# Noise? with mosfet and high powered LEDs

I'm working on a project were I am pulsing an array of 3w LEDs . The LEDs have an adjustable pulse from 1HZ two 120hz and at the same time i can PWM dim the LEDs from 0 to 100% intensity. I am not really having an issue with functionality but more of a curiosity around the wave from. Is the ripple at the bottom of the wave form noise? The ripple at the bottom scrolls too.

50% Duty 30HZ

25% Duty 30HZ

MOSFET SPECS
FQP30N06L N CHANNEL
Continuous Drain Current Id: 32 A
Drain Source Voltage Vds: 60 V
On Resistance Rds(on): 0.027 ohm
Rds(on) Test Voltage Vgs: 10 V
SVHC: No SVHC (16-Dec-2013)
Threshold Voltage Vgs Typ: 2.5 V
Transistor Polarity: N Channel

Hi, the ripple is the volt drop across the drain to source of the FQP30N06L.

https://www.fairchildsemi.com/ds/FQ/FQP30N06L.pdf

This device is not a logic input type MOSFET and 5V is not completely turning it ON.
The 2.5V gate threshold is just that, the point at which significant gate influence occurs.
Also what are you using to power the LEDs, the ripple is 60Hz which I would say is your mains frequency(USA?).
Can you show us a trace of your 12V supply to the LEDs while this is occurring.
The ripple will scroll until you have the pulsing frequency equal to 60Hz or a sub or major multiple. ie 30Hz, 120Hz.

Tom.......

Where are you measuring and where is the ground of the scope?
That waveform looks like it is just noise picked up because at that part of the cycle your scope input is floating.

By the way that is not the way to power a 3W LED you need much more than a simple resistor for a power LED.

When the FET is off its high impedance. When the voltage across the LED is below the
threshold of conduction it too is high impedance.

You then see the mains waveform picked up from the environment (perhaps via your
finger) - a few microamps is enough to be picked up by the scope if the rest of the
circuit is high impedance. As soon as the FET is on it overwhelms the tiny capacitive
current picked up from the mains and controls the voltage.

I'm surprised you see this with that 10k resistor in place, it ought to keep mains
pick up down at a very low level.

Hi, it looks as though the input of the CRO is in AC mode not DC.
The low part of the waveform, if it is looking at the drain to source voltage, is the FET turned ON, the high part of the waveform is the FET turned OFF.

The ripple is when the FET is turned ON, which is when high current is being drawn from the power supply. So the ripple could be from power suppy with not enough filtering and FET with not enough gate drive.
(That is not fully turned ON, some drain to source resistance)

Tom.....

Thanks for the replies and I apologize I am a newbie to MOSFETs. I thought I was getting a logic level MOSFET, I bought this one from sparkfun and their description said it was I just now went back to the webpage and the first comments someone left, they said that it is not a logic level :0

The measurements were taken across the LEDs with DC coupling on.

Sorry the previous schematic was incorrect their are 3 LED in series.

The power supply is a Meanwell switching 12v 100w http://www.meanwell.com/search/NES-100/NES-100-spec.pdf
I even tried the circuit with a car battery instead of the power supply. I still had the same result.

So I guess I need to find a new mosfet? Or try a LTC1155 as recommend by the commenter on Sparkfun who pointed out the fet was not logic level. Can anyone recommend a logic level fet.
Thanks

Yes that is just as I said, By only measuring across the LEDs the scope is floating when the LEDs are turned off. So what you see is correct.
It matters not about logic level FETs.

It matters not if there are three LEDs in series to drive power LEDs you need a constant current supply. That resistor is too small to do much as temperature variations change the forward voltage drop of the LEDs.

http://m.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/

Would this work?

Sorry for my ignorance

Yes that circuit is fine for a constant current source.

No need to appoligise for lack of knowlage.

Thanks for your help i appreciate it

lol here is the measurement done correctly.