Power supply is a reference to a source of
electrical power. A device or
system that supplies
electrical or other
types of
energy to an output
load or group of loads is called a
power supply unit or
PSU. The
term is most commonly applied to electrical energy supplies, less
often to mechanical ones, and rarely to others.
Electrical power supplies
A power supply may include a power distribution system as well as
primary or secondary sources of energy such as:
For large-scale power supplies, see
electricity generation.
Constraints that commonly affect power supplies are the amount of
power they can supply, how long they can supply it without needing
some kind of refueling or recharging, how stable their output
voltage or
current is under varying load
conditions, and whether they provide continuous power or
pulses.
A regulated power supply or
stabilized power supply is one that
includes circuitry to tightly control the output voltage and/or
current to a specific value. The specific value is closely
maintained despite variations in the load presented to the power
supply's output, or any reasonable voltage variation at the power
supply's input.
Power supply types
Power supplies for electronic devices can be broadly divided into
linear and switching power supplies. The linear supply is a
relatively simple design that becomes increasingly bulky and heavy
for high current devices; voltage regulation in a linear supply can
result in low efficiency. A switched-mode supply of the same rating
as a linear supply will be smaller, is usually more efficient, but
will be more complex.
Battery power supply
A
battery is a type of linear
power supply that offers benefits that traditional line-operated
power supplies lack: mobility, portability and reliability. A
battery consists of multiple electrochemical cells connected to
provide the voltage desired.
The most commonly used
dry-cell battery is
the
carbon-zinc dry cell battery.
Dry-cell batteries are made by stacking a carbon plate, a layer of
electrolyte paste, and a zinc plate alternately until the desired
total voltage is achieved. The most common dry-cell batteries have
one of the following voltages: 1.5, 3, 6, 9, 22.5, 45, and 90.
During the discharge of a carbon-zinc battery, the zinc metal is
converted to a zinc salt in the electrolyte, and magnesium dioxide
is reduced at the carbon electrode. These actions establish a
voltage of approximately 1.5 V.
The
lead-acid storage battery may be used.
This battery is rechargeable; it consists of lead and lead/dioxide
electrodes which are immersed in sulfuric acid. When fully charged,
this type of battery has a 2.06-2.14 V potential. During discharge,
the lead is converted to lead sulfate and the sulfuric acid is
converted to water. When the battery is charging, the lead sulfate
is converted back to lead and lead dioxide.
A
nickel-cadmium battery has become
more popular in recent years. This battery cell is completely
sealed and rechargeable. The electrolyte is not involved in the
electrode reaction, making the voltage constant over the span of
the batteries long service life. During the charging process,
nickel oxide is oxidized to its higher oxidation state and cadmium
oxide is reduced. The nickel-cadmium batteries have many benefits.
They can be stored both charged and uncharged. They have a long
service life, high current availabilities, constant voltage, and
the ability to be recharged.
Linear power supply
An
AC powered linear power
supply usually uses a
transformer to
convert the voltage from the wall outlet (mains) to a different,
usually a lower voltage. If it is used to produce
DC, a
rectifier is
used. A
capacitor is used to smooth the
pulsating current from the rectifier. Some small periodic
deviations from smooth direct current will remain, which is known
as
ripple. These pulsations
occur at a frequency related to the AC
power frequency (for example, a multiple
of 50 or 60 Hz).
The voltage produced by an unregulated power supply will vary
depending on the load and on variations in the AC supply voltage.
For critical electronics applications a
linear regulator will be used to stabilize
and adjust the voltage. This regulator will also greatly reduce the
ripple and noise in the output direct current. Linear regulators
often provide current limiting, protecting the power supply and
attached circuit from overcurrent.
Adjustable linear power supplies are common laboratory and service
shop test equipment, allowing the output voltage to be set over a
wide range. For example, a bench power supply used by circuit
designers may be adjustable up to 30 volts and up to 5 amperes
output. Some can be driven by an external signal, for example, for
applications requiring a pulsed output.
The simplest DC power supply circuit consists of a single
diode and
resistor in series
with the AC supply. This circuit is common in rechargeable
flashlights.
AC/ DC supply
In the past, mains electricity was supplied as DC in some regions,
AC in others. A simple, cheap linear power supply would run
directly from either AC or DC mains, often without using a
transformer. The power supply consisted of a rectifier and a
capacitor filter. The rectifier was essentially a conductor, having
no sudden effect when operating from DC.
