AC Power Supply Technologies Input Circuits Inductive Capacitive Resistive

When commercially generated power first became available, nearly all power supplies working off the standard AC (Alternating Current) line power used to employ a transformer. Transformers are simple, easy to make, and when properly made, very long life devices. They work without moving parts, and offer many great benefits. The most important of this is the isolation from the dangerous direct connection path of the life threatening high voltages of the power line.

Transformer type power supplies as shown on Figure 1 always present an inductive load to the input power source. Power companies are unhappy about inductive loads on their distribution lines, as the inductive current causes more heat loss for their systems.

Figure 1. Typical inductive power supply input circuit

Transformers store energy in the iron core and they are surprisingly efficient. Larger transformer units used by the power suppliers in distribution networks, often referred to as the power grid, exhibit efficiencies often as high as 99%. Unfortunately smaller transformers are much less efficient.

Transformers have two major losses. In the iron core there is a magnetizing loss, and in the wire coils are an ohmic loss which is proportional to the wire resistance. In a well-designed transformer, which is now a (lost) art, it is possible to optimize these losses. Small transformers have significant losses. The output voltage of a transformer is proportional to the primary (the input side) and secondary coil (output) winding's turn ratio. By properly selecting this ratio, nearly any output voltages and currents are easily generated. At the vacuum tube anode voltages used to be hundreds of volts and the tube heaters required low voltages with high current. With separate windings, transformers easily supplied all of the many different required voltages.

Currently, almost all power supplies are made without the "costly" transformer. As shown on Figure 2 this type of circuits present a capacitive loading toward the power line, much to the delight of the power companies whose transmission lines, and the loads on them are mostly inductive. Eliminating the power transformer increases the overall efficiency of power supplies.

Figure 2. Typical capacitive power supply input circuit

Capacitive input power supplies often called universal or switching power supplies, as they can work equally well on both AC and DC (Direct Current) line voltages.

Capacitors store power in their dielectric. As the AC line voltage is a 60 Hz sinusoidal wave, the input current changes direction sixty times every second. Thus when a capacitor is connected across the power line, it is repeatedly changing it's polarity. That relentless action results in a loss in the capacitor's dielectric in a form of heat. After more or less time the capacitors would fail as their dielectric breaks down. In most capacitive input power supply circuits there is a semiconductor diode rectifier. Because of this device the capacitor charges up to the maximum line voltage in the first few cycles and it will experience only a small AC component, an AC ripple current.

Thomas A. Edison was convinced of the superior merit of the DC power distribution. DC power supplies are presently making a comeback, still currently DC power is very rare. At present power generation is a terribly inefficient process. About 35 to 60 percent of the input energy heats up the environment, but even the remainder is not completely available to us, as the power grid loses another 20 to 40 percent.

DC power used to be available in larger (inner) cities. Back then with DC, there was no easy and efficient method available to obtain lower or higher voltages. It was not an efficient or easy to use system. Often motor generators or rotary converters or in more modern times DC-to-AC converters were employed. "Universal" radios were made with all their vacuum tube filaments connected in series.