Excellent !! Finally, calculating resistor value for LEDs is no longer a mystery to me. Thanks so much, Bob. You have a nice day. Davis Bob Ammerman wrote: Yes, Blue LEDs tend to have a high Vf. (Vf is the voltage across the LED when current is flowing through it in the forward direction. In reality it is not a constant, but rather depends to some small extent on the current flowing. Practically, however you can treat it as a constant for normal ranges of current.) You can always hook them up with a relatively high resistance (1K on a 12V supply) and then measure the voltage across the LED. You will then be able to compute the correct resistance for a specific current. Bob Ammerman RAm Systems ----- Original Message ----- From: "techy fellow" To: Sent: Tuesday, January 13, 2004 2:36 AM Subject: [PIC:] BEGINNER'S QUESTIONS - LED > 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 > -- 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