1.2Vdc 1N4002. Anode V+

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1 ECE 2274 Pre-Lab for MOSFET Night Light and Voltmeter 1. Night Light The purpose of this part of experiment is to use the switching characteristics of the MOSFET to design a Night Light using a LED, MOSFET, photocell, resistors, and power source. For our LTspice design we will use a variable resistor to simulate the photocell and the in LTspice use QTPL690C for the LED. In the lab we will use a photocell and LED Schematic If no LED model LED Anode V+ Cathode V- DC 1.2Vdc 1N4002 Anode V+ Flat Cathode V- Page 1 of 7

2 For the night light circuit below use the following specifications: Calculate Rg1 such that the MOSFET is on when Rphoto (photocell) is equal to 2k Always place a (Rd) resistor in series to limit the diode (LED) current to approximately 10mA with Rd (LED spec: Typical, Vf =1.8V@ If = 10ma). The gate voltage should be biased to Vg = 2.3V when it switches. Show all work. Then simulate your design with LTspice plotting the switching voltages. Plot: Vds vs. Rphoto Id vs. Rphoto Vgs vs. Rphoto Turn in your plots and circuit diagram LTspice tips: Page 2 of 7

3 LTspice add.include 2N7000.sub for 2N7000 MOSFET that we will use in the lab. Use QTPL690C for the LED 1. Variable Resistor for Photocell Rphoto 2. Instead of Rphoto (photocell) having a numeric value for its resistance, it has the name of a parameter, {Rphoto}. We define the parameter by adding the.step PARAM Rphoto command to the schematic. 3. Use DC operating point.op with Step sweep Rphoto from 500Ω to 5000Ω by 5Ω 4. Place appropriate probes to produce necessary plots. 2. Voltmeter Build a MOSFET-controlled voltmeter in LTpice. The MOSFETs in the voltmeter act as switches to turn on the MOSFET assume Vg = 2.3V when it switches on. Design the Rd1, Rd2, and Rd3 values, such that the maximum drain current is 10mA for Rd values. Set Vdd = 5Vdc. Design the input voltage divider resistors values such that M1 Vout1 turns on at approximately Vin = 10V, M2 Vout2 turns on at approximately Vin = 15V, and M3 Vout3 turns on at approximately Vin = 20V. Choose input resistor values that are in the 10kΩ to 150kΩ range. Tip: Remember that you can split this circuit into three circuits that are similar to the first stage of the voltmeter. To find the voltage divider resistors (Rg1, Rg2) (Rg3, Rg4) (Rg5, Rg6) you can create three circuits when Vin=10V, Vin=15V, and Vin=20V. The calculation steps will be identical to the first voltmeter, but the input voltage (Vin) will be the only difference. Vin Rd1 Rd2 Rd3 DC Vin Rg1 Rg3 Rg5 Vout1 Vout2 Vout3 M1 M2 M3 Vdd DC Rg2 Rg4 Rg6 Show all work Page 3 of 7

4 V G1 Voltage of gate of M1 Rg1 Rg2 Rd1 Drain current limit resistor V G2 Voltage of gate of M2 Rg3 Rg4 Rd2 Drain current limit resistor V G3 Voltage of gate of M3 Rg5 Rg6 Rd3 Drain current limit resistor Run a DC Sweep simulation on Vin from 0V to 25V in increments of 100mV. Plot VDS for each MOSFET (M1, M2, & M3). Print out your plot. Remember that a MOSFET is considered to be on after V DS voltage is approximately 0.2 volts. From the Pspice plot measure Vds of M1, M2, and M3 when the MOSFET is fully on. What input voltage did each on the MOSFETs turn on at? Required Attachments: Night Light: VDS vs. Rphoto: Place cursors when the MOSFET is on and off Night Light: Id vs. Rphoto: Place cursors when the MOSFET is on and off Night Light: VGS vs. Rphoto: Place cursors when the MOSFET is on and off Voltmeter: DC Sweep, varying Vin measuring VDS of M1, M2, and M3 Voltmeter: DC Sweep, varying Vin measuring Id of M1, M2, and M3 Page 4 of 7

5 Lab Exercise Night Light and Voltmeter Part I Night light 1. You must test your 2N7000 MOSFET with the curve tracer before build your experiment. Set curve trace to N-FET, Is Max = 10ma, Vds max =10V, Vg/step = 0.1V, Offset = 1.8V, Rload=.10, N Steps = Build the Night Light you designed in the pre-lab. Use the 2N7000 MOSFET, LED, Photocell, resistors, and power supply. 3. Measure Vds, Id, and Vgs in the on and off mode. Compare with your design. (To make on hold hand over photocell) 4. Verify to your instructor that your night light works. Lab Instructor must sign off. Part II Voltmeter Build the voltmeter you designed in pre-lab. Manually adjust Vin to find the turn on voltages of each MOSFET. Record the voltage at which the MOSFET turn on. Sweep the circuit using the Basic DC sweep Vin from 0 to 25V. Add the other two voltmeters into the Basic DC Sweep loop. Be sure to add them in the Output section of the Sweep step Run a DC sweep of Vin from 0Vdc to 25Vdc and turn in the following plots for each on voltage (10V, 15V, 20V) range with your lab report: Vds vs. Vin, Id vs. Vin. Measure the voltage across a known resistance and calculate the scale factor for the current plot, write the scale factor on the plot. You will have a total of six plots to be turn in with your lab report. Remember to label each plot. Also, remember Id is the current. How does it compare with your design? MAKE SURE ALL PLOTS ARE LABELED CORRECTLY. Page 5 of 7

6 DATA SHEET Name: Name: Instructor: Class day and time: Date: Bench number: MOSFET Night Light and Voltmeter 1. You must test your 2N7000 MOSFET with the curve tracer before build your experiment. Set curve trace to N-FET, Is Max = 10ma, Vds max =10V, Vg/step = 0.1V, Offset = 1.8V, Rload=.10, N Steps = 10 Part I. Night light Vdd = 5Vdc (LED on) Design Value (LED on) Measured Value Vds Id (measure V across known R) Vgs (LED off) Design Value (LED off) Measured Value Vds Id (measure V across known R) Vgs Instructor: Page 6 of 7

7 Part II. Voltmeter Vdd= 5Vdc Voltage Vin when MOSFET turns on 10V M1 Vout1 15V M2 Vout2 20V M3 Vout3 2. Compare the DC sweeps of your Pre-Lab with the results of your experimental DC Sweeps. What conclusions can you make? Required Attachments: DC Sweep V DS vs. Vin Vds of M1, M2, M3 (3 voltmeters) DC Sweep Id vs. Vin (voltage across a resistor Rd): write the scale factor to convert to voltage current on the plot. Id of M1, M2, M3 (3 voltmeters) Page 7 of 7