Diodes. Diodes, Page 1

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1 Diodes, Page 1 Diodes V-I Characerisics signal diode Measure he volage-curren characerisic of a sandard signal diode, he 1N914, using he circui shown below. The purpose of he back-o-back power supplies is o make i easy o make measuremens near zero supply volage. Plo he V-I characerisic on graph paper o show he rapid rise in forward curren when he forward volage approaches 0.7 vols. I should look somehing like he figure below. Noice ha he curren in milliamps is given by he volage across a resisor as measured in vols. Therefore, you can measure V vs. I by connecing a digial volmeer alernaely across he diode and he resisor. Repea he measuremen using he LabView program, I_vs_v using a resisor for he sandard. Make a prinou of he curve and inser ino your lab noebook. I D (ma) 5V Var. + - V D -18V 0 V D 0.7V Figure 1: Diode Circui. Figure 2: Diode V-I Plo. Temperaure dependence Bias he diode o ge a reverse curren of abou 0.2 microamps. Pu a soldering iron close o he diode and noe he percenage change in reverse curren. There is a big effec because he minoriy carriers ha ranspor he reverse curren are hermally generaed. Now bias he diode in a forward direcion o ge a forward curren of abou 0.2 ma. Repea he hea reamen and observe he change in curren.

2 Diodes, Page 2 The Half-Wave Recifier V 0 10K V 0 Figure 3: Half-wave Recifier. Figure 4: Inpu ( and Oupu ( waves. The basic half-wave recifier wih resisive load looks like he circui above. The Inpu and Oupu waveforms are shown on he righ. The half-wave recifier wih filer capacior and resisive load is shown in Figure 6 wih he inpu and oupu waveforms in Figure µf Figure 6: Recifier wih Capacior. Figure 5: Inpu and Oupu forms. The capacior is elecrolyic. You mus obey he polariy markings when you connec his capacior in he circui. Use an 8 VRMS 60-Hz ransformer secondary as a source. ( Begin wih he capacior disconneced. Observe he half-wave recified paern on he oscilloscope. Verify ha he frequency of he paern is 60 Hz. ( Add he 10-µF capacior, geing he polariy righ, bu remove he load resisor. Observe how he capacior inegraes he recified power o make a smooh DC source. We migh imagine using such a source o power some elecronics ha need a sable source. (c) Add he resisive load back ino he circui and see how he load causes he oupu of he power supply o ripple. You may need o AC couple he scope and increase he

3 Diodes, Page 3 gain o see he ripple well. Verify ha he frequency of he ripple is 60 Hz. Use he dual race feaure of he scope o find he phase of he ripple wih respec o he phase of he inpu AC volage. On wha phase of he AC inpu does he capacior charge? The Full-Wave Recifier The full-wave recifier, wih filer capacior and resisive load, is shown in Figure 8 wih he inpu and oupu waveforms in Figure µf Figure 8: Full Wave Recifier. Figure 7: Inpu and Oupu Shapes Build he circui above. Use an 8-VRMS, 60-Hz supply. ( Begin wih he capacior disconneced. Observe he full-wave recified paern on he oscilloscope. Verify ha he frequency of he paern is 120 Hz. [Noe: Because he wo channels of your oscilloscope have a common ground, i is no possible o view boh he inpu and he oupu of he recifier a once.] ( Add he 10-µF capacior, geing he polariy righ, bu remove he load resisor. Observe how he capacior inegraes he recified power o make a smooh DC source. (c) Add he resisive load back ino he circui and see how he load causes he oupu of he power supply o ripple. Verify ha he frequency of he ripple is 120 Hz. (d) Reduce he resisive load by making he resisor 5 K insead of 1 K. Observe ha he ripple decreases. Explain he advanage of he full-wave over he half-wave recifier in making a sable power supply. In fac, neiher supply is very good. Boh are unregulaed. Regulaed power supplies will be sudied in a laer lab. The Zener Diode Use he measuring seup of Par 1 above o measure he V-I characerisic for a Zener diode, he 1N4734, which has a Zener volage of 5.6 v. (Noe: In he forward

4 Diodes, Page 4 condiion, curren flows ou of he erminal labeled wih he band.) Plo he V-I characerisic using he LabView program and noe he dramaic increase in reverse curren when he volage reaches 5.6 vols. A Zener diode, biased a his breakdown condiion, is a sable volage reference because he volage does no change much even hough he curren may change. The PIN Diode The PIN diode has inrinsic (no doped) maerial (I) in he juncion beween P and N ype maerial. I makes a fas phoo diode, wherein minoriy carriers are generaed by phoons. Because he phooeffec depends upon minoriy carriers, he diode is biased in he reversed condiion for normal operaion as a ligh-sensiive device. Use he PN323B pin diode in he circui below o show a large increase in diode curren when a brigh lamp is brough close o he diode. Wha is he effec of elevaed emperaure on he reverse curren? Reverse he diode o aemp o find he effec of ligh on he diode curren in he forward-biased condiion. 5V The Seven-Segmen Display Long Lead Figure 9: PIN Diode Circui. The Seven-segmen display uses 8 ligh-emiing diodes, including one for he decimal poin. The LN516RK is a common cahode device, as shown in Fig11. The anodes are conneced as shown in Fig Com Pin 3 Pin 8 Figure 11: Common Cahode LN516RK. Use he circui in Fig. 12 o urn on segmens and verify he diagram in Fig Figure 10: LN516RK Pinou.

5 Diodes, Page 5 5V Figure 12: Measuremen Circui. The inerruper The inerruper consiss of an LED and a phooransisor separaed by a gap (see Figure 13). When he gap is empy he ligh (infrared) from he LED excies carriers in he phooransisor, which hen conducs. When he gap is filled, he ligh is inerruped and he phooransisor does no conduc much Figure 13: The H22A1. can be deermined. 2 3 Build he circui in Figure 14 using an H22A1 from Harris Semiconducor. Can you inerrup he ligh pah so fas ha you es he speed of he response? Beer be speedy. The urn-on ime is 8µs. The urn-off ime is 50µs. Inerrupers are used wih spinning disks wih holes ha allow he ligh o pass. A simple couner on he oupu of he phooransisor can coun he number of revoluions, and wih he addiion of a iming circui he speed of he disk 10V 10K Figure 14: The Inerruper Circui.