EET2222

 

Lab 2

 

Diode Characteristics

 

Part I – Diode Testing

 

Digital Multimeters can be used to test diodes.  The DMM is set to the resistance measurement mode on the appropriate scale.  In this mode, the red lead has a positive potential relative to the black lead.  Obtain a zener diode with a Vz = 9.1 V.  The diode test procedure is as follows:

 

1.      Set the DMM to either the special diode position or on the 2 kΩ scale.

 

2.      Connect the red lead to the anode and the black lead to the cathode.  What value does the DMM display?  (hint:  this value should be the forward-bias voltage drop for the type of diode being tested.

 

3.      Reverse the connections.  What does the DMM display?  What does this mean?

 

4.      Re-connect the leads per step 2 and change the scale setting on the Tektronix DMM to the 200 Ω scale.  Describe what happens.

 

Part II – Diode Characteristic Curve

 

5.   Set up the circuit shown in Fig. 1 to measure the voltage across the zener diode (Vd) with one DMM voltmeter and the current through the diode (Id) with the other DMM in the Tektronix console.  Use a value of 1 kΩ for R.

 

 

Fig. 1

 

 

6.  Vary the voltage source (Vs) to obtain 0.05 V increments of Vd starting at 0 V and measure Id as it varies from 0 to 10 mA in the forward biased direction.  Now reverse the polarity of the source voltage and increase the voltage until you see the current increase sharply from avalanche breakdown.  Take enough measurements of Vd and Id to plot the zener knee. Do not exceed a zener (reverse) current of 20 mA.

 

7.   Plot the diode current vs. the diode voltage (voltage on horizontal axis).  Use a whole page of paper for your plot.  Label each axis with suitable units and calibration numbers.

 

Part III – Simple Diode Voltage Regulator

 

The circuit of Figure 2 is a voltage regulator.  The diode is reverse biased so the voltage across it is close to the zener (VBR) voltage.  This results in a fairly constant voltage across the load (the 820 ohm resistor) as the supply voltage to the circuit changes.

 

8.   Determine the value for Rs.  The maximum zener diode power dissipation is 400 mW. Calculate a value for Rs which will result in a zener power dissipation of 10 to 25% of the maximum value for a supply voltage of 20 V.

 

9.   Set up the circuit of figure 2.  Measure the zener current to confirm your calculations (record your measured value).  Now measure the voltage across the 820 Ω load resistor as the source voltage varies from 15 to 20 V. 

 

 

Fig. 2

 

10. What is the % voltage regulation of your circuit?  Use the load voltage which corresponds to a supply voltage of 15 and 20 V in the equation, rather than the "no load" and "full load" values (i.e. Vno load = the load voltage when the supply voltage is 20 V).

 

 

 

Your report is due at the end of the lab period.  Please use the green engineering paper for your report.