ledled-driverpotentiometer
I am designing a circuit to control a high current LED string. Most of the time the led string will be ON. Where should I place a potentiometer to control the brightness of the LED when it is on by limiting its current. I am thinking at the source to ground of the fet? LED has a forward voltage of 12V and If of 100mA at max brightness
Please excuse if I am misunderstanding anything as this is my first project.
Few things to check:
If you still can't find the issue, can you post an image of your board layout (pics/gerber) and measured voltages if possible.
Just poke around with a 30V power supply with a 1K resistor in series, as Starblue suggests and you'll quickly figure it out (I suggest the resistor because the output capacitor of even a current-limited power supply could damage the LEDs conceivably)
Probably 3 strings of series or series/parallel with a common. Whether it's common anode or cathode is unknown.
Chances are the top strip in your photo is the common, and the bottom left is red and the other two are green and blue (maybe not that order for green and blue).
simulate this circuit – Schematic created using CircuitLab
You need to limit the operating current into the LEDs to less than the smaller of the rated current and the maximum allowed by the power dissipation. As in the comments if you power this up at anywhere near the claimed 20W input power it will see perhaps 18W of heating and will expire in short order.
Best Answer
You are on the right track headed in the right direction.
To do this correctly you will need more than a potentiometer.
In the following image I circled in red where the potentiometer (33KΩ) would go.
The MOSFET is highlighted in yellow.
The easy way is to replace your MOSFET with an On-Semi CAT4201.
The PWM controller in the block diagram has a switching frequency of up to 1 Mhz.
The switching frequency is an operating parameter which affects nearly characteristic of the LED driver design including efficiency and cost. Inductor value is inversely proportional to switching frequency for equal peak-to-peak ripple current.
See Effects of High Switching Frequency on Buck Regulators
LED drivers should run in Continuous Conduction Mode. This is the purpose of the inductor. With slow PWM (under 10,000 khz) the inductor is of little use.
See also Section 2.1 Buck Steady-State Continuous Conduction Mode Analysis in this app note Understanding Buck Power Stages in Switchmode Power Supplies