## Final Exam

Chem 498A/C Fall 1997

Name _______________________________________

Time limit: 2 hours.

1. a. What resistance will be measured in the following circuit from point A to point B (i.e. if an ohmmeter is connected between point A and point B)?

b. If a battery is connected between points A and B, which single resistor in the above circuit has the largest voltage across it?

c. In the following circuit, which single resistor has the largest current flowing through it? Explain your reasoning.

2. Suppose you need a line-operated regulated power supply that is capable of supplying 5 volts DC with both line and load regulation for load currents from 0 to 100 mA. You have on hand a power transformer with a 115 VAC primary and a 6 VAC secondary, a large number of rectifier diodes (1 amp current rating, 100 volt peak reverse breakdown voltage), a 2000 µF electrolytic filter capacitor, a 5 volt, 5 watt Zener diode, and a full supply of resistors in all standard values up to 5 watts power dissipation. Design and draw the schematic diagram of a circuit that will meet the above requirements.

3. In the following Wheatstone bridge, all the resistors are 100 Ohms , M is a voltmeter whose input resistance is very much higher that 100 Ohms , and the battery supplies 12.0 volts.

If the voltmeter can detect a minimum voltage change of 1 mV, then what is the smallest resistance change in R4 (in ohms) that could be detected, assuming that the other 3 resistors do not change?

4. When planning to acquire data from an instrument into a computer, how do you determine when to use an RS-232 interface and when to use a analog-to-digital converter?

5. Suppose an instrument 's analog output has a 0 - 100 mV DC voltage range and an output resistance of 1000 ohms. You want to digitize this signal with a CIO-AD08 12-bit ADC that has a 0 - 5 volt range with full 12-bit full-scale resolution?. Design a circuit that will accomplish this, showing all component values.

6. In the lab you used this QuickBASIC program to read the CIO-AD08 12-bit ADC in the IBM-PC:

channel% = 0
OUT baseplus2%, channel%
OUT strobe%, 0
seeifdone:IF INP(baseplus2%) >127 THEN GOTO seeifdone

Suppose the output voltage of instrument A is connected to channel 1 and the output voltage of instrument B is connected to channel 2. The ADC is set to the monopolar 0 to +10 volt range. Both instruments outputs fall within this range of voltages. Write a simple QuickBasic program that will open two files ("inst1.txt" and "inst2.txt") for output, read both of the instrument signals and the TIMER repetitively, convert the ADC readings to voltage, and save the channel 1 data to inst1.txt and the channel 2 data to inst2.txt in the format: time, signal. Construct the program so that the readings are continuously read and saved until the Q key on the keyboard is pressed, then close both files and end the program.

7. (a) In the digital signal processing exercise (using SPECTRUM), for what purposes did you utilize the fourth derivative operation?. What is the relationship between the amplitude of the nth derivative of a peak-shaped signal (e.g. Gaussian or Lorentzian peak) and its (b) peak height and (c) peak width?

8. In the experiment involving the stepper motor connected to the monochromator, what is the purpose of the transistor switches between the computer's digital outputs and the remote inputs of the stepper controller box? That is, why not connect the computer and the controller box directly without the transistor switch?

9. Explain the operation of the following program, assuming that it runs on a computer that is correctly hooked up to a PROTEK digital multimeter that has been set to RS-232 and the temperature mode.

```DIM temp(1000)
DIM time(1000)
TIMER ON
interval=10
OPEN "com1:1200,E,7,1" AS #1
j =0
repeat: key\$=INKEY\$
IF j=1000 THEN GOTO done
IF key\$<>"q" THEN GOTO repeat
done: FOR k=1 TO j
PRINT  time(k), temp(k)
NEXT k
END
PRINT #1," "
INPUT#1,A\$
j=j+1
time(j)=TIMER
temp(j)= VAL(MID\$(A\$,6,4))
RETURN```

10. Suppose you are measuring a signal with the Vernier Universal Lab Interface and Data Logger software, like you used in the laboratory, to digitize and record the signal from some instrument.

a. How do you know what what data rate and time axis range to use?

b. Is there any disadvantage to use the largest available data rate and time axis range available?