My color vision isn't too hot, especially to distinguish certain greens from browns. When I get resistors out of a bin (even/especially at home, where I've got several bins & there's a good chance for something to get "mis-filed"), I take a multi-meter along & check for myself. Radio Shack sells a nice, compact little multi-meter that's also cheap: http://www.radioshack.com/product/index.jsp?productId=2104114&cp=&sr=1&origkw=multimeter&kw=multimeter&parentPage=search A lot of people who aren't EEs still know & understand the basic SI scalars. For instance, "k" is in common usage to mean "thousands" when talking about weights, distances, speeds, & plenty else. Whenever you see "k" behind a number, it means that you should multiply the number by 1,000. So, please answer your own question, if for no other reason than to show that you're following along OK so far: 2.8k ohms = ______ ohms. The wattage rating should be determined by how much power (expressed in watts) your circuit uses. As a practical matter, what you're trying to do is figure out how much power you can cram through the resistor, before it heats up & smokes. You want to pick a rating that's high enough so that it won't do that. This is a secondary consideration. The primary consideration, which you've already figured out, is that you need 2.8k of resistance. You already know the voltage of your power source. Using Ohm's Law, you can then calculate the current of the circuit. Looking back at prior messages, we see where it was posted that P=I*E = that is, Power (expressed in watts; "W") is equal to Intensitãt (Georg Ohm was German; we use Amperes or "Amps"; "A") times Electromotive force (expressed in volts; "V" for the non-Germans among us). Since you know the current (I) & voltage (E), you can easily calculate P (in watts) by multiplying voltage by resistance. You want your resistor to be able to handle at least the amount of power you've calculated, plus a safety margin. Using a higher-rated resistor won't hurt anything; it'll just be larger & probably a little more expensive. Example: Assume that your calculations show that your circuit will consume 80mW (80 milli-watts, or 0.08 W) of power. "Rounding up", a 1/8W resistor (0.125W) ought to be plenty, however using a 1/4W or 1W resistor won't hurt anything unless space & weight are big factors. JM
Well thanks to all. I was finally able to figure it out. In the end it was a really fast calculation at the electronics store that helped out and it is starting to make sense. I learned to look at the package and determine the wattage from the ma rating. Most of the ones I have are 25ma which equates to 1/4 watt and if the voltage is 6v I subtract the 2.4v max and then a calculation is made ( I am still fuzzy on that) and I got 144 ohms. I installed them and I got this: Question: on the back of my led the package says forward current 25ma which I believe I understand as the 1/4 watt, the max voltage is 2.4v so it seems that the only calculation need is the ohms. Is that correct. Question: Based on the calculations below From John E = I * R E = Electromotive Force; in other words, Volts. I = Current; measured in Amperes, also called Amps. R = Resistance, measured in Ohms So, Ohm's Law establishes the relationship between Volts, Amps, & Ohms. If you have a 6V power supply, & you want to limit current to 25 mA (milli-Amps, or 1000ths of 1 Amp; 0.025A), then what resistance do you need? From Ohm's Law we know that E = I * R. Our value for E is 6 (Volts). Our value for I is 0.025. We need to solve for R. 6V = 0.025A * R R = 6 / 0.025 R = 240, or 240 Ohms. Many times, a "ballpark value" is all you need. Also, it's often a good idea to "pad" it a little, for safety. This makes sense but when I went to: http://www.quickar.com/noqbestledcalc.htm, and used the single led it said to use 144 ohms. The guy at the electronics store told me to use 144 ohms. How do that arrive at that number given that I have a 6v battery, 2.4v max, and 25ma current? Thanks for all the help as I really wanted to light my ship. I got a blinking green light for the bow and a blinking red light for the stern (just have to figure out how to mount it so it wont get shot off). I really appreciate all the help. Thanks to all!
