high bright LED

[mahinda jayasinghe]

There are high bright LEDs available. Specification says brightness is 8000 mcd (ithink milly candles)

I replaced torch bulb with hibright one. This 3 AA bateries. ie 4.5 V.
According to the specification The max voltage is 4V. I use a Zenner to control the voltage. It gives considerbal enough light foe me to work in dark.
Muy question is How would be the life lime of above LED under the above condition. What is the brightness od ordinary torch bulb (4.5V).

Your idea will be highly appreciated.

 

[bloody-orc]

do you know how much current your LED takes maximum? if you do, then yiou coould also use a resistor and the brightness increases.

but leds lifetime is 100 (if not more) times longer.

 

[audioguru]

LEDs are rated in max average current as well as typical forward voltage. If the current exceeds the max, then the lifetime of the LED could be only nanoseconds.
You definately need a current limiter, which could be a series resistor to the LED instead of a zener.

 

[mahinda jayasinghe]

Specification sheet says DC forward current 30 mA max
peak pulse forward current 100mA.

If I connect a series resister, will the brightness reduced(because of voltage drop).

 

[audioguru]

The LED will be brighter with a continuous DC forward current more than 30mA but might burn out at any time. You calculate the value of the series resistor so that the current does not exceed 30mA, but the calculation is difficult unless you know the exact forward voltage of the LED by measuring it at the current you wish to use. The current and brightness will decrease as the battery runs down so to keep it bright, a constant current circuit could be used instead of a series resistor.

 

[mahinda jayasinghe]

Typical Vf=3.6 for I(f)=20mA.

If I connect 4.5V battery

the V drop across series resistor =4.5-3.6= 0.9

Then V=IR

gives R=0.9/20mA
=45 Ohm


Is this calculation is correct

if so series resistor would be 45 Ohm.

 

[David Bridgen]

do you know how much current your LED takes maximum? if you do, then yiou coould also use a resistor and the brightness increases.

I would love to know how to do that.