Switched-mode power supply
A
switched-mode power
supply (SMPS) works on a different principle. AC mains input is
directly rectified without the use of a transformer, to obtain a DC
voltage. This voltage is then sliced into small pieces by a
high-speed electronic switch. The size of these slices grows larger
as power output requirements increase.
The input power slicing occurs at a very high speed (typically 10
kHz — 1 MHz). High frequency and high voltages in this first stage
permit much smaller
step down
transformers than are in a linear power supply. After the
transformer secondary, the AC is again rectified to DC. To keep
output voltage constant, the power supply needs a sophisticated
feedback controller to monitor current draw by the load.
Modern switched-mode power supplies often include additional safety
features such as the
crowbar
circuit to help protect the device and the user from harm. In
the event that an abnormal high current power draw is detected, the
switched-mode supply can assume this is a direct short and will
shut itself down before damage is done. For decades PC power
supplies have also provided a
power
good signal to the motherboard which prevents operation
when abnormal supply voltages are present.
Switched mode power supplies have an absolute limit on their
minimum current output. They are only able to output above a
certain power level and cannot function below that point. In a
no-load condition the frequency of the power slicing circuit
increases to great speed, causing the isolated transformer to act
as a
Tesla coil, causing damage due to
the resulting very high voltage power spikes. Switched-mode
supplies with protection circuits may briefly turn on but then shut
down when no load has been detected. A very small low-power
dummy load such as a ceramic power
resistor or 10-watt light bulb can be attached to the supply to
allow it to run with no primary load attached.
Power factor has become a recent issue
of concern for computer manufacturers. Switched mode power supplies
have traditionally been a source of
power line harmonics and have a very
poor power factor. Many computer power supplies built in the last
few years now include power factor correction built right into the
switched-mode supply, and may advertise the fact that they offer
1.0 power factor.
By slicing up the sinusoidal AC wave into very small discrete
pieces, a portion of unused alternating current stays in the power
line as very small spikes of power that cannot be utilized by AC
motors and results in waste heating of power line transformers.
Hundreds of switched mode power supplies in a building can result
in poor power quality for other customers surrounding that
building, and high electric bills for the company if they are
billed according to their power factor in addition to the actual
power used. Filtering capacitor banks may be needed on the building
power mains to suppress and absorb these negative power factor
effects.
Programmable power supply
Programmable power supplies
Programmable power supplies are those in which the output voltage
can be varied remotely. One possible option is digital control by a
computer interface. Variable properties include voltage, current,
and frequency. This type of supply is composed of a processor,
voltage/current programming circuits, current shunt, and
voltage/current read-back circuits.
Programmable power supplies can furnish DC, AC, or AC with a DC
offset. The AC output can be either single-phase or three-phase.
Single-phase is generally used for low-voltage, while three-phase
is more common for high-voltage power supplies.
When choosing a programmable power supply, several specifications
should be considered. For AC supplies, output voltage, voltage
accuracy, output frequency, and output current are important
attributes. For DC supplies, output voltage, voltage accuracy,
current, and power are important characteristics. Many special
features are also available, including computer interface,
overcurrent protection, overvoltage protection, short circuit
protection, and temperature compensation. Programmable power
supplies also come in a variety of forms. Some of those are
modular, board-mounted, wall-mounted, floor-mounted or bench
top.
Programmable power supplies are now used in many applications. Some
examples include automated equipment testing, crystal growth
monitoring, and differential thermal analysis .
Uninterruptible power supply
An
Uninterruptible Power Supply (UPS) takes its
power from two or more sources simultaneously. It is usually
powered directly from the AC mains, while simultaneously charging a
storage battery. Should there be a dropout or failure of the mains,
the battery instantly takes over so that the load never experiences
an interruption. Such a scheme can supply power as long as the
battery charge suffices, e.g., in a computer installation, giving
the operator sufficient time to effect an orderly system shutdown
without loss of data. Other UPS schemes may use an internal
combustion engine or turbine to continuously supply power to a
system in parallel with power coming from the AC mains. The
engine-driven generators would normally be idling, but could come
to full power in a matter of a few seconds in order to keep vital
equipment running without interruption. Such a scheme might be
found in hospitals or telephone central offices.
High-voltage power supply
High voltage refers to an output on the order of hundreds or
thousands of volts. High-voltage supplies use a linear setup to
produce an output voltage in this range.
Additional features available on high-voltage supplies can include
the ability to reverse the output polarity along with the use of
circuit breakers and
special connectors intended to minimize
arcing and accidental contact with human
hands. Some supplies provide analog inputs (i.e. 0-10V) that can be
used to control the output voltage, effectively turning them into
high-voltage
amplifiers albeit with very
limited
bandwidth.
