Fig 1719a and b illustrate how the regulators are

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Fig 17.19A and B illustrate how the regulators are used in the conventional mode. Several regulators can be used with a common-input supply to deliver several voltages with a common ground. Negative regulators may be used in the same man- ner. If no other common supplies operate from the input supply to the regulator, the circuits of Fig 17.19C and D may be used to regulate positive voltages with a nega- tive regulator and vice versa. In these con- figurations the input supply is floated; neither side of the input is tied to the sys- tem ground. Manufacturers have adopted a system of family numbers to classify three-ter- minal regulators in terms of supply polar- ity, output current and regulated voltage. For example, National uses the number LM7805C to describe a positive 5-V, 1.5-A regulator; the comparable unit from Texas Instruments is a UA7805KC. LM7812C describes a 12-V regulator of similar characteristics. LM7905C de- notes a negative 5-V, 1.5-A device. There are many such families with widely var- ied ratings available from manufacturers. Fixed-voltage regulators are available with output ratings in most common val- ues between 5 and 28 V. Other families include devices that can be adjusted from 1.25 to 50 V. Regulator Specifications When choosing a three-terminal regula- tor for a given application, the most impor- tant specifications to consider are device output voltage, output current, input-to-out- put differential voltage, line regulation, load regulation and power dissipation. Output voltage and current requirements are deter- mined by the load with which the supply will ultimately be used. Input-to-output differential voltage is one of the most important three-terminal regulator specifications to consider when designing a supply. The differential value (the difference between the voltage ap- plied to the input terminal and the voltage on the output terminal) must be within a specified range. The minimum differen- tial value, usually about 2.5 V, is called the dropout voltage. If the differential value is less than the dropout voltage, no regulation will take place. At the other end of the scale, maximum input-output dif- ferential voltage is generally about 40 V. If this differential value is exceeded, de- vice failure may occur. Increases in either output current or dif- ferential voltage produce proportional in- creases in device power consumption. By employing a safety feature called current foldback, some manufacturers ensure that maximum dissipation will never be ex- ceeded in normal operation. Fig 17.20 shows the relationship between output current, input-output differential and cur- Fig 17.19 — Parts A and B illustrate the conventional manner in which three- terminal regulators are used. Parts C and D show how one polarity regulator can be used to regulate the opposite-polarity voltage.
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Power Supplies 17.17 rent limiting for a three-terminal regula- tor nominally rated for 1.5-A output cur- rent. Maximum output current is available with differential voltages ranging from about 2.5 V (dropout voltage) to 12 V.
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  • Fall '15
  • Rectifier, output voltage, Power supplies

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