|DC/AC Circuit Reference|
We increasingly conceive of circuits in terms of terminal behavior. That is, we label the source signal and the potential difference between some other pair of nodes , and we think of the network of devices between the input and output terminals to be a filter that transforms into .
You have been provided with a breadboard, a resistor, a capacitor, an inductor, a function generator, and an oscilloscope. First, measure the resistance of the resistor by any means you like. Then construct the series LC and RC circuits shown in Figure 5. Drive the circuits with a square wave signal from the function generator.
For each circuit, pick a good frequency, and find the capacitance/inductance by measuring the time constant of the response with the oscilloscope. Explain what made your choice of frequency ``good.'' Use one channel of the oscilloscope to measure , and trigger on that channel. Measure with the other channel to look at the natural and step response of the circuit. Measuring the half time and converting to the time constant may be easier than trying to measure the time constant directly. Use the time base magnification feature of the oscilloscope to improve the precision of your time measurements. Give the uncertainties in your results.
Compare and at various frequencies. How well do your measurements compare with your theoretical calculation and spice simulation? Remember that . Use a measurement of in your theoretical calculation to find . Report an uncertainty with your result. Be sure to measure your value at good frequency. What does ``good'' mean here?2 How far off were you in your measurement of of Section 1? Can this discrepancy be attributed to ignoring the internal resistance of the inductor? Explain.
In time base mode, you can look for the frequency at which the voltage across the resistor is maximal. Report an uncertainty in your result. Compare your result with your theoretical prediction using your best measured values of and .
Derive general expressions for and the phase angle for the band pass filter in Figure 7 as a function of the resistances, capacitances, and signal frequency. Use this expression to produce a plot of vs. frequency. Predict the cutoff frequencies (3 dB points) of the filter.
An important step in converting the 120 V AC power supplied by a standard wall outlet into 5 V DC power for your computer is voltage reduction. You have been provided with a variable transformer that can step the 120 V signal from the wall socket down to 7 V across the two red terminals, or 3.5 V across either black/red pair. In the following exercises, use the 7 V option.
In order to convert an AC current that spends equal amounts of time flowing in opposite directions into a DC current flowing in only one direction, we need a device that responds differently to applied voltages of different polarities. The standard device used for this purpose in modern electronics is the semiconductor diode. The circuit of Figure 8 is called a half wave rectifier, because the diode simply blocks the segments of the signal with negative polarity.
Set up the circuit of Figure 8 using a 10 k load resistor, and compare the voltage across the load resistor to the input signal. Are you surprised by the peak voltage across the load? Make note of this. Also compare your observations with a Spice simulation (a transient analysis). If they agree, just hand in a graph of the simulation. Otherwise, also sketch your observed oscilloscope traces.
In your Spice simulation, do not explicitly include the transformer. Simply treat it as a sinusoidal voltage source. In order to use diodes in a circuit, include the following statement somewhere in your circuit file.
.MODEL D1 DThis tells Spice that device type D1 is a generic silicon diode. Then, diodes are specified with the syntax
[Name] [+] [-] D1where the [Name] field must begin with a D, and current is allowed to flow from the [+] node to the [-] node.
You should have found that the load is not being driven by a particularly steady DC signal, although the current always moves in one direction. (If you did not, perhaps we should look at what you discovered together.) The next step is to use a large capacitor to filter out most of the unwanted variation or ripple in the signal across the load resistor. The circuit of Figure 9 includes a filter capacitor in parallel with the load. Since the diode does not allow current to flow back through to the transformer, the capacitor discharges through the load resistor with a time constant . Assuming derive an expression for the peak to peak magnitude of the ripple in the output signal in terms of and . Assuming a 10 k load, find the capacitances you would need to use to reduce the ripple to 10% and 1% of the DC output voltage.
Using the best filter capacitor you can find 3, measure the DC level and the magnitude of the ripple across the load. Use the DC (direct coupling) mode of the oscilloscope or a DVM to measure the the DC level, and use the AC (active coupling) mode of the oscilloscope to remove the DC component of the signal and zoom in on the ripple. Then change the load resistance, and determine whether the ripple changes as expected.
The configuration of four diodes in the circuit shown in Figure 10 is called a bridge rectifier. It is also a full wave rectifier, because it does not throw away any part of the input signal. Consequently, the filter for this circuit requires roughly half the time constant of the half wave rectifier.
Build the circuit of Figure 10 using a 10 k load resistor and a suitable filter capacitor, and compare your observation of the voltage across the load resistor, with and without the filter capacitor, with Spice simulations. 4 If they agree, just hand in a graph of the simulation. Otherwise, also sketch your observed oscilloscope trace. Measure the magnitude of the ripple, change the load resistance, and determine whether the ripple changes as expected.
You have been given an open power supply constructed as shown in Figure 11. This supply is a full wave rectifier with a filter and a device called a voltage regulator that removes the remaining ripple. This device is an integrated circuit (IC) that contains lots of very small resistors, capacitors, diodes and transistors manufactured on a single silicon wafer. For the moment, we will treat it as a ``black box.'' Its function is to deliver or accept current as needed to maintain a constant output voltage. The 78XX series of voltage regulator maintains an output voltage of XX Volts. Your supply has a 7806, so the DC output voltage should be close to 6 V.
Poke around carefully with the oscilloscope. The filter capacitor is physically the largest capacitor. It also has the largest capacitance. (The other two capacitors, labelled and are recommended by the manufacturer of the voltage regulator.) Measure the DC and ripple voltages both at the output and at the filter capacitor.
For each circuit shown in Figure 12:
|Copyright © 2001-2004, Lewis A. Riley||Updated Mon Jan 19 13:29:10 2004|