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resistors for leds why?

i know i need resistors for leds, i know how to calculate what i need, etc. etc. but what im wondering about is why.
why arent leds like glowing bulbs, what makes the difference in needing the resistor.

also, why cant 1 resistor take care for multiple leds at the same time?

please, if ur answering my questions, make it understandable english. im not comfortable enough in the world of science to understand that kinda stuff... (otherwise i wouldnt be asking these questions :D )


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godofal (author) 7 years ago
thhnx everyone for ur answers.
i understand it now, in theory 1 resistor could be used for multiple leds in series, but in reality, it cant becouse there are always small differences.
and leds just work in a different way than bulbs (no resistance due nearly no heat)

just one quick question; leds have VERY long life-spans (100.000 hour or so) so, if u use 1 resistor for multiple leds in series, one of them will have a shorter life span, but with that enourmous lifespan as it is, how much would you really know about it? does it affect the lifespan by minutes, hours, thousands of hours?
godofal (author)  godofal7 years ago
oh, and i forgot about paralel, whats up with that+1 resistor? does that overload the resistor?
saastamo7 years ago
Two reasons
1) to adjust the voltage suitable for the led

2) to "soften" the power-up. LEDs do not like too sudden power rises. The resistor makes it slower => LED will work longer time.

You CAN have several LEDs in paraller & 1 resistor or in series & 1 resistor. It only affects the calculations of the resistor needed. In this case the LEDs should be identical.
Running leds in parallel with 1 resistor is bad form because even identical leds have slight variances in their construction.  You will likely not have an obvious failure immediately but that tiny tiny variance will mean one diode compared to the others will get a slightly higher current, not all equal.  That slightly higher current will cause that led to run a tiny bit hotter, which reduces the internal resistance even further.  That thermal run-away WILL lead to shorter than expected life on the led.  If the led fails as a short circuit all the others will turn off or become dim.  If it fails as an open circuit (most likely with semiconductors) all the other leds will each have to bear the extra load of the dead led, and you get a cascade effect of overcurrent and doomy gloom poop.

Will the world stop turning if you use leds in parallel with no resistors on each?  no, but it's bad design when resistors are so cheap and easy to use.
lemonie7 years ago
Bulbs glow because they are hot, like other hot things glow. LEDs emit light due to physics which you don't know about I'm not going to attempt to explain.

Resistors are used to limit the flow of current, because as steve' says LEDs don't give much resistance to current themselves. Like bulbs, if you push too much electricity through them, they "break".

One resistor can be used with multiple devices (LED or otherwise).

L
A filament bulb could be shown on a circuit as a little resistor, its a bit more complicated, because the resistance is a function of current/temperature too, but essentially, a bulb "looks like" a resistor. You can't put too much current through it, because its resistance intrinsically stops you.

An LED is a more complicated beast. It has a resistance, but that is VERY small, and it ALSO has a tiny "battery " in series with that resistor. Its not a real battery, it can't power anything, but it "looks like" there is something that stops a current flowing when you attach a battery. The forward voltage of the LED and the internal battery voltage are the same.

Until you have enough voltage to overcome the "internal battery", little or no current flows, but once you are over the hump, the internal resistance of the LED WON'T stop very large currents flowing.

Current x Vf = power dissipated in the diode, so too much current and you burn the LED.

Brightness is also proportional to current, so you have to balance off two competing things - lots of light, and not too much heating !


Re-design7 years ago
A regular bulb can take an over voltage better than an led can.

1 resistor can take care of multiple leds at one time but it must be bigger to handle the power load.