DIODE CHARACTERSITIC AND THE HALF-WAVE RECTIFIER
The objective of this experiment is to study the DC characteristic of a silicon junction rectifier diode. Also, the performance of a half-wave rectifier will be studied and measured.
A semiconductor junction diode is a very useful device that is used in many electronic circuits. The diode is a unidirectional device that allows for the flow of electric current in one direction with little resistance, while at the same time it will provide relatively much higher resistance when the current flows in the opposite direction. The diode is in the turn-on state when it exhibits low resistance, and turned-off when it is in the high resistance mode.
Current will flow in the forward direction when the forward bias voltage reaches the turn-on voltage of 0.7 V in case of silicon diodes, and about 0.3 V in case of germanium diodes. The 0.7 V turn on voltage is due to the equilibrium barrier potential, also known as the built in voltage. Complex electron energy considerations and equations are used to define the equilibrium barrier potential. However, it is readily measurable in the form of the forward turn-on voltage of the silicon junction diode. A typical volt-ampere characteristic of a low power silicon junction diode is shown in Figure 1. The circuit used to obtain this curve consists of a variable DC voltage applied across the two terminals of the diode. The voltage is increased from a negative value to a positive value. The forward, and reverse currents are measured and are plotted against the voltage.
The non-linear characteristic of a diode can be utilized to convert alternating current into unidirectional, but pulsating current in a process called rectification. Rectifier circuits employ one, or multiple diodes to provide various degrees of rectification effectiveness and efficiency. When analyzing the pulsating currents at the rectifier output, it will be evident that this current consists of a DC component in addition to many harmonics which are integer multiple of the fundamental harmonic of the input Alternating current. These harmonics are reduced through the use of some reactive components that will filter out the harmonics and allow the DC component to pass to the load.
In this lab session, the V-I characteristic of the silicon diode will be measured and studied. The half-wave rectifier will be tested and the impact of a simple filter on the rectified waveform will be demonstrated.
Figure 1 - Voltage-Current characteristic of the silicon rectifier diode
The half-wave rectifier circuit is as shown in Figure 2. The diode in this circuit will provide low resistance when the input AC power goes through the positive polarity. The diode resistance will be significantly higher when the AC cycle goes through the negative polarity. Consequently, the input and output waveforms in this circuit are as shown in Figure 3.
Figure 2 - Circuit diagram of simple half-wave rectifier
The rectified output voltage can be related to the input AC voltage as,
Vp-out is the peak voltage of the rectified signal. Vp-in is the peak voltage of the input AC voltage. rd is the forward resistance of the rectifier diode, Vton is the turn-on voltage of the diode, and RL is the load resistance. Refer to Figures 2 and 3.
Figure 3 - Input and output waveform of the half-wave rectifier
The average DC component in the half-wave rectified signal is given as,
In fact, the rectifier diode converts the alternating current into a train of one polarity voltage applied to the load impedance is in fact a train of periodic pulses as clearly displayed in Figure 3. These uni-polar pulses contain a DC component in addition to a large number of undesirable harmonics. A significant improvement in smoothing out these pulses can be achieved by placing a large capacitor across the load impedance, as illustrated in Figure 2. The output waveform with the capacitor connected across the load is as shown in Figure 4.
Figure 4 - Effect of smoothing capacitor on the rectified power delivered to the load
- Measurement of the DC characteristic of the junction diode
- Set the DC voltage of the power supply to 0 V.
- Connect the diode circuit as shown in Figure 5.
- Change the DC supply voltage by 0.3 V step or larger steps. Use a digital voltmeter, measure Vin, VR, and VD as defined in Figure 5. Tabulate your measurement data.
- For each step, calculate the DC current through the diode, which is equal to (VR / 10).
- DO NOT LET Vin TO EXCEED 9 V DC.
- Reverse the polarity of the DC power supply in Figure 5 and change the resistance to 4.7 MΩ. Repeat the measurement steps from 1.a to 1.d.
- DO NOT LET Vin TO EXCEED 30 V DC.
Figure 5 - Connection diagram for measuring the DC characteristic of the junction diode
- Characteristic of the Half-wave Rectifier
- Connect the half-wave rectifier circuit as shown in Figure 2 in which RL = 1 kΩ. Do not connect the capacitor C across the load.
- Monitor both Vs and Vo (see Figure 3) on the oscilloscope simultaneously. Measure Vp-in and Vp-out. Sketch the displayed waveforms. Use digital voltmeter, measure the DC voltage across RL.
- Calculate the value of the forward resistance of the diode rd knowing that the diode used in the measurement is a silicon diode in which Vton = 0.7 V.
- Connect 47 μF across RL. Monitor Vs and Vo on the oscilloscope and sketch both waveforms as accurately as possible. Repeat the measurement with 10 μF capacitor. Compare the two rectified waveforms obtained with the different capacitors.
Results and Discussions:
- Plot the V-I characteristic the rectifier diode.
- What is the value of the forward resistance of the diode?
- What is the value of the reverse-bias resistance of the diode? (Hint: use the measured V-I curve to calculate this value).
- Estimate the turn-on voltage of the diode from the V-I characteristic curve.
- When a capacitor is used across the load resistance, did you notice any change in the output DC voltage? If the answer is yes, how much?
- Compare the measured DC voltage across RL with that predicted by equation . What is the difference between the two values? Why there is a difference (if any)?