Collection Schematic

Many of the readers have time and again requested for inverter circuits. Given below is one such system, rated at 150 watts, complete in all respects.
The circuit works automatically when mains supply fails and switches back to charging mode when the supply is resumed. A prototype of the same has been constructed by me and is in operation for  the last 4% years, power in a 122 cm (4 feet) ceiling fan and a tube light.  The circuit shown here may be divided into two sections: The first, charger section, includes step-down transformer Xl to provide l2V—0—l2V AC. When switched on through Sl, relay RLI gets energised by the DC voltage supplied by one set of full-wave rectifier comprising D3 and D4. This  rectified DC voltage smoothed by C1 is again brought down to the required RLl coil voltage through R2 and C2. When RL1 gets energised, it connects the mains live line to the output socket through RLl’s N/O contacts.  Diodes D1 and D2 form another set of full-wave rectifier which charges the battery through resistor R1 and the N/ C  contacts of relay RL2.

The value of Rl should be so chosen as to give the desired current level. lt is advisable to have two or three resistors in parallel, each rated 25W. A 20 ampere meter may be connected between the battery`s negative terminal (after the fuse) and the RL2’s main  contact to monitor the charging rate as well as the consumption during inverter operation. When the AC mains supply fails, relay RL1 gets deenergised and connects the live line of the secondary of the inverter transformer X2 to the output socket through its N/ O contacts. At the same time, RLl causes RL2 to operate  through its N /C contacts. (lt is assumed that S2 is in on position.) ln this circuit the transistors operate as switches. Resis tors R3 through R6 are used to provide the requisite base drive through the feedback winding for starting the oscillations. , Slight imbalance in the two halves of the primary winding of X2 as well as in the component values, causes one set of transistors to conduct. Say, for example, T1 is conducting  while T2 is off thus the supply to the top half of the primary winding through the RL2 contact  (Of course it is assumed the mains supply is off and that switch S2 is in ON position) When T1 is in full Conduction, heavy current flows in the transformer which saturates the core causing the magnetic field to collapse as also the polarities of all the windings in X2 to reverse and also the switching positions of the transistors too. Because of the auto-transformer action, a potential of twice the supply voltage is developed across the total primary winding of X2, which is stepped up by the secondary winding. This process continues. R7, C4, D5 and D6 are used to protect the transistors from spikes while C3 and C5 are used to improve the oscillations. Resistors R8 and R9 have been incorporated to facilitate distribution of current equally amongst the transistors. The value of R8 and R9 is around 0.05 ohm each; these can be made by taking about 1.75 metres of 20S WG enamel wire and winding it over a pencil to give it the shape ·of a coil.


Some useful hints regarding the proposed inverter with charger changeover circuit

 1. On switch on (S2), if the circuit does not function, reverse the feedback winding connection of one set (or both, if required). In other words, if the beginning of L2 is connected to emitter of Tl , the other end of feedback winding  L1 should be connected to the base of T2.

2. Connection from the battery terminals to the chassis tag should be as short as possible, and the wire used must be rated to carry 13 to 14 amperes.

3. Wherever heavy current flows, i.e. from the tag to the RL2 contacts and from there to emitter resistors (R8 and R9) through the centre—tap of X2, wiring must be done by 15SWG wire.

4. The no—load current is 5.5 to 6.5 amperes, depending upon the base drive. For a 122 cm ceiling fan load, it will rise to 8.5 to 9 amperes, and for both fan and tube light it will rise to around 12 to 13 amperes. lf it is more or less, adjust the values of R4 and R5 suitably.

5. Efficiency will be around 80 per cent on full load.

6. Sometimes the tube light may not light immediately on switch on, or it may start blinking when the fan is working. In such an event, a 0.68 F paper condenser rated at 400V in series with a push button switch may be connected (as shown by dotted lines) across A and B terminals of X2. However this is not necessary when it automatically takes over upon failing of mains. provided S2 and tube light switch are kept closed.

 A 12V 5 plate lead-acid battery used in automobiles is the minimum requirement for this system. This battery can be used for about four hours continuously at full load. Since the battery is used in inverter system, it never attains full charge, even when put under charge for long hours. As these batteries are not intended for cyclic operation, only 75 per cent of the stored energy can be used. Deep discharge or over-charge should be avoided.