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ECE 231 Laboratory Exercise 5Diode Characteristics

ECE 231 Laboratory Exercise 5 – Diode Characteristics

Laboratory Group (Names) ____________________ ___________________ __________________

OBJECTIVES Validate the Schottky diode equation. Learn how to use the XY mode on an oscilloscope to observe and measure the

characteristics of a diode. Learn how to calculate the dc and dynamic (ac) resistance of a diode at a specific

operating point on a diode curve. Observe how the diode curve varies with temperature. Observe the rectifying characteristics of a diode in both a ½ wave (one diode) and

full wave configuration( four diodes). That is, it passes current in only one direction. Measure the breakdown voltage of a zener diode at a specific operating point. This

is the point where the diode has current in the reverse direction. Learn how to design a simple zener diode power supply.

EQUIPMENT REQUIRED ECE 231 Circuit Board or diode board. Three BNC (Bayonet Neill–Concelman) cables. One is for the Function/Arbitrary waveform

generator and two are for the oscilloscope. two banana cables. One for the DMM (Digital Multimeter) and one for the DC power supply. One lot of clip leads and/or jumper wires. DMM One arbitrary function generator and one oscilloscope. Several resistors for loads

BACKGROUNDThe Schottky diode equation (1) is a very good approximation of how an actual diode behaves in the laboratory. The plot of this equation is shown in Figure 1. In this experiment you will investigate the properties of a diode in quadrants I and III. Most diodes if operated in the breakdown region (quadrant III) they will be destroyed. There are however diodes made to operate in this region, and they are called zener diodes. Their breakdown voltage is used as a reference voltage in many electronic circuits. The next region (left of center) is the reverse bias region. This is the normal region when a diode is reverse biased and it does not allow the diode

1R. Frank Smith, Cal Poly Pomona University, 2017


to carry any current when reverse biased. The next region, quadrant I, is the normal forward biased region.

I diode=I s (eV D/(nV T )−1 ) (1)

Some definitionsIs is the reverse saturation current and is approximately equal to 10-12A. It is sometimes referred to as Io or Ir. This current is proportional to the area of the diode. You will not measure this current in this experiment.VD = diode voltage, n is approximately 1 to 2. Use 1 for this lab. VT is 26 mV at 300oK.VD should be in the range of 0.6 to 0.75 V for silicon diodes and 0.3 V for germanium diodes.

Some industrial diodes can be several inches in diameter and are capable of carrying over 1000 amperes and some are rated at several thousand volts and some are water cooled. Common ones used by students are shown in Figure 2. The black one on the right of the figure is a bridge rectifier and it consists of 4 diodes in a bridge configuration. The ones used on circuit boards with surface mounted components are only about 1 or 2 mm across. See Figure 3 for typical electronic schematic symbols for diodes. Photodiodes may receive light (solar cells or light sensors) or output light (Light Emitting Diode – LED). Zener diodes are designed to operate in the breakdown region. Their breakdown voltage can range from several volts to tens of volts.

The Schottkey diode has a low forward voltage drop (0.15 to 0.45 V). This is much less than the 0.6 volts for silicon diodes. These are used to reduce the power used in some electronic circuits.



Figure 1. Plot of diode characteristic equation. Source: Wikimedia Commons

Figure 2. An assortment of typical diodes.


http://upload.wikimedia.org/wikipedia/commons/a/a5/Diode-IV-Curve.svg


Z1 1N2804D1 1N1183 LED1 CQX35ASD1 1N5817

Schottkey Diode Diode Zener Diode Photodiode or LED

Figure 3. Electronic symbols for diodes.

PROCEDURE

You are going to observe a diode characteristic curve then analyze and calculate operating parameters of a diode. In addition you are going to measure the operating characteristics of a zener diode. You will then observe the rectifying characteristics of diodes in a ½ wave (one diode) configuration and then in a full wave configuration ( 4 diodes) which is often called a bridge circuit.

Part 1

1. Connect the test circuit shown in Figure 4 to the oscilloscope and the Function Generator.

R1

1k

R2

1kD2

D1 Diode

Diode

Channel 1 (X) of oscilloscope

Channel 2 (Y) of the oscilloscope

Function Generator

Figure 4. Diode Test Circuit using the Oscilloscope XY Mode.



2. Set the signal generator output amplitude to a ramp wave form 50% symmetry that goes from -4 to + 4 volts at a frequency of 100 Hz. You can also use asinwave output (8 VPP). See Figure 5. Make sure that you have the output impedance set to high (1 megohm) not 50 ohms. Select “high Z”” on the front panel (left-hand corner).

