An Inverter circuit converts DC to high volt AC for driving high power devices. The principle of simple emergency lamp circuit and the heavy load inverter circuit is the same. The DC voltage is converted into AC using an oscillator circuit. A driver transformer then step up this low voltage AC to 230 volt AC. The output waveform from the oscillator depends on the type of circuit. In simple inverter, it is square wave but in microprocessor based types, it may be quasi sine wave or pure sine wave. Pure sine wave AC is similar to the AC available in the mains power supply so all the instruments usually running in Mains power can be connected in Pure sine wave inverter. The appliances connected to Square wave and quasi sine wave may not work well as seen in the humming of fan, flickering of tubes etc. This circuit is meant for explaining the method of an inverter.
The circuit has an Oscillator consisting of a High power NPN transistor TIP 3055, 120 ohms resistor and 0.1 polyester capacitor. R1 provides base bias to T1 and C1 couples the positive AC feedback from the winding of the transformer. Battery used is 6 volt 4.5 Ah emergency light battery. When S1 is closed, T1 oscillates and the 6V DC will be converted into low volt AC. To step-up this low volt AC, A Step-down transformer is used in the reverse direction. Usually the primary of the step-down transformer is connected to Mains. Here by reversing the connection, the 6-0-6 transformer works as a step-up transformer. You can also use a driver transformer shown in the image. The 6 watt tube ( one used in emergency lamp) can be connected to the primary of the transformer. The quality of output is not so good, but still you can experiment.
Two things are important.
1. Do not touch the primary of transformer in the charged state. It will give shock.
2. Do not switch on the inverter without load. It will damage the transistor.
The tube has two pins on each side. Short the pins on each side and connect to the primary.
Pins of TIP 3055 are Base Collector and Emitter from front side. Its Tab is also collector.