Computer & Electronic Systems Year 1, Semester 2 Electronic Building Blocks 1 Diode Applications Diode Applications • • • • Circuit Protection Half-Wave Rectifier Full-Wave Rectifier Zener Voltage Reference Diode Applications 2 Circuit Protection • Protect circuit from being damaged due to a reversed power supply • When power supply is correctly connected, the diode is forward biased • Reversed power supply connection means diode is reverse biased, no current passes and no damage occurs to the circuit Diode Applications 3 Half-Wave Rectifier • AC voltage converted to DC voltage • Load is represented by the resistor R • Positive half-cycles of the input voltage forward-bias the diode which conducts current into R • Negative half-cycles reverse-bias the diode, meaning no current and the voltage across R=0 • VAVG=VPeak/π Diode Applications 4 Bridge Full-Wave Rectifier • Uses 4 diodes. Current in the same direction through R on both half-cycles • Positive input cycle: D1, D2: forward-biased, conduct current D3, D4: reverse-biased Current path A, D1, R, D2, B • Negative input cycle: D3, D4: forward-biased, conduct current D1, D2: reverse-biased Current path B, D4, R, D3, A • VAVG=2*VPeak/π Diode Applications 5 Zener Diodes • Zener diode is designed for use in the reverse breakdown region • Voltage across diode remains constant over wide range of reverse currents, useful in a power supply to keep voltage steady under various current loads • Breakdown voltages available from 1V to >100V Diode Applications 6 Zener Voltage Regulation • Provides an output reference voltage which is stable despite changes in input voltage • Used to regulate voltages in reference circuits • Use with 9V battery (whose voltage falls with use) to keep output steady at 5.1V Diode Applications 7 Zener Application • Keep bulb voltage at 6V as battery increases to 9V – see diagram on board • Battery 3V-6V: Zener not conducting, lamp brightness gradually increases, all current through lamp • Battery 6V-9V: Zener conducts and keeps voltage across the lamp fixed at 6V Diode Applications 8 Power Supply • Convert 240V AC mains voltage to 9V DC • 4 main blocks: 1) Transformer converts 240V AC to 12V AC 2) Bridge full-wave rectifier converts 12V AC into pulsating DC 3) RC low-pass filter to smooth out pulsating DC 4) Zener diode fixes 9V • For mains applications, buy the power supplies, don’t make them!! Diode Applications 9 Typical Diode Calculations • What is the average value of a half-wave rectified voltage with a peak value of 200V? • What is the average value of a full-wave rectified voltage with a peak value of 75V? • What is max current a Zener diode can pass without damage if breakdown voltage is 10V and power rating is 5W? Diode Applications 10 Review • PN Junction • Diodes • Diode Application Circuits Diode Applications 11