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Contents
7.1 Power Processing
7.2 AC-AC Power Conversion
7.2.1 Fuses and Circuit Breakers
7.3 Power Transformers
7.3.1 Volt-Ampere Rating
7.3.2 Source Voltage and Frequency
7.3.3 How to Evaluate an Unmarked Power
Transformer
7.4 AC-DC Power Conversion
7.4.1 Half-Wave Rectifier
7.4.2 Full-Wave Center-Tapped Rectifier
7.4.3 Full-Wave Bridge Rectifier
7.4.4 Comparison of Rectifier Circuits
7.5 Voltage Multipliers
7.5.1 Half-Wave Voltage Doubler
7.5.2 Full-Wave Voltage Doubler
7.5.3 Voltage Tripler and Quadrupler
7.6 Current Multipliers
7.7 Rectifier Types
7.7.1 Semiconductor Diodes
7.7.2 Rectifier Strings or Stacks
7.7.3 Rectifier Ratings versus Operating Stress
7.7.4 Rectifier Protection
7.8 Power Filtering
7.8.1 Load Resistance
7.8.2 Voltage Regulation
7.8.3 Bleeder Resistors
7.8.4 Ripple Frequency and Voltage
7.8.5 Capacitor-Input Filters
7.8.6 Choke-Input Filters
7.9 Power Supply Regulation
7.9.1 Zener Diodes
7.9.2 Linear Regulators
7.9.3 Linear Regulator Pass Transistors
7.9.4 Three-Terminal Voltage Regulators
7.10 “Crowbar” Protective Circuits
7.11 DC-DC Switchmode Power Conversion
7.11.1 The Buck Converter
7.11.2 The Boost Converter
7.11.3 Buck-Boost and Flyback Converters
7.11.4 The Forward Converter
7.11.5 Parallel, Half and Full-Bridge Converters
7.11.6 Building Switchmode Power Supplies
7.11.7 Switchmode Control Loop Issues
7.12 High-Voltage Techniques
7.12.1 High-Voltage Capacitors
7.12.2 High-Voltage Bleeder Resistors
7.12.3 High-Voltage Metering Techniques
7.12.4 High-Voltage Transformers and Inductors
7.12.5 Construction Techniques for High-Voltage Supplies
7.12.6 High-Voltage Safety Considerations
7.13 Batteries
7.13.1 Choices of Secondary Batteries
7.13.2 Lead Acid Batteries
7.13.3 Nickel-based Batteries
7.13.4 Lithium-based Batteries
7.13.5 Charging Methods
7.13.6 Discharge Methods
7.13.7 Battery Handling
7.13.8 DC-AC Inverters
7.13.9 Selecting a Battery for Mobile Operation
7.14 Glossary of Power Source Terms
7.15 References and Bibliography
7.16 Power Source Projects
7.16.1 Four-Output Switching Bench Supply
7.16.2 12 V, 15 A Linear Power Supply
7.16.3 13.8 V, 5 A Linear Power Supply
7.16.4 Adjustable Resistive Load
7.16.5 Inverting DC-DC Converter
7.16.6 High-Voltage Power Supply
7.16.7 Reverse-Polarity Protection Circuits
7.16.8 Automatic Sealed Lead-Acid Battery Charger
7.16.9 Overvoltage Protection for AC Generators
7.16.10 Overvoltage Crowbar Circuit
Chapter
7
Power Sources
Acknowledging the changing nature of amateur power requirements, the title of this chapter
has updated from the traditional
Power Supplies
to
Power Sources
. More mobile and portable
operation relies on power from batteries, for example. Hybrids of ac and dc power sources are
becoming more common, blurring what has traditionally been known as a “power supply.”
In response, the scope of this chapter now includes more of the changing power environment
in the amateur station. (Generators are covered in the Portable Installations section of the
chapter on
Building a
Station
.)
Our transceivers, ampliiers, ac-
cessories, computers and test equip-
ment all require power to operate.
This chapter illustrates the various
techniques, components and sys-
tems used to provide power at the
voltage and current levels our equip-
ment needs. Topics range from basic
transformers, rectiiers and ilters
to linear voltage regulation, switch-
mode power conversion, high voltage
techniques and batteries. Material
on switchmode conversion was con-
tributed by Rudy Severns, N6LF and
Chuck Mullett, KR6R. A new section
on batteries was contributed by Isidor
Buchmann from his book
Batteries
in a Portable World
. Alan Applegate,
KØBG contributed the section on se-
lecting batteries for mobile use.
