experimental method 3.pdf

8/11/2019 Experimental Method 3.pdf

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CHAPTER 1 DIODES

1 Diode

Objectives of this Experiment

Measure the DC parameters of a PN junction diode

Recognize the characteristics of the reverse bias current.

Identify typical values of DC parameters for PN junctiondiodes

Equipments required for this Experiment

Circuit #1 of D3000 - 2.1 Semiconductors-1 Module Two multimeters Shorting links and connecting leads

Exercise 1.1 Diode Forward Characteristic

The physical properties of a semiconductor junction preventOhm's Law having control over the current that flows both in theforward and reverse bias modes.

In the forward bias mode, the barrier potential must first beovercome before any appreciable current can flow.

In reverse bias the very small number of minority carriersavailable is the controlling influence on the amount of current flowing.

Figure 1.1The circuit diagram for taking the measurements to plot the forward

characteristic is shown in Fig 1.1 above.

The special power supply used for plotting the forward bias conditions of thediode in Circuit #1 is mounted on the module panel near to the top right hand corner,marked 0-2V DC SUPPLY.


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Figure 1.2

Steps for doing the experiment1. Locate the 0-2V DC supply and turn the control to MIN.2.

Connect a shorting link between sockets 1.3 & 1.8, as shown in Fig1.2 above.

3. Connect multimeter 1 on DC current range to sockets 1.1 (positive)and 1.4 (common).

4. Connect multimeter 2 on DC voltage range to sockets 1.9 (positive)and 1.10 (common).

5. Switch ON the Module Power Supplies.6. Adjust the 0-2V supply to give 100mV across the diode.7.

Read the current from multimeter 1 and record in Table 1.1.8. Reset the voltage to 200mV and repeat the current reading.9.

Continue taking readings at the voltages indicated in Table 1.1,

noting that the interval is changed to 50mV as soon as the currentstarts increasing rapidly. Watch the current reading as the voltage isincreased and change the range setting of multimeter 1 as required.

If your multimeter has auto-range facility then this should beselected

Beware - as the range is changed on multimeter 1 you may need to reset

the voltage.


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Table 1.110.Plot the forward characteristic of the diode on the axes provided.

Note that the low current readings are too small to be plotted

You will observe that the steps of current are not linear against voltage,showing that the diode does not follow Ohm's Law.


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Exercise 1.2 Diode Reverse Characteristic

It is necessary to get the reverse bias current (leakage current) flow of thediode into proportion. For practical purposes this current is negligible. This isconfirmed in the first part of the experiment. However the leakage current of a p-n

junction can be significant in some semiconductors, so it is worth spending a littleextra time investigating this more carefully.

Figure 1.31.

Plug a shorting link between sockets 1.7 & 1.82. Connect multimeter 1 on DC current range between sockets 1.5

(positive) and 1.4 (common).

3. Set the 0-12V Variable DC supply control to the MIN (0V) positionand switch ON the Module Power Supplies.


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Note: The 0-12V DC variable power supply is connected with its negativeterminal uppermost, so that the diode can be conveniently reverse biased usingshorting links.

4. Watch multimeter 1 as the 0-12V DC variable supply is increased to

maximum.You will note that there is no response, indicating that the current is of

negligibly small proportions. However, we know that there is a small amount ofminority carrier current, and by monitoring the voltage dropped across R16

(1Mresistor in Circuit #8, Standard Amplifier Configurations), we will be able todetermine the leakage current.

5. Remove multimeter 1 from the circuit and replace it by a shorting linkbetween sockets 1.4 & 1.5.

6. Remove the shorting link between sockets 1.7 & 1.8. Connect leads

between sockets 1.7 & 8.14 and 8.15 & 1.8.7. Connect multimeter 2 on DC voltage range to sockets 1.9 (positive) and

1.10 (common), to measure the anode voltage of diode D1 with respectto the cathode. Adjust the variable supply until the measured anode

voltage is -1V.

Figure 1.4


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8. Move the positive lead of multimeter 2 (still on DC voltage range) tosocket 1.7, as shown in Fig 1.4.

Note that you cannot use two multimeters, since one of them would be

shunting the diode and giving false readings on the other.

9.

Record the potential difference (voltage) across R16 (in mV) in Table1.2.

