LED's not at its full brightness when refreshing

[Murugesh_89]

Hi,
I am using 8 tricolor LED's in a PCB which obviously require 24 I/O pins. But i connect it as
a. All the anodes 1 of each LED are connected to one I/o pin
b. All the anodes 2 of each LED are connected to another I/o pin
c. All the anodes 3 of each LED are connected to another I/o pin
d. Channel 0 of decoder connected to cathode of Led 1 and so on as channel 7 of decoder connected to cathode of led 8

Hence i require only 6 I/O's as 3 for anodes and 3 for decoder channel select.

The code is as follows.
for(decoder_channel = 0;decoder_channel < 8;decoder_channel++)
{
select_channel_led(decoder_channel);

switch (led_status[decoder_channel])
{
case 0x00:
{
LED_OFF
select_channel_led(decoder_channel);
LED_OFF
delay_us(800);
break;
}

case 0x01:
{
LED_OFF
select_channel_led(decoder_channel);
RED_ON
delay_us(800);
break;
}

case 0x02:
{
LED_OFF
select_channel_led(decoder_channel);
BLUE_ON
delay_us(800);
break;
}

case 0x03:
{
LED_OFF
select_channel_led(decoder_channel);
GREEN_ON
delay_us(800);
break;
}
LED_OFF
}
}
The above is continues as the LEd's are refreshing.
But on doing this, i can not able to get the complete brightness of LED. LED looks some what dull.

What can i do so that i can get the full brightness of LED?

 

[iukhan]

Since the duty cycle is around 5% for each LED, the average current in an LED is around 5%, this is why the LEDs looks dull, so you need to decrease the value series resistors of LEDs by 20-25 times compare to normal value.

 

[Murugesh_89]

Thanks for your reply.

Now i am using 150 ohms resistance. Since red led needs 2V as forward voltage and 20mA current, 150 ohms will drop 3V.
If reduce the resistance to 7.5 ohms (150/20 times), the current drawn by the led will be 400 mA (3V / 7.5 ohm). Will it have any effect on life of LED or will it over heat the LED?

 

[KonstanU]

You will fast destroy your led. Look in the datasheet parameter Imax.

 

[Murugesh_89]

The pulse forward current is given as 100mA. The dc forward current is given as 30mA. So what value of resistance will increase the brightness which does not affect the life. or is there any other way to do this?

 

[KonstanU]

Question. Can input/output pins conduct current of value 100mA or not? Look on datasheet.

Light Flux is dependence of led current and junction temperature. I belive that in the case of 100mA you have less efficiency that leds to increasing conduction current if you want to get the same light flux.

 

[Murugesh_89]

The I/O can source up to 25mA. Can i accomplish this using a transistor, if yes, can you please tell me how?