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# 3 LEDs in series

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#### daviddoria

##### New Member
so... i can't have more than 2 leds in series?? I have 3 hooked up, and none of them turn on... unless i short 1, then the other 2 work. what the heck?

i am running this on 5v.

any ideas?
david

Insufficient voltage.

look at....

for a simple explanation of this.

:lol:

LEDS in series

Look at the electrical specification of the LEDs (Vf voltage forward). Generally they require about 2.5 to 3.1 volts and 20mA.

Do the math.

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If you want all 3 of them to work simultaneously, connect them in parallel with a series current limiting resistor.

the thing is simple.
leds, unlike lamps, wich for voltages much smaller than the nominal voltage can light bit, are semiconductors. that means that they have a well determined voltage range for operation. below tha votage they dont function, above the max voltage they get destroied very easily.
thats why you nedd to put a series resistor. sometimes if you power a led and a series resistor from a voltage source much highter than the led voltage, you get low efficency on the circuit, so the resistor is not always the best solution to use. sometimes we can use other type of circuits to power leds.

LED without current limiting resistor? Never used in my life!: I have never come across any circuit which supplies exact operating voltage to the LED and that too without current limiting resistor. Most of the circuit I saw till now operated LED at either 5V or 12V or sometimes 230V with appropriate current limiters.

I am eager to see what other circuit you use to lit a LED.

what i was talking about was that if you have highter voltage, 24, 36, 50V you have a very low efficency, so you can use a simple switcing circuit or something esle, like a capacitor and a zenner for a.c.
but it is true as you say, most of the circuits use 12V ore less., so the preffered way is the resistor.
and you can say "A LED and its current limitong resistor were ment for eachother" :lol:

zenner for a.c.
What is that? Never heard or seen before!
Remember Zener in forward bias works as normal diode and in reverse bias it acts a a voltage source with voltage equal to its reverse cut-in voltage or zener breakdown voltage.

well i use a capaacitor a resistor of a few ohma and a zenner diode and you get a nice suply for ac for a led, you have some power loss, but not so much as with a simple resistor.

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