6.4.1: Reverse Curve
1. Consider the circuit of Figure 6.3.1 using R = 2.2 k\(\Omega\). For any positive value of E the Zener is reverse biased. Until the Zener potential is reached, the diode resistance is effectively infinite and thus no current flows. In this case the voltage across R is zero due to Ohm’s law. Consequently, all of E should appear across the Zener. Once the source exceeds the Zener voltage, the remainder of E (i.e. E minus the Zener potential) drops across R. Thus, as E increases, the circulating current increases but the voltage across the zener remains steady.
2. Build the circuit of Figure 6.3.1 using R = 2.2 k\(\Omega\). Set E to 0 volts and measure both the diode's voltage and current and record the results in Table 6.5.1. Repeat this process for the remaining source voltages listed.
3. From the data collected in Table 6.5.1, plot the current versus voltage characteristic of the reverse biased diode. Make sure \(V_D\) is the horizontal axis with \(I_D\) on the vertical.
6.4.2: Practical Analysis
4. Consider the circuit of Figure 6.3.2 using R1 = 2.2 k\(\Omega\) and R2 = 4.7 k\(\Omega\). In general, to analyze circuits like this, first assume that the Zener is out of the circuit and then compute the voltage across R2 using the voltage divider rule. If the resulting voltage is less than the Zener potential then the Zener is inactive (high resistance) and does not affect the circuit. If, on the other hand, the resulting voltage is greater than the Zener potential then the Zener is active and will limit the voltage across R2 to \(V_Z\). Via KVL, the remainder of the voltage drops across R1 and from this the supply current may be determined. This current will then split between R2 and the Zener. The R2 current is found using Ohm’s law. The Zener current is then found via KCL. Note that for higher and higher values of E, the voltage across (and therefore the current through) R2 does not change. Instead, all of the “excess” current from the source passes through the Zener.
5. Build the circuit of Figure 6.3.2 using R1 = 2.2 k\(\Omega\) and R2 = 4.7 k\(\Omega\). Set E to 2 volts. Compute the theoretical diode voltage and current, and record them in the first row of Table 6.5.2. Then measure the diode current and voltage and record in Table 6.5.2. Finally, compute and record the deviations.
6. Repeat step 5 for the remaining source voltages in Table 6.5.2.
6.4.3: Computer Simulation
7. Repeat steps 5 and 6 using a simulator, recording the results in Table 6.5.3.