Resistor and their wattage help?

[snazi]

Hi,

I bought this ready made led circuit for some aquarium lighting **broken link removed** (the circuit is labeled)
- I want to use it with a 12v power supply so using 30 Ω resistors.
- A total of 120 leds can be mounted on the circuit, I want to use white leds so looking at the following figures
supply voltage: 12v
diode fwd voltage: 3.3v
forward current: 20 mA

I need to know if my choice of 30Ω resistors is correct also what wattage of resistors would be fine for this application? I have a few 1 watt 30 Ω resistors will they work ?

Any help would be appreciated.
Regards.

 

[Ian Rogers]

Can you jot down / draw an intended circuit?

 

[snazi]

sorry, Can you please explain what you mean by jot down /draw an intended circuit?

 

[Pommie]

That is a very bad circuit board. It has 3 lots of 40 LEDs, The LEDs are in parallel and share a resistor. However, 40*20mA = 800mA so R = (12-3.3)/0.8 = 11 ohms. Wattage = VI = 8.7*.8 = 7 Watts.

Mike.