LIC–Special Function IC (Unit 5)–Two marks

Anna University





1. What are the operating modes of a 555 timer?

a. Monostable mode

b. Astable mode

2. List out the applications of 555 timer?

a. Oscillator

b. pulse generator

c. ramp and square wave generator d. mono-shot multivibrator

e. burglar alarm

f. traffic light control.

3. Define sink current and source current?

Sink current: When the output is low, the load current that flows through the load connected between Vcc and o/p terminal is called sink current.

Source current: When the output is high, the load current that flows through the load connected between ground and o/p terminal is called source current.

4. Define normally ON load and normally OFF load?

Normally ON load: The load connected between VCC and output terminal. Normally OFF load: The load connected between output terminal and ground.

5. What is the use of reset pin of 555


This is an interrupt for the timing device when pin 4 is grounded, it stops the working of device and makes it off.

6. What is the purpose of control voltage pin (5) of 555 timer?

This pin is the inverting input terminal of comparator. This is reference level for comparator with which threshold is compared. If reference level is other than 2/3 VCC, then external input is to be given to pin 5. Pulse width modulation is possible due to pin 5.

7. List out the major blocks of 555 timer functional diagram?

The IC 555 timer combines the following elements.

1) A relaxation oscillator

2) RS flip-flop

3) Two comparators

4) Discharge transistor

8. Define duty cycle?

It is defined as the ratio of on time to the total time of one cycle. D = W/ T

W – time for output is high = TON

T – total time of one cycle.

9. Write the expression for pulse width of 555 timer in monostable mode?

Pulse width W = 1.1 RC seconds

R – resistor in ohms, C – capacitor in farads

10. Write the expression for total time period of 555 timer in astable mode?

T = 0.693 (RA + 2 RB) C seconds

11. What is the frequency of oscillation of free running mode of 555 timer?

F = 1.44/ (RA + 2 RB) C Hz

12. List out the applications of 555 timer in astable mode.

a. missing pulse detector b. Linear ramp generator c. Frequency divider

d. Pulse width modulation.

13. List out the applications of 555 timer in monostable mode.

a. FSK generator

b. Pulse-position modulator

14. Define voltage regulators and give the types?

A voltage regulator is an electronic circuit that provides a stable dc voltage independent of the load current, temperature, and ac line voltage variations.

The classification of voltage regulators:

*Series / Linear regulators

*Switching regulators.

15. What do you mean by linear voltage regulators?

Series or linear regulator uses a power transistor connected in series between the unregulated dc input and the load and it conducts in the linear region .The output voltage is controlled by the continuous voltage drop taking place across the series pass transistor.

16. Define switched voltage regulators?

Switching regulators are those which operate the power transistor as a high frequency on/off switch, so that the power transistor does not conduct current continuously. This gives improved efficiency over series regulators.

17. What are the advantages of adjustable voltage regulators over the fixed voltage regulators?

i) Improved line and load regulation by a factor of 10 or more.

ii) Because of the improved overload protection, greater load current can be drawn.

iii) Improved reliability.

18. List out the parameters related to the fixed voltage regulators?

1) Line regulation

2) Load regulation

3) Ripple rejection

4) Output impedance

5) Maximum power dissipation

6) Rated output current

19. Define dropout voltage of a fixed voltage regulator?

It is the minimum voltage that must exist between input and output terminals. For most of regulators, it is 2 to 3 volts.

20. What is an opto-coupler IC? Give examples.

Opto-coupler IC is a combined package of a photo-emitting device and a photosensing device.

Examples for opto-coupler circuit : LED and a photo diode,

LED and photo transistor, LED and Darlington.

Examples for opto-coupler IC : MCT 2F , MCT 2E .

21. Mention the advantages of opto-couplers.

*Better isolation between the two stages.

*Impedance problem between the stages is eliminated.

*Wide frequency response.

*Easily interfaced with digital circuit.

*Compact and light weight.

*Problems such as noise, transients, contact bounce,.. are eliminated.

22. What is an isolation amplifier?

An isolation amplifier is an amplifier that offers electrical isolation between its input and output terminals.

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