rybitski Posted January 9, 2006 Report Share Posted January 9, 2006 So I recently bought a lot of leds off ebay, here are the specs:Quantity: 100Luminous Intensity Typ Iv (mcd) : 4000(Typical) ~ 5000(Max)Size: 5mm / 0.2 inchesOperation Voltage: 3.2V - 3.6VReverse current (uA): Less than 30uALife Rating: 100,000 HoursViewing angle: 25 - 35 degreeAbsolute Maximum Ratings (Ta=25 Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 9, 2006 Report Share Posted January 9, 2006 Hi Rybitski,Welcome to our forum. ;DDoes 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. Quote Link to comment Share on other sites More sharing options...
rybitski Posted January 10, 2006 Author Report Share Posted January 10, 2006 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 Hi Chris,This circuit will work. The pot might not last long. Quote Link to comment Share on other sites More sharing options...
rybitski Posted January 10, 2006 Author Report Share Posted January 10, 2006 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 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 Quote Link to comment Share on other sites More sharing options...
rybitski Posted January 10, 2006 Author Report Share Posted January 10, 2006 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..)Fast internet is always good... Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 10, 2006 Report Share Posted January 10, 2006 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 Quote Link to comment Share on other sites More sharing options...
rybitski Posted January 11, 2006 Author Report Share Posted January 11, 2006 I was thinking some where alont those lines... 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? Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 11, 2006 Report Share Posted January 11, 2006 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 Quote Link to comment Share on other sites More sharing options...
rybitski Posted January 12, 2006 Author Report Share Posted January 12, 2006 Stick a pot in there and adjust it to the dimness you want, measure its resistance and wire-in a resistor of that value. GrinVery good idea, thank you for all your help.I'll keep you posted on how it goes. Quote Link to comment Share on other sites More sharing options...
rybitski Posted February 12, 2006 Author Report Share Posted February 12, 2006 I have completed the project finally (it was a while before I found the time and patientce to finish)...Again, thank you for all your help and guidance.Recently my project was hosted on Hackaday.comHere is a link to the finished project: http://wishi-waspecial.livejournal.com/77678.html ;D Quote Link to comment Share on other sites More sharing options...
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