• DURING THE PAST FEW YEARS
selenium rectifiers have come into wide use as a means for obtaining d-c output voltage from a-c supply lines. They are extremely simple, rugged, compact, and efficient, and they have practically unlimited life. Being conservatively rated, they will carry heavy overloads without damage. In most of its applications the selenium rectifier is chosen to do a particular job, wired permanently into a circuit, and then forgotten. However, there are many cases, especially in standardizing laboratories and in development work, when a continuously variable source of d-c voltage is very useful. Storage batteries are not too convenient and usually require charging just when they are needed. Furthermore, the output voltage of a storage battery is not easily adjustable. The development engineer often needs varied d-c voltage
supplies. His requirements may be 1.0 volt at 4.5 amperes filament supply for a power oscillator today, 10.0 volts at 20 ma for a bridge supply tomorrow, 6.0 volts at 5 amperes to test a small d-c constant-speed motor the next day, and so on for a large number of relatively short-time applications, no two of which have the same requirements.
It was to meet these needs that the Type 1260-A Variac-Rectifier was developed. This power supply is a combination of a Type 200-B Variac, a step-down transformer, a selenium rectifier, a filter, and an output voltmeter arranged electrically as shown in the name-plate wiring diagram (see Figure 1).
By supplying continuously adjustable primary voltage between 0 and 115 volts to the step-down transformer, the Type 200-B Variac provides extremely smooth control of the d-c output voltage from 0—15 volts. A maximum power of 40 watts may be drawn from the power supply between 7 and 10 volts, but the maximum current of 6 amperes should not be exceeded at any voltage. There are many applications where the unfil-tered d-c output from the rectifier would be adequate; however, the possible uses of the power supply are increased many fold if the d-c output is filtered so that the a-c ripple is a small part of the output. The output filter of the Type 1260-A Variac-Rectifier reduces the ripple or hum voltage to a value low enough for most applications. The d-c internal output impedance of the power supply is approximately 0.6 ohm. This means that there will be an increase in output voltage of about 2.4 volts if a load drawing a current of 4 amperes is removed.
The Type 1260-A Variac-Rectifier is a compact ami convenient power unit. It is mounted in a sturdily constructed steel case; it is light in weight and easily portable. The various components have been carefully selected to obtain maximum output ratings and yet insure long life and trouble-free operation. Several of these units, built a few years ago, have been used in many ways in our standardizing laboratory and in our engineering laboratories as replacements for storage batteries. They have been operated for long periods at maximum ratings with no appreciable drift in output voltage. These power supplies were designed to be convenient, useful sources of low-voltage d-c power, and our experience with them over the past few years has proved that the design objectives were well met.
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