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led problem


rybitski
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So I recently bought a lot of leds off ebay, here are the specs:

Quantity:  100
Luminous Intensity Typ Iv (mcd) : 4000(Typical) ~ 5000(Max)
Size: 5mm / 0.2 inches
Operation Voltage: 3.2V - 3.6V
Reverse current (uA): Less than 30uA
Life Rating: 100,000 Hours
Viewing angle: 25 - 35 degree
Absolute Maximum Ratings (Ta=25

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Hi Rybitski,
Welcome to our forum. ;D
Does your transformer have a 12VDC output or an AC output? Without a rectifier, AC destroys LEDs.

Your problem is that the LEDs in your circuit operate at more than their rated max of 30mA.
Your "transformer", which is probably an AC to DC adapter is rated to have a 12V output when it has a 300mA load. It is probably cheap and not regulated so with less load current its output voltage is higher, maybe 15V or more. If its output voltage is 15V and the voltage of your LEDs are 3.3V then with the pot set for max brightness, 11.7V will be across the 100 ohm current limiting resistor and the current calculates to be 117mA. The 100 ohm resistor will be cooking at 1.4W and if it is too small maybe is burned and a lower value, causing more current.
If the LEDs are exactly the same, each will share the total current and operate at 117mA/4= 29.3mA. Leds are not exactly the same, Each LED has a slightly different voltage. Therefore the LED with the highest voltage will operate at a higher current than the others in parallel and since the average max current in your circuit is so close to the max current rating of the LEDs then it will burn out. The other LEDs in parallel will try to share the current but now there are only 3 LEDs so each operates at 117mA/3= 39mA and will quickly burn out one after the other.

LEDs should never be operated in parallel at high current. You have about 15V so 3 LEDs could be wired in series, and in series with a current limiting resistor and a pot.The values of the resistor and pot will need to be re-calculated. In series, the LEDs have exactly the same current which is much lower than if they were in parallel. The lower current might cause the adapter's output voltage to be high enough for 4 LEDs in series. If the voltage isn't high enough for 4 LEDs then two strings of 2 LEDs in series with a current limiting resistor can be in parallel and in series with a pot.

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Thank you for your quick reply...

My transformer has an output of 12vdc.

Being the novice that I am, I don't have a full understanding for electronics so could you elaborate on what I need to do to fix this circuit and make it work with 4 leds without burning out, please?

Thank you again for your quick response.
-Chris Rybitski

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Again, thank you for your quick reply and of course the time you took to make the schematic for me.
Alas, I have more questions...
Is there anything that could be done to increase the life of the pot?
...and being a junior in high school and planning on at least minoring in electrical engineering in college my question to you is how did you get to be where you are today? (tips, tricks, advice)
Thank you again,
-Chris Rybitski

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Hi Chris,
When the pot is turned almost to max brightness, a tiny portion of its resistance will be very hot. It is too small to dissipate the heat and will fail. A physically larger pot would last longer but is very expensive and hard to find. I would use a transistor to dissipate the heat and a pot with a much higher value to control it and be cool.

Do you know what I am today? I am a retired bum. I was an electronics geek all my career and got it mainly due to good luck. I am very happy and fairly well off, with a nice family and home. And a nice high-speed connection to the internet! ;D

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So, if I remove the pot what other method could I use to dim the leds. (I would at least like to have two different brightness settings..)

Simply a single-pole switch and a resistor. That's what I did in my new Sound Level Indicator project when I discovered that a pot wouldn't fit:
http://www.electronics-lab.com/projects/motor_light/009/index.html
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So if I took out the 10k pot would I have to recalculate the value for the two resistors?
If so what is the formula?

No, The 330 ohm resistors set the current at max for max brightness when the switch bypasses the dimming resistor.
I don't know how dim you want it to be. 3.3k ohms would be pretty dim. Stick a pot in there and adjust it to the dimness you want, measure its resistance and wire-in a resistor of that value. ;D
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