It's my first time with LED's and I am a complete noob on electronics. I want to make a circuit, pic related. 12 rows of 3 leds. I have a 12 V battery pack and the leds are with forward voltage(V) min 3.2 and typ 3.4 and 20 mA.
What I can't figure out what resistors to use, because all the info I found on wiring leds was only with parallel or series and I am confused what to do if I have this mixed circuit where series of 3 are wired parallel.
Each branch will get 12v as they are in parallel.
Voltage drop across the resistors = 12 - (3*3.4) = 1.8v
Resistance = Voltage/Current = 1.8/0.02 = 90 ohms
Of course, since you're using a battery the supply voltage will drop slightly but it shouldn't be much of an issue.
You just calculate the resistor for one of the series arms, then put as many as you like of these arms in parallel.
Exactly like in your circuit diagram.
You don't need any extra resistors, just the ones that're already in the serieses.
The other thing you should be aware of its the power rating of the resistor, resistor drops 2.4v@20mA so the power out dissipates is 48mW. Normal resistors are 1/4W so this is fine but if you used higher power leds (100mA possibly?) then you would need a fatter resistor.
36mW is the power dissipated in each resistor since 20mA through 90 ohm is 1.8v. I^2*R
Still your point is valid as like you say heat generated in a dropper resistor can become serious business once you move onto higher power LED's/higher supply voltages.