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# Reverse Polarity

Electric current, the flow of electrons that powers our lives, can be classified into two broad categories. Direct current (DC) is a unidirectional flow of charged particles. The current and voltage can be time variant or invariant, but the polarity remains fixed. Plotted versus time, DC often appears as a flat line, though it can change with time so long as it never crosses zero. For example, the DC found in modern digital circuits might oscillate millions of times per second.

A 60 Hz AC signal (top) and a DC signal (bottom)

In contrast, the flow of electrons in an alternating current (AC) periodically reverses direction. That is, the polarity of the current reverses cyclically. Plotting an AC waveform results in a repeating pattern that crosses zero with each cycle. The number of cycles per unit time is known as the current’s frequency. In North America, the power grid operates at 60 Hz; it reverses polarity 60 times every second. The shape of an AC waveform is most typically sinusoidal, but other profiles are possible and can be utilized for specific applications.

Although both AC and DC can be found in everyday appliances, the electricity supplied by the electrical grid to outlets in your home is AC. In contrast, the battery powering your laptop computer supplies DC. The convention of using AC for power distribution arose in the latter decades of the nineteenth century. Thomas Edison, credited with the invention of the incandescent light bulb, was an early and vehement proponent of DC for power distribution. Nikola Tesla, inventor of the induction motor, along with backer George Westinghouse, advocated for the adoption of AC for this purpose. Through a series of public demonstrations, a bitter rivalry arose from this conflict, with Edison in particular insisting that his competition’s product was fundamentally unsafe, a principle he demonstrated by electrocuting various animals with high voltage AC.

Eventually it became clear that the fundamental advantage of AC was indisputable; the voltage of an AC system can be efficiently and economically scaled up or down using a transformer. An electrical grid utilizing AC can, with relative ease, operate at a variety of voltage levels. In order to move beyond small-scale local grids, this feature was requisite. High voltage is needed for the transmission of electrical energy over significant distances, while for safety and practicality, a relatively low voltage is required in homes to power appliances. The technology of the day made it impossible for such a system to be built using DC. During this time period, the conversion between DC and AC was fraught with difficulty and highly inefficient in both directions.

A variety of innovations and technologies, most notably the rise of semiconductors and integrated circuits, has made the conversions between AC and DC both cheap and efficient. While some electrical power sources, such as batteries or photovoltaic (solar) cells produce only DC power, others such as generators produce AC. Therefore, even today, it is frequently necessary to convert between AC and DC to suit a specific electrical load’s requirements. An inverter is used to convert DC to AC; using electronic switches, the polarity of a DC supply is rapidly reversed with respect to the load, providing alternating current. Inverters are used for a wide variety of applications, including photovoltaic installations and electric vehicles.

The conversion from AC to DC is comparatively uncomplicated. An arrangement of diodes, devices that allow current to pass only in one direction, are used to convert the AC into a lumpy DC signal. Such a device is known as a rectifier. The output of the rectifier is subsequently connected to an array of smoothing capacitors to achieve a constant DC voltage, which can then be connected to the desired load. Rectifiers are found in almost every electrical device we use today as they are an integral part of the switch mode power supplies that provide electricity to most modern consumer electronics. Rectifiers and inverters are available to suit an extremely broad range of applications and power levels.

At the Solar Initiative, we believe that the electrification of our personal energy footprint is one of the keys to combatting the climate crisis. Both AC and DC have played and will continue to play critical roles in this process.