Analog Diode Circuits

The circuits in this experiment represent some of the most common and practical uses of diodes. Every electrical engineer should be familiar with the operation and analysis of these circuits. The fundamental concepts of these circuits are covered at length in your text and will not be reviewed here.

You should do the following in preparation for this experiment.

1. Assume the following circuit values:
   1. V_S = 10sin(w*t), where w = 120pi (except circuit b)
   2. C = 10uF,
   3. R_L = 27K,
   4. R_X = 1K (or greater)
   5. V_ON (forward) = 0.6 Volts
   6. V_Z (reverse) = -5.8 Volts (Or, use what you found for the zener breakdown value for the zener diode in your kit.
   7. R_S = 50 Ohms (Function generator internal resistance.)

2. Analyze the circuits in the Figures a to e.

3. Sketch the expected graphs of V_S(t), zener the V_g(t), and V_L(t).

4. Find the peak value and the average DC value of the load voltage V_L.
5. For the DC Power Supply circuit in Figure b find the ripple voltage V_r and average DC value of V_L with
   (a)w = 120pi and
   (b)w = 1200pi

6. For the Zener Regulator circuit in Figure c., calculate the minimum value allowable for R_L and still have good voltage regulation. diode is "just ON reverse" at the voltage V_Z.
7. Be sure you do all the above before coming to lab.

Note that in every case you must first find the range of values of V_S that keep the diode(s) OFF. (i.e. What range of V_S guarantees V_d < V_ON ?) Then, assume that range, use the open circuit model for diode, and, do circuit analysis. Next, assume V_S takes on the remaining range of values. Therefore the diode must be ON. Replace the diode with the required ON model and do circuit analysis.

Do the following in lab:

Note: basically, the data is the sketches (right off the scope) of the voltages.
1. Set V_S = 10sin(120pi*t) = 7.07 RMS Volts.
   Do this by measuring V_g without anything connected to the function generator.

2. Construct the circuits and measure (i.e. sketch) the graphs of V_S(t), V_g(t), and V_L(t). Compare the sketches to your predictions of your preliminary work. If they are extremely different maybe your circuit is not setup correctly.

3. For each circuit use the DMM as a DC voltmeter and measure the DC value of V_L. Compare this to your calculated values.

   foltage or the voltage

4. For the DC Power Supply circuit in Figure b be sure to measure V_r and V_{L_DC} at frequencies 60Hz and 600Hz and compare the values to your preliminary calculations.

5. For the Zener Regulator circuit try attaching smaller and smaller values of R_L until you find the minimum R_L allowable and still have good regulation. Compare to your calculated minimum value.

6. For all circuits. What happens when you remove R_L and just leave an open circuit. Does the circuit performance improve? doubler circuit try different values of R_X and observe the effect on V_L. What value of R_X drives V_L to a maximum value?

Include the following in your written report:

1. Include all preliminary calculations and sketches.

2. Include your data. i.e. the sketches from the scope of the voltages.

3. Explain the discrepancies between calculated and actual experimental values. Example: Why is V_g so distorted when the circuits are attached to the function generator?
4. Any conclusions about all of this?

The Circuits

The Half-Wave Rectifier This circuit clips off the negative peaks of the pure AC sine wave. The result is a voltage with a positive average value. So, we've converted AC to DC!	Figure a.
The DC Power Supply This circuit is the half-wave rectifier with a filter capacitor. The capacitor "holds up" the voltage so that the average DC value of the output voltage VL is almost the same magnitude as the magnitude of the input AC sine wave!	Figure b.
The Zener Voltage Regulator CIRCUIT CORRECTION: Please note the addition of resistor RX to the circuit. It is required or the output cannot be pure DC (our goal.) Note that: [RL/(RL + RX)](Vgmax - Vd(on)) > |VZ| to have pure DC output. So, RL >> RX is required. The Zener Diode removes the ripple voltage that "rides on the DC" of the power supply circuit above. We loose some magnitude but we gain a pure DC output as long as RL is not to low a value.	Figure c.
The Voltage Doubler In this circuit we get the AC sine wave input to "ride on a DC" value equal to the magnitude of the sine wave input. The result is a "peak output voltage" that is twice the magnitude of the AC input peak. Note that the capacitor charges when the diode goes on and (if RL is sufficiently large) does not discharge. That's our free ride, the charge on the cap. Also called a "diode clamper."	Figure d.
Voltage Doubled DC Supply Here we first double the input voltage with a voltage doubler circuit. Next, we connect that output to the input of a half-wave rectifier with filter html >	Figure e.

End Analog Diode Circuits