Tecnico

circuit

ideas

TripLe power SuppLy


s.c. dwiv

edi

Sandip Trivedi and p LeLe .d.

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his low-cost, multipurpose power supply fulfils the requirements of almost all laboratory experiments. Nonetheless, it can be easily fabricated by hobbyists. A single transformer is used to build this triple power supply. Regulator IC LM317 generates variable power supply of 1.25 to 20V, 1A.The dual ±12V, 1A power supply is generated by regulators 7812 and 7912. Similarly, dual ±5V, 1A power supply is generated by regulators 7805 and 7905. ‘On’/‘off’ switches (S2 through S4) select the required power supply. Variable power supply is used to study the characteristics of devices. Fixed +5V power supply is used for all digital, microprocessor and microcontroller experiments. Dual ±12Vpower supply

is used for op-amp-based analogue circuit experiments. Fig. 1 shows the circuit of the triple power supply, while Fig. 2 shows the pin configuration of the regulators used in the circuit. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. The transformer output is rectified by full-wave bridge rectifier BR1, filtered by capacitors C1, C2, C3, C7and C8, and regulated by IC1 through IC5. Regulator IC1 (LM317) provides variable voltages (1.25 to 20V), while IC2 and IC4 provide regulated +12V and –12V, respectively. The output of IC2 is fed to regulator IC3 (7805), which provides fixed +5V. Similarly, the output of IC4 is fed to regulator IC5 (7905), which provides fixed –5V. Capacitors C4 through C6, and C9 through C11, are used for furtherfiltering of ripples
HEAT SINK IN S2

in positive and negative regulated power supplies. LED1 glows to indicate that +5V is available, while LED2 indicates that –5V is available. Switch S1 is used for mains ‘on’/ ‘off’. Using switches S2 through S4, any of the three supplies can be independently turned off when not required in a particular experiment. This reduces unnecessary power dissipationand increases the life and reliability of the power supply. Since the circuit uses three terminal regulators, only capacitors are required at the input and output. The use of few components makes the circuit very simple. The three terminal regulators have heat-sink provision to directly deliver 1A output current. To ensure the maximum output, do not forget to

S2 = FOR VARIABLE VOLTAGE S3 = FOR+12V AND +5V S4 = FOR –12V AND –5V S1-S4 = ON/OFF SWITCH C3 0.1µ

3

IC1 LM317 2
1 ADJ.

OUT R1 120

+1.25 TO 20V

VR1 2.2K POT HEAT SINK IN HEAT SINK OUT IN

GND +12V

S1 ON/OFF SWITCH

F1 1.5A FUSE L 230V AC 50Hz N

X1
BR1 W04
BR1 W04 C1 1000µ 35V

1

S3

IC2 7812
2

3

1

IC3 7805
2

3

OUT

GND C4 100µ 25V

GND C5 10µ 16V

R2 330 C6 0.1µ +5V LED1C2 0.1µ

GND X1 = 230V AC PRIMARY TO 18V-0-18V, 1.5A SECONDARY TRANSFORMER C7 1000µ 35V C8 0.1µ GND 1 BR1-W04 1.5A, BRIDGE RECTIFIER S4 IN 2 C9 100µ 25V GND 1 3 OUT IN 2 LED2 3 OUT C10 10µ 16V R3 330 –5V

C11 0.1µ

IC4 7912

IC5 7905

HEAT SINK

HEAT SINK

–12V GND

Fig. 1: Tripple power supply w w w. e f y M ag . co M e l e c t ro n i c s f o r yo u • M a r c h 2 0 0 9 • 8 1circuit

ideas

Fig. 2: Pin configurations of regulators

use heat-sinks for the regulators. The three-terminal regulators are almost non-destructible. These have inbuilt protection circuits including the thermal shutdown protection. Even if there is overload or shorting of the output, the inbuilt overload protection circuit will limit the current and slowly reduce the output voltage tozero. Similarly, if the temperature increases beyond a certain value due to excessive load and heat dissipation, the in-built thermal shutdown circuit will reduce the output current and the output voltage (gradually) to zero. Thus complete protection is provided to the circuitry. Assemble the circuit on a generalpurpose PCB and enclose in a box as shown in Fig. 3. The step-by-step procedure to...