Page 151 - CITS - Electrician - Trade Practical

ELECTRICIAN - CITS

EXERCISE 64 : Design Simple circuit containing power
diode & power transistor

Objectives

At the end of this exercise you shall be able to
• design circuit diagram of 12V,3A DC power supply
• test the components
• assemble the circuit of general-purpose PCB
• test the power supply.

Requirements

Tools/Equipments/Instruments
• Side cutting pliers (150mm) -1 No. • Rectifier Diode (SB 550) - 4 Nos.
• Combination pliers (150mm) -1 No. • Zener Diode (12V, 1N4742A) - 1 No.
• Electrician knife (75mm) -1 No.
• Tweezer (75mm) -1 No. • Heat Sink - 1 No.
Materials/Components • Resistor (680 ohm) - 2 Nos.
• Stepdown transformer (230V/15V) - 1 No. • Capacitors (5000μF, 50V) - 2 Nos.
• NPN Power Transistor (2N3055) - 1 No. • Capacitors (0.01μF,20V) - 1 No.
• General purpose PCB - 1 No.

Procedure

TASK 1 : Design circuit diagram of 12V,3A DC power supply
1 Calculate power output P = V X I , =12 X 3 = 36 watts.

2 Select a suiTable transformer. [considering the output DC of 12V, transformer of 15V,5A is selected, which will
supply the losses too]
3 Select diode for rectification, which should handle 3A. [SB550 is a Schottky power diode which can handle
50V and 5A]
4 Select a regulator diode for the circuit and assess. [A 12V Zener (1N4742A)can regulate 12V,however can not
act as a current source due to its power limitation]
5 Select a power transistor to amplify current. [Common base power transistor 2N3055 can handle 5A, refer
Fig 1]

6 Select a current limiting resistor to zener regulator and current to base of the transistor. [For 1N4742A the
standard test current is 21mA for 1 watt. To limit current to safe value 680ohms can be used. (DC rectified
expected 21V approx. and 12V, effectively give 9V drop across resistor.)]

7 Draw the circuit diagram. (Fig 2)

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