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I have device with one PCB with 6 parallel LEDs (30mA and 3.8V forward voltage each). This very small PCB is connected via 2 vires to main PCB where 555 timer is generating PWM signal based on voltage input (potentiometer). Everything is supplied with 5V DC. And now's question:
It's better to add resistor for each LED or lower voltage to 3.8V on main PCB (with voltage divider maybe)?
Also, on LEDs PBC I have space only for 0603 resistors, which power is lower than LEDs power.
Thanks for help :)
Add a resistor for each LED.
It's better to drive an LED with a constant current, since a small change in voltage will resut in a big change in current. Ideally you want to keep your LED at the 3.8V/30mA point in it's IV-Curve. But if there are some changes you want it to stay close to that.
Your resistor should have the value: $$ R = \frac{U}{I} = \frac{5V-3.8V}{30 mA} = \frac{1.2V}{30mA} = 40 \Omega $$ due to resistor tolerance of probably 20% you want to go higher than that, I would try 68 Ohm, if it's important to you that the light is bright choose a lower value, if your main concern is lifetime choose a higher value.
Also keep in mind the Power Dissipation in the Resistors: $$P =\frac{\Delta U^2}{R} = \frac{1.2V^2}{68 \Omega} = 0,021 W$$
But according to http://www.resistorguide.com/resistor-sizes-and-packages/ a 0603 package can disspate 0.1W so you should be good.
Also if you were to use a voltage divider, it would be hard to make the voltage stay constant, because you are loading it with the LED.
Also I am sure you can find a few related questions here, that explain different aspects more in detail like this one: Resistor selection for LED
