Autotransformer-Variac AC-DC Power Supply
Known as an autotransformer or variac this is a single winding transformer. It can be step-up or step-down. A variac can have taps or a slider. In the past these were also referred to as rheostats. Variac is a trademark of General Radio from the 1930s.
Here I am interested only in a single phase slider type variac. (Fig. 4 below.) It is useful for producing a variable AC voltage that can be rectified-filtered for a variable direct current output.
Fig. 1 upper left corner illustrates a B&K Model 1653 power supply. It incorporates a variac and an isolation transformer.
That eliminates the greatest problem with an autotransformer - lack of electrical isolation from the "hot" side of the AC line.
This not only creates a shock hazard but could damage my oscilloscope or other power line connected test equipment.
I'm doing a number of higher voltage test circuits and need higher variable DC voltage up to 180-volts DC.
Fig. 2 is a schematic for building your own isolated variable AC-DC power supply.
T2 and T1 exist within the B&K Model 1653 while the other parts are on the small green circuit board in the center of Fig. 1.
The B&K Model 1653 was sold in the early 1980s and similar boxes today are expensive, may not have an actual autotransformer.
One can buy separate parts as shown in Fig. 3 upper right corner.
Any bridge rectifier of sufficient current and voltage will work and the capacitor rating must be at least 200-volts.
If one buys the unit with no case ($30-$50+ lower right corner) and the separate isolation transformer all can be installed in a single metal case.
For more technical information I quote,
An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which have no metallic conducting path between them.
The autotransformer winding has at least three taps where electrical connections are made. Since part of the winding does "double duty", autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass.
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