Voltage Regulators, Physics tutorial


Voltage regulators include the class of extensively utilized ICs. Regulator IC units have circuitry for reference source, comparator amplifier, control device, and overload protection all in single IC. Though internal construction of IC is rather different from that of discrete voltage regulator circuits, external operation is much same. IC units give regulation of the fixed positive voltage, fixed negative voltage or adjustable set voltage.

Power supply can be built using the transformer connected to ac supply line to step ac voltage to desired amplitude, then rectifying that ac voltage, filtering with the capacitor and RC filter, if desired, and finally regulating dc voltage using the IC regulator. Regulators can be chosen for operation with load current from hundreds of milli- amperes to tens of amperes, corresponding to power ratings from milli- watts to tens of watts.

Three-Terminal Voltage Regulators:

Fixed voltage regulator has the unregulated dc input voltage, Vi, applied to one terminal, regulated output dc voltage, V0, from second terminal, with third terminal connected to ground. For the selected regulator, IC device specification list voltage range over which input voltage can differ to maintain regulated output voltage over range of current load. Specifications also list amount of output voltage change resulting from the change in load current (load regulation) or in input voltage (line regulation).

1870_Three Terminal Voltage Regulator.jpg

Fixed Positive Voltage Regulators:

Series 78 regulators give fixed regulated voltages from 5 to 24V. Unregulated input voltage Vi is filtered by capacitor C1 and connected to IC's IN terminal. IC's OUT terminal gives the regulated +12V that is filtered by capacitor C2(generally for any high-frequency noise).

The third IC terminal is connected to ground (GND). While the input voltage may vary over some permissible voltage range and the output load may vary over some acceptable range, the output voltage remains constant within specified voltage variation limits. These limitations are spelled out in the manufacturer's specification sheets.

The ac line voltage (120Vrms) is stepped down to 18V rms across each half of center-tapped transformer. Full- wave rectifier and capacitor filter then gives unregulated dc voltage, shown as a dc voltage of approx 22V, with ac ripple of few volts as input to voltage regulator. 7812IC then gives output which is a regulated +12Vdc.

Positive Voltage Regulator Specifications:

Some consideration of important parameters must be made:

Output voltage: Specification for 7812 shows that output voltage is typically +12V but could be as low as 11.5V or high as 112.5V.

Output regulation: Output voltage regulation is seen to be typically 4mV to maximum of 100mV (at output current from 0.25 to 0.75A). This information specifies that output voltage can typically differ only 4mV from rated 12V dc.

Short-circuit output current: Typical amount of current is restricted to typically 0.35 if output were to be short- circuited (presumably by accident or by another faulty component).

Peak output current: while rated maximum current is 1.5 A for this series of IC, typical peak output current that might be drawn by the load is 2.2A. This shows that although the manufacturer rates the IC as capable of providing 1.5A, one could draw rather more current (possibly for the short period of time).

Dropout voltage: Drop out voltage, typically 2V, is minimum amount of voltage across the input-output terminals which must be maintained if IC is to operate as regulator. If input voltage drops too low or output rises so that at least 2V is not maintained across IC input-output, IC will no longer give voltage regulation. One thus maintains the input voltage enough to make sure that dropout voltage is given.

Fixed Negative Voltage Regulator:

Series 7900ICs give negative voltage regulators, like those giving positive voltages. IC regulators are available for the range of fixed negative voltages; selected IC providing rated output voltage as long as input voltage is maintained greater than minimum input value. For instance, 7912 gives output of -12V as long as input to regulator IC is more than negative -14.6V.

Adjustable Voltage Regulators:

Voltage regulators are also available in circuit configurations which permit user to set output voltage to desired regulated value. LM317, for instance, can be operated with output voltage regulated at any setting over range of voltage from 1.2 to 37V.

Resistor R1 and R2 set output to any desired voltage over adjustment range (1.2 to 37 V). Output voltage desired can be computed using:

V0 = Vref(1 + R2/R1) + IadjR2

With typical IC values of:

Vref = 1.25V and Iadj = 100μA.

647_Connection of adjustable-voltage regulator.jpg

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