EXPERIMENT 10

Voltage Limitation and Regulation with Diodes

This experiment is a follow up to EXPERIMENT 9 where we looked at a simple half-wave rectifier. A serious problem with simple rectifiers is that the DC voltage they produce is dependent on the load. A common way to make the rectifier less sensitive to the load is to add some regulation. This we can do by utilizing the avalanching effect that occurs if we reverse voltage the diode too much. First, we will see what kind of voltage limitation can be achieved with a forward biased diode. Such limitation of voltages is usually applied to protect circuit components.

Part A PN Junction Voltage Limitation

When you performed the triangular wave sweep of the 1N4148 diode in EXPERIMENT 9, you should have observed that the voltage across the diode remained near 0.5 volts when it was on. We can take advantage of this effect in the following circuit which permits small voltages to pass without distortion, but clips any voltage outside the range of about -0.5 to 0.5 volts.



  1. Observe the diodes functioning as limiters (or clippers)

  1. Draw the circuit shown. Use VSIN for the source (VAMPL = 10, FREQ = 1k, everything else = 0).

  1. Perform a transient analysis in increments of 50us up to 5ms.

  1. Use PROBE to plot the input and output voltages. The input is the voltage source while the output is taken across the diode pair, as shown.

  1. Print your PROBE plot.

  1. Change the amplitude of the sinusoidal input, Vin, to 0.1 volt. Repeat the transient analysis.
  2. Why do the two plots look the way they do? Why is this circuit called a limiter? When and why might this circuit be useful?













  1. Assemble the diode limiter circuit on your proto-board

  1. Use the function generator to supply a 1 kHz, 10V, sinusoidal input.

  1. Observe Vout and Vin with the oscilloscope. You will need to use both channels and to make sure that you connect your grounds to only one place. Be careful to adjust the function generator output such that the signal you observe on the scope looks as much like the PSpice output as possible. You might have to adjust the amplitude of the signal and the voltage and time scales on the scope.

  1. Add six experimental points to your PSpice graph.

  1. Repeat the measurements with a 1 kHz, 0.1V sinusoidal input.

  1. Comment on the differences in your results.




Part B Zener Diode Voltage Regulator


In the circuit shown above, D1 is a Zener diode. Be sure you have its orientation correct.

  1. Obtain Zener diode characteristic.

  1. Draw the circuit shown.

  1. Perform a DC sweep analysis from -10 to +20 volts in increments of 0.1 volts. Be sure that you enter V1 for the name of the device that is swept.

  1. Plot the current through the Zener diode I(D1) vs the voltage across the diode -V(D1:2) to obtain the I-V characteristic of the Zener diode in the same manner as we addressed the standard diode in the last experiment. Note that your plot should look like a typical Zener diode characteristic. If, for some reason, the current or the voltage look backwards, reverse their signs until the plot looks correct. Print this plot.

  1. Now plot Vout vs Vin for your DC sweep. Print this plot.

(2) Hardware Implementation

  1. Assemble the circuit on your protoboard.

  1. Use the DC source that can produce an output up to 25 volts.

  1. Using the Digital Multimeter (DVM), observe the voltage across the Zener diode at the following source voltages: -10, -5, 0, 5, 10, 15, 20 volts. Put the seven experimental points on your PSpice plot of Vout vs Vin.

  1. Connect the DVM to measure the current through the diode. Current measurements are made in series using the two DVM inputs labeled with the letter "I." Adjust the DC source voltage until the current reads 5mA. Measure the voltage across the diode for this condition. Plot this point on your PSpice I-V characteristic. Repeat this for a current of -5mA.


  1. Sinusoidal Response

  1. Repeat the PSpice simulation of the Zener circuit with a sinusoidal source like the one you used in Part A (VSIN with VAMPL=10 and FREQ = 1k). Do the same kind of transient analysis. Print the plot of Vout vs Vin.

  1. Replace the DC source on your protoboard with the function generator set to produce the same conditions as in your simulation. Observe Vout and Vin on the oscilloscope. Again, take pains to make the scope image look as much like the PSpice simulation as possible.

  1. Discuss the similarities and differences between the simulated and experimental results.












  1. Discussion/Calculation/Simulation of Results

We have seen that the voltage across the Zener diode will remain equal to the Zener voltage, as long as we provide enough voltage and current from the source. However, the circuit configuration we have studied does not include a load. Discuss how this circuit will perform for a load that is much smaller than 1k ohm (eg 50 ohms), equal to 1k ohm, and much larger than 1 k ohm (eg 1 megohm). That is, under what conditions will it produce the desired regulated voltage? Support your discussion with calculations, simulations, and/or experimental results. Hint: If you choose to simulate the circuit, you might want to try adding the load resistor to your PSpice simulation while you are in class.























Last Updated on 21 February 1998