10.Repeat the previous two steps for anode voltages of -2V, -3V, and soon, up to -11V.

Note: Because the voltage across R16 is so very small multimeter 2 may havedifficulty in giving a steady reading when measuring across R16. If you encounterthis difficulty you should take the average of the highest and lowest indications andrecord that value.

Table 1.2

The value of the leakage current can now be calculated by dividing the voltageacross resistor R16 by its value (1M).

Note that dividing millivolts (10-3) by megohms (106) gives nanoamps (10-9).

For example, 10mV divided by 1Mgives 10nA.

11.Calculate the value of leakage current for each of the voltage steps andadd to Table 1.2 opposite.


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2 Diode Rectifier Circuits

Objectives of this Experiment

Determine by measurement and calculation Vp-p for an AC wave

Measure Vp and Vave for a half-wave rectifier output

Measure reservoir capacitor voltages for a half-wave rectifier output Determine by measurement and calculation Vave across a load for

positive and negative rectifier outputs Calculate Vp and Vave for half-wave rectifier circuits

Recognise the function of a reservoir capacitor Diagnose a fault in a half-wave rectifier circuit

Equipments required for this Experiment

Circuit #2 of D3000 - 2.1 Semiconductors-1 Module Multimeter

Signal generator Oscilloscope Shorting links and connecting leads

Exercise 2.1 Halfway Rectifier

The diode only permits current to flow in one direction. If alternating voltageis applied to a circuit containing a diode, conventional current will only flow in thedirection of the arrowhead that is part of the diode symbol. Hence the current is

directional or direct (DC).

The value of the direct current flowing will be proportional to the peak valueof the applied alternating voltage, which is itself proportional to the RMS value ofthe applied voltage.


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Figure 2.1The relationship between alternating voltage values is shown in Fig 2.1. If the

peak voltage is taken as the initial reference, the peak-to-peak, RMS and average

values can be related to it. Vpeak-to-peak (Vp-p) is twice the peak value and theRMS value is 0.707 times the peak value (= /2; = 2 ). Notethat the average value of a complete cycle is zero, and this will be the result if you

try to measure an alternating quantity on the DC range of a meter.

The average value over one half cycle is also shown, and this leads to theaverage of full- and half-wave rectified sinewave quantities which are also shown in

Fig 2.1 above.

Figure 2.2

Steps for doing this experiment:1.

Connect the multimeter, on ACvoltage range between sockets 2.1 and

2.22.

Set the frequency of the signal generator to main supply frequency, andadjust the output amplitude to give 5VRMSinput to the circuit as

indicated on the multimeter.


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Exercise 2.2 Reservoir Capacitor

With an alternating voltage applied, the instantaneous value will fall to zeroat the start and finish of each half cycle. This is not a very satisfactory form of directcurrent supply. The ability of a capacitor to store and then release charge allows this

disadvantage to be overcome.There is a small variation of the voltage provided to the load, falling as the

capacitor discharges when the diode is non-conducting (reverse biased), and risingquickly again when the diode conducts to restore the charge during the peak of the

applied voltage.

This is called the ripple voltage, the amplitude of which increases with loadcurrent, and causes a reduction in the average value of load voltage.

Figure 2.3

Steps for doing this experiment

1.

Remove the shorting link from sockets 2.4 & 2.7 and connect a leadbetween sockets 2.4 & 2.9, and a shorting link between sockets 2.8 &2.11.

2. Connect the multimeter positive on DC voltage range to socket 2.10.3.

With the input from the signal generator adjusted to main supplyfrequency and 5VRMSas in the previous exercise, measure the directvoltage across the reservoir capacitor C1.

Note: Resistor R3 (10k) simulates the load. This is not connected at all atthe moment and so there is no current being drawn from the reservoir capacitor. The

voltage across the capacitor will therefore be the full peak voltage from the diode.


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Figure 2.4

4. Remove the lead from sockets 2.4 & 2.9, and connect shorting linksbetween sockets 2.4 & 2.7, 2.6 & 2.9, and 2.8 & 2.11.

5. Set up the oscilloscope as follows:Timebase to 5ms/div, trigger selector to AC, dual trace operation.Set the ALT/CHOP to CHOP.

Centralize both traces at the middle of the display.CH.1 Y amplifier gain to 2V/div, DC input.

