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GVSU EGR 214 - Transistors

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EGR214 Laboratory Activities 1School of EngineeringGrand Valley State UniversityEGR 214 – Laboratory #7TransistorsObjectives• To investigate the behavior o f the N-channel MOSFET transistorPre-Lab Assignment1. Read through the entire labor atory procedure.Pre-Lab DeliverablesThese deliverables are due at the beginning of your laboratory period. Your instructor will verifythese deliverables as you enter the laboratory and will use them to construct your laboratory grade.1. None for this lab.IntroductionThere are several types of transistors available. In this lab you will study the transistor known as the metal-oxide semiconductor field effect transistor, or MOSFET. Unlike the two-terminal devices you have studiedso far (re sistors, capacitors, inductors), a MOSFET is a three-terminal device. Its electrical symbol is shownin part (a) of Figure 1. Its three terminals are known as the Gate, Drain, and Source.DrainGateSourceDrainGateSource+-Resistance varies basedon VgsRDSVGS(a) (b)Figure 1: (a) MOSFET symbol, (b) simplest MOSFET electrical modelThe simplest explanation of the MOSFET’s behavior is that the voltage from Gate to Source, VGS,controls the va lue of a re sistance from the Drain to the Source (this resistance is r e ferred to as RDS), asshown in part (b) of Figure 1.One way to think of the MO SFET is as a semiconductor p otentiometer, i.e., a potentiometer without aphysical knob, but instead it has an “electrical knob”, namely the voltage between Gate and Source.Note that there is no current flowing into the Gate terminal of the MOSFET. The Gate terminal is verywell modelled as an open circuit, so that it is only the voltage at this node (r e lative to the Source) thatcontrols the Drain- to-Source resistance RDS.Copyrightc 2010 Padnos College of Engineering & ComputingEGR214 Laboratory Activities 2NOTE: MOSFET’s are e asy to destroy with electro-static discharge (ESD). Since the Gate terminal is anopen circuit, any amount of charge that is induced there tends to accumulate and build up a high voltage.Most MOSFET’s cannot tolerate a VGSvoltage any higher than 20V, and ESD events often involve thousandsof volts. It is imperative that you:• touch the MOSFET pins as little as possible• always try to “ground” yourself before handling the MOSFETPart I – The 2N7000 MOSFET (30 minutes)1. Construct the following circuit using the 2N7000 MOSFET. The pinout of this MOSFET is as s hownbelow. The pins for a MOSFET are labelled G for Gate, D for Drain, and S for Source. When thedevice is placed on a breadboard, the flat side of the transistor should be pointing right and sho uldlook like the diagram below on the left.DrainGateSourceVin5V+-+-100Ω2N7000VGSVDSiDGSD2. The goal is to draw a graph of RDS(the MOSFET’s re sistance from Drain to Source) as a function ofVin= VGS. Prior to turning the circuit on, accurately mea sure the value of the 100Ω resistor. WithVin= 0, turn the circuit on and accurately measure the voltage of the 5V s ource.3. Redraw the circuit above using the electrical model shown in par t (b) of Figure 1. Note that RDSandthe 100Ω resistor form a voltage divider and the voltage across RDSis VDS.4. Vary Vinfrom 0V to 5V in steps of 0.2V in order to construct the graph of RDSas a function of Vin.For each value of Vin, measure the voltage VDS. This measurement will allow you to compute RDS.Do you know how?NOTE: You CANNOT use a DMM to measure RDS! Resistance can only be measur e d on anunpowered circuit. Trying to measure resistance on a powered circuit can damage the DMM.• Report your table of values from the above experiment.• Include a graph of RDS(Y-axis) against VGS(X-axis). Properly la bel the graph with axis titlesand appropria te units (Ω and V).• Include a graph of iD(current flowing through the 100Ω r e sistor) against VGS. Properly labelthe graph.• A MOSFET is often used a n electronic switch: sometimes RDSis very low hence the MOSFETswitch is “on” and current is allowed to flow, and s ometimes RDSis very high hence the MOSFETswitch is “off” and very little current flows. Looking at your graph of iDas a function of VGS:– For what range of VGSvalues would you say the MOSFET is clearly “on”?– For what range of VGSvalues would you say the MOSFET is clearly “off”?SHOW YOUR RESULTS TO YOUR INSTRUCTOR FOR VERIFICATION!Copyrightc 2010 Padnos College of Engineering & ComputingEGR214 Laboratory Activities 3Table 1: Comparison of operating parameters for two MOSFET’sParameter 2N7000 IRF540Maximum VDS60V 100VMinimum RDS1Ω − 5Ω 0.077ΩMaximum iD0.2A 23AVGS(th)threshold ~2V ~3VMaximum Power 0.4W 100WPart II – The IRF540 (45 minutes)Not all MOSFET’s are c reated equa l. Some o f the main parameters that distinguish MOSFET’s are:• the ma ximum VDSvoltage they can handle befor e breaking down• the minimum value of RDS• the ma ximum value o f iD• the value of VGSat which the MOSFET starts to turn “on” (this is known as the threshold voltageVGS(th))• how much power can the MOSFET dissipateTable 1 compa res the 2N700 0 from Part I with another MOSFET, the IRF540. Note that the 2N7000 isreally only suitable for low-voltage low-current operation. It can, for example, be used to turn LED’s onand off. To switch higher currents a nd voltages, a higher-power MOSFET is needed like the IRF5401. Thistype of MOSFET is much more suitable for switching high-current loads like motors, pumps, so le noids, etc.1. Turn the power off.2. Repeat the experiment o f Part I with the following mo difications:• Use an IRF540 instead of a 2N7000. Its pinout is shown below – you will have to rewire yourbreadboard slightly:• Replace the 100Ω resistor with a 10Ω 10W power resistor. DO NOT USE a “regular” resistor(these are o nly rated for 1/4W of power diss ipation).• Allow VGSto go as high as 9V instead of only 5V.Make s ure the 5V power supply current limit knob allows at least 500mA of current flow (i.e., if thered cur rent-limit LED comes on, you’re not allowing enough current).• Report your table of values from this experiment.• Include a graph of RDS(Y-axis) against VGS(X-axis). Properly la bel the graph.• Include a graph of iD(current flowing through the 10 Ω resistor) against VGS. Properly label thegraph.• For what range of VGSvalues would you say the MOSFET is clear ly “on”?• For what range of VGSvalues would you say the MOSFET is clear ly “off”?SHOW YOUR


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