Electronic Transformers

Although no exact line of demarcation can be drawn between power transformers and electronic transformers, in general electronic transformers are smaller. The source of power on a 60-cycle network is extremely large and may be the combined generating capacity of half a continent. Power in electronic equipment is limited to the capabilities of electron tubes, of which even the largest is small compared to a power station generator.

Transformers are needed in electronic apparatus to provide the different values of plate, filament, and bias voltage required for proper tube operation, to insulate circuits from each other, to furnish high impedance to alternating but low impedance to direct current, and to maintain or modify wave shape and frequency response at different potentials. The very concept of impedance, so characteristic of electronics, almost necessarily presupposes a means of changing from one impedance level to another, and that means is commonly a transformer.

Impedance levels are usually higher in electronic, as compared with power, equipment. Consider the connected kva on an 11,000-volt power line; it may easily total 1,000,000. Compare this with a large broadcast transmitter operating at the same voltage and drawing 70 kva. The currents in the two cases are 90,000 amp and 6 amp, respectively. For the power line, the load impedance is 11,000/90,000, or slightly more than 0.1 ohm; for the transmitter it is 11,000/6, or nearly 2,000 ohms. Source impedances are approximately proportional to these load impedances. In low-power electronic circuits the source impedance often exceeds the load impedance and influences the transformer performance even further.

Weight and space are usually at a premium in electronic equipment, and reliability is of paramount importance. Transformers account for a considerable portion of the weight and space, and form a prime component of the reliability.

These and other differences of application render many power transformers unsuitable for electronic circuit use. The design, construction, and testing of electronic transformers have become separate arts, directed toward the most effective use of materials for electronic applications.