Utilizing the LT1074CT Switching Regulator IC
The 3-volts Car Adapter circuit is constructed around the LT1074CT switching regulator IC. A standard component widely used for such applications. A more detailed and elegant design explanation can be found in Application Note AN35, published by Linear Technology. Interested readers are strongly encouraged to obtain a copy of AN35 for comprehensive insights. In this schematic, the LT1074CT functions as a positive step-down or ‘buck’ converter, reducing a +12-volt car battery voltage to +3 volts. This transformation is specifically intended for powering personal hi-fis and handheld games, catering to the author’s lively children during extended car journeys. It’s worth noting that young children, typically under ten years of age, are seldom hi-fi enthusiasts and are generally unconcerned about any noise generated by the circuit.
Connection and Operation via Car +12-V System
The circuit is linked to the car’s +12-V system through the cigarette lighter socket, and it is advisable to use a fused version of the plug for safety. The +12-V supply enters the board through screw-terminal block J2. Diode D2 offers reverse voltage protection, and C3 acts as a decoupling element for the input to the LT1074CT. The LT1074CT rapidly switches the supply voltage on and off, responding to the signal at its F/B input. This modulation ensures that the average output voltage aligns with the desired +3 volts.
Resistors R1-R3, configured as a potential divider, attenuate the output voltage to maintain 2.5 V at the F/B pin. The difference between this attenuated voltage and the internal 2.5-V reference controls the modulation effect of the switcher. Components R2 and C2 stabilize the frequency within the feedback loop. Inductor L1, coupled with the LT1074CT, constitutes the primary switching components, and C1 provides decoupling for the output load. The 3-V output voltage is accessible via screw terminal J1.
With this circuit built, boxed up and installed in your car, you can look forward to possibly your first ‘quiet’ long car journey.