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Mosfet Question Answered

Hey guys, I am doing the arduino controlled rgb leds (1 watt leds) project and I am going to use mosfets for the leds. I ordered 3 IRF540 mosfets, 1 for each color of leds (Red, Green, Blue). the arduino is going to use pwm to control the leds but the arduino can only supply 5V at 40mA max. The mosfet have gate-source voltage of +/- 20V and a gate-threshold voltage of 2.0V to 4.0V. Do i need a driver circuit between the arduino and the mosfets and if not is 5V sufficient to run the mosfets in pwm? If i do need a drive circuit, does someone have a schematic since i am inexperienced at electronics? Would i need a heatsinks for the mosfets? thankx for any insight...


so how do i up the voltage on the pwm signal? transformer?

. You may not have to. Try what you have and see if it works. . Don't know if a xformer will work or not. I'm guessing not, but that's just a guess. . An op-amp may do the job (another guess).

np, thankx... i also have another question, how can i use a current limiting resistor? i though the resistor brings down voltage, how do you bring down current? i am trying to make sure that my leds are safe since they consume 350mA each and i plan of having 15 of them in each color...

. There will be a voltage drop (if you have resistance and current, there's gonna be a voltage drop), but if your resistor is sized properly, it won't be much. Better to lose a little brightness than burn out your LED. ;)
. Look at some of the other LED topics on Ibles for info on sizing current-limiting resistors (someone posted a link to a page that will do the calcs for you in one of them) and using series/parallel to wire the LEDs.

thankx, i have been going thru the ibles and i found out that if i am going to wire leds in parallel (like i am) you have to put a properly (i will calculate it myself and check back later) sized resistor on each led. but i am getting conflicting meanings on anodes and cathodes of an led. i thought cathodes are suppose to be positive and anodes to be negative, some people have said the complete opposite, whats with that?

cathode -
anode +

the resistor can be placed on any of them. if you use common-side A leds then place the resistor on side B

. Gate-threshold voltage is where the MOSFET starts to turn on. 5V may not be enough to turn the 540 fully on - most of the circuits I found used a 12V supply. Should work well enough for proof-of-concept, but your LEDs may be dim.
. But I'm no expert.