OBJECTIVE

The objective here is to build a dual power supply that generates regulated +12 volts and -12 volts from 230VAC mains. Such a supply is a very common requirement in all those circuits that use opamps. Since opamps are very widely used in a variety of circuits of hobbyists’ interests, construction of this project could serve as a very useful tool in testing all those circuits that need a dual supply. Each of the outputs in the circuit shown in Fig.8.1 has a current delivering capability of 250mA. You would also discover this circuit to be an integral part of the more complex circuits that are mains operable.

 

CIRCUIT DESCRIPTION

The unregulated AC/DC power supply part of the circuit consists of a transformer (T-1) that steps down 230VAC to 15 volts across a center tapped secondary winding i.e., 15VAC individually across the two halves of the secondary winding with opposite polarities, diodes (D1) to (D4) that rectify the AC appearing across the secondary with (D1) and (D3) providing ‘full wave rectification to produce a positive output, (D2) and (D4), providing full wave rectification to produce a negative output, capacitors (C1) and (C2) providing the filtering action. C3to C8 are decoupling capacitors. lC-1 is a fixed output positive three terminal regulator whereas IC-2 is a fixed output negative three terminal regulator

PARTS LIST

Capacitors

C1, C2 : 1000  50V (electrolytic)

C3, C4, C7, C8 : 0.1  (ceramic disc)

C5, C6 :  50V (electrolytic)

Diodes

D1 to D4 1N4001 or equivalent

ICS

IC-1: 7812

IC-2: 7912

Miscellaneous

TransformerT-l:15 -0-15voIts, 250mA mains transformer.

Fuse F-1:500 mA tubular type with holder

Switch SWI : Mains ON/OFF switch

Solder metal, wires, power supply terminals, mains cord Figs. 8.2 and 8.3 respectively show the PCB layout as seen from component side and We components layout.

TESTING GUIDELINES

Testing part of this circuit is almost self evident.

You have to only switch ON the-AC power and then measure the two output regulated voltages with a millimeter. The load delivering capability of the supply can also be verified by connecting a resistance of 47 ohms, 5W across each of the two outputs as shown in Fig.8.4 and keeping the circuit ON for ten to fifteen minutes.

See that the transformer and IC regulators do not get excessively heated up and also that the regulated output voltages stay put. The regulation part can also be checked by feeding the AC input to the circuit from a VARlAC (Variable Autotransformer).

You can verify that the DC outputs stay put at +12V and -12V for AC input right from 160V to 320V The test set-up is shown in Fig.8.5.

IDENTIFYING THE LEADS

Refer to Figs.8.6 (alto (c) for lead identification of IC-1, 1C-2 and diodes D1 to D4.

OTHER VARIATIONS OF THE CIRCUIT

The same circuit configuration can be used to produce (+5V, -5V), (+9V, -9V), (+15V, -15V), (+18V, -18V) and (+24V, -24V).What you needto do is to use different type nos. for the two regulators and a different transformer. The table Table 8.1 gives the details.

Desired Output                         T-1                                                     IC-1                                                 IC-2

+5V,-5V                                 7.5-0-7.5,250mA                                7805                                                 7905

+9V,-9V                                  12-0-12 ,250mA                                  7809                                                 7909

+15V,-15V                            15-0-15 ,250mA                                    7815                                                  7915

+18V,-18V                            18-0-18 ,250mA                                    7818                                                 7918

+24V,-24V                           24-0-24,250mA                                    7824                                                  7924

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