In this article we are going to study a simple full-bridge inverter circuit using IC 555, which can be built by any new hobbyist within few hours and see it running successfully.
What is Full Bridge Inverter
A full bridge inverter is also referred to as an H-bridge inverter circuit usually requires a relatively complex circuitry for accomplishing its working conditions. It cannot be achieved through simpler means because this type of topology demands a two way push pull operation of the connected transformer, which is a two wire transformer and not a center tap one.
A full bridge topology is often the preferred topology and is quite popular because in this configuration the inverter works with only a single primary winding and is able to utilize it with maximum efficiency through a full end-to-end push-pull conduction of the winding.
This enables the transformer to be smaller and compact in size and yet deliver better efficiency compared to the center tap topology where only 50% of the primary is utilized during each magnetic switching cycle, which results in a 50% less efficiency than a full bridge counterpart.
Normally it is thought that full bridge inverter can be built only with special ICs or using complex circuit configurations. In this article we will try to figure out how a full bridge inverter can be built using a simple IC 555 circuit as shown below.
However this high efficiency full bridge topology comes with a drawback, here we have to employ two batteries instead of one. But two batteries also means higher amount of backup, so in fact two batteries become beneficial and not a drawback in the long run.
How the Circuit Functions
As you can see the shown IC 555 full bridge inverter circuit is extremely simple and yet is capable of delivering some serious amount of output power with a relatively smaller transformer
The IC 555 is rigged as a standard astable multivibrator, for generating a constant 100 Hz frequency at its pin#3.
This frequency is applied to a buffer transistor stage using 2N2222.
The collector of this buffer transistor is connected to a push pull driver power transistor stage using a couple of Darlington transistor TIP142 and TIP147.
The TIP142/147 are high gain Darlington power transistors capable of supplying around 10 amp current to a connected load.
Here the load is in the form of transformer primary winding, and therefore the power BJTs deliver the full 10 amp across the primary of the transformer in a push pull manner, quite similar to a H-bridge inverter circuit.
You can also see a 1000uF/25V capacitor connected in series with the transfromer, this is to ensure that the power transistor do not get overdriven or overloaded under any circumstance and thus protect the BJTs from a catastrophic situation.
This capacitor also determines the maximum amount of current that can pass into the transformer and thus consequently decides the power rating of this Full bridge inverter circuit.
The user may feel free to increase the value of this capacitor in order to upgrade the power of this IC 555 based simple full bridge inverter circuit.
Simplifying the Circuit
A careful inspection of the circuit suggests that the BJT 2N2222 has no crucial role in the operation of the inverter and therefore can be simply eliminated.
The simplified version of the circuit can be witnessed below, which now looks a truly easy to build IC 555 H-bridge inverter circuit.
Prototype Circuit and Working Results