112 - Lab 8 Purpoe Oberve one-way diode behavior Ue ome L in conventional and non-conventional way Ue JT tranitor a amplifier and witche Part/tool needed: oldering iron and hand tool Part available in lab Power upply MM Note: Thi lab i a little longer than other. Read through the tep and do what you can before coming to lab. Oberve one-way diode behavior In general, diode only conduct current in one direction. In fact, almot all emiconductor device exhibit ome degree of polarity. In ome cae emiconductor uch a tranitor may be damaged when exceive voltage i applied in the incorrect direction. ecaue of thi, electronic device that ue batterie how explicitly how to put the batterie in to avoid damaging the device. An L exhibit polarity a well. It poitive terminal i indicated by being lightly longer than the other lead. Figure 1: L - Poitive lead (anode) i lightly longer Let firt oberve the diode one-way behavior by contructing the circuit below on a big pad protoboard. Ue your power upply et to output and a red L for. 1
4 10k green) ion" R4 (enitivity) g d L (white) Q2 L (red) Figure 2: iode xperiment One Once powered up, the L hould light. Meaure the forward voltage V d acro the regular diode and the L, with your MM. forward voltage: Red L forward voltage: The i a normal ilicon diode and exhibit a V d of roughly 0.7V. However, L exhibit much higher forward voltage. The V f for red L average about 2.2 volt. reen and yellow exhibit more. Meaure the forward voltage of a yellow, green and white L by replacing the red L with each of the following: Yellow L V f : reen L V f : White L V f : (Note: Alway have a reitor of at leat Ω in erie with your L. They will be permanently damaged otherwie.) Now, with any color L till in place and illumined, revere the polarity of the power upply. I the L till illuminated? I any current flowing in the circuit? xplain what you are oberving. 2
The wizzle circuit It i poible to ue ome diode oriented o that it make no difference which way batterie are intalled. In thi experiment, we will ue four diode to allow a battery to power the L circuit regardle of how the battery i connected by uing the one-way behavior in a clever way. The incomplete chematic below how a circuit where a 5 volt ource could be applied in either direction to a circuit that light an L. Actually, the L can withtand coniderable revere voltage without damage, but let allow it to repreent ome polarity enitive circuit. If voltage i applied in a backward fahion, a wizzle circuit orient the poitive and negative upply voltage o that it alway applied to the circuit correctly. omplete the circuit chematic given to produce a wizzle circuit and then build it on your big pad protoboard. e ure to clearly how your connection dot on the chematic. There are two miing wire. Your circuit drawing hould be clear. The cathode end of the diode are marked with a black band. 3 k OR note revered polarity 5 L(red) 4 Figure 3: wizzle chematic to be ompleted touch pad L MPA Once your circuit i complete, try to light the LR4 with both polaritie. If you have it deigned and wired it correctly, the L will light under both polaritie. If not, figure out your mitake, redraw the circuit and fix your circuit until it work properly. 10k L (white) Meaure the voltage between the right ide of (+(enitivity) lead of MM) and the cathode of the L (- lead of MM). d L (green) g Q2 I it equal to the upply voltage? How much le i it? "air connection" L (red) xplain the difference. 3
JT a a enitive amplifier We have een in cla how a JT can operate a a current amplifier. ome JT tranitor have exceptionally high current gain due to a pecial internal tructure. We will ue a uper-beta tranitor with a current gain (beta) of about 5000 to build a touch enor. The touch enor ue the tiny current path through the pad of your finger to provide the current path from the poitive upply to the bae terminal of a JT. The JT will amplify the current and turn it fully on. If an L i placed in the collector to upply branch, a touch actuated L witch i formed. A finger touch on the two pad will upply the current, and the L will turn on. 3 k 5 L(red) L (white) 4 touch pad 1 0k en) 1 " ottom view) R4 uild the circuit on the blue protoboard. Ue a white L for. You can ue two unconnected wire or the protoboard pad a your touch pad. Make ure your circuit operate correctly. L (white) (enitivity) oe the L d brightne change when you puh harder on the touch pad or ue moitened finger on g the touch Q2 pad? L (red) xplain why: Figure 4: Touch enor chematic Have your TA check off the operation of your touch witch: 4
Uing an L a a photo-detector (not a photo-emitter!) Mot all emiconductor device are enitive to light, infrared through X-ray. Thi i why mot are encapulated in black epoxy or metal cae. L are alo enitive to light. ince they are in tranparent cae, they can both emit and detect light. L are optimized to emit light, but they can till allow a mall leakage current to pa when they are revere biaed and light i applied to their PN junction. Look at the chematic in figure 5. It i imilar to the touch enor circuit except a green L i put in the place of the touch pad for your finger. If light i applied to L, a mall leakage current flow into the bae of, turning it on. When fully on, the 3collector of will be at about 0.2 volt. ince the collector of i connected to the gate of MOFT Q2 which ha a threhold voltage of about 2.5 volt, Q2 will be off. When Q2 i off, it pae no current from drain to ource, thu L will remain off. For thi circuit configuration, you can ee that when i on, k Q2 i off. When i off, Q2 will be on. 5 OR However, if no light fall on the enor L, there L(red) i no bae current into and it will be off. note revered Under thi ituation, the collector polarity of a well a the gate of Q2 i at 5 volt. Thi turn on Q2 touch pad allowing it to pa current from drain to ource, thu illuminating L. What we have created i a light ource that come on only when the light go off; an automatic 4 night light. L MPA 10k L (green) "air connection" R4 (enitivity) g d L (white) Q2 L (red) Figure 5: Automatic Night Light chematic 5
uild the circuit on the blue big pad board. However, the connection between the anode and the bae of the mut be made in the air. Uing the pad on the board allow the tiny diode current to leak away from the bae onto the board and to ground. ee figure 6. Figure 6: onnection Made in the Air Once built, ee that it work correctly by covering with your finger. You can ue the white L you already ued for. You can vary the enitivity of the circuit by adjuting. You can alo tailor the light wavelength the circuit i enitive to by uing yellow or red L. You can ue multiple enor L in parallel to make it more enitive to different wavelength of light. how your working circuit to your TA. ircuit work? 6