I am trying to figure out how long the circuit will last, but I know I cannot just figure I am using up 20mah, the resistor must be using up something. Is it a little, a lot? I am guessing it likely is a tiny overhead amount and calculating it wouldn't matter much?
I am actually running 14 LEDs and trying to determine how large a power source I need for it to run undiminished for 5 hours.
2007-10-25 18:32:30 · 5 answers · asked by star_surfer84 2 in Science & Mathematics ➔ Engineering
Firstly, yes. Resistors waste energy as heat. It's inevitable.
Secondly, l.e.ds aren't voltage operated devices, they're current operated.
mah is the unit used to express the capacity of a cell or a battery, and it is written as mAh.
I assume you mean that your l.e.d's maximum current is 20mA? So at 20mA the l.e.d's forward voltage will be about 3.5. Change the current and the voltage across it will change, but not in proportion. See the graph I will point you to in a moment.
With a 9V battery and a 330 ohm resistor in series .......
your l.e.d. has about 3.5 volts across it. That means there must be 5.5V across the resistor. 5 volts across 330 ohms results in 16.7mA through the whole circuit.
The power dissipated in the resistor will be a little less than be 0.1W
You do know Ohm's Law don't you?
The l.e.d. will light quite satisfactorily and, as it is running well below its maximum current, it will live longer than you.
To simultaneously light 14 of those l.e.ds you have two choices. Either series connected, which would necessitate a source which was a few volts greater than 14 x 3.5, probably not very practical, although the current would be no more than 20mA, or series/parallel.
A third possibility, all in parallel, is to be strictly avoided.
A series/parallel arrangement means you have more than one series chain of l.e.d.s, each chain with its own current-limiting resistor. The chains being connected in parallel across the voltage source.
The disadvantage of this is that the total current will be 20mA multiplied by the number of paralleled chains.
I wonder what value of voltage source you have available?
Are you determined to use, or limited to, a 9V battery?
If you can let me know I can suggest one or two alternatives for the wiring.
Now, that graph. Have a look at this treatise which I wrote some time back and which should help you http://www.davidbridgen.com/leds.htm
Make contact through Yahoo Answers if you wish.
2007-10-25 19:06:32 · answer #1 · answered by dmb06851 7 · 0⤊ 0⤋
330 Ohm Resistor
2016-10-05 00:43:34 · answer #2 · answered by ? 4 · 0⤊ 0⤋
yes, resistor cosume power. you can light that LED even 24 hours . connect resistor and led together, the long pin of LED is the positive. if you have 14 LED to light, you need also 14 resistor. you can not use many LED for one resistor for some will not light. my recommendation for the value of resistor is 450 ohms, 1/4watts. using 330 ohms connected to led would draw a current of 27ma which is above the safe current of LED 20ma. A minimum 300ma power supply is enough to that size. tnx
2007-10-29 18:17:13 · answer #3 · answered by bongards 2 · 0⤊ 0⤋
forget the resistor. connect 3 LEDs in series. 20 ma is about the max you would want to draw off a 9 volt battery, so 3 LED in series would be the max load you should use. They would be quite bright running that close to max specs, using 4 in series would probably improve the running lifetime.
2007-10-26 09:13:47 · answer #4 · answered by lare 7 · 0⤊ 0⤋
It waste energy at the rate of i^2*r. The wattage of a resister is how much energy that is thrown off by heat.
2007-10-25 18:36:09 · answer #5 · answered by eric l 6 · 0⤊ 0⤋