First Exam

CHEMISTRY 595

February 1997 Due 24 hours after pickup

 

You may use as reference your copy of Malmstadt, Enke, and Crouch (MEC), your problem set answers, your lab reports, and the class notes. Any other published material is acceptable as well. Unpublished work by others (such as anyone else's homework or old exam solutions) is not. If you make use of any material from a published source, please use normal referencing practices and give credit where it is due.

 

SHOW ALL YOUR WORK so partial credit can be assigned if necessary. All seven (7) problems are weighted equally.

 

Turn in your exam in Gilbert 8; if the door is locked, slide the exam gently under the door. Questions during the exam: Schuyler's office 737-6735; home 758-3395 (if not home, leave message with your phone number and time of call).

 

1. Consider the circuit shown to the right. If the meter's sensitivity is 100 µA (fs) and its resistance is 1000 , what percent of full scale will the meter be indicating if the 5000 pot is set at 2225 ? What will be the voltage drop across the 1000 resistor?

 

2. Consider the case of a half wave rectifier operating off a transformer with a 6.3 vac (rms) secondary. Assume a silicon diode is being used.

A.) What peak inverse voltage (PIV) must the diode be able to withstand if no filter capacitor is used? Clearly explain your reasoning.

B.) What PIV is needed if a large filter capacitor is used? Explain.

3. The Texas Instruments model 350 voltage regulator is designed to produce regulated voltage over an adjustable range if a circuit similar to that shown here is used. The part will adjust the output, VO, to whatever voltage is necessary so that the center ADJ terminal is maintained at 1.25 V (nominal) above ground.

a.) Derive an expression that shows how the output voltage, VO, is related to the values of R1 and R2 and to the ADJ terminal's 1.25 V reference voltage. Show clearly how your expression is derived.

b.) What circuit would produce the lowest regulated output voltage, 1.25 V?

c.) Speculate about the roles of the two capacitors in this circuit.



4. A Weston standard reference voltage cell that is supposed to output 1.0182 V is "measured" with a voltmeter of uncertain specifications, and a reading of 0.936 V is obtained.

a.) What might be the source(s) of the discrepancy?

b.) How could you confirm your hypothesis(es) experimentally? How can you determine whether the cell or the meter is at fault? (Don't simply say, "Get a meter known to be good." Explain clearly that you know what is happening.)

 

5. The diode bridge clipper circuit shown at the very bottom of page 12 in the lecture notes is designed to clip any voltage outside a specific range (in either polarity). For instance, if a 30 VP-P, bipolar sinusoidal input is supplied, a pseudo square wave is the result.

a.) Describe how this circuit does its job, indicating what the maximum and minimum output voltages would be if the Zener is a 6.2 V part. (Hint: trace the path of current for either polarity input.)

b.) Similarly predict the behavior of the circuits shown here. Describe briefly how each would treat the sinusoidal input waveform and why each behaves that way. Each Zener shown is a 6.2 V; each other diode is 100 PIV germanium.

c.) Suppose a circuit (such as a TTL digital circuit) must be fed a waveform derived from the 60 Hz line but never exceeding the range +5.0 V to ±0.4 V (resembling a unipolar square wave, similar to that from the FG 503 TRIG OUT). The frequency of this derived waveform could be any reasonable ratio to the line frequency (such as 30 Hz, 60 Hz, 120 Hz) and must be in phase with the 117 vac line. Suggest a circuit which could modify the bipolar line wave form and produce such a unipolar square wave.

6.

a.) What is the difference between the AC and DC modes of an oscilloscope?

b.) Give a concrete example of a measurement that requires the use of the AC mode because the DC mode would be ineffective.

c.) Does the AC mode use a high pass or a low pass filter? How about the DC mode?

 

7. On page 399 of MEC, the internal circuitry of an Op Amp is shown. The following question deals with the way in which such a device behaves and is not about Op Amp circuits of the type used in Experiments 9-11.

a.) Describe in general, qualitative terms how the input stage of the Op Amp works. In particular, point out why it is the difference between the input voltages that matters rather than the absolute (signed) voltage of the input voltages. You may want to refer to the prior page in MEC for help.

b.) Describe why it is that the Op Amp output can be of either polarity. Furthermore, explain why the output cannot exceed the range of the power supplies, and in fact can get no nearer than about a half a volt of either supply.

c.) What ultimately limits the current that such an Op Amp can source or sink at its output?