Suppose the power goes off, and all you have on hand is a 12 V car battery. Can you use it to power your refrigerator so the food doesn't go bad? Unfortunately the answer is no, because you're missing something important, and we're not just talking about a receptacle for the plug. You need a device that will convert the DC power from the battery to AC power that can operate the refrigerator's compressor.
This DC to AC converter is called an inverter. It's fairly easy to convert AC current to DC – all you need to do is feed the current through a diode, which only passes current in one direction. Converting from DC to AC is more complicated, because you need some kind of oscillator that reverses the current direction at the frequency you need. There's a way to do this mechanically, but most inverters rely on resistors, capacitors, transistors and other circuit devices.
An inverter needs one more thing: a way to alter the voltage of the current source for use by the device that will be using the power. In other words, it needs a transformer. For example, if you're powering your 120 V refrigerator with a 12 V battery, the inverter needs a step-up transformer that increases the voltage by 10 times. Since it only works with AC current, the transformer goes in the circuit after the components that alter the current from DC to AC.
What Are AC and DC Current?
Most people learn about DC current in their introduction to electricity, and the best way to visualize it is to think of a battery. If you connect the battery terminals with conducting wire, electrons flow from the negative terminal to the positive one, much like ants following each other as they forage.
If you place a load such as a light in the circuit, the electrons flow through the load and do work on their way to the positive terminal. In the case of a light bulb, the work is to heat the filament so that it glows.
Instead of flowing in a single direction, AC current reverses direction many times per second, and that's due to the way it's generated. Making use of electromagnetic induction, a phenomenon whereby a changing magnetic field produces an electric current in a conducting wire, an AC generator makes electricity with a spinning rotor and a coil of conducting wire. In one version, the rotor is a permanent magnet, and as it spins, it generates a current in the coil that changes direction with every half spin of the rotor.
AC current doesn't move through the wire in the same way DC current does. The best way to think of it is as if the electrons in the wire are vibrating in place. During the first half-spin of the rotor, electrons move in one direction, and during the second half spin, they move the other way.
If you plot the movement of a single electron versus time, it will generate a waveform known as a sine wave. The frequency of the wave is governed by the speed of rotation of the generator rotor.
A Simple Mechanical DC to AC Converter
A device that can change DC to AC current must be able to switch off the current going in one direction and send it the other way, then reverse the process at regular intervals. A way to do this would be to place a rotating wheel between a pair of terminals and arrange the contacts so the wheel alternated the battery connections with every spin. The current would flow one direction when the wheel was at its starting point and in the opposite direction when the wheel had rotated 180 degrees.
Such a crude setup would produce an all-or-nothing current in each direction, and if you graphed the movement of an electron in the circuit, you would get what's known as a square wave. This wouldn't be a good power inverter for the home. The current might be able to perform simple tasks, such as making a heating element glow, but it wouldn't work for sensitive electronic equipment. Moreover, you'd need an accurate way to control the rotation of the wheel to make the resulting AC power useful.
Inverters Use Circuit Components to Change Current Direction
Rather than spinning wheels, commercial inverters make use of circuit components such as capacitors, resistors and transistors. A common DC to AC inverter schematic shows parallel circuits with transistors in series with resistors and cross circuits with capacitors and power transistors, or MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). Another type employs a Wien bridge oscillator, which is constructed with resistors and capacitors.
Both of the inverters described above are pure sine wave (PSW) inverters, and the signal they generate can be used by all electronic devices. If you're looking for a power inverter for the home, you need a PSW inverter, because it will work with the electronic components in your stove, dryer, washing machine and other appliances.
The other type of DC to AC converter is a modified sine wave (MSW) inverter. It employs cheaper components, such as diodes and thyristors, which are similar to transistors. The signal from an MSW inverter is like a square wave with its corners slightly rounded, and while it can power large appliances, it isn't suitable for electronic equipment. It would be the best power inverter for a car, making the battery available for power tools and car repair equipment.
One More Thing: the Transformer
Even if you convert the signal from a DC power source, such as a battery or solar panel, to AC, the voltage won't be large enough to power a 120 V appliance. Fortunately, it's easy to step up AC voltage. All you need is a transformer, which also operates on the principle of electromagnetic induction.
The operation of a transformer is simple. Two conducting coils are placed side by side – or one inside the other – and the current passing through one coil, called the primary coil, induces a current in the other, which is the secondary coil. The ratio of the currents in the two coils as well as their voltages is governed by the difference in the number of turns in the coils.
If the secondary coil has more turns than the first, the transformer will step up the voltage by an amount equal to the number of turns in the secondary coil divided by the number of turns in the primary coil.
You can design an inverter to supply any voltage you want, but if you want a DC to AC converter that will turn your 12 V car battery into a 120 V power source for your home, you need to make the ratio between the primary and secondary 1 to 10. Commercial inverter transformers have hundreds of turns, and the wires generate resistive heat, so the inverter needs fins – and possibly a fan – to keep cool. Moreover, the coils are sometimes wound around a solid core to make for more effective induction, and that can make the inverter very heavy.