Purpose: 1) to investigate the electrical properties of a diode; and 2) to use a diode to construct an AC to DC converter.

Transcription

1 Name: Partner: Partner: Partner: Purpose: 1) to investigate the electrical properties of a diode; and 2) to use a diode to construct an AC to DC converter. The Diode A diode is an electrical device which acts like a one-way valve. In an ideal diode, current flows with zero resistance in one direction, but can not flow in the opposite direction due to infinite resistance. For the diode shown (along with its symbol) current flowing down encounters zero resistance, while current flowing upward encounters infinite resistance. Thus for an ideal diode the current will be infinite for positive values of V d, and the current will be zero for negative values of V d. In Project 1 you will measure how the current depends on V d for a real diode. Project 1: Current versus Voltage for a Diode Use the Pasco Circuit Board to set up the following circuit. For each value of R you will measure the source voltage V 0 and the current I. It is easiest to measure V 0 first for each R, then go back and measure I for each R. In this way you will only have to switch the multimeter from voltage mode to current mode once. Remember to break the circuit and place the multimeter in series to measure current. Record the values in the table below. After you have measured V 0 and I, calculate the voltage across the diode V d for each R. Make sure you have the diode properly oriented. Look at the previous figure to determine which way the white band goes. Page 1 of 6

2 R V 0 I V R = IR V d 100 Ω 1 kω 10 kω 100 kω Now repeat the above procedure with the diode reverse biased. In order to do this, turn the diode around. Note, since the diode is turned around, the voltage V d is now negative, so enter the values for V d as negative numbers. Think about what the current should be in this case before you make the measurements. R 100 Ω 1 kω 10 kω 100 kω V 0 I V R = IR V d Using data from your two tables make a graph of I (y-axis) versus V d (x-axis) for the diode. Use the Graphical Analysis program on the computer. Print graphs for each member. Q. How do your data compare to the predictions you would make for an ideal diode? Page 2 of 6

3 Project 2: Basics of AC to DC conversion The one-way valve characteristic of a diode is the basis of converting alternating current (AC) to direct current (DC). To understand how this works consider the circuit: Vout 2V + - Ra Rb ground Ra represents a diode oriented as shown above. Current from the source will go clockwise through the circuit. Ra 0 since it is the resistance of the forward biased diode. What will be the voltage at Vout with respect to ground? Show your work. Now suppose the voltage source has changed direction so that it tries to drive current counterclockwise. - 2V + V Ra Rb Vout ground The diode is now reverse biased, hence Ra. What will be the value of Vout now? If the voltage source is AC that means it is sinusoidal, so that it alternates between driving current clockwise and counterclockwise. You should now be able to predict what Vout will look like for an AC voltage source, Ra a diode, and Rb a resistor. Page 3 of 6

4 Now you will construct a simple AC to DC converter using the diode on a Pasco Circuit Board. Step1. Set up the following circuit. Note there is no diode yet. 1. Set the function generator to produce a 2 volt amplitude (4 volts peak to peak) 1000 Hz sine wave. Use the oscilloscope to display this voltage V in on Channel 1 in the upper half of the screen. Note that the oscilloscope voltage probes are 10 probes. This means that the voltage scale on the oscilloscope is one tenth the actual voltage. For example, if you use the 0.1 VOLTS/DIV setting, it is really 1.0 VOLTS/DIV. 2. You should adjust the trigger, position, Volts/ Div, Time/ Div, etc. until you get a stable sine wave. The screen should look similar to this. 3. Display V Out on Channel 2 in the lower half of the screen. (For this circuit V in and V Out are the same. Why? Explain below. ) The screen should look like this. Page 4 of 6

5 Step 2. Insert a diode to make the following circuit. Note that V in and V Out are no longer attached to the same point of the circuit. Draw graphs of V in and V Out as they appear on the screen. Clearly indicate where zero volts is for each channel. Also indicate the Volts/ Div and Time/ Div. Q. Are these graphs substantially different than in Step 1? If so, explain why. Use your results from Project 1 and think about how the current through the diode depends on the voltage across it. Also remember that the current then goes through the resistor. Step 3. Add a capacitor in parallel with the resistor to make the following circuit.. Page 5 of 6

6 Draw graphs of V in and V Out. Clearly indicate where zero volts is for each channel. indicate the Volts/ Div and Time/ Div. Also Q. Are these graphs substantially different than in Step 2? the capacitor can charge and discharge in this circuit. If so, explain why. Think about how Homework 1. Compare your I versus V graph for the diode to the I vs. V graph for a 1 kω resistor. Sketch each below making it clear how they differ. 2. Draw a graph for V in and V Out for the AC to DC converter in Step 2 if the diode were turned around. Page 6 of 6