How do you create an AC current that changes directions using transistors?
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We build an inverter for this purpose. The basic principle is that we need to build a circuit where the electricity flows one way and then back the other.
For a basic inverter, we start with a DC power source. Attach the high voltage end to two switches in parallel.
Connect the output of one switch to the top of a coil of wire. Connect the output of the other switch to the bottom of that coil of wire. Connect the center of that coil back to ground. That coil is an inductor.
Close – but not connected – to that coil is another coil. The paired coils form a transformer.
If you set the switches to opposite positions (one on, one off), every time you flip both switches, the current flowing through the coil will switch and you’ll get a corresponding current through the paired coil.
Since flipping both switches also switches the direction of the current, this is translated through our transformer to the far coil and we get something akin to a square wave on the other end of the transformer.
Since transistors are really just electronically controlled switches for our purposes, we can just attach them to a timing circuit to ensure they switch on schedule to produce AC at the other end.
If you’re not particularly concerned about power, you could also make a simpler circuit with a single transistor. You simply have it turn a DC voltage on and off at a timed interval – think of rapidly flicking a light on and off – and your steady DC voltage becomes an AC waveform (albeit one that you’d need to fiddle with quite a bit to get into the shape/power that you want).
If you have two wires that you what to have AC in but you only have a battery you could do that by connecting the battery in one direction and physically rotating it so what wire the battery poles are connected do are reversed. Repeat that and you have AC from DC.
You can simplify it by having switches that change the connection direction when you flip it up and down. You need two 3 poles switches and connect them so both are switched at the same time. By continuing flipping the switch you create AC from DC
A transistor can be looked at as an electrically controlled switch. So replace the switch with four transistors and add a control signal that opens and closes them in the appropriate sequence.
You have now created AC from DC. It will be a square wave, not a sine wave but it is still AC and show the general idea.
You can look at it as a [full bridge rectifier](https://lastminuteengineers.com/the-full-wave-bridge-rectifier/) used to make DC from AC. but run in reverse. That requires diodes to be replaced by transistors and an extra control signal is needed but it is the same idea in reverse.
One way is an oscillator, wchich is a circuit that feeds back its output inverted (positive to negative or vice versa) to its input, with a small time delay. So a circuit with a positive input produces a negative output which feeds back to the input and switches it over again (and again….) The output is then switching positive to negative etc all the time and so the current goes back and forth.
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