A relay power inverter converter simple and easy inverter make circuit to convert 12V to AC voltage that can drive some small 230V AC loads like some low power lamps (LED or fluorescent energy saving bulbs). It is often advertised as a simple and easy electronics diy projects make at home.
The basic idea of relay inverter is to use a relay as oscillator and switching components. The voltage is converted up with a mains transformer, typically 12V to 230V. When relay switches the power on 12V coil on and off, the 230V coil gives around mains voltage out. Relay based design are the very basic of inverter, that output square wave (with potentially some high voltage spikes without load), works only with some bulbs. This crude inverter output is not suitable for driving any expensive electronics, because the crude output waveform can damage sensitive electronics. The output power is typically pretty low few watts, but it still can give a nasty shock or kill if touched.
Most relay inverter oscillator designs are based on using the relay hysteresis: A relay that has tripped on at 100% won’t go off until the driving signal drops below 30%, while a relay that is off at 0% won’t trip on until the signal reaches 70%. Some designs use just transformer characteristics and some design use an additional timing capacitor.
Relays tend to be quite reliable in benign environments, however they have a limited lifetime. Typically something like 50,000-100,000 operations at full rated load. At lighter loads, the life will increase, generally up to many millions of operations with a negligible load (the so-called mechanical life). All this information will be clearly given in any decent datasheet. At the power inverter application I would expect the lifetime to be quite limited, maybe just hours tens of hour of use! Not very practical. Expect that sooner or later the relay contacts welded shut or relay in other ways damaged because the relay contacts will generate lots of sparks in this application (sparks will also generate radio frequency interference).
Here are some relay inverter plans links I found on internet (no guarantee that they work well or at all):
How to Make Inverter Using Relay
How to make Relay Inverter dc 12 to 230v, truth behind relay inverter
using relay inverter is safe?