Reverse Engineering a 12V Fluorescent Driver: From BJT to MOSFET

Prerequisites

• Understanding of DC voltage (12V) and polarity.
• Basic soldering and PCB cleaning techniques.
• Familiarity with reading electronic schematics.
• Knowledge of transistor types (BJT vs. MOSFET).

Key Libraries & Tools

• Acetone & Cotton Buds: For removing protective PCB paint to identify component markings.
• Multimeter: Essential for checking capacitor values and transistor continuity.
• Schematic Capture: To map out traces and understand the 555 timer chip logic.

Circuit Walkthrough

The heart of the board is the 555 timer chip. It generates a fixed frequency to drive the primary side of a step-up transformer.

// Logic representation of the 555 output driving the transistor
void drive_inverter() {
    while(power_on) {
        set_pin(3, HIGH); // Pulse to transistor base/gate
        delay_us(frequency_constant);
        set_pin(3, LOW);
        delay_us(frequency_constant);
    }
}

A critical pre-heat stage occurs via a PTC thermistor. At startup, current flows through the heaters; as the thermistor warms and its resistance climbs, voltage is forced across the tube to strike the arc.

Syntax Notes

• Zener Configuration: The use of 7.5V and 2.4V Zener diodes in series on Pin 5 is a hardware "syntax" for voltage stability, likely countering vehicle alternator fluctuations.
• Transistor Logic: While the original used an NPN BJT, a logic-level STP36NF06L MOSFET serves as a drop-in replacement with higher efficiency.

Tips & Gotchas

• Capacitor Fatigue: A 100nF capacitor that drops to 40nF will cause heat issues and eventual failure. Always check for bulging or "doming."
• Polarity Protection: This circuit uses a shunt diode. If you reverse the 12V leads, the diode shorts the supply to blow the fuse rather than frying the timer chip.