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FOUNDATION EXP 5 – DIODES EXPERIMENT 5 DIODES 1.0 INTRODUCTION A diode is a semiconductor device that has a non-linear characteristic and exhibits a low resistance to current flow when connected in one direction, and a high resistance in the opposite direction. It has no single value for its voltage:current ratio, and is thus significantly different from a linear resistor. The symbol for a diode is shown below in Figure 1.1 (+) Anode (-) Cathode Figure 1.1 The cathode of a diode is normally indicated by a band around one end of the device. If the positive terminal of a power supply is connected to the anode and the negative terminal to the cathode, then the diode will be forward biased and a current will flow. In practice this current may be very large and a series resistor must normally be used to limit it to a safe value. If the power connections were reserved (i.e. positive to cathode and negative to anode) then ideally no current will flow. In practice a very small leakage will flow. If the reverse voltage is made too large, then the diode will break down and a very large reverse current will flow. Again if this is not limited, the diode will be damaged and possibly destroyed. 2.0 TESTS 2.1 PROCEDURE 1 The first test is to observe that a diode has no single, meaningful value of resistance because of its nonlinear characteristic. Set the DMM to measure resistance, and use it to measure the resistance of the diode in both directions (forward and reversed biased). Draw circuit diagrams in your lab book for both directions, noting down the diode resistance next to each. Next connect up the circuit as shown in Figure 2.1. The value of R1 should be between 310 and 400. Use the LCD display to the left of the power supply terminals to take measurements for VS, which should be varied from 0V up to a maximum of 10V. Use the DMM set up as an ammeter (break and make method) to measure ID, and then reconnect and reconfigure it to take Dr. Daniel Nankoo 1 of 5 FOUNDATION EXP 5 – DIODES respective readings of VD, where it is set up as a voltmeter (in parallel to the diode). ID R1 + VS - + VD - Figure 2.1 Draw a table in your lab books with three columns, headed by VS, VD, and ID. As you increase VS, take the corresponding readings for VD and ID. Once you have enough readings (you decide on an appropriate number), draw a control graph of ID versus VD. Once you have drawn your graph, examine closely the points where there is the greatest change in the slope, and decide whether you need to take finer readings at these points to obtain a smoother curve. 2.2 PROCEDURE 2 ID R1 + VS - + VD - Figure 2.2 Repeat the procedure outlined in Section 2.1, but for the reversed biased connection, as shown in Figure 2.2 and again plot ID versus VD, where VD has negative values. Then combine both sets of results for both the forward and reversed biased connections and plot the results on one single ID versus VD graph. When plotting graphs, do not forget to clearly label your axes, and to use the appropriating scaling. 2.3 PROCEDURE 3 You will now conduct tests on another type of diode called a Zener diode. A Zener diode is very similar to a conventional diode, except that the reverse breakdown voltage is adjusted to occur at some lower voltage than normal. The voltage remains almost constant once breakdown has occurred, which makes Zener diodes extremely useful in circuits where a voltage reference source is needed (e.g. in power supplies). Dr. Daniel Nankoo 2 of 5 FOUNDATION EXP 5 – DIODES Replace the conventional diode in the circuit of Figure 2.1 with the Zener diode provided. Measure and record the values of VS, IZD and VZD (Zener current and voltage respectively) in tables drawn in your lab books, and thus repeating the procedure outlined in Sections 2.1 and 2.3. Once you have drawn the three control graphs for the Zener diode (forward bias, reverse bias and a combined graph), estimate the Zener (reverse breakdown) voltage for your diode from the results. 2.4 PROCEDURE 4 You will now use the oscilloscope in “COMPONENT TESTER” mode. On the older CRT scopes, there was a COMPONENT TESTER button which was used to display the voltage-current characteristic of a passive device. However, on the digital storage scopes in the lab, this function does not exist, and thus the scope will have to be configured manually in order for this mode to be operational. Using the appropriate cables, build the circuit shown in Figure 2.3 on your breadboard. CHANNEL 1 OF SCOPE FOR X RLOAD 680Ω 1KhZ MAX AMPLITUDE PLACE DEVICE TO BE TESTED HERE CHANNEL 2 OF SCOPE FOR Y RSHUNT 10Ω → 100Ω Figure 2.3 Set the Function Generator to produce a sine wave of 1kHz, with maximum amplitude (i.e. the AMP dial pushed in and turned fully clockwise). Ensure that the 50Ω output is selected. Set up the scope by first pressing the DISPLAY button, and switching the “Format” to “XY”. This disengages the horizontal time axis. Next, place a resistor (330 R 820 ) in the part of the circuit where a device can be tested. Adjust the VOLTS/DIV CH1 for the X axis and VOLTS/DIV CH2 for the Y axis to obtain suitable traces. Once a suitable trace has been achieved, draw what is displayed into your lab books. Dr. Daniel Nankoo 3 of 5 FOUNDATION EXP 5 – DIODES Now connect a conventional diode, and then a Zener diode, again drawing what you see for both cases of biassing. Do the traces look familiar? What are they the same as? 2.5 DIODES IN SERIES-PARALLEL Connect four conventional diodes in the series-parallel combination as shown in Figure 2.4. Figure 2.4 Carry out a voltage/current test, following the same procedure as laid out in Sections 2.1 and 2.2, tabulating your results in your lab books, and drawing the relevant control graphs of diode current against diode voltage. Then connect the full diode combination to the circuit of Figure 2.3, and observe and draw the trace that you see. Dr. Daniel Nankoo 4 of 5 FOUNDATION EXP 5 – DIODES 3.0 COMMENTS AND CONCLUSIONS As usual it is important to examine your results and see how and why they do or do not correspond, e.g. are there, or are there not, any connections between the tests on a single diode and on the series-parallel combination? What is the range of voltage:current ratios for points on the forward biased part of the characteristic for a single diode? What is the largest power dissipation in the diode and the resistor used in the circuits you have investigated? Dr. Daniel Nankoo 5 of 5