Chemistry 498A Laboratory, Spring 1990
A. Basic characteristics of relays
Use the resistance ranges of your digital multimeter to locate the coil connections on the relay supplied by the instructor and to measure its resistance. Connect the relay coil to your variable power supply and slowly turn up the voltage until you can hear and/or see the relay trip. Record this voltage and calculate the current required to trip (turn on) the relay.
B. Transistor switch
The objective of this part of the experiment is to design a transistor switch that will allow the relay to be operated from a 3 volt signal -- that is, relay off when the signal voltage is zero and relay on when the signal voltage is 3 volts. You will use a 2N2030 transistor, a general-purpose, medium power switch and amplifier transistor, which has the following characteristics:
Type NPN silicon
Current gain (ß ) 50 minimum
Maximum collector-emitter voltage 40 volts
Maximum collector current 700 mA
Maximum power dissipation 5 watts
Maximum frequency 100 MHz.
Set up this circuit on your breadboard:
Turn down the input voltage Ein until the collector voltage is 1/2 of Vcc. Measure the voltages across R1 and across Rc, calculate the currents through these resistors, and calculate the current gain, ß . _______________
Measure the collector-emitter voltage when the transistor is conducting, that is, when Ein is 3 volts. __________________ volts.
Question: What would happen to the collector current if the ß of the transistor were larger than 50 or if the input voltage were larger than 3 volts (say, 5 volts)? Why?________________________________________
C. Transistor AC amplifier
1. Set up the following circuit on your breadboard, where Rc is 10 KOhm and Re is 1 KOhm (both 10%, 1/2 watt). Obtain the +12 volts from the 12 volt power supply that is built in to the breadboard, and the input voltage from the sine-wave output of your signal generator, Set the frequency to 1000 Hz and turn the output level all the way down -- counterclockwise -- to start with. Pay attention to the polarity of the 25 µ f electrolytic capacitor. Use the box-mounted ten-turn Helipot for the 20 KOhm pot and set it all the way down -- counterclockwise -- to start with. Observe the output voltage Eout on the oscilloscope.
Measure the AC voltage gain of this amplifier (the ratio of the AC output voltage to the AC input voltage). ____________________________
Note: It should be equal to the ratio of Rc to Re (that is, about 10).
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