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Quick LED question

Discussion in 'Electronic Basics' started by Jason S, Sep 11, 2006.

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  1. Jason S

    Jason S Guest

    I want to connect 8 white high brightness LED's together.
    What is the safest way to connect them? I've thought about 1 resistor per
    LED, but is it really necessary if all the LEDs have the same specs and
    power requirements? I want to save as much room on my PCB as possible.

    Power supply = 12V DC
    LED type = White, 7000mcd, 3.5V Forward V, 30mA typical current.

    If I connect them all in parallel, and use one single 3 or 5W resistor at
    say, 39ohms, would that be ok?
    Or would it be better if I just do two groups of 4 LEDs instead (with
    separate 82ohm or so 3W/5W resistor for each group)?

    The reason for the question is because as you know, the high brightness LEDs
    are still quite expensive.

    Thanks in advance,
  2. SDC

    SDC Guest


    1. Reducing the number of resistors won't reduce the cost of the LEDs.

    2. The forward voltage drops of the LEDs will not all be equal, so a
    different current will flow in each, resulting in varying brightness. You
    have 2 alternatives - Either connect the LEDs in series with a
    constant-current source or use 1 resistor per LED. (This is not expensive,
    at a couple of cents/pence each.)
  3. Jason S

    Jason S Guest

    Sorry, but you must have misunderstood me about cost. I meant the reason I
    am asking is to avoid ruining my LEDs if I connect them up wrong. It had
    nothing to do with the resistors =)

    Thanks for your reply anyway
  4. SDC

    SDC Guest

    No problem Jason - glad I could help.

    .... Steve C.
  5. John Fields

    John Fields Guest

    Since the LEDs will all have slightly different forward voltage
    drops, you may _not_ connect them in parallel and share a current
    limiting resistor.

    What you _may_ do, though, is connect them in series and use a
    single resistor for each series group. However, since the Vf of the
    LEDs is 3.5V and you have a 12V supply, the largest number of LEDs
    you can connect in series is three, since voltage drops in series
    add and 4 * 3.5V = 14V.

    If you've got a stiff supply you could connect the LEDs in two
    groups of three and one group of two, like this:

    | | |
    [51R] [51R] [180R]
    | | |
    |A |A |A
    [LED] [LED] [LED]
    | | |
    |A |A |A
    [LED] [LED] [LED]
    | | |
    |A |A |
    [LED] [LED] |
    | | |

    That is, if the _typical_ forward current is 30mA. Check it again
    and make sure it isn't 20mA, which I believe is more common, and if
    it is, recalculate the resistor values.
  6. davidlaska

    davidlaska Guest

    I would think each led set of three with a resistor in pretty standard
    for consumer stuff. Naturally the one resister per led is better, as
    it is with all electrical stuff to be isolated rather than share.
    White Led,s are less than a dollar now.

    But john, I have a question about 12 volts. The assumption is using a
    quality source for power. These wall adapters rated at 12 volts are
    only good for the device it goes with. Otherwise the voltage can be as
    high as 20 vdc.
  7. You are exactly right john, That is the way I did on a board holding over
    1000 white led's and it works well. You can't imagine the brightness of
    that many white led's in a 8" by 10" space.
    If anyone want my pcb artwork I will send it to A.B.S.E. group. JTT
  8. Lord Garth

    Lord Garth Guest

    John Fields warns you about that in his sentence that begins:
    "If you've got a stiff supply ..."

    You will need to use a voltage regulator to stiffen the supply.
    You have 10.5 volts across the LEDs, the regulator will require
    some as well. Use one of the LDO regulators available from
    National Semiconductor:
  9. Lord Garth

    Lord Garth Guest

    Sure, send away!
  10. John Fields

    John Fields Guest

    That's not necessarily true, and particularly not true in this case.


    If you have an LED with a forward voltage of 3.5V and a supply of
    12V, then you'll need a resistor:

    Vcc - Vf 12V - 3.5V
    R = ---------- = ----------- = 283.3 ohms
    If 0.03A

    The closest safe standard 5% resistor is 300 ohms, so the current it
    will allow in the circuit will be:

    Vcc - Vf 12V - 3.5V
    I = ---------- = ------------ = 0.028 ampere
    R 300R

    And the power it will dissipate (waste) will be:

    P = I²R = 0.028A² * 300R = 0.235 watts.

    Now, since there'll be one resistor for each LED and there'll be 8
    LEDs in the circuit, the total power wasted by the resistors will be
    1.882 watts.

    With 3 LEDs in series, they will drop a total of 10.5 volts, which
    means that, with a 12V supply, the current limiting resistor for
    that string will need to be:

    Vcc - (3Vf) 12V - 10.5V
    R = ------------- = ------------- = 50 ohms
    If 0.03A

    The closest safe 5% value in this instance will be 51 ohms, which
    means the current in the string will be:

    Vcc - (3Vf) 12V - 10.5V
    I = ------------- = ------------- = 0.029 amperes
    R 51R

    and the power the resistor will waste will be:

    P = I²R = 0.029A² * 51R = 0.043 watts.

    Since there will be two 51 ohm resistors in the circuit, with each
    one dissipating 43 milliwatts, that's a total of 86 milliwatts
    wasted for 6 LEDs, plus whatever will be wasted in the resistor for
    the 2 LED string, which you can figure out.

