Connect with us

LED fixture build help

Discussion in 'LEDs and Optoelectronics' started by bmplank86, Dec 17, 2013.

Scroll to continue with content
  1. bmplank86

    bmplank86

    6
    0
    Dec 17, 2013
    Hello! I am currently building an LED fixture for my 5 gallon saltwater fish tank. I have experience in electronics, but engineering something is a little new to me.

    I have 6 leds, each has a max voltage of 3.4v and I want them run at 650mah. The supply voltage is a DC power adapter of 24v at 2amps. These LEDS will be ran in series.

    Total voltage of the LEDs is 20.5v. What size resistor do I need to lower the supply to spread 650mah through each LED. This is ran in series, so current is constant, correct? According to my calculations I need a 32ohm resistor. Can someone help me out?
     
  2. mursal

    mursal

    75
    0
    Dec 13, 2013
    24VDC - 20V5 = 3V5

    3V5/650mA =

    3V5/0.65A = 5R3 (5.3 Ohm) resistor

    Resistor size (wattage) = 3V5 x 0.65 ≈ 3watt
     
    Last edited: Dec 17, 2013
  3. duke37

    duke37

    5,341
    753
    Jan 9, 2011
    I presume you mean 650mA (0.65A). mAh is a measure of energy such as in a battery.

    The current in a series circuit doesn't need to be spread across the LEDs, it will be constant throughout the circuit.

    My calculations are different from yours.
    6 * 3.4 = 20.4
    Voltage across resistor = 24 - 20.4 = 3.6
    R = V/I = 3.6/0.65 = 5.5

    This seems to be a bit low for current stabilisation and variations in PSU and LED voltages. Others may be able to advise.
     
  4. bmplank86

    bmplank86

    6
    0
    Dec 17, 2013
    Okay, I understand. You take the sum of the LEDs and subtract from supply voltage to get the left over voltage, which is what the resistor needs to limit. But what about current? Does it matter whether I use a 2amp or a 4amp power supply? If the diodes have a forward current of 650ma, and I supply it with more than that, what limits it?
     
  5. bmplank86

    bmplank86

    6
    0
    Dec 17, 2013
    Would it be safer to out the DC power supply w/ resistor and use a Meanwell driver? Constant current of 700ma and 9-48v
     
  6. mursal

    mursal

    75
    0
    Dec 13, 2013
    The resistor (≈6R) will limit the current flow to 650mA at that supply voltage, regardless of the available current from the supply. So it doesn't matter whether its a 2A or a 4A supply.
     
  7. bmplank86

    bmplank86

    6
    0
    Dec 17, 2013
    Right, but the resistor was calculated to make up for the extra voltage coming from the LEDs...how does that coorespond to the forward current for the LEDs? I guess I just don't understand how it lowers to the 650ma and makes up for the extra voltage.
     
  8. duke37

    duke37

    5,341
    753
    Jan 9, 2011
    The resistor was calculated to drop the required voltage at 650mA (not 650ma !). If you want less current, the resistor will be higher.

    If the power supply can maintain its voltage, it does not matter whether it is rated at 1A or 100A. It just needs to be powerful enough for the job.
     
  9. bmplank86

    bmplank86

    6
    0
    Dec 17, 2013
    Ding. Got it. Thank you very much
     
  10. davenn

    davenn Moderator

    13,558
    1,854
    Sep 5, 2009
    :)

    You need to remember that the primary function of the resistor is to limit the current, its voltage dropping effect is a secondary effect

    Dave
     
    Last edited: Dec 17, 2013
  11. BobK

    BobK

    7,680
    1,685
    Jan 5, 2010
    +1 on the constant current supply. For that kind of current, a simple resistor is not a good solution. They are not really all that expensive.

    Bob
     
  12. bmplank86

    bmplank86

    6
    0
    Dec 17, 2013
    How much safer is a driver vs the resistor with DC power adapter?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-