80 General Properties of Diodes 80 - Page 1 of 5

General Properties of Diodes

Equipment

1 AC/DC Electronics Laboratory EM-86562 Voltage Sensors UI-51001 Short Patch Cords (set of 8) SE-7123

Required but not included:1 850 Universal Interface UI-50001 PASCO Capstone

Introduction

The purpose of this experiment is to investigate the characteristic curves (I vs.V) of various types of diodes and to determine their “turn-on” voltages.

Theory

A diode (or p-n junction rectifier) is an electronic device which allows current to flow in only one direction. As can be seen in the characteristic curve of the diode in Figure 1, when the voltage drop across the diode is negative (called reverse bias), no current flows (except for a very small reverse current). When the voltage becomes positive, no current flows until a “turn on” voltage is exceeded (for the case shown at about 0.5 V). From then on a very small increase in voltage results in a very large increase in current. Note that here we have defined positive voltage to be that which causes current flow. In terms of a circuit diagram an arrow is used to indicate a diode as in Figure 2, and current flows only in the direction of the arrow. So current would flow if the red side of VA was positive with respect to the black side by more than about 0.5 V. From Figure 1 it is clear that the “turn on” voltage is somewhat arbitrary and we take it to be the voltage where the current exceeds 2.0 mA.

A light-emitting diode emits light as current passes through the diode. A bicolor LED (here yellow green) is actually two diodes in parallel but in opposite directions so the yellow allows current flow in one direction and the green allows current flow in the opposite direction. A bicolor LED is an example of a Zener diode. The Zener diode has a “turn on” voltage for both positive and negative voltages (for a Zener the “turn on” voltage is called the “breakdown voltage”) This is useful in circuits since the voltage across the Zener remains constant independent of the current providing it is above the “breakdown voltage”.

Written by Chuck Hunt

Figure 1: Characteristic Curve

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Figure 2: Diode Circuit

Setup

1. Setup the circuit shown in Figures 2, 3 & 4 using the 1000 Ω (brown-black-red-gold) resistor between the spring clips. The gold band on the resistor means it is accurate to within 5% (1000 ± 50 Ω), but it is probably much better. The gray band (cathode end) on the diode should be to the left away from the positive (red) terminal.

2. Attach patch cords from Output 1 on the interface the banana jacks on the Electronics Laboratory circuit board. Observe the polarity shown so the red patch cord attaches to the jack by the right end of the diode.

Written by Chuck Hunt

Figure 3: Setup

Figure 4: Close-up of Diode Setup

Figure 3: SetupFigure 3: SetupFigure 3: SetupFigure 3: Setup

Figure 4: Close-up of Diode SetupFigure 4: Close-up of Diode SetupFigure 4: Close-up of Diode SetupFigure 4: Close-up of Diode Setup

80 General Properties of Diodes 80 - Page 3 of 5

3. Attach a Voltage Sensor to Analog Input A on the interface. Its red end should attach to the red patch cord from Output 1. Attach an alligator clip to the black end, clip it to a white wire and attach the white wire to a spring clip at the left end of the diode. Caution: if you attach the alligator clip directly to the spring clip, you will tend to pull the spring loose from the circuit board.

4. Attach a Voltage Sensor to Analog Input B on the interface. Its black end should attach to the black patch cord from Output 1. Attach an alligator clip to the red end, clip it to a white wire and attach the white wire to a spring clip at the left end of the resistor. Note: this will be used to measure the current in the circuit since the voltage drop across the resistor divided by 1000 Ohms equals the current.

5. In PASCO Capstone, click open the Signal Generator at the left of the screen. Set Output 1 for a Triangle Waveform with a Frequency of 0.1 Hz and an amplitude of 10 V. Click Auto. Click the Signal Generator again to close the panel.

6. Set the common sample rate to 200 Hz.

7. In the calculator, create the following calculation:

i = [Voltage, Ch B] Units of mA

The current is calculated using Ohm’s Law: i = V/R = [Voltage, Ch B]/(1000 Ω) in units of Amps. In the above calculation, the units are mA because the voltage is not divided by 1000. Note that if the actual resistance of the resistor is not 1000 Ω, this calculation will not be accurate.

