In electronics, a linear regulator is a system used to maintain a steady
voltage. The resistance of the regulator varies in accordance with the load
resulting in a constant output voltage. The regulating device is made to act
like a variable resistor, continuously adjusting a voltage divider network to
maintain a constant output voltage, and continually dissipating the difference
between the input and regulated voltages as waste heat. By contrast, a switching
regulator uses an active device that switches on and off to maintain an
average value of output. Because the regulated voltage of a linear regulator
must always be lower than input voltage, efficiency is limited and the input
voltage must be high enough to always allow the active device to drop some
voltage. Linear regulators may place the
regulating device in parallel with the load or may place the regulating device
between the source and the regulated load. Simple linear regulators may only
contain a Zener diode and a series resistor; more complicated regulators
include separate stages of voltage reference, error amplifier and power
pass element. Because a linear voltage regulator is a common element of many
devices, integrated circuit regulators are very common. Linear regulators may
also be made up of assemblies of discrete solid-state or vacuum tube
components. Overview
The transistor is used as one half of a potential divider to establish the
regulated output voltage. The output voltage is compared to a reference
voltage to produce a control signal to the transistor which will drive its gate
or base. With negative feedback and good choice of compensation, the output
voltage is kept reasonably constant. Linear regulators are often inefficient:
since the transistor is acting like a resistor, it will waste electrical
energy by converting it to heat. In fact, the power loss due to heating in
the transistor is the current multiplied by the voltage difference between input
and output voltage. The same function can often be performed much more
efficiently by a switched-mode power supply, but a linear regulator may be
preferred for light loads or where the desired output voltage approaches the
source voltage. In these cases, the linear regulator may dissipate less
power than a switcher. The linear regulator also has the advantage of not
requiring magnetic devices which can be relatively expensive or bulky, being
often of simpler design, and being quieter. Some designs of linear
regulators use only transistors, diodes and resistors, which are easier to
fabriacate into an integrated circuit, further reducing their weight, footprint
on a PCB, and price. All linear regulators require an input
voltage at least some minimum amount higher than the desired output voltage.
That minimum amount is called the dropout 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, before the output voltage
begins sagging below the rated output. Its dropout 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 must be used. When the output regulated voltage must
be higher than the available input voltage, no linear regulator will work.
In this situation, a switching regulator of the "boost" type must be used. Most
linear regulators will continue to provide some output voltage
approximately the dropout voltage below the input voltage for inputs below the
nominal output voltage until the input voltage drops significantly.
Linear regulators exist in two basic forms: shunt regulators and series
regulators. Most linear regulators have a maximum rated output current. This is
generally limited by either power dissipation capability, or by the
current carrying capability of the output transistor.
= Shunt regulators= The shunt regulator works by providing a
path from the supply voltage to ground through a variable resistance. The
current through the shunt regulator is diverted away from the load and flows
uselessly to ground, making this form usually 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. A shunt regulator can usually only sink current.
= Series 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 power dissipated by the
regulating device is equal to the power supply output current times the voltage
drop in the regulating device. A series regulator can usually only source
current. Simple shunt regulator
The image shows a simple shunt voltage regulator that 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 as well as the load current IR2. R1 can
be calculated as , where is the Zener voltage, and IR2 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. Once R1 has been
calculated, removing R2 will allow the full load current 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 because the Zener current will
vary depending on and inversely depending on the load current. In some
designs, the Zener diode may be replaced with another similarly functioning
device, especially in an ultra-low-voltage scenario, like several
normal diodes or LEDs in series. Simple series regulator
Adding an emitter follower stage to the simple shunt regulator forms a simple
series voltage regulator and substantially improves the regulation of
the circuit. Here, the load current IR2 is supplied by the transistor whose base
is now connected to the Zener diode. Thus the transistor's base current 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
and is determined as where, VZ is the Zener voltage, IB is the transistor's
base current, K = 1.2 to 2 and where, IR2 is the required load current and is
also the transistor's emitter current and hFE(min) is the minimum acceptable
DC current gain for the transistor. This circuit has much better regulation
than the simple shunt 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 VBE drop. Although this
circuit has good regulation, it is still sensitive to the load and supply
variation. This 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.
