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Model Forum / General / Railroads / August 2008Formula for figuring series resistor values for LEDs
Don't ask me what prompted me to post this. Guess it's because the question often comes up here, what value of resistor do I need to run my LEDs off my 12-volt power supply? So here's the generalized formula which should work for every situation. Just plug the numbers in and turn the crank. Assuming you have a DC power supply, a single LED with a single resistor in series with it: Vt - Vled R = ----------- Iled where Vt = total voltage (power supply voltage) Vled = operating voltage of LED Iled = operating current of LED (in amperes) For example, with a 12-volt power supply and a 2-volt, 20mA LED, you'd get R = (12 - 2) / 0.02 = 500 ohms.  Signature"In 1964 Barry Goldwater declared: 'Elect me president, and I will bomb the cities of Vietnam, defoliate the jungles, herd the population into concentration camps and turn the country into a wasteland.' But Lyndon Johnson said: 'No! No! No! Don't you dare do that. Let ME do it.'" - Characterization (paraphrased) of the 1964 Goldwater/Johnson presidential race by Professor Irwin Corey, "The World's Foremost Authority". > Don't ask me what prompted me to post this. Guess it's because the > question often comes up here, what value of resistor do I need to run my [quoted text clipped - 19 lines] > > R = (12 - 2) / 0.02 = 500 ohms. The Model Railroad System includes a program, Resistor, that will do this computation (you can select the supply voltage, the LED voltage, and the LED current). It will also let you know what wattage resistor you need and will also find the nearest stocked value and display the color bands for such a resistor. SignatureRobert Heller -- 978-544-6933 Deepwoods Software -- Download the Model Railroad System http://www.deepsoft.com/ -- Binaries for Linux and MS-Windows heller@deepsoft.com -- http://www.deepsoft.com/ModelRailroadSystem/ On 8/28/2008 11:13 AM Robert Heller spake thus: >> Don't ask me what prompted me to post this. Guess it's because the >> question often comes up here, what value of resistor do I need to run my [quoted text clipped - 25 lines] > you need and will also find the nearest stocked value and display the > color bands for such a resistor. That's nice. The little formula above requires only paper and pencil (or a calculator to those who prefer automation), and will work in a power outage without a UPS. Signature"In 1964 Barry Goldwater declared: 'Elect me president, and I will bomb the cities of Vietnam, defoliate the jungles, herd the population into concentration camps and turn the country into a wasteland.' But Lyndon Johnson said: 'No! No! No! Don't you dare do that. Let ME do it.'" - Characterization (paraphrased) of the 1964 Goldwater/Johnson presidential race by Professor Irwin Corey, "The World's Foremost Authority". >On 8/28/2008 11:13 AM Robert Heller spake thus: > [quoted text clipped - 9 lines] >to those who prefer automation), and will work in a power outage without >a UPS. How are you going to light the LED when the power is out? ---john. >>On 8/28/2008 11:13 AM Robert Heller spake thus: >> [quoted text clipped - 13 lines] > > ---john. Well, I don't know about you, but my truck could easily provide enough power to keep my Digitrax system happy for MONTHs. If the system's got power, then my White LED headlights can light. :-) The mains are out, and the residents of my model railroad are happy because they've still got electricity. Imagine that. lol Puckdropper SignatureIf you're quiet, your teeth never touch your ankles. To email me directly, send a message to puckdropper (at) fastmail.fm > > In article <48b6f35a$0$31930$82264...@news.adtechcomputers.com>, > >>On 8/28/2008 11:13 AM Robert Heller spake thus: [quoted text clipped - 29 lines] > > - Show quoted text - ---------------------------------------------------------------------------------------------------------------- If one loses power during a thunderstorm, they could always send up kites to get the power they need for their model railroad. <g> Bill http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm r > Don't ask me what prompted me to post this. Guess it's because the > question often comes up here, what value of resistor do I need to run my [quoted text clipped - 30 lines] > presidential race by Professor Irwin Corey, "The World's Foremost > Authority". ---------------------------------------------------------------------------------------------------------------------- These might help... Resistors: http://www.merg.org.uk/resistor/index.htm LEDs: http://www.merg.org.uk/led/index.htm Bill Bill's Railroad Empire N Scale Model Railroad: http://www.billsrailroad.net Brief History of N Scale: http://www.billsrailroad.net/history/n-scale Bill's Store--Books, Trains, and Toys: http://www.billsrailroad.net/bookstore Resources--Links to 1,200 sites: http://www.billsrailroad.net/bills-favorite-links > > Don't ask me what prompted me to post this. Guess it's because the > > question often comes up here, what value of resistor do I need to run my [quoted text clipped - 51 lines] > > - Show quoted text - This calculator even shows you the colors on the resistor. Very neat. http://led.linear1.org/1led.wiz |
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