel as you are. There is a good reason to have a resistor in each string, and an EE explained it to me in the past, but I cannot remember the explanation well enough to tell you exactly why, but it is very useful. If you want your LED strings to last, the resistor in each string is needed.

    It might be worth buying some precision resistors to work with. That way, you can test the actual current in each string by measuring the voltage drop across each resistor in each string as you build it up.

    Just so you know, there is no perfect LED plus electronic driver setup. When using resistors, some power is just going to be lost in the resistors. In a driver, they also need some headroom voltage to make it all work, so more strings are needed. Switching power supplies like you are using sometimes benefit from having a big capacitor across the output to stabilize them, but if you do this, add a high resistance “bleed off” resistor across the cap so that it discharges when turned off. Those big power caps can knock you on your butt.
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  3. #3
    Flashaholic* uk_caver's Avatar
    Join Date
    Feb 2007
    Central UK

    Default Re: resistor question(noob needs help)!

    If you're using a resistor for current control, there's a tradeoff between efficiency and stability.
    The more efficient the setup (the less voltage 'headroom' between what the LEDs want to run at your target current and what the supply provides, the more the current will change for a given change in LED Vf (such as when the LEDS heat up and they Vf drops).

    If you're looking at running the LEDs at ~700mA, one other possibility, at least for strings which are pretty close to 12V is to use a driver with pair of AMC7135s for each string of LEDs.
    They're pretty cheap (about a dollar for a board with 2x7135s on), and don't need much headroom to regulate the current, so they would allow you to get your total string Vf close to 12V while still having a stable setup.

    They're designed for running from up to a 6V supply, but there's a way of wiring them up to work with higher-voltage strings as long as the overall voltage headroom doesn't result in them getting too hot.

    As far as the LED Vfs go, I'd be tempted to get hold of some and do a quick check to find out what they actually are at 700mA when cold (likely they'll be a little lower that that in actual use), and to see what kind of variation there is between devices.

    VF checking is pretty quick/easy to do in relative bulk with a 2xAMC7135 driver board and a cheap DMM - just wire the driver to a suitable power supply (like 4xNiMH) cells, and have the LED outputs from the board wired to a couple of croc clips. Clip the croc clips to the probes of a DMM set on a voltage range, then you can prod the pins of various LEDs and see what voltage reading you get, making sure you don't look at the LED when it's lighting up.

    It might be that you could get some particularly efficient wiring system once you know what the Vfs of the LEDs really are in practice - the blues could well be meaningfully lower than 3.6V, and maybe you could get a regulated 2B+2R string running from 12V.
    It's even possible that 3B+1R might just work.

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