The circuit is readily made adjustable by adding a potentiometer across the
Zener, moving the transistor base connection from the top of the Zener to
the pot wiper. It may be made step adjustable by switching in different
Zeners. Finally it is occasionally made microadjustable by adding a low value
pot in series with the Zener; this allows a little voltage adjustment, but
degrades regulation. Fixed regulators
"Fixed" three-terminal linear regulators are commonly available to generate fixed
voltages of plus 3 V, and plus or minus 5 V, 6V, 9 V, 12 V, or 15 V, when the
load is less than 1.5 A. The "78xx" series regulate positive
voltages while the "79xx" series regulate negative voltages. Often, the
last two digits of the device number are the output voltage. There are variants
on the 78xx series ICs, such as 78L and 78S, some of which can supply up to 2
Amps. = Adjusting fixed regulators=
By adding another circuit element to a fixed voltage IC regulator, it is
possible to adjust the output voltage. Two example methods are:
A Zener diode or resistor may be added between the IC's ground terminal and
ground. Resistors are acceptable where ground current is constant, but are
ill-suited to regulators with varying ground current. By switching in
different Zener diodes, diodes or resistors, the output voltage can be
adjusted in a step-wise fashion. A potentiometer can be placed in series
with the ground terminal to increase the output voltage variably. However, this
method degrades regulation, and is not suitable for regulators with varying
ground current. Variable regulators
An adjustable regulator generates a fixed low nominal voltage between its
output and its adjust terminal. This family of devices includes low power
devices like LM723 and medium power devices like LM317 and L200. Some of the
variable regulators are available in packages with more than three pins,
including dual in-line packages. They offer the capability to adjust the
output voltage by using external resistors of specific values.
For output voltages not provided by standard fixed regulators and load
currents of less than 7 A, commonly available adjustable three-terminal
linear regulators may be used. The LM317 series regulates positive voltages while
the LM337 series 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.
Single IC dual tracking adjustable regulators are available for
applications such as op-amp circuits needing matched positive and negative DC
supplies. Some have selectable current limiting as well. Some regulators
require a minimum load. Protection
Linear IC voltage regulators may include a variety of protection methods:
Current limiting such as constant-current limiting or foldback
Thermal shutdown Safe operating area protection
Sometimes external protection is used, such as crowbar protection.
Using a linear regulator Linear regulators can be constructed
using discrete components but are usually encountered in integrated
circuit forms. The most common linear regulators are three-terminal integrated
circuits in the TO-220 package. Common solid-state series voltage
regulators are the LM78xx, LM79xx, and the AMS1117 series. Common fixed
voltages are 1.8V, 3.3V, 5 V and 12 V. 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.
See also Voltage regulator
Bandgap voltage reference LM317
Brokaw bandgap reference Switched-mode power supply
Low-dropout regulator References
External links "Zener regulator" at Hyperphysics
Linear voltage regulator tutorial video in HD Includes practical examples.
ECE 327: LM317 Bandgap Voltage Reference Example — Brief explanation of the
temperature-independent bandgap reference circuit within the LM317.
ECE 327: Procedures for Voltage Regulators Lab — Gives schematics,
explanations, and analyses for Zener shunt regulator, series regulator,
feedback series regulator, feedback series regulator with current limiting,
and feedback series regulator with current foldback. Also discusses the
proper use of the LM317 integrated circuit bandgap voltage reference and
bypass capacitors. ECE 327: Report Strategies for Voltage
Regulators Lab — Gives more-detailed quantitative analysis of behavior of
several shunt and series regulators in and out of normal operating ranges.
"7A SPX1580 regulator"