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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.

enter image description here

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  • \$\begingroup\$ What is driving the MOSFET? PWM? \$\endgroup\$ Oct 9, 2018 at 19:33
  • \$\begingroup\$ needs more components... \$\endgroup\$
    – dandavis
    Oct 9, 2018 at 19:42
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    \$\begingroup\$ I strongly advise you to look into how this is normally done. So look at how PWM works and linear current regulation. What you have now is just "slapping some components together" and that will end in disappointment. Your "circuit" suggests it is part of a switched regulator and then you would not be using a potentiometer directly with this circuit, to control brightness. \$\endgroup\$ Oct 9, 2018 at 19:59

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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.

enter image description here



The easy way is to replace your MOSFET with an On-Semi CAT4201.


enter image description here

I am driving the mosfet with a PWM ic

The PWM controller in the block diagram has a switching frequency of up to 1 Mhz.

enter image description here

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

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  • \$\begingroup\$ Thanks for the help! I am driving the mosfet with a PWM ic... In the first image, any reason why that lower resistor is so low? ie. 1 ohm \$\endgroup\$ Oct 9, 2018 at 20:32
  • \$\begingroup\$ The first image is the block diagram from the CAT4201. It is the current sense resistor which you want as low as possible because any power dissipated is wasted energy and produces heat. A current sense resistor is often less than 1 Ω. \$\endgroup\$ Oct 9, 2018 at 20:54

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