Whether you’re just getting into electronics or could use a refresher on some component or phenomenon, it’s hard to beat the training films made by the U.S. military. This 1965 overview of transformers and their operations is another great example of clear and concise instruction, this time by the Air Force.
It opens to a sweeping orchestral piece reminiscent of the I Love Lucy theme. A lone instructor introduces the idea of transformers, their principles, and their applications in what seems to be a single take. We learn that transformers can increase or reduce voltage, stepping it up or down through electromagnetic induction. He moves on to describe transformer action, whereby voltages are increased or decreased depending on the ratio of turns in the primary winding to that of the secondary winding.
He explains that transformer action does not change the energy involved. Whether the turns ratio is 1:2 or 1:10, power remains the same from the primary to the secondary winding. After touching briefly on the coefficient of coupling, he discusses four types of transformers: power, audio, RF, and autotransformers.
Power transformers are designed to operate at a specific frequency to step AC voltage up or down. They usually have iron cores and one or more secondary windings. Audio transformers operate over the spectrum of audible frequencies, between ~20 to ~20,000 cycles per second, as they are called here. Radio Frequency or RF transformers are used at frequencies north of 20KHz, and are prone to hysteresis and eddy currents. Therefore, they usually have air cores.
Autotransformers use a single winding for both coils. In a step-up transformer, the turns of primary winding constitute a percentage of the windings in the secondary, and vice versa for step-down units. Some autotransformers are of variable action. These operate similarly to potentiometers, and are commonly referred to as variacs.
In conclusion, the instructor applies this knowledge by using voltage and resistance measurements to identify unknown turns ratios. Finally, he checks for open and short windings on a PSM-6 mulitmeter.
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