Need help with led circuit.

Discussion in 'The Projects Forum' started by JasonAyersR, Aug 25, 2009.

  1. JasonAyersR

    Thread Starter New Member

    Aug 25, 2009
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    0
    I have put together a circuit with a power supply of 3.3VDC 2.5A. I have used 18AWG to feed each of the six branch circuits. Each branch circuit is wired with 24 AWG with variable numbers of 5mm led's wired in parallel with forward voltage between 3.2 and 3.7 and operating at 20ma DC.
    I would like to know what problems I might run into with this setup. What i am wiring requires that I use limited amount of space and more complicated design was impractical. Parallel wiring with set voltage was ideal for my application however i was wondering if i am asking for trouble with this many leds. The power supply has over current protection and is self switching/adjusting designed for input anywhere between 90VAC to 240VAC >5sec at max with a continuous 3.3vdc output. My question is: if I have a source voltage that is an ideal nominal for my led application with enough current what are the concerns if any to the integrity of my design. In my diagram the resistor symbol represents the diodes in each branch circuit. Please excuse the error. I am using EDRAW. Any suggestions would be great. Thanks.
     
  2. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    > My question is: if I have a source voltage that is an ideal nominal for my
    > led application with enough current what are the concerns if any to the
    > integrity of my design.

    Too much current --- you want to make sure that the current through
    each LED never gets much higher than 20 ma. While LED's limit their own
    voltage, they do not limit their own current.

    > Parallel wiring with set voltage was ideal for my application however I
    > was wondering if i am asking for trouble with this many leds.

    When you wire LED's in parallel, you need to limit the current to each
    LED separately; it's not enough to just limit the current to the whole
    parallel circuit because the current may not be evenly distributed between
    LED's and because, should one LED burn out, the others will receive more
    current, so pretty quickly the whole string will go kaput. A simple way to
    do this is to wire a suitable resistor in series with each LED.

    Your circuit diagram is confusing because it doesn't follow the usual
    conventions for diagrams, so I have to guess to figure out what you mean.
    In particular, it took me a while to figure out where the LEDS were because
    you used a resistor symbol instead of a diode symbol --- this is all the more
    confusing because I take it that the component labelled "1 Ohm" is really
    a resistor.
     
  3. JasonAyersR

    Thread Starter New Member

    Aug 25, 2009
    3
    0
    Yes as I said the program used to draw this diagram is lacking in symbols. I understand that I have more than needed current. I have 55 led lights in total is there any way to limit the current without placing a resistor in series with each led light. I always understood from linework that current available does not equal total current used. If the forward voltage is correct why would an led diode draw more than it uses to operate. These lights work now and I am just afraid of the lifespan especially after what you told me. What would be the most simple way to correct the layout I have now. Can I place a 1 ohm resistor in line with each of my six branch circuits. Will this help at all. Thank you for your time responding to me.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Jason,
    If your LEDs are rated at 20mA with Vf's from 3.2v to 3.7v, they're just not going to work well at all from a 3.3v supply. Some will be very dim or not lit at all (due to their higher Vf) and some will be very bright (due to low Vf) and the latter will rapidly burn up due to overcurrent.

    An LEDs' Vf will change over temperature. As it warms up, the Vf becomes lower. If there is no current limiter, the LED will pass more current, making it hotter, so that it passes more current, making it hotter yet, etc. This is a thermal runaway situation.

    A much better arrangement would be to use several LEDs in series with a resistor. If your typical Vf is 3.5v @ 20mA, you could wire two, three or more in series with a suitable resistor for the regulated voltage supply that you are using. Sadly, 3.3v is not adequate.

    Let's say your typical Vf is 3.5v@20mA, and you're going to use strings of three LEDs.
    3 x 3.5= 10.5v. You'd need a regulated 12vdc supply.
    The current limiting resistor for each string would need to be (Vsupply - VfLED)/DesiredCurrent = (12 - 10.5)/20mA = 1.5/.02 = 75 Ohms.

    There isn't an "easy fix" for how you have things connected right now.
     
  5. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    > If the forward voltage is correct why would an led diode draw
    > more than it uses to operate.

    That's true, but the problem is that diode current depends quite
    sensitively on forward voltage, so you would need to regulate the
    voltage quite precisely, to within a few thousandths of a volt.
    Raising the voltage by about a fortieth of a volt is enough to
    double the amount of current.

    Even if you did nail down the forward voltage precisely, that still
    would not be good enough because the current also depends on
    temperature. What makes this especially bad is that, if you keep
    the voltage constant, the current will increase as you increase
    temperature, which leads to the problem of thermal runaway.
    That means that, if a LED connected to a voltage source gets a bit
    too warm, then the extra current flowing through will heat it
    further, which will make even more current flow, etc. so pretty
    quickly it will overheat and burn out.

    If, as you propose, the LED's are connected in parallel so they all
    are at the same voltage drop, there is the problem that the correct
    voltage drop will vary from unit to unit. Just because the LED's
    you have say 3.3 V at 20 mA doesn't mean that each LED in the
    batch will drop exactly 3.300 V. As explained above, even small
    differences in voltage will lead to big changes in current, so some
    of the lights in the string would be brighter than others (and
    might very well burn out from too much current).

    The bottom line is that, whilst regulating the voltage looks fine in
    principle, it doesn't work in principle, so you need to regulate the
    current to operate LED's reliably.

    My suggestion would be that, unless you are wedded to that
    particular power supply, to consider getting a higher volatge
    power supply, wiring the LED's in series in strings with a single
    dropping resistor for each string, then wiring the strings
    together in parallel.
     
  6. JasonAyersR

    Thread Starter New Member

    Aug 25, 2009
    3
    0
    Thanks very much for the advice. i guess Im going to have to take it apart and start over. at least I hadn't started soldering yet. I should have read up on this before I began. Thank you both for the help.
     
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