7.1 Power Processing
Fig 7.1
illustrates the concept of a power processing unit inserted between the energy
source and the electronic equipment or load. The
power processor
is often referred to as the
power supply
. That’s a bit misleading in that the energy “supply” actually comes from some
external source (battery, utility power and so forth), which is then converted to useful forms
by the power processor. Be that as it may, in practice the terms “power supply” and “power
processor” are used interchangeably.
The real world is even more arbi-
trary. Power processors are frequently
referred to as
power converters
or
simply as
converters
, and we will see
other terms used later in this chapter. It
is usually obvious from the context of
the discussion what is meant and the
Fig 7.1 — Basic concept of power processing.
Chapter 7 —
CD-ROM Content
Projects
•
Four Output Bench Supply
•
12 V, 15 A Power Supply — Article
and PCB Template
•
13.8 V, 5 A Power Supply — PCB
Template
•
28 V High Current Power Supply —
Article and PCB Template
•
Dual Output Power Supply
•
Micro M+ PV Charge Controller
•
Revisiting the 12 V Power Supply
•
Series Regulator Power Supply —
Article and PCB Template
•
Build an Inverting DC-DC Converter
Supplemental Articles
•
Testing and Monitoring Batteries
— Excerpts from
Batteries in a
Portable World
by Isidor Buchmann
•
Vacuum Tube and Obsolete
Rectiiers
Fig 7.2 — Four power processing schemes: ac-ac, dc-dc, ac-dc and dc-ac.
Power Sources
7.1
glossary at the end of this chapter gives some
additional information.
Power conversion schemes can take the
form of: ac-to-ac (usually written ac-ac),
ac-dc, dc-ac and dc-dc. Examples of these
schemes are given in
Fig 7.2
. Specific names
may be given to each scheme: ac-dc => recti-
fier, dc-dc => converter and dc-ac => inverter.
These are the generally recognized terms but
you will see exceptions.
Power conversion normally includes volt-
age and current regulation functions. For ex-
ample, the voltage of a vehicle battery may
vary from 14 V when being charged down to
10 V or less when discharged. A converter and
regulator are required to maintain adequate
voltage to mobile equipment at both over- and
under-voltage conditions. Commercial utility
power may vary from 90 to 270 V ac depend-
ing on where you are in the world. AC power
converters are frequently required to handle
that entire voltage range while still providing
tightly regulated dc power.
7.2 AC-AC Power Conversion
In most US residences, three wires are
brought in from the outside electrical-ser-
vice mains to the house distribution panel.
In this three-wire system, one wire is neu-
tral and should be at earth ground potential.
(See the
Safety
chapter for information on
electrical safety.) The neutral connection to
a ground rod or electrode is usually made at
the distribution panel. The voltage between
the other two wires is 60-Hz ac with a poten-
tial difference of approximately 240 V RMS.
Half of this voltage appears between each of
these wires and the neutral, as indicated in
Fig 7.3A
. In systems of this type, the 120 V
household loads are divided at the breaker
panel as evenly as possible between the two
sides of the power mains. Heavy appliances
such as electric stoves, water heaters, central
air conditioners and so forth, are designed for
240 V operation and are connected across the
two ungrounded wires.
Both hot wires for 240 V circuits and the
single hot wire for 120 V circuits should be
protected by either a fuse or breaker. A fuse or
breaker or any kind of switch should
never
be
used in the neutral wire. Opening the neutral
wire does not disconnect the equipment from
an active or “hot” line, possibly creating a
potential shock hazard between that line and
earth ground.
Another word of caution should be given
at this point. Since one side of the ac line is
grounded (through the green or bare wire —
the standard household wiring color code) to
earth, all communications equipment should
be reliably connected to the ac-line ground
through a heavy ground braid or bus wire of
#14 or heavier-gauge wire. This wire must
be a separate conductor. You must not use
the power-wiring neutral conductor for this
safety ground. (A properly-wired 120 V outlet
with a ground terminal uses one wire for the
ac hot connection, one wire for the ac neutral
connection and a third wire for the safety
ground connection.) This provides a measure
of safety for the operator in the event of ac-
cidental short or leakage of one side of the ac
line to the chassis.
Remember that the antenna system is fre-
quently bypassed to the chassis via an RF
choke or tuned circuit, which could make
the antenna electrically “live” with respect
Fig 7.3 — Three-wire power-line circuits. At A, normal three-wire-line termination. No
fuse should be used in the grounded (neutral) line. The ground symbol is the power
company’s ground, not yours! Do not connect anything other than power return wiring,
including the equipment chassis, to the power neutral wire. At B, the “hot” lines each
have a switch, but a switch in the neutral line would not remove voltage from either
side of the line and should never be used. At C, connections for both 120 and 240 V
transformers. At D, operating a 