12W Dimmer with Soft Start for 12V LEDs

Rajkumar Sharma
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SKU: EL97570
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The circuit presented here is a LED dimmer with a soft-start function. The board can drive a LED up to 12W and the circuit can manage a load up to 1A continuous with 12Vsupply. The circuit is built using SG3525 PWM IC and the IRFR120 MOSFET, SG3525 generates the PWM, and IRFR120 MOSFET drives the 12W max load. BC847 is used to invert the PWM signal from SG3525 and create a soft-start circuit. SG3525 has a soft-start function and the soft-start timing can be slowed down by increasing the capacitor C2 value to 22uF/16V. Trimmer pot PR1 provided to adjust the PWM frequency from 210 Hz to 6.5Khz. Keeping PWM frequency lower is advisable for LED load. The intensity of the LED can be adjusted using the slider potentiometer P1. Optional small trimmer pot is provided in case slider pot is not available. I have tried a LOG slider pot which is used in audio applications and a linear pot will have better linear dimming. The duty cycle upper limit can be restricted by changing R2. Use R2 0 Ohms or 1 Ohm for normal 0-100% duty output.

The circuit is designed to drive 12V LED or LED panels, so don’t connect directly any LED to this board, as LED forward voltage is 3.5V and they need a series resistor to control the current. Any LED, LED Panel, Light Pad of 12V, and current from 10mA to 1A can be connected to this board directly.

Direct drive of LED without a series resistor is possible by limiting the maximum current, choose the appropriate resistor value of R2 to achieve this.