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LEDs - am I doing something wrong???

Discussion in 'LEDs and Optoelectronics' started by Lance Mannion, Jun 27, 2014.

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  1. Lance Mannion

    Lance Mannion

    37
    2
    Jun 9, 2014
    I purchased the following LED product from Radio shack

    http://www.radioshack.com/product/index.jsp?productId=2062568

    I bought 4 of these LEDs and had to return 3 of them after a few hours of testing a circuit.

    I have a power supply that is 12V and 1A. The LED is supposed to have a resistor built in and someone in the comments noted that is was a 680ohm resistor (??). I thought that should be OK.

    I also dropped the voltage down to 9V and still experienced an outage with one of the LEDs.

    I'm tried of going back and forth to Radio Shack ;) Am I doing something wrong in the wiring? Should I throw another resistor in for good measure??

    Thanks.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
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    Jan 21, 2010
    How bight do the LEDs appear, and how did they fail?

    Can you measure the current draw? Ddo you have a multimeter, and do you know how to measure current?

    You might like to read this for some background (the first 2 sections are probably all you need): https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

    From the description the assembly sounds like it has a resistor included.

    I suspect that your power supply is either a) significantly higher than 12V (measure it) or it's 12V AC. 12V AC will kill the LEDs very fast.
     
  3. Lance Mannion

    Lance Mannion

    37
    2
    Jun 9, 2014
    The LEDs seemed normal - not too bright and not dim. At one point they worked several times in the circuit when I tripped a switch. At one point I tested it again by tripping the switch and the LED did not light. In all tests, I also have a buzzer wired into the circuit and the buzzer goes off when the switch is tripped.

    Well, I dropped the voltage down to 7.5 and the amps read 11 mA. That means the resistor is about 680ohms which agrees with what someone mentioned in the comments about this component.

    Based on that, at 12V, the amps should only be 17.6mA which is below the 20mA advertised for this component on the Radio Shack package.

    The DC power supply measured 12.05V on the 12V setting and 7.55 on the 7.5V setting.
     
  4. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Can you please clarify on your working Voltage being AC or DC?

    I understand that the LEDs seem to work normally at first, but then don't light at all during a subsequent test?
    I would very much like to see the circuit you are working on as well.

    I used to sell those parts at the Canadian version of the store (The Source) and found them to work every time when I hooked them up to an old 12V DC power supply (We used it to test car chargers). I never used a series resister for these as they had one built in.


    *Please also double check your math for the built-in resistor. LEDs have a forward voltage drop, so your calculations should be:
    (7.5V - 2.2V) / 11mA = 482 Ohms
    (12V - 2.2V) / 20mA = 490 Ohms
    Red LED forward voltage drops range in the 1.8 to 2.2 mark
     
    Last edited: Jun 27, 2014
  5. Lance Mannion

    Lance Mannion

    37
    2
    Jun 9, 2014
    I have a DC power supply. The circuit is simple. From the power supply to a switch, and then an LED and buzzer in parallel.

    At 7.5V DC, the current draw was 11mA - both the LED was lit and the buzzer was buzzing.
     
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    Here is where my lack of experience in the field hurts :S
    I am very doubtful that the piezo buzzer would have any ill affect on the paralleled LED, but would still encourage you to try the LED on it's own.
    Perhaps someone here can fill in the missing blank.

    Can you also confirm if the 11mA is for the LED branch side of the circuit, or does this account for the buzzer as well?
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yes, I would try it without the buzzer.

    If that doesn't help, try a reputable source for your LEDs.

    It might also be worth trying a reverse biased diode across the LED to prevent issues with reverse voltage spikes.
     
  8. BobK

    BobK

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    Jan 5, 2010
    Can a piezo buzzer create voltage spikes? I would think yes.

    Bob
     
  9. kpatz

    kpatz

    311
    82
    Feb 24, 2014
    The OP didn't say if it was a piezo buzzer or not. If it's an electromechanical buzzer, it's putting all sorts of spikes into the LED while buzzing as well as when it's shut off. Adding a diode in inverse parallel with the buzzer should do the trick.

    A piezo element could conceivably generate spikes as well, though I would think they wouldn't be as strong as an inductive load would produce.

    Do you have a schematic of your circuit?
     
  10. Lance Mannion

    Lance Mannion

    37
    2
    Jun 9, 2014
    The 11mA was for both LED and piezo buzzer.
     
  11. Lance Mannion

    Lance Mannion

    37
    2
    Jun 9, 2014
    Well, the control box has been installed. I have to see how it does in the field. If the LED burns out again, I'll measure the current without the buzzer. Also, putting in a diode can't hurt so I might as well do that.
     
  12. Lance Mannion

    Lance Mannion

    37
    2
    Jun 9, 2014
    I don't have a schematic of my circuit. Is there a decent freeware program out there that you could suggest that I could you to draw one?

    Thanks
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
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    Jan 21, 2010
    I often use the one digikey has. Schemeit or something.

    I think you have to sign in to save stuff, but that's optional from memory.
     
  14. BobK

    BobK

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    Jan 5, 2010
    Get LTSPICE, it is free, you can draw schematics, and then you can simulate the circuit!

    Bob
     
    Gryd3 likes this.
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