Voltage multipliers
Voltage multipliers, as the name implies, are circuits designed to
multiply the input voltage. The input voltage may be doubled
(voltage doubler), tripled (voltage tripler), quadrupled (voltage
quadrupler), etc. Voltage multipliers are also power converters. An
AC input is converted to a higher DC output. These circuits allow
high voltages to be obtained using a much lower voltage AC
source.
Typically, voltage multipliers are composed of half-wave
rectifiers, capacitors, and diodes. For example, a voltage tripler
consists of three half-wave rectifiers, three capacitors, and three
diodes. Full-wave rectifiers may be used in a different
configuration to achieve even higher voltages. Also, both parallel
and series configurations are available. For parallel multipliers,
a higher voltage rating is required at each consecutive
multiplication stage, but less capacitance is required. The voltage
capability of the capacitor limits the maximum output
voltage.
Voltage multipliers have many applications. For example, voltage
multipliers can be found in everyday items like televisions and
photocopiers. Even more applications can be found in the
laboratory, such as cathode ray tubes, oscilloscopes, and
photomultiplier tubes.
Power supply applications
Computer power supply
A modern computer power supply is a switched-mode supply designed
to convert 110-240 V AC power from the mains supply, to several
output both positive (and historically negative) DC voltages in the
range + 12V,-12V,+5V,+5VBs and +3.3V. The first generation of
computers power supplies were linear devices, but as cost became a
driving factor, and weight became important, switched mode supplies
are almost universal.
The diverse collection of output voltages also have widely varying
current draw requirements, which are difficult to all be supplied
from the same switched-mode source. Consequently most modern
computer power supplies actually consist of several different
switched mode supplies, each producing just one voltage component
and each able to vary its output based on component power
requirements, and all are linked together to shut down as a group
in the event of a fault condition.
The most common modern computer power supplies are built to conform
to the
ATX form
factor. The power rating of a PC power supply is not officially
certified and is self-claimed by each manufacturer.A common way to
reach the power figure for PC PSUs is by adding the power available
on each rail, which will not give a true power figure. The more
reputable makers advertise "True Wattage Rated" to give consumers
the idea that they can trust the power advertised.
Welding power supply
Arc welding uses electricity to melt the
surfaces of the
metals in order to join them
together through
coalescence.
The electricity is provided by a
welding power supply, and
can either be
AC or
DC. Arc welding typically requires high
currents typically between 100 and 350
amps.
Some types of welding can use as few as 10 amps, while some
applications of
spot welding employ
currents as high as 60000 amps for an extremely short time. Older
welding power supplies consisted of
transformers or
engines driving
generators. More recent supplies use
semiconductors and
microprocessors reducing their size and
weight.
AC adapter
Switched mode mobile phone
charger
A linear or
switched-mode
power supply (or in some cases just a transformer) that is
built into the top of a
plug is known as a
"
wall wart", "power brick", "plug pack",
"plug-in adapter", "adapter block", "domestic mains adapter" or
just "power adapter". They are even more diverse than their names;
often with either the same kind of
DC
plug offering different voltage or polarity, or a different
plug offering the same voltage. "Universal" adapters attempt to
replace missing or damaged ones, using multiple plugs and selectors
for different voltages and polarities. Replacement power supplies
must match the voltage of, and supply at least as much current as,
the original power supply.
The least expensive AC units consist solely of a small
transformer, while DC adapters include a few
additional diodes. Whether or not a load is connected to the power
adapter, the transformer has a magnetic field continuously present
and normally cannot be completely turned off unless
unplugged.
Because they consume
standby power,
they are sometimes known as "electricity vampires" and may be
plugged into a
power strip to allow
turning them off. Expensive switched-mode power supplies can cut
off leaky electrolyte-capacitors, use powerless
MOSFETs, and reduce their working frequency to get a
gulp of energy once in a while to power, for example, a clock,
which would otherwise need a
battery.
This type of power supply is popular among manufacturers of low
cost electrical items because:
- Devices sold in the global marketplace don't need to be
individually configured for 120 volt or 230 volt operation, just
sold with the appropriate AC adapter.
- The device itself doesn't need to be tested for compliance with
electrical safety regulations. Only the adapter needs to be
tested.
- Product development becomes faster and cheaper, because the
heat produced by the power supply is outside of the product.
- The device itself can be smaller and lighter, since it does not
contain the power supply.
