EXP4: Rectifiers

OBJECTIVES.

The general objective of this experiment is for you to become familiar with the response of a in a circuit, whether the circuit is supplied with a DC, AC or AC-DC . In this part we will also see how to couple AC and DC sources, and finally we will illustrate the use of in rectifying circuits.

INTRODUCTION AND TEST CIRCUITS.

The circuits we will be studying are the basic limiting circuit, half-wave and full-wave rectifiers. We will be doing the analysis of the circuits using aproximation, such as assuming we know the diodes’ voltage Vf . Also we will make use of the graphical method to find the voltage and current in the diode. We will run a bias analysis using PSpice which is the same as using an iterative method. And finally we will measure in lab the actual voltage and current values. That way we will be able to evaluate the accuracy of the different methods.

We will also get the transfer characteristic, and use it to predict the output waveform for a given input. We will check our estimate using PSpice and in lab.

For theory on the behavior of the circuits we will be studying please refer to the textbook or some book that covers the topics, limiting and rectifying circuits.

PREPARATION.

For the first two circuits, the limiting and the half-wave rectifier we will do the analysis using different methods. If you are not familiar with any of these methods please refer to the textbook or some book that covers this topic.

Limiting circuit:

1. Approximation:Use the circuit in figure 4-1 with V1 = 5V . Using the assumption Vf = 0.7V. find the resistor you need to have a current of 5mA. flowing through the diode, this resistor value will be used for the following points. So our first set of data is going to be [0.7V, 5mA].

2. Iterative solution:Now do the schematics on PSpice, run a bias analysis and find out the value for the current and voltage in the diode. That will give you the second set of data.

3. Graphical solution:Use a DC sweep to plot the VI characteristic of the diode, plot on the same graph the load line and find the voltage and current for your diode. This will give a third set of data.

4. Transfer characteristic:Use a DC sweep to get the transfer characteristic of the circuit. Use that plot to estimate the output waveform if your input is a 5V. triangle wave. ± 5. Graphical ”moving load line”:Use PSpice to run a transient analysis for the same circuit where V1 is a 5V. triangle wave source. ±

19 Figure 4-1: Basic limiting circuit.

6. Piece-Wise linear model:Using the first set of data and n = 1 and VT = 25mV , build a piece-wise linear model for the diode. Add a V sin source to your schematic in additive series with V 1 use an amplitude of 1mV and a frequency of 1KHz. Run a transient analysis. Repeat the analysis for an amplitude of 6v. Print both outputs. In a piece of paper do the analysis, for both cases, using the piece-wise linear model you found. Compare your results and explain the differences.

Half-Wave Rectifier:

Figure 4-2: Half wave rectifier.

1. Approximation: Analyze the circuit in figure 4-2 using the assumption Vf = 0.7V., for V 1 = 5V and R1 = 3.3k. First set of data.

20 2. Iterative solution: Use Pspice to find the current and voltage using the same V 1 as in last step. Second set of data.

3. Graphical solution: Now your load is the resistor, using PSpice plot the resistor’s VI characteristic, then plot the load line and find the voltage and current of the diode. Third set of data.

4. Transfer characteristic: Using PSpice plot the transfer characteristic of the circuit. Use it to sketch the output if the input is a 5V. triangle wave. ± 5. Graphical ”moving load line”:Use PSpice to run a transient analysis for the same circuit where V1 is a 5V. triangle wave source. ±

Full-Wave Rectifier:

Figure 4-3: Full wave rectifier.

1. Use PSpice to get a plot of the transfer characteristic of the circuit shown in figure 4-3. Use the plot to estimate the output if the input is sine wave with 10V. amplitude and 60Hz of frequency.

2. Use PSpice to plot the output for the same input source as in the last step.

Figure 4-4: Voltage regulated power supply.

