An AC to DC Converter

In one of my classes, E E 331 Devices and Circuits I, our final project was to build an AC to DC converter. Our design specifications were to take a 10 Vpp 60 Hz DC input from a center-tap transformer and have an adjustable 10 V to 20 V DC output. It was expected to have output noise of maximum 100 mV, and able to deliver 1 mA current for all voltage settings. Also, we were graded on how cheap the circuit was to build.



Above is a block diagram of our design. The circuit works by rectifying an AC input, creating a high frequency square wave, and using that wave to drive a boost converter to amplify the rectified input signal to a level dictated by a differential amplifier you could control using a potentiometer.



And above is the circuit schematic as viewed in PSPICE schematic and simulation software. The circuit was designed mostly on computer, then built later when the simulated circuit met specifications. The square wave was provided by a 555 timer, and the differential amplifier was built using a LM741 operational amplifier. The amplifier compares the voltage at the anode of a 5 V zener diode with the output voltage, and increases its output voltage when the output voltage falls too low, and decreases its output when the output voltage gets too high. This feedback system maintains a constant output voltage, which is calibrated using a network of resisters to operate within the specified range.



Above is the circuit built on a breadboard. The total cost of all the electrical components is a little under $8. Our design had barely any noise in the output, and it met all performance requirements within a narrow error margin.