Polarity
AC-to-DC adapters have polarity; even if the plug fits into a
device, the positive and negative connections may be oriented the
wrong way. It is necessary to use an adapter with the correct
polarity to avoid damage. Standardized
polarity symbols are usually used to label
polarity.
Overload Protection
Power supplies should have some type of overload protection.
Overload protection is important to protect the electronic
equipment hooked up to the power supply and to also prevent
overheating, which could potentially lead to an electrical fire.
Fuses and
circuit breakers are two of the more
frequent mechanisms used for overload protection.
Fuses
A piece of wire is connected between two metal ends. The two metal
ends of the fuse are connected by either a tube of glass or plastic
which surrounds the wire. If too much current flows, the wire
overheats and melts. This interrupts the power supply, and the
equipment stops working until the problem that caused the overload
is identified and the fuse is replaced.
There are two types of fuses, slow-blow and fast-blow. In a
fast-blow fuse, the wire inside the fuse will melt if the current
exceeds the rated current, even if it is just for a fraction of
second. This concise process is important in electronic equipment
where even a small spike in the current could damage the equipment.
A slow-blow fuse is designed to only melt when there is a
continuous overload. Slow-blow fuses are ideal for motor
systems.
Circuit Breakers
One benefit of using a circuit breaker as opposed to a fuse is that
it can simply be reset instead of having to constantly replace the
blown fuse. A circuit breaker works once an overloaded current
causes some element to heat and trigger a spring which shuts the
circuit down. Once the element cools, and the problem is identified
the breaker can be reset and the power restored.
Power conversion
The term "
power supply" is sometimes restricted to
those devices that
convert some other form of energy into
electricity (such as solar power and fuel cells and generators). A
more accurate term for devices that convert one form of electric
power into another form (such as transformers and linear
regulators) is
power converter. The
most common conversion is from
AC to
DC.
Mechanical power supplies
Terminology
- SCP:Short circuit protection
- OPP:Overpower (overload) protection
- OCP:Overcurrent protection
- OTP:Overtemperature protection
- OVP:Overvoltage protection
- UVP:Undervoltage protection
- UPS:Uninterruptable Power Supply
- PSU:Power Supply Unit
- APSU:Alarm Power Supply Unit
See also
References
- Malmstadt, Enke and Crouch, Electronics and Instrumentation for
Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981,
ISBN 0-8053-6917-1, Chapter 3.
- Malmstadt, Enke and Crouch, Electronics and Instrumentation for
Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981,
ISBN 0-8053-6917-1, Chapter 3, pg. 66
- Malmstadt, Enke and Crouch, Electronics and Instrumentation for
Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981,
ISBN 0-8053-6917-1, Chapter 3, pg. 68
- Quoting US patent #4937722, High efficiency direct coupled
switched mode power supply: The power supply can also
include crowbar circuit protecting it against damage by clamping
the output to ground if it exceeds a particular voltage.
http://www.patentstorm.us/patents/4937722-description.html
- Quoting US Patent #5402059: A problem can occur when loads
on the output of a switching power supply become disconnected from
the supply. When this occurs, the output current from the power
supply becomes reduced (or eliminated if all loads become
disconnected). If the output current becomes small enough, the
output voltage of the power supply can reach the peak value of the
secondary voltage of the transformer of the power supply. This
occurs because with a very small output current, the inductor in
the L-C low-pass filter does not drop much voltage (if any at all).
The capacitor in the L-C low-pass filter therefore charges up to
the peak voltage of the secondary of the transformer. This peak
voltage is generally considerably higher than the average voltage
of the secondary of the transformer. The higher voltage which
occurs across the capacitor, and therefore also at the output of
the power supply, can damage components within the power supply.
The higher voltage can also damage any remaining electrical loads
connected to the power supply.
http://www.patentstorm.us/patents/5402059-description.html
-
http://electronic-components.globalspec.com/LearnMore/Electrical_Electronic_Components/Power_Supplies_Conditioners/Programmable_Power_Supplies
-
http://electronic-components.globalspec.com/LearnMore/Electrical_Electronic_Components/Power_Supplies_Conditioners/Voltage_Multipliers
- Miller, Rex. Electronics The Easy Way, 4th ed. Barron's
Educational Series, 2002 p. 88-89.
- http://www.silentpcreview.com/article28-page3.html SPCR: Power
Supply Fundamentals: POWER SHMOWER or How PSU Power Ratings Mean
Almost Nothing
- Malmstadt, Enke and Crouch, Electronics and Instrumentation for
Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981,
ISBN 0-8053-6917-1, Chapter 3.
External links
Wikis
Quiz
Map
Search
