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Genre: Science & Technology
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Shared June 25, 2026
In this video I build a constant current LED driver from scratch using a BC337 transistor, IRF540N MOSFET, and 1Ω sense resistor.
This circuit keeps LED current stable even if battery voltage drops. I power it with a 3.7V 3000mAh Li-ion battery and drive a 3W LED.
What I cover in this video:
1. How the constant current circuit works - transistor + MOSFET current sink explained simply
2. Step by step wiring and how to set LED current using a potentiometer
3. Live multimeter measurements of input voltage, LED voltage, and current
4. How to calculate power loss and efficiency η with the formula
5. Real battery runtime test - how many hours this 3000mAh cell can run the LED
Measurements shown in video:
Input: 3.35V, 0.183A
LED: 2.88V, 0.17A
Efficiency η = 79.9%
Power loss =20.1%
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