hello all i have 30 RGB LEDs that i want to be able to switch them using a transistor. all leds are connected in parallel. now if each RGB LED has all diodes on and conducting 20mA each that will make my total current: 30 LEDs* 3 LEDs each * 20mA = 1.8 A we know that P = I*R, the mosfet's Rds is 0.2ohm @ Vgs ~4.5, so 1.8 * 0.2 = 0.36 Watt am i correct ? is this mosfet (NDT014L) an overkill ? what do you guys recommend. thanks for you help
I'm a bit unsure what you are trying to do, but using one transistor to power 90 LEDs in parallel sounds like a very strange thing to do. Is there any reason why you can't make small groups of something like 3. This will make a much more efficient circuit. Remember that you also need to calculate in the resistors for all the LEDs as well. More LEDs in series means less wasted energy at the resistors. It's a bit difficult to make sane recommendations without knowing the circuit.
The thing is P=UI and by Ohms law R=U/I thus giving P=I^2*R. What you want to make sure is that current passing through the transistor doesn't exceed it's max current rating. If you can't draw the circuit could you at least describe it more.
The mosfet you have suggested is fairly hefty and will easily handle a continuous current of 1.8A. I dont think this is overkill at all, better to be overspec just in case However as previously suggested I would consider running your LEDs in groups(I would split into 3), each switched by a single transistor.
the circuit is a bike POV and the LEDs are individually controlled by serial to parallel shift regs.. and all the common ground connects to the transistor drain, and the transistors switches one of the two banks of the LEDs (front & back) . what i want is to find answers to these questions just to be safe : if the power the transistor needs to dissipate is 0.36 Watt, is it safe to assume a transistor the is rated at say 0.4W would do the job just fine ? it would be cost effective too is the way i calculated the power is correct? what other factors i need to consider ? "Remember that you also need to calculate in the resistors for all the LEDs as well."...@Smilodon: please explain more..