CH.2 Y amplifier gain to 2V/div, DC input.6.

With CH.1 of the oscilloscope connected to socket 2.1, check that thewaveform still indicates an input of 5VRMSat main supply frequency.

7. Connect CH.2 of the oscilloscope via a lead to socket 2.10.8.

Make an accurate sketch of the oscilloscope display, marking in thevoltage and time scales on the graticule provided.


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Exercise 2.3 Bridge Rectifier

Full-wave rectification allows both half cycles of the input to cause current toflow in the load, but always in the same direction.

Figure 2.5

Circuits #2 and #3 have been located close to each other to facilitatecomparisons between half- and full-wave rectifier circuits.

9. Connect shorting links between sockets 2.4 & 2.7 and 2.8 & 2.11 onCircuit #2 and a lead to provide a common ground line between Circuits

#2 and #3 from socket 2.5 to 3.4.Make sure that you connect the common ground line to the correct socket in

Circuit #3.

10.

Set up the oscilloscope as follows:Timebase to 5ms/div, trigger selector to AC, dual trace operation.Set the ALT/CHOP switch to CHOP.Centralize the CH.1 trace at the middle, CH.2 at the bottom of thedisplay.CH.1 Y amplifier gain to 5V/div, DC input.

CH.2 Y amplifier gain to 2V/div, DC input.11.

Use leads to connect CH.1 of the oscilloscope to socket 3.5, and CH.2

to socket 2.6.12.

Switch ON the Module Power Supplies.13.

Set the signal generator to main supply frequency, and adjust the output

amplitude to give a 6V peak voltage. Fine tune the frequency to get bothtraces on the oscilloscope as stationary as possible.


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Note: It is not possible to use the signal generator or the main supply to feedboth circuits because of the ground line which would short out one of the diodes inthe bridge circuit.

14.Sketch the oscilloscope traces on the graticule provided, marking in

voltage and time scales.Note: It will be necessary to have two different voltage scales, one for the

upper half of the screen and another for the bottom.

Figure 2.6


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Exercise 2.4 Effect of Reservoir Capacitor

With the Bridge Rectifier circuit the reservoir capacitor is recharged everyhalfcycle, instead of just on the positive half-cycles.

Figure 2.7

Steps for doing this exercise

1. Connect shorting links between sockets 2.4 & 2.7 and 2.8 & 2.11 onCircuit #2 and a lead to provide a common ground line between Circuits#2 and #3 from socket 2.5 to 3.4.

2. Set up the oscilloscope as follows:Timebase to 5ms/div, trigger selector to AC, dual trace operation.

Set the ALT/CHOP switch to CHOP.Centralize the CH.1 trace at the middle, CH.2 at the bottom of thedisplay.CH.1 Y amplifier gain to 5V/div, DC input.CH.2 Y amplifier gain to 2V/div, DC input.

3.

Use leads to connect CH.1 of the oscilloscope to socket 3.5, and CH.2to socket 2.6.

4.

Switch ON the Module Power Supplies.5. Set the signal generator to main supply frequency and adjust the output

amplitude to give a 6V peak voltage. Fine tune the frequency to get both

traces on the oscilloscope as stationary as possible.6.

Plug a lead into socket 2.10 to act as a flying lead.7.

Plug the loose end of the flying lead to socket 3.3.8. Sketch the upper oscilloscope trace on the graticule provided below,

marking in voltage and time scales.9. Transfer the CH.2 oscilloscope lead from socket 2.6 to 2.9 and transfer

the flying lead from socket 3.3 to socket 2.6.10.Sketch the lower trace.


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Note: It will be necessary to have two different voltage scales, one for theupper half of the screen and another for the bottom.

Figure 2.8

Note: Do not attempt to monitor the waveforms on both sides of the diode

bridge at the same time. Connecting ground returns to both sockets 3.2 and 3.4 willalmost certainly result in destroying one of the diodes in the bridge circuit.

HOMEWORKS

1. Simulate the actual laboratory experiment in Exercise 1.1 (DiodeForward Characteristic) by using Electronic Workbench software(Multisim)

2. Simulate the actual laboratory experiment in Exercise 1.2 (Diode

Reverse Characteristic) by using Electronic Workbench software(Multisim)

experimental method 3.pdf

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