Zero Crossing Detector

  Aim

  To design and simulate a Zero Crossing Detector circuit.

  Components

  Theory

  As the name indicates the zero crossing detector is a device for detecting the point where the voltage crosses zero in either direction. As shown

  in the above circuit diagram the first section is a bridge rectifier, which provides full wave rectified output. This is applied to the base of the transistor

  through a base resistor, R2. The capacitor charges to maximum of the bridge rectified output through the diode,D5. This charge is available to

  the   transistor as V_CC. The capacitance value is kept large in order to minimize ripple and get perfect dc. The transistor remains OFF until the

  Cut-in   voltage V_BE is reached. During the OFF period of the transistor the output will be high and approximately equal to V_CC. Once the transistor

  is ON and I_B increases according to the input wave, the transistor moves slowly towards saturation where the output reduces to the saturation

  voltage of the transistor which is nearly equal to zero.

  Initially V_BE = Cut-in voltage of diode, the capacitor will charge through the diodeVm where Vm is the maximum amplitude of the rectified wave.  

  Now the diode is reverse biased and hence does not provide a discharging path for the capacitor, which in turn has two effects.

1. Variation in V_CC.
2. It will provide base current to the transistor in the region where both diode and transistor are OFF.

  Thus an output square wave is produced whenever the input voltage crosses zero thereby acting as a zero crossing detector.

  Procedure

  EDWin 2000 -> Schematic Editor: The circuit diagram is drawn by loading components from the library. Wiring and proper net assignment

  has been made. The values are assigned for relevant components.

  EDWin 2000 -> Mixed Mode Simulator: The circuit is preprocessed. The waveform markers are placed at transformer primary, bridge output and

  detector output. The Transient Analysis parameters have been set. The Transient Analysis is executed and output observed in the

  Waveform Viewer.

  Result

 The output waveform may be observed in the waveform viewer.