how to set IR LED current

[ulaska]

hello,

building a nightvision IR LED module..
(6 IR LED + 47 ohm serial ) x 8 Row.

in the datasheet of IR LED, the continuous LED current is max 50mA.
i set the resistors to flow 35mA current in each LED. but the board heats with resistors(1/2 watts) and LEDs.

is it normal or harmful ?

do i have to increase the LEDS numbers and decrease the current to ~25 mA.

thanks.

 

[VVV]

The power dissipation is going to be high, because you have 48 LED's running at 35mA each. Assuming about 2V across each LED, we get 70mW per LED. Multiply that by 48 and you get 3.36W in the LEDs alone. That is already significant power.
Plus you will dissipate some power in the series resistors. I cannot tell how much, because Vcc is not given.
In any case, the 3.36W alone will cause the board to get hot.

 

[PaulHolland]

If this was a perfect world and we would understand everything we would be able to make LED's with an efficiency of 100% unfortunately we are barely able to get 15% for IR LED's. So 85% of the electrical power put into the LED's is converted into heat A less perfect world.

Paul.

PS: Completely normal, make sure you do not burn you fingers when that is happening you should use something to cool your IR LED's.

PS2: Better use a pulsed current of 100 - 200mA through the LED's and puls with a frequency higher than 100Hz. This will give more light and you do not see the flickering.

 

[ulaska]

hello ,
VCC = 12 V
Forward voltage of LED is 1.4V

it is a good idea to drive LEDs with 100 Hz.
in the data sheet, it says 100Hz %1 duty cycle to drive LEDs with 100mA ~ 1 A

its OK to use 555 to get that pulse signals?

thank you.

 

[VVV]

Of course it is OK. Just make sure you adjust the duty-cycle correctly

 

[ulaska]

hello,
how can we pulse the transistor?
can we use 555 ?
if 555 works wrong and gives always "HIGH ", transistors will be turn on and of course all IR leds will be burn.

do you have any idea, what kind of circuit , or what kind of 555 circuit can we use for this protection?

thank you.