ECEN 313

The Semiconductor Diode

Objective

The purpose of this lab is to acquaint you with the operation and use of a semiconductor diode. A low power diode will be used, but large diodes are available to use in applications involving thousands of amps. A large signal application will first be studied followed by a small-signal application. In addition, we will be looking at an LED as a diode.

Preliminary Work

Familiarize yourself with the operation of a half-wave rectifier.

Laboratory Work

Part 1--Approximate Characteristics of the Diode

  1. Signal diode PSPICE Simulation
  2. A diode is a nonlinear device with an exponential relationship between the current and voltage. Because of this relationship a small change in the voltage applied across a diode can result in a large current. The measurement of the diode characteristics is done by putting a resistor in series with the diode.
  3. Simulate the schematic shown in Figure 1. In order to simulate this circuit we will be using the DC Sweep Analysis in PSPICE. Place a voltage marker on the diode and sweep the voltage source from 0V to 10V. Notice how the voltage changes rapidly up to a particular point and then is fairly flat. The voltage can be modeled as being linear for large voltages and linear for small voltages.

Figure 1. A diode circuit.

  1. Calculate the slope of the two linear regions.
  2. Extrapolate the two linear regions to find the intercept. This intercept is essentially the diode voltage and is called the forward voltage or Vf.
  3. Signal diode Measurement
  4. Built the circuit shown in Figure 1.
  5. Measure enough values to find the two linear regions.
  6. Extrapolate the two linear regions to find the intercept.
  7. LED Measurement
  8. Built the circuit shown in Figure 1 with the diode replaced with your red LED.
  9. Measure enough values to find the two linear regions. (DO NOT GO OVER AN LED CURRENT OF 20 mA.)
  10. Extrapolate the two linear regions to find the intercept.
  11. If we want an LED current of 15 mA with an applied DC voltage of 10V calculate the required resistor.
  12. Built the LED circuit and verify that the LED current is 15mA

Part 2—Full Wave Diode Rectifier

The electricity used in your home is an ac signal that contains no dc component. While many appliances operate on ac voltage, all electronic devices in the home require dc voltages. Radio, stereo, and TV must operate on the ac power input voltage and create a dc signal that is used to power the electronics in the system. A popular circuit used to create a dc voltage from an ac input is a diode rectifier circuit. (See http://en.wikipedia.org/wiki/Rectifier for more detail.) While the output of this circuit contains a dc voltage, there is also a time-varying component remaining in the output voltage. A capacitor filter circuit can eliminate most of the time-varying component to create an almost pure dc output voltage.

  1. Design a half-wave rectifier.
  2. Use PSPICE to simulate the half-wave rectifier. Use the transient analysis with a voltage source of Vin= 5 sin(2p 1000 t). This is 10 Vpeak-to-peak.
  3. In your PSPICE simulation add a large capacitor in parallel with your diode. (Use a large capacitor that is available in your lab kit like 10 mF or 100 mF.)
  4. Change the frequency of the input signal to f=100Hz and f=10kHz. Describe the change in the maximum voltage change.
  5. Build the circuit that you designed.
  6. Measure the response with and without the capacitor in parallel with the load.
  7. Measure the average voltage with the multimeter (on the DC setting).
  8. Measure the maximum change in the signal using the oscilloscope.
  9. Change the frequency of the source to f=100 Hz and f=10 kHz and redo the measurements from part 6.
  10. Describe any differences between the circuit that you built and the simulation.

Part 4—Half-Wave Diode Rectifier to light an LED

  1. Build a half-wave diode rectifier to light up your LED. The input signal to your rectifier should be 5V amplitude and a frequency of 1 kHz.
  2. Be sure to place a resistor in series with your LED so that the peak current is below 20 mA.
  3. Start with your input signal small and increase it to 5V while monitoring the current through the series resistor to make sure that current never exceeds 20 mA.
  4. Measure the output voltage with and without the LED applied. Describe any differences. (The difference caused by adding the LED and resistor is called circuit loading.)
  5. Measure the average current through the LED.