Hi Bob, Thanks so much for the detailed explaination. I am "clearer" now. BTW, may I know where (web address) can I obtain general specs about LED's voltage ? I am using 3mm Ultra-bright Blue LEDs. But, I foresee, I will be using different types in the future. I recalled, different voltages and amp for different types and even color of LEDs ! Once again, thanks in advance for the help. cheers, Davis Bob Ammerman wrote: Ohms law is your friend: I = E/R --- current = voltage / resistance or E = IR --- voltage = current timers resistance or R = E/I --- resistance = voltage / current The current ratting of your source is simply is maximum capability, so you just pay attention to its voltage. So, to figure our your LED current: Supply = 12V Forward voltage of LED (Vf) from spec = 2.1V (for example) Voltage to drop across the resistor = 12V - 2.1V = 9.9V So, to get the correct resistance: R = E/I or R = 9.9V/25ma = 396 ohms. A 390 ohm resistor will do nicely here. I've also answered your specific questions inline below. Bob Ammerman RAm Systems ----- Original Message ----- From: "techy fellow" To: Sent: Tuesday, January 13, 2004 1:16 AM Subject: [PIC:] BEGINNER'S QUESTIONS > Hi, > > Pls bear with me, I am new to EE. I have couple of questions about lighting up LEDs. I understood amperage and LED goes hand in hand. Hence, > > Q1. Does it mean voltage is not a concern at all. That is, 3V and 12V will give the same brightness assuming the ampere for both 3V and 12V is the same at 25ma ? The LED brightness depends only on the current through it. > Q2. If my power source produces 12V at 1amp, do I calculate the resistor value based on this 1 amp (using Ohm's law) or I simply use say, 330R resistor ? See the note above. > Q3. Say, my power source (transformer) besides driving a LED but, is also driving 2 x 0.17amp fans, does it mean I should first deduct the 0.34 amps from the 1 amp and use the balance to calculate the resistor value for the LED ? Since the power source is rated at 1A, it will still be providing (at least close to) the 12V specified. Thus, you do not need to take into account other loads on the supply when calculating the resistor value. > Thanks in advance for the help. > > best rgds, > Davis -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics --------------------------------- Do you Yahoo!? Yahoo! Hotjobs: Enter the "Signing Bonus" Sweepstakes -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics