the voltage is 12v and this will drive 2 5v leds...or the input is 12v but when I measure the voltage at the led its 3.8v.. does this help?It is a lot easier to figure out when you know what the load is and what voltages the input will be.
In fact, even now, we at AAC don't know what the input voltages will be, but we can see that you might put 30 milliamps through the LED, and most small Leds are rated at 20 ma, maximum. With 1.1 ma through the base of the transistor, it will not be in full saturation and might get a bit hot.
The 330 ohm resistor should be at least 500 ohms to keep the LED current at or below .02 amps and the input resistor should be about 5600 ohms.
wow,, I need to learn to be more specific..lol.. ok.. the leds are blue, they are in a series. the package they came in said 5v max on it. I see no resistors on them, the leds are in a case separate from the board. I measured the voltage at the output on the board with the leds plugged in and on. I haven't measured the voltage "unloaded?" with the leds unplugged.., im assuming the leds don't come with their own limiting resistors because I hooked one straight up to the 12v supply and it popped right away..Two LEDs of 3.8 volts each, in series, will leave 4.4 volts to be used by the resistor.
4.4V/.02A = 220 ohms minimum.
Of course, I'm guessing again because I don't remember using any 1.9 volt leds and you did not say whether you have them in series or in parallel with each other, or what color they are, or if the LEDs are labeled 5 volts because they come with their own limiting resistors, or why there is a numerical difference between, "2 5V leds" and, "measured the voltage at the led".
That's the best I can do right now.
thank you very much for the lesson.. that went straight in the notebook.. that's what this is all about to me, I want to learn how how the components do what they do and how they act.. Im an installer in a car audio shop.. we have leds on the shelf that do what the circuit I just built does and I could've just grabbed one, But I want to "know" how things do what they do.. It might be kinda hard to understand but ive always had a thirst for understanding these things.. So im sure you will see many more newbie questions from me until I get a firm grasp on things..lol Thank You for your patience and explaining things to me in dummy terms...It looks like I guessed pretty well. Let's assume that "5V" is just sloppy labeling because 3.8 volts is pretty typical of blue LEDs. What I want you to understand is that the voltage the LEDs don't need is used to place a resistor that will limit the current to the ability of the LEDs. In this case, 220 ohms or greater is a good answer.
Then, there is the input resistor. The, "right" way to do it is to have the transistor running at ten times as much collector current as it has base current. To get .002 amps through the base, you need about 5600 ohms. That will slam the transistor hard on and minimize its heating.
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by Luke James
by Luke James