You’ve seen them when you’re driving. Mailmen use them, snow plows have them. This is a schematic of the circuit inside.This was taken from a popular unit available at NAPA auto parts stores. It’s amazing how simple it is for a PWM circuit.
It uses a UC3842 (here a TL3842 clone) pulse width modulator to regulate the charge on the main storage capacitor. In this respect is similar to a computer power supply. It has an interesting Zener booster circuit using an NPN transistor, shown on the left of the drawing. This keeps the supply voltage at or below 15V at all times. One great feature that this affords is a great variety of input voltages. Itâ€™s rated at 12-80V.
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