I'm going to make some LED circuits, but I'm not sure if it will work. The problem is that I want to use Red and Blue leds, but they have different drop voltage. Will this circuit work?
1\$\begingroup\$ Very closely related: electronics.stackexchange.com/questions/159810 \$\endgroup\$– The PhotonNov 16, 2015 at 17:13
No, it won't work, or at least not very well.
But not because you have red and blue LEDs.
It won't work because even LEDs with the same color will have slightly different forward voltages, especially if they are at (even slightly) different temperatures.
So your three blue LEDs in parallel won't light up equally, and your three red LEDs in parallel won't light up equally. Possibly the difference in characteristics will even be enough to damage the LEDs.
The better solution is just to use a separate resistor for each LED.
simulate this circuit – Schematic created using CircuitLab
Or, if you have a higher-value voltage source available, place all the LEDs of the same color in series so that they are sure to all get the same current.
\$\begingroup\$ Thanks, so if I use different color leds with resistors for each led(39ohm for blue, 100ohm for red), every led will light equally if I'm correct. What about current, every led needs 20mAh, so will every led get 20mah current? Btw, I'm thinking of soldering 20blue and 20reds all in parallel. \$\endgroup\$– wFcNov 16, 2015 at 17:29
\$\begingroup\$ (4.2-2)/100 = 22 mA, so each of your red LEDs will get about 22 mA. But since the Vf ranges from 1.8 to 2.2 V, there will be some variation (mostly depending on temperature, but also device-to-device). But that's as accurate as you're going to get without using a true constant-current driver. Similar considerations for the blue LEDs, but the variation will actually be higher because the higher Vf forces you to a lower resistor value. \$\endgroup\$ Nov 16, 2015 at 17:36
\$\begingroup\$ Also, the human eye doesn't respond equally to different colors so you will need to adjust the current differently between the red and blue LEDs if you want them to appear equally bright. \$\endgroup\$ Nov 16, 2015 at 17:36
\$\begingroup\$ If I will use 3528 SMD LED's which needs max 20mAh current, is it safe to use them at 22mAh or I should get 110+ohm resistors? \$\endgroup\$– wFcNov 16, 2015 at 17:45
\$\begingroup\$ It's safer to make sure they are below 20 mA, even when the Vf is at its minimum. Even safer would be to use 4 of each color and run them at 15 mA instead of 20 mA. \$\endgroup\$ Nov 16, 2015 at 17:52