Simple LED Circuit Trouble

Discussion in 'The Projects Forum' started by MM Trigger, Jul 3, 2007.

  1. MM Trigger

    Thread Starter New Member

    Jul 3, 2007
    5
    0
    I'm designing a rather simple LED circuit as shown here:

    [​IMG]

    I believe I have the resistor set properly and the LEDs configured properly too. Specs for the LEDs are here:

    http://www.led2go.com/J800SE4LD.htm

    When I hook everything up on a breadboard and connect a 12v power supply, none of the LEDs seem to light. Individually using a 3v PSU (it is the only thing close to 3.3v I have hiding around here), each LED lights properly.

    There has to be something simple I'm overlooking...
     
  2. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    I would suggest using at least three resistors, one in each column of LEDs. This will help to ensure the available current is shared between the three columns of LEDs.

    By my calcs that 33 should be more like 11 ohms, and with 180mA (20mA / LED) through it, that's dissipating almost half a watt!! Besides, it's just not a good way to do this.

    Loose the 33 ohm. Put a 33ohm resistor at the top of each LED column instead. Bear in mind the forward voltage drop of the LEDs which could be up to 3.4V per LED!
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    What is the formula for series leds. It should be easy to find or figure but I'm not sure.
    I get 40 ohms per 3 diode string if the diode voltage is 3.2 and current is 20ma.
     
  4. MM Trigger

    Thread Starter New Member

    Jul 3, 2007
    5
    0
    The formula I had been using for each of the series sets is

    U = RI + V*(# of LEDs)

    U = input voltage
    V = forward voltage drop of each LED

    So I ended up with 12v = R(0.02A) + (3.4v)(3) ==> R = 90 ohms
    (I assume I will only draw 0.02A per series set.)

    I then simplified the 3 resistors I would need and ended up with a single 30 ohm resistor. The closest one I could find being a 33 ohm.

    Oh... I see what you mean about going over a half-watt on the resistor. I have to change the design. In that case would a ~90 ohm resistor be correct for each set, or is there something else I'm omitting in the calculation?
     
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    I might add that there is no problem in using one resistor if all LED's have the same type and if each series association has the same number of LEDs.

    I think the problem there might be the resistor or a bad contact somewere in the breadboard (check the rails). Also, considering about 20mA per LED (no safety factor, I would use 16mA), we need 60mA total. The voltage drop for each series of LEDs is 9.9V (3.3V per LED - but biasing curves are lacking and I cannot adjust the values better). So:
    R = (12V - 9.9V) / 0.06A = 35 Ohm

    I would recommend a 39 Ohm resistor. Also, if it is for a car, you should consider that you can reach 18V. I would advice you to use shorter series of LED's (two LEDs in series), so the light intensity won't vary much, in case this is used in a car.
     
  6. MM Trigger

    Thread Starter New Member

    Jul 3, 2007
    5
    0
    I finally got the design to work! Borrowing the idea of how to modify it for car use, I shortened each series by one LED and expanded it to 5 parallel sets of 2 LEDs each (I originally had 10 LEDs I could use anyway). I dropped in a 150 ohm resistor on each series set and now it works nicely.

    This circuit will eventually be used in a PC on the 12v line. I was testing it using a power adapter from an external hard drive enclosure that also supplies 12v.

    Thank you all for your assistance!
     
  7. MM Trigger

    Thread Starter New Member

    Jul 3, 2007
    5
    0
    Actually, I do have one more question about this project. Here is my revised design:

    [​IMG]

    I need to have a working assembly by Sunday, and I've run into a problem. I don't know what would be the best way to distribute power to each of the series sets. I was considering making my own PCB, but I lack the materials nor experience in producing one. A solder splice is another alternative, but I don't know if soldering 6 wires to one joint is a very good idea... Any advice would be appreciated!
     
  8. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Try a stripboard (also Veroboard) or a tripad board. They are good for most projects and will be the ideal for yours.
     
  9. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Six or sixty it will work for a temporary solution. If you have perfboard, strip board or the like it's neater. I've used various materials and drilled my own holes.

    I have an LED light with a current regulator IC, transistor, resistor and LED that is made using their respective wires. It's simple and I can claim it's art not laziness. I stuffed all of it in a small plastic bottle (ha, do that and not get a short!) where the LED fits through the lid. It's all connected to a battery pack. I just tested how long it worked on two D cells and it only started getting dim after two weeks.
     
  10. MM Trigger

    Thread Starter New Member

    Jul 3, 2007
    5
    0
    60 huh? I'd like to see that!

    I managed to find a supplier locally that sells perfboard that is close to strip board. It has copper strips I can use for power anyway. Now I just have to assemble everything and put it into the housing.

    Once finished, it'll be a lighting box for a computer. Since the LEDs were too large to use in clear fans or the like, I'm just throwing them together to make a kind of accent light for the inside of the case. Its a gift for someone, hence the deadline.

    Hopefully from here on out everything will go smoothly. Funny, I'm a computer engineer in training but haven't actually assembled a physical circuit yet (let alone analog) outside of prototyping. Guess I have to start somewhere. Thanks for all your help once again!
     
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