which resistor should i use with a white LED ?

Question

i am making a circuit in which 26 LED's are connected in parallel. i am supplying current from 2 AA size (1.5 v) batteries. which resistor should i use to prolong the life of the LEDs?

Answer 1

it all depends on the values of the circuit. You can figure by using the formula V/I x r voltage / current will give you the value of the resistor

Answer 2

If only one LED will be lit at a time you can use just one resistor, otherwise you will need one resistor for each LED since the current going through the resistor must remain the same.

You need to know the power rating of the LED. Power = Current x Voltage. 1 watt = 1 amp x 1 volt. The voltage drop across the resistor will be close to 1.5 V, check it when it is lit fairly brightly, it will not change much from that value.
If you have a 1/4 Watt LED the equation would be 0.25 W = Current x 1.5 .
Use trial and error until, in the above case you get a current of.... Current = .25 / 1.5 = 0.166666667. which is about 160 mA

You can get the current by measuring the voltage across the resistor... current = Voltage / Resistance

Answer 3

You need to put a resister on every led to cause the current to be dispersed evenly to all 26 LED’s

Sense you did not give any information on the LED’s You can do this by trial.
start with about 200 ohm resistor in series with one led.
determine if this is the level you expect from all LED’s then put a 200 ohm in series with each and every led.

Then I also suggest using d cell as the overall current will be about 130 ma or .13 amps or more

each led may need about 5 ma or more depending on the type of diodes you have.

John

Answer 4

while i could assume you intend to use this with a 12 volt battery system, you did not specify the operating current of the LED. White and Blue LEDs typically draw more current than Red or Green types. There is no way to size a resistor without knowing the current draw.

Answer 5

LED's are self limiting on current with 1-1.5volt drop. With a 5volt source 220 ohms is used. So with 3 volts, I would try 110 ohms