Is it really necesary cumpulsrw to add a resistor in series to led?

no not compulsory, there no led police thow in jail if don't that. heck, can connect led directly car battery or ac outlet or tesla coil (careful, don't electrocuted). because can make such connection, not mean meaningful (yes, listed destroy led).

now, leds non-linear devices , sensitive voltage change. thereore using voltage spec makes hard design cricuit leds, @ least such work more 1 specific led (bunch of leds same batch have pretty wide tolerances). nobody wants spend time measuring each , every led , calculating circutis every single 1 of them. replacing 1 led rather difficult task.

see resistors considered linear. if have 100 ohm , connect 0.5v, 0.5/100=0.005a. if double voltage 1.00 v, double current: 1.0v/100 ohm = 0.010 etc. when increase in voltage produces proportional increase in current, relationship linear.

diodes non linear. means if have diode , connect 0.5v, tiny current such 0.00001a. if connect double voltage (1v), not double of current, more 10a , diode may not survive long time. because threshold normal diode conducts 0.7v. below threshold, there little current. once exceed threshold, diode conducts slight increase in voltage means huge change in current.

leds diodes threshold different, on order 1.6 - 3v depending on used material , process , "knee" of nonlinear curve not quite sharp principles still same. since using fixed voltage touchy, how using fixed current? turns out leds happy 5-10ma. 20ma listed max continuous current. means add series resistor led, can "linearize" leds characteristic (entire circuit consisting of led , resistor more similar linear, resistor circuit led itself). means changes in supply voltage have less impact on led current , therefore performance of led more consistent.

not have have resistor in series led need have current relatively constant (say 15ma). how accomplish you. simplest , cheapest way use resistor. other ways use led driver ic (with additional components).

calculate series resistor led can use:

r=(vcc - n*vf)/if


vcc supply voltage (such 5, 9 or 12v example)
vf forward voltage of led(use value datasheet or if unknown, assume 2.2v)
n number of leds.
if forward current led such 10-20ma

example:

vcc=12v
vf=2.5v
if=0.020a (20ma)
n=3 (number of leds in series)

r=(12-3*2.5)/0.020= 225 ohm

closest common value 220 ohm
power of resistor i^2*r=0.02*0.02*220=88mw 1/4w or 1/8w fine.

i m confused mixed answers plz clarify

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