# help electrical prep for exam2

1. Use thevenin’s theorem to calculate the current flowing through the 10 ohm resistor in figure 1.

2.1. define the following terms:

2.1.1. Q-factor

2.1.2. Band Width

2.2. Consider the phasor diagram in furgure 2 and calculate the values of ZA, ZB, and ZC, if the supply  voltage is given as 220V.

3.1. match each of the following:

 Column A Column B 3.1.1 PN-Junction Diode A It operation depends on the capacitance that exists at the PN-Junction which is reverse biased. 3.1.2. Varactor Diode B Permits current to flow in the forward bias direction but it will also flow in the reverse bias when the voltage is above breakdown voltage 3.1.3 Tunnel Diode C Has a negative resistance region where an increase in terminal voltage results in reduction in Diode current 3.1.4 Zener Diode D Has heavily doped p-type and n-type regions separated by an instinsic region

3.2. a diode has a forward resistance of 0,11 ohm. Calculate the current flor through the diode at a room temperature of 30 deg C.

3.3. figure 3 shows a waveform obtained from a half-wave rectifier.

Calculate the following:

3.3.1. transformers secondary peak voltage

3.3.2. transformers average output voltage

3.3.3. transformers secondary RMS voltage

3.4. calculate the output voltage of an LC-π- filter with a resistance of 200 ohms and a 1,5 kΩ load resistance. The voltage across the input capacitor is 200V.

4.1. true or false?

4.1.1. when transistor voltage and current are constant, the transistor is operating in its dynamic condition.

4.1.2. when the transistor operates in the active region its base – emitter junction is reverse biased and base-collector junction is forward biased.

4.1.3. power amplifiers are designed to handle large signals.

4.1.4. the uni-junction transistor is a single junction device, which has negative resistance characteristics.

4.1.5. field-effect transistors are relatively unaffected by radiation.

4.2. study the characteristic curves in figure 4 and figure 5. Using H-parameters, calculate the following dynamic values:

4.2.1. the input impedance

4.2.2. the reverse voltage gain

4.2.3. the forward current gain

4.2.4. the output impedance

5.1. indicate the properties of an ideal operational amplifier.

5.2. explain how the internal gain of an operational amplifier can be changed.

5.3. calculate the rate of change of the output voltage for an integrator amplifier if

C=100µF , Rin=15kΩ, and Vin=3V

5.4. calculate the gain and expected output voltage for an amplifier whose output voltage is 280 deg out of phase with the input voltage if:

Feedback resistance= 100kΩ

Input resistance=200kΩ

Input voltage=20mV

5.5. make a neat, labelled circuit symbol of a comparator amplifier.

6.1. give two differences between the SCR and the TRIAC

6.2. explain how LASCR can be used in a circuit to replace SCR.

6.3. draw by means of a neat sketch, how a TRIAC by transistors. All the terminals must be shown.

6.4. show by means of a neat sketch, how TRIAC is constructed, All the terminals must be shown.

6.5. a quadrac is basically a triac and a diac combined together in a single package. True or False? Motivate your answer.

7.1. Describe, with the aid of a circuit diagram how strain gauges are used in a Wheatstone Bridge.

7.2. Which formula describes the principle on which a strain guage operates?

8.1. what is a cathode-ray oscilloscope (CRO)?

8.2. what is the function of the time base generator as used in the oscilloscope?

8.3. which wave forms would you use for the following:

8.3.1. digital electronic switching

8.4. an oscilloscope displays a sine wave over 6 vertical divisions and 4 horizontal divisions. The vertical amplifier setting is 2V/div and the time base is at 15µs/div.

Calculate:

8.4.1. the amplitude

8.4.2. the period

1. a network is arranged as shown in Figure 1 below. Calculate the value of the current flowing through the 5Ω resistors by using Kirchoffs law.

2. a network having a resistance of 10 Ω, an inductor of 0,2H and a capacitor of 150µF in series, is connected across a 100V, 50 Hz supply.

Calculate the following:

2.1. the total impedence

2.2. the total current

2.3. the voltage across the resistor

2.4. the voltage across the inductor

3.1. draw a neat, labelled characteristic curve of a tunnel diode.

3.2. calculate the forward current of a diode if it has a forward resistance of 0,103 Ω at room temperature of 30 deg C.

3.3. a full wave bridge rectifier with a capacitor filter has a turns ratio of 5:1. The supply voltage is 200V, 50Hz.

Calculate the following:

3.3.1. the peak secondary voltage

3.3.2. the average secondary voltage

3.3.3. the peak inverse voltage of the diodes

3.4. State two disadvantages of a half-wave rectifier.

4.1. Use a graph to explain what crossover distortion is and how it can be minimised.

4.2. where are the push-pull amplifiers used?

4.3. A transistor has the following input and output specifications:

Output current = 5mA

Output impedance = 6kΩ

Input voltage = 1,5mV

Input impedance = 12kΩ

4.4. draw a sketch showing the basic connection of CMOS by means of enhancement PMOS and NMOS transistors.

4.5. Name and explain two types of feedback known to you.

5.1. calculate the rate of change of the output voltage for an integrating amplifier if C = 10µF, Rin=15kΩ and Vin=10V. also draw the input and expected output waveforms if a sine wave of 3600 is used as an input.

5.2. what is the purpose of the feedback resistor in an amplifier?

6.1. Draw a neat, labelled block diagram of an open loop motor speed control system and explain its operation.

6.2. Draw a neat, labelled characteristics curves of the following:

6.2.1. Triac

6.2.2. SCR

6.2.3. Diac

7.1. Draw the IEC symbols of the following transducers:

7.1.1. photo diode

7.1.2. photo transistor

8.1. Draw a block diagram of basic function generator

8.2. there is a horizontal trace on the screen of an oscilloscope, but no vertical deflection when a sine wave is applied. Which circuit would you investigate?

8.3. Explain the purpose of the time base generator in an oscilloscope.

3.1

Match the tablular form.

Solution :

3.1.1 - A.

3.1.2- D.

3.1.3- C.

3.1.4- B.