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You must use 12V 20W bulb, with 4.2V your current is 0.7-1A, then you don't need big heatsink! With feedback LED (right side) when current flowing (bulb is not damaged), LED is lightening. this is important, because this 20W bulb has no light with 4.2V 1A, also 4W power, only heat comes out!
LED in center showing, must balancing, right showing, current is flowing. Left side you must use TL431 instead zener diode.
Left potentiometer: you muss the balancing voltage set, 4.20-4.25V. Right potetiometer: the feedback LED muss on, when 1A flowing, and muss off, wenn smaller than 100mA flowing trough the bulb.
P-CH FET is AP3401.
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