PSpice implementation of 4-diode Bridge Rectifier circuit.
A full-wave, 4-diode Bridge Rectifier circuit with load operates from a 17V, 60-Hz source. It uses silicon diodes that one can model to have a 0.7-V drop for any current.
1.) PSICE simulation of the circuit, and plot Vout vs time and Vs vs time.
Figure 1: PSpice implementation on 4-diode bridge rectifier circuit without capacitor.
Figure 2: Simulation waveform at input (red) and Output (green) of circuit in figure 1.
2.) Using the results from the simulation, Describe how the Bridge Rectifier operates. Explain and state which diodes are ON and OFF.
During the first half cycle (Positive half cycle)
During first half cycle of the input voltage, the diodes D1 and D2 are forward biased and current flows through load resistance RL. During this half of each input cycle, the diodes D3 and D4 are reverse biased and current across them are zero. The current enters in diode D1, then pass through resistance RL and return back through diode D2, thus completing the circuit. Since we are measuring the output voltage on a node through which current enters into RL. Thus, the measured voltage is positive.
During the second half cycle (Negative half cycle)
During second half cycle of the input voltage, the diodes D3 and D4 are forward biased and current flows through load resistance RL. During this half of each input cycle, the diodes D1 and D2 are reverse biased and current across them are zero. The current enters in diode D4, then pass through resistance RL and return back through diode D3, thus completing the circuit. In this cycle the current direction across RL is same as positive half cycle current direction and again, we are measuring the output voltage on a node through which current enters into RL. Thus, the measured voltage is positive.
3.) What is the peak voltage of the rectified output?VoutPeak =VsPeak - 2*VDON = 17 - (2*0.7) = 15.6 V
4.) Circuit b), plot Vout vs time. Explain what is happening after adding the capacitor.
Figure 3: PSpice implementation on 4-diode bridge rectifier circuit with capacitor.
Figure 4: Simulation waveform at input (red) and Output (green) of circuit in figure 3.
The average DC output of the rectifier can be increased by reducing the AC variation of the rectified output by using smoothing capacitors to filter the output waveform. Smoothing capacitors connected in parallel with the load across the output of the full wave bridge rectifier circuit increases the average DC output level even higher as the capacitor acts like a storage device.
For the increasing voltage of the waveform in point 1, capacitances charges to the peak value of waveform i.e. VoutPeak. When decreasing voltage, capacitor starts to discharge and again starts to charge as soon as increasing part of the waveform starts. The capacitance value determines the amount of ripple that will appear superimposed on top of the DC voltage. Too low a capacitance value and the capacitor has little effect on the output waveform. But if the smoothing capacitor is sufficiently large enough, the output voltage will be almost as smooth as pure DC.
Ripple voltage is calculated as:
Vripple = Iload/(f*C) volts
Where:
I = DC load current,
f = frequency of the ripple waveform (twice the input frequency)
C = load capacitance.
