The scenario
A single-cell lithium battery powers an LED flashlight. A BJT current mirror sets the LED current, while a protection transistor senses the emitter current and limits it to prevent LED damage.
What is in this drawing
Read the decisions behind it.
01
Use a BJT current mirror (Q1/Q2) for constant current drive
02
Add sense resistor Rsense and protection transistor Q3 to clamp LED current
03
Power from a single-cell 3.7V battery with SPST switch
Reusable for any low-voltage LED constant current source needing overcurrent protection, such as flashlights, indicators, or battery-powered lighting.
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