Ripple voltage:The voltage is a parameter associated to rectifier circuits with capacitive loads. Such circuits are better known as power supplies. The ripple voltage is defined as the peak to peak voltage of the waveform at the capacitive load when an additional load is connected. The power supply is considered to

21 have no load if the rectifier has only a capacitive load. In order to have a load for the power supply we need to have either an inductor or a resistor or a combination of both attached in parallel to the . When that happen and depending on the impedance of the load the level of ripple voltage will be set. The ripple voltage is inversely proportional to the impedance. The ripple voltage is seen by the load as . In order to get rid of that noise we can add an additional circuit, a . The most simple circuit is a combination of a resistor and a zener. Using the voltage regulator we can assure a delivery of a more pure DC voltage to the load, for a range on impedance value. The circuit shown in figure 4-4 shows a circuit that combines the rectifier with the capacitive load to make a power supply and the power supply with a voltage regulator to get a circuit called voltage regulated power supply.

Use PSpice to find the minimum resistor value you can use as a load to the circuit such that your DC voltage do not decay more than 10% of its value without load.

You can start working on the preparation section. I will have the last two sections ready on monday.

PROCEDURE.

You will use a program called tranchar.vi to get the transfer characteristic of all the circuits but the voltage regulated power supply and the limiting circuit which combines the DC and AC.

1. Use the program iv curve.vi to get the VI characteristic for the diode you will be using. Remember you will be using a VDC of 5V. so make sure your x axis has that value on it. hint: this has to be done by manipulating the axis range, not the maximum voltage you apply to the diode.Warning: do not apply more than 1V to the diode. 2. Build the circuit shown in figure 4-1, with the values used in the preparation section; using a voltmeter and an amperemeter measure the diode’s current and voltage. 3. Build the circuit shown in figure 4-5, with the values shown. Monitor the ac signal in the diode for different values in the DC source. Measure the peak to peak value (ac) for 0VDC , 0.5VDC , 0.7VDC (DC), both measured at the diode’s terminals.Note that the DC values have to be measured at the diode’s terminals as well as the AC values, use any source DC value needed.

Figure 4-5: Basic limiting circuit with DC and AC.

22 4. Build the circuit shown in figure 4-2, with the values used in the preparation section; using a voltmeter and an amperemeter measure the diode’s current and voltage. Also save the data from the output for this circuit if the DC source is replaced by a sine wave with the same parameters used for the full-wave rectifier in the preparation section.

5. Build the full-wave rectifier shown in figure 4-3, with the values used in the preparation section; using the program tranchar.vi get the transfer characteristic of the circuit. Then use a sine wave, same as in preparation section, and save the data from the output.

6. Using the same circuit as in the last step, add a capacitor in parallel to the 1k resistor and a 10Ω resistor in series with any diode. Measure the peak to peak voltage (Ripple voltage) and save the data from the output. Repeat this step for C = 1µ and C = 100µ. Save the data for the waveform seen in the 10Ω resistor for the different cases, C = 1µ, 10µ, 100µ,no capacitor. 7. Build the circuit shown in figure 4-4, and measure the DC and AC voltage at the zener terminals. Use a resistor box in parallel to the zener, start with a big value 220k, and reduce the resistor value until you see an output voltage (DC) less than the voltage without load minus its 10%.

ANALYSIS.

Use the VI characteristic, you found in lab, and graphical methods to find the diode’s voltage and current for the two limiting, and the half wave rectifier circuits.

For the limiting circuit which combines DC and AC, analyze the circuit using a piecewise linear model to find the peak to peak voltage for all cases seen in the procedure section. Compare this theoretical results with your lab measurements.

Use the transfer characteristic, found in lab, and graphical methods to find the output voltage, using the same input parameters as in the preparation section.

Explain the waveform you observed in the 10Ω resistor you placed in series with the diode in the full-wave rectifier. hint: you are seeing the current waveform for the diode which is in series with the resistor. What percentage of the time are the diodes conducting, on all different cases? What happend with that time when we increase the capacitance?.

Use pspice to find the ripple voltage for different values of capacitor as in the step 6. Use at least two capacitor values in between the values we used in lab, plot the ripple voltage variations versus capacitance. Mark the values you measured in lab. Do the measured values agree with the plot found using PSpice?.

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