8. Create a graph of the calculation “i” vs. Voltage, Ch A.

9. Create a table with three columns (as shown below) and create user-entered data sets called “System”, “Voltage at +2 mA” with units of V, and “Voltage at -2 mA” with units of V.

Table I: Diode Data

System Voltage at + 2mA(V)

Voltage at - 2mA(V)

DiodeRed LEDYellow LEDGreen LEDBicolor LED Green 2ndBicolor LED Yellow 2nd

Written by Chuck Hunt

80 General Properties of Diodes 80 - Page 4 of 5

Procedure

1. Click RECORD. Data collection will stop automatically after the entire curve has been traced (5 s).

2. Click on the Scale-to-Fit icon on the graph toolbar.

3. Click open Data Summary and re-label this run as “Diode”.

4. Find the red, yellow, green, and bicolor LEDs. The bicolor is actually clear when unpowered. It is the one shown in Figure 5. If the wires have not already been bent so they will bridge between the two springs as in Figure 5, do so now. Be careful not to break the wires. When you are done, leave them bent, since repeatedly bending them will break them.

5. Identify the cathode (negative) side of the red, yellow, and green LEDs (doesn’t matter for the bicolor). There are four ways to identify the cathode (refer to Figure 6): 1) the wire on the cathode side is shorter, 2) there is a flat spot on the plastic by the cathode (shows clearly on the right of the green LED in Figure 6), 3) if you hold the diode up in front of a light, you will see a triangle…the wide side is attached to the cathode, and 4) if you make a mistake you will find that the diode conducts when the voltage is negative rather than positive.

6. Replace the diode with the red LED so its cathode is to the left. Repeat steps 1-3. Label the run “Red LED”.

7. Repeat for the other yellow and green LEDs, labeling them appropriately.

8. Repeat with the bicolor LED. Watch to see which color is second and label the run “Bicolor Green 2nd” or “Bicolor Yellow 2nd”.

9. Reverse the bicolor LED and repeat.

Written by Chuck Hunt

Figure 6: The LED Cathode wire is shorter.

Figure 5: LED SetupFigure 5: LED SetupFigure 5: LED SetupFigure 5: LED SetupFigure 5: LED Setup

Figure 6: The LED Cathode wire is shorter.Figure 6: The LED Cathode wire is shorter.Figure 6: The LED Cathode wire is shorter.Figure 6: The LED Cathode wire is shorter.

80 General Properties of Diodes 80 - Page 5 of 5

Analysis

1. Click on the black triangle by the multicolored Run Select icon in the graph toolbar and select the “Diode” run. If necessary, click the Scale-to-Fit icon.

2. Click on the Coordinate icon.

3. Move the Coordinate tool to the point where i = 2.00 mA or as close as possible. Read the Voltage to two decimal places and record it in Table I in the “Voltage at +2 mA) column.

4. Repeat for the other runs. For the bicolor LED runs, also record the Voltage at -2 mA.

Conclusions

1. Do the diodes obey Ohm’s Law? How do you know?

2. Click the Run Select icon to allow display of multiple runs. Select the “Diode”. “Red LED”, “Yellow LED”, and “Green LED” runs. Click on the Selection icon and adjust the handles on the Selection box to select the positive voltages data. Click the Scale-to-Fit icon.

3. Why is the maximum voltage across the diode above 9.5 V while the maximum voltage across the LEDs is only about 8.0 V?

4. Now add both the bicolor runs and adjust the selection box to select only the data between 1.5 V and 2.2 V. Click Scale-to-Fit. Click Remove Active Element in the graph toolbar to remove the selection box.

5. Do the yellow and green LEDs have the same characteristic curves and the bicolor LED? Are all green (or yellow) LEDs the same?

6. Turn off all except the two bicolor runs. Click Scale-to-Fit.

7. In the circuit, replace the bicolor LED with the yellow LED (cathode to the left) and add the green LED so it is in parallel with the yellow LED but has its cathode to the right.

8. Click Record. Watch which diode is the second to glow. Does its curve match what you would expect?

Written by Chuck Hunt