Figure 5. View of Function/Arbitrary Waveform Generator

3. On the oscilloscope go to the Horizontal controls area and press the Horz key to open the Horizontal menu. Now press it until you see Time Mode XY You can also select the trigger time reference point. Set the oscilloscope sensitivity of each channel to 200 mv. This is a suggested starting operating point. Your oscilloscope trace should look similar to Figure 6. Place a photograph of your oscilloscope trace in your report and record your oscilloscope settings.



Figure 6. Diode Characteristic Curve using the XY mode on the oscilloscope

4. Measure the forward voltage drop of the diode at approximately 1 mA. Use the cursors to measure the voltage and current at the operating point that you wish to investigate. R dc= Vdc/ Idc. The resistance will vary depending on what you select as the operating point for the diode.

5. Measure the vertical slope of the diode curve to obtain the ac dynamic resistance of the diode using equation (2).

rac=∆V∆ i

=V diode@12V−V diode@8V

idiode@12V−idiode@8V (2)

6. Now let’s observe the temperature sensitivity of a diode. Take a heat gun and apply heat to the diode only and observe that the slope of the diode curve increases. This property of a diode permits it to be used as a temperature sensor. It also causes problems in the operation of many electronic circuits.



7. Connect your zener diode (1N5226B) in series with a 1 K resistor as shown in Figure 7. Its characteristics are shown in Table 1.

R1

1k

Z1 1

N52

26B

Zener diode

DC Pow er supply

Voltmeter

Figure 7. Zener diode test circuit

8. Slowly increase the DC power supply until the voltmeter read approximately 3.1 volts. See Figure 8. You have three power sources from which to choose. The center one in the figure is probably your best choice. It is digital and easily set to a more precise value.

Figure 8. Power supplies on your workbench.

9. Calculate the zener diode current using ohm’s law equation (3). The voltmeter reading is the zener voltage.

(3)

10. Increase the DC power supply voltage until the zener current increases by about 5ma.



11. Now calculate the zener dynamic resistance Zz or Rz using ohms law. This will be the slope of the diode curve. See Figure 9. This the same procedure as used for a forward biased diode, equation 2. Zz = ________ ohms

12. Compare your results to the manufacturer’s values shown in Table 1.

Figure 9. Typical zener diode curve.

Table 1. Zener diode electrical characteristics.

A good zener power supply should not draw any current from the zener because this will change the reference voltage (zener voltage) as the load varies. To avoid loading the zener insert a voltage follower circuit between the zener and the load. See figure 10.



R1

1k

Z1 1N5226B

-

+

IOP1

Load 1k

Zener diode

DC Pow er supply

Voltmeter

Figure 10. Simplified zener diode power supply schematic

13. Construct the circuit shown in Figure 11 with no capacitor in parallel with R2. The output of this circuit is called a ½ wave rectified signal. All of the negative going signals are blocked by the diode as shown in Figure 11.. 14. Connect a capacitor in parallel with R2 as shown in Figure 12. The capacitor releases its energy to the load during the diode off part of the cycle. This is called filtering. If you increase the frequency to 100 KHz you will see that the capacitor changes the pulses into a DC voltage with a small ripple voltage. Increasing either the frequency or the capacitor size will reduce the ripple. See equation (4).



Figure 11. Half wave rectifier circuit.



Figure 12. Halfwave rectifier with filter capacitor. The input is a 10 sinewave (CH 2) and the output (CH 1) is a DC value with about a 10% ripple.

As you increase the frequency, the ripple becomes less because the capacitor has to discharge for a shorter period. You can also reduce the ripple by installing a larger capacitor. The minimum size capacitor is

Cminimum=iload∗(discharge timeof capacitor )Ripple voltage peak−¿− peak

(4)

This is a typical power supply design calculation. Diodes act like one way check valves in many electronic circuits. They allow current to go only one way.



Write a professional comprehensive lab report using a word processor. Your lab report should discuss what you did and what you observed. Show your results and include a comprehensive conclusion. There are lots of sample lab reports on the internet. The following are a couple of examples.

http://writing.engr.psu.edu/workbooks/laboratory.html http://ecp.engineering.utoronto.ca/online-handbook/types-of-documents/lab-reports/ .

Questions:

a) What was the waveform output from your function/waveform generator?b) What was the forward voltage drop of your diode at approximately 1 ma?c) What was the dc resistance of your diode at your selected Q point?d) What was the ac resistance of your diode at your selected Q point?e) What was your zener breakdown voltage?f) What was rz for the zener?


http://ecp.engineering.utoronto.ca/online-handbook/types-of-documents/lab-reports/

http://writing.engr.psu.edu/workbooks/laboratory.html

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