Linear regulator

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In electronics, a linear regulator is a voltage regulator based on an active device (such as a bipolar junction transistor, field effect transistor or vacuum tube) operating in its "linear region" (in contrast, a switching regulator is based on a transistor forced to act as an on/off switch) or passive devices like zener diodes operated in their breakdown region. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage.

Contents 1 Overview 2 Simple zener regulator 3 Simple series regulator 4 Using a linear regulator 5 Cautions 5.1 Fixed regulators 5.2 Adjustable regulators 5.3 Other devices

[edit] Overview

The transistor (or other device) is used as one half of a potential divider to control the output voltage, and a feedback circuit compares the output voltage to a reference voltage in order to adjust the input to the transistor, thus keeping the output voltage reasonably constant. This is inefficient: since the transistor is acting like a resistor, it will dissipate heat. In fact, the power loss due to heating in the transistor is the current times the voltage dropped across the transistor. The same function can be performed more efficiently by a switched-mode power supply (SMPS), but it is more complex and the switching currents in it tend to produce electromagnetic interference. All said, most modern SMPS's have a performance that can be as good as or exceed that of linear regulators. Most computers have a SMPS to power them. A SMPS can easily provide more than 30A of current at voltages as low as 3V, while for the same voltage and current, a linear regulator would be very bulky and heavy.

Linear regulators exist in two basic forms: series regulators and shunt regulators.

• Series regulators are the more common form. The series regulator works by providing a path from the supply voltage to the load through a variable resistance (the main transistor is in the "top half" of the voltage divider). The power dissipated by the regulating device is equal to the power supply output current times the voltage drop in the regulating device.

• The shunt regulator works by providing a path from the supply voltage to ground through a variable resistance (the main transistor is in the "bottom half" of the voltage divider). The current through the shunt regulator is diverted away from the load and flows uselessly to ground, making this form even less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltage-reference diode, and is used in very low-powered circuits where the wasted current is too small to be of concern. This form is very common for voltage reference circuits.

All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the drop-out voltage. For example, a common regulator such as the 7805 has an output voltage of 5V, but can only maintain this if the input voltage remains above about 7V. Its drop-out voltage is therefore 7V - 5V = 2V. When the supply voltage is less than about 2V above the desired output voltage, as is the case in low-voltage microprocessor power supplies, so-called low dropout regulators (LDOs) must be used.

When one wants a voltage higher than the available input voltage, no linear regulator will work (not even an LDO). In this situation, a switching regulator must be used.

[edit] Simple zener regulator

The image shows a simple zener voltage regulator. It is a shunt regulator and operates by way of the zener diode's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the zener breakdown region. The resistor R1 supplies the zener current I_Z as well as the load current I_R2 (R2 is the load). R1 can be calculated as -

where, V_Z is the zener voltage, and I_R2 is the required load current.

This regulator is used for very simple low power applications where the currents involved are very small and the load is permanently connected across the zener diode (such as voltage reference or voltage source circuits). Once R1 has been calculated, removing R2 will cause the full load current (plus the zener current) to flow through the diode and may exceed the diode's maximum current rating thereby damaging it. The regulation of this circuit is also not very good due to the fact that the zener current (and hence the zener voltage) will vary depending on V_S and inversely depending on the load current.

[edit] Simple series regulator

Adding an emitter follower stage to the simple zener regulator forms a simple series voltage regulator and substantially improves the regulation of the circuit. Here, the load current I_R2 is supplied by the transistor whose base is now connected to the zener diode. Thus the transistor's base current (I_B) forms the load current for the zener diode and is much smaller than the current through R2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load. R1 sets the zener current (I_Z) and is determined as -

where, V_Z is the zener voltage, I_B is the transistor's base current and K = 1.2 to 2 (to ensure that R1 is low enough for adequate I_B).

where, I_R2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and h_FE(min) is the minimum acceptable DC current gain for the transistor.

This circuit has much better regulation than the simple zener regulator, since the base current of the transistor forms a very light load on the zener, thereby minimising variation in zener voltage due to variation in the load. Note that the output voltage will always be about 0.65V less than the zener due to the transistor's V_BE drop. Although this circuit has good regulation, it is still sensitive to the load and supply variation. It also does not have the capability to be adjustable. Both these issues can be resolved by incorporating negative feedback circuitry into it. This regulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits.

See also: Capacitance multiplier

[edit] Using a linear regulator

Linear regulators can be discrete as well as in integrated circuit form. The most common linear regulators are three-terminal integrated circuits in P1d packages/TO220 package. (The TO-220 package is the same kind that many medium-power transistors commonly come in: three legs in a straight line protruding from a black plastic molded case with a metal backplate which has a hole for bolting to a heatsink).

After one connects the appropriate pins to 0v and incoming power, the regulated output voltage appears on the output pin.

Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages), and common fixed voltages are 5 V (for transistor-transistor logic circuits) and 12 V (for communications circuits and peripheral devices such as disk drives). In fixed voltage regulators the reference pin is tied to ground, whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider (such as a potentiometer) allows the user to adjust the regulated voltage.

[edit] Cautions

It's important to follow all the datasheet's recommendations:

• Use a heat-sink on the regulator if it is going to dissipate substantial power or get very hot without one.

• Use the recommended capacitors on the input and output lines to avoid oscillations and poor regulation.

• Remember the regulator metal case is connected to one of the circuit pins on almost all regulators. The pin varies, the case is usually connected to the ground pin on most positive voltage regulators, connected to the output pin on most negative voltage regulators, but check each time to make sure.

[edit] Fixed regulators

"Fixed" three-terminal linear regulators are commonly available to generate fixed voltages of plus 3 V, and plus or minus 5 V, 9 V, 12 V, or 15 V when the load is less than about 7 amperes.

The "78" series (7805, 7812, etc.) regulate positive voltages while the "79" series (7905, 7912, etc.) regulate negative voltages. Often, the last two digits of the device number are the output voltage; eg, a 7805 is a +5 V regulator, while a 7915 is a -15 V regulator.

[edit] Adjustable regulators

For output voltages not provided by standard fixed regulators and load currents of less than 7 amperes, commonly-available "adjustable" three-terminal linear regulators may be used. An adjustable regulator generates a fixed low nominal voltage between its output and its 'adjust' terminal (equivalent to the ground terminal in a fixed regulator). The "317" series (+1.2V) regulates positive voltages while the "337" series (-1.2V) regulates negative voltages.

The adjustment is performed by constructing a potential divider with its ends between the regulator output and ground, and its centre-tap connected to the 'adjust' terminal of the regulator. The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.

[edit] Other devices

More complex regulators are available in packages with more than three pins, including dual in-line packages.