Hi everyone,
I just built an rgb "panel" consisting of 27 common anode rgb LEDs each with their own resistors. I did the calculations for 12v and used 470 ohm 330mW 1206 resistors which should be more than enough to give me 20mA per colour. Now I've got all of them running and the pcb gets quite warm. I can touch the resistors without burning myself, but it's still uncomfortably hot. Is this normal for smd resistors? This is my first time using any surface mount components.

Here is a basic diagram (one on the right) of what I'm doing... x 27 of course ( and using 470 ohm for the resistors.

 

At a voltage of 12 V on the resistors, the power dissipates 4 to 5 times more than the LEDs.
I advise you to reduce the supply voltage (for example, up to 5 V). Resistors have to be corrected to get 20 mA. Overheating of the resistors will decrease 3 to 5 times. If the temperature of the resistors was 150 degrees Celsius, it would not be more than 65 degrees.

 

Assuming that the Vf of led is 3V then the If = 20mA and just use its 80% as 16 mA, and the
Rf = (Vcc - 3V)/If
= 9V/16mA
= 562
so choosing a 560Ω

The Watts = Vf * If
= 9V * 16mA
= 0.144W
Choosing watts at least 3 times of 0.144W = 0.432W, so choosing a 0.5W is better, or using a 5V to supply the power.

 

Thanks for the replies guys. Yeah I was afraid of that. I'll change all the resistors to 150 ohm and run the circuit from 5v. They should only dissipate around 40mW each then.

 

"hot" to a human hand isn't necessarily "hot" in the electronics world..
Its not uncommon for parts of circuit boards to run "hot to the touch"...
100 deg+ C temperatures is not uncommon... But not necessarily good either..

There are choices that can be made to dissipate less heat (lower voltage/current,etc...).. Less heat should/will prolong the life of the circuit and its components so its something a good designer/Engineer will always be thinking about..

 

Yeah understandable. The main reason I want the heat down is the rgb panel will be inside a very small itx PC case. I really don't need that kind of heat being generated in that case.