Trouble controlling multiple LEDs with MOSFETs

Hello,

The goal of my project is to individually control 10 LEDs in order to display patterns. The LEDs I got for this project require about 300-350 mA in order to be bright enough for my application (they are rated to withstand a maximum current of 1 Amp).

Because these LEDs require far more current than my Arduino Uno can provide, I decided I'd try to use N-channel MOSFETs to control the LEDs and an external power supply to power them. I'm using the FQP30N06L: N-Channel MOSFET 60V 30A - COM-10213 - SparkFun Electronics

I pulled the schematic seen below off the internet because my circuit is set up very similarly. For the sake of testing, I have five MOSFETs set up the same way, each with their own digital pin. All five LEDs are wired in parallel and powered by an external supply.

The problem I'm encountering is that I can only light up three LEDs at a single time. When I try to light up a fourth simultaneously, three of them light up very dimly and one either doesn't light up at all or does so extremely dimly. When I try to light up all five, only three light up dimly, one is either off or barely showing any light, and one is completely off.

The only limitation I can possibly run into on the Arduino is the maximum output current, correct? I measured the gate current to peak at 6-7 mA, therefore the Arduino isn't even close to hitting its 200 mA maximum. However, looking at the FQP30N06L datasheet, I calculated the theoretical gate current to be 66.67 mA. The turn-on delay time is typically 15 ns with a rise-time of 210 ns, and its total gate charge is 15 nc. So the gate current should be (delay time + rise time)/(total gate charge) = 15 nc/225 ns = 66.67 mA. It's just odd that I'm measuring 6 or 7 mA for the gate current using a multi-meter.

Either 1) my multi-meter is wrong and the current actually is ~67 mA, which would perfectly explain why I can only light up 3 LEDs before hitting the 200 mA maximum output current. I would try to power the Uno through the barrel jack to increase the maximum output current and see if it can now power more MOSFETs and light up more LEDs BUT I somehow managed to fry a bunch of the LEDs and will have to wait a few days until more arrive.

Or 2) my math is wrong and I could use some help to figure out the real cause of this problem.

Any help is appreciated!

Tom

What is the specification of the "external power supply"? High power LEDs should normally be run from a constant current supply - is that what you have? Or do the LEDs at least have current limiting resistors?

Steve

slipstick:
What is the specification of the “external power supply”? High power LEDs should normally be run from a constant current supply - is that what you have? Or do the LEDs at least have current limiting resistors?

Steve

To save you all from reading an extremely long post, I omitted the list of troubleshooting attempts I have already made. Among these, I successfully powered all 5 LEDs in parallel simultaneously without the MOSFETs, therefore I did not see any issue with my power supply. I was simply using a 9V battery into a lm7805 voltage regulator. In my tests, I was actually keeping the LEDs closer to 150 mA (using resistors) both to stay within the lm7805’s current rating and to save my eyes from the insane brightness of these LEDs.

arduino's recommended current per pin is 20ma. You might need another transistor to drive the gate of the mosfet. What I did for my 5050 12v LED light strips was use a s8050 transistor. Those are cheap and beefy, up to 1.5A in some variations. I have the s8050 running 6m of rgbs on the back of my tv right now.

swagguy8:
arduino's recommended current per pin is 20ma. You might need another transistor to drive the gate of the mosfet. What I did for my 5050 12v LED light strips was use a s8050 transistor. Those are cheap and beefy, up to 1.5A in some variations. I have the s8050 running 6m of rgbs on the back of my tv right now.

Thanks for the reply. I think that suggestion is going to help me in a different part of this project!

I do have my MOSFETs working correctly now. I missed that the Arduino's ground needs to be connected to the ground of the external power source. Apparently my multi-meter was correct and my math was wrong. I'll have to go review how to calculate the gate current.