    If/ when you do, you'll see that the benefits for connecting the
    LEDs in series are threefold:

    1. You get to use fewer, smaller wattage resistors. (Less $)

    2. You get to use a smaller power supply. (Less $)

    3. You waste less power. (Less $, less heat pumped into the
    Why is that relevant?
    That's not true.
    As long as the current delivered into the load is the current the
    wall-wart was designed to supply, everything will be fine if the
    load can also stand the ripple.
    But, the OP specified 12V, so that was the basis for the solution I

    If you'd like to come up with something which would cover every
    unmentioned eventuality, I'm sure it would make interesting reading.
  11. davidlaska

    davidlaska Guest

    This is why I referred to the cost near the end, He was concerned
    about the expensive LEDs.
    I was in the mind set that he wanted to whatever it takes to not damage
    the led,s regardless of efficiency. But your approach is the correct
    approach for getting maximum use of all resources. I did not know a
    stiff power source is, but I had a feeling that I should have known
    that it was a given based on your experience regardless.
    Again I was looking at safeguarding the led's however unreasonable the
    method is.
    I know it is a simplified view, I usually tell people that just in case
    they read the adapter incorrect.That is something I should have not added to the solution.
    That is my fault for reading 12v as other voltages.
    I made my own led board once and I did like the 3 led one resistor
    setup you showed here. So I skipped some parts of your post, answered
    only with ideas to treat his led as the main concern. But things should
    be going better by now. ( I learned what stiff power source means by
    the way)
  12. ehsjr

    ehsjr Guest

    For some reason, I couldn't reply over on ABSE.

    Nice layout! There's a possibility of a mis-scan
    26 columns in going from left to right. Looks like
    some of the traces are touching. Since it works well
    for you, I'm assuming it scanned improperly.

    How did you determine that you have sufficient cooling?

  13. davidlaska

    davidlaska Guest

    How much did that project cost or how much would it cost a consumer in
    parts? I only ask because I saw a unit of 1000 leds set into a board
    and sealed in epoxy or such. They were on sale for 400 dollars a unit
    a few year ago. Usage was for lighting streets and pathways with
    something vandals would have a difficult time with.
    I bought 500 yellow siemons branded leds and soldered them wire to wire
    and no resistors for running on 2 volts. Worked with one lead acid
  14. The led's I got were on eBay from a vender in china. For 1000 led's it cost
    a little over $100. I may try out some 10mm led's rated for 130,000 mcp I
    saw on eBay for 10.00 for 100 of them + 25.00 shipping just for curiosity.
    The pcb I used is an fr4 material with 1 oz copper. Mounted the pcb in a
    standard wood picture frame to keep it from warping.
  15. Thanks ed. Maybe I missed that part on the traces touching, I scanned it
    from the printed output of the layout program into Photoshop. I'm going to
    have to take another look over my artwork and make sure now.
    As far as determining cooling, I just old fashioned touch to see if there
    were hot areas. With a nice air current flowing along the bottom of the
    board, the led's stay fairly cool.
  16. Jason S

    Jason S Guest

    Thanks John. Terrific help as usual.
    See, I told you you would help me again in the future! Remember that motor
    controller about 3 months ago? =)

    One more question though, about your 'series' connection: What is the
    likelihood of one LED failing for some strange reason and causing the rest
    of the LEDs in the string to get affected? Kind of reminds me of those
    fairy lights at Christmas time... you know, one bulb fails causes a whole
    lot of them to go out as well. Not a huge concern, but just a little

  17. John Fields

    John Fields Guest

    My pleasure. :)
    LEDs are very reliable if you don't beat 'em up, so the likelihood
    of failure is remote.


    If one should fail open the remaining LEDs in that string will go
    out. If one should fail shorted, then the current in that string
    will increase because the voltage drop across the resistor will

    In essence, a 3 LED string will become a 2 LED string and the
    current through the string will increase to:

    Vcc - (2Vf) 12V - 7V
    I = ------------- = ---------- = 0.098 ampere
    Rs 51R

    So that'll probably fry the other two LEDs in short order.

    If one of the LEDs fails shorted in the 2 LED string the current
    will increase to 47mA, which is still bad if the absolute maximum
    current rating for the LED is less than that.

    The way to sidestep the whole issue, though, would be to use
    constant-current sources/sinks to feed the strings. Want a
  18. davidlaska

    davidlaska Guest

    How bright is this 1000 led panel you put together ( did you do it one
    by one or discovered a short cut?) in real life. What wattage
    florescent light would be the same, or if you used white leds, halogen
    or softwhite bulbs. Volts X Amps = Watts could be a common ground.
    Or just something simple like it is as bright as a x bulb.


    PS, what angle are the leds? wide or narrow?
  19. I don't have a meter to check the brightness output, but it is more then
    enough to replace the lamp in an lcd projector. I did a small board of 96
    led's running on a 12 volt NiCad battery pack and took it outside at night,
    just to see how it stood up to a flashlight. That small panel would light
    the trees across the street from my house. The light it makes also seems to
    make everything in high contrast. Give it a try and you will see what I mean
    by that.

    As far as wattage, (350 strings drawing .025 ma.) = 8.75 amp = (12v * 8.75
    amp) = 105 watt on the panel. But you can judge the led brightness by that,
    as Led's are more efficient then Incandescent lamps.

    The angle on the led's were 20 degree if I remember correctly.
  20. Jason S

    Jason S Guest

    No, schematic not necessary.
    Thanks John. I think I'll be right from here. =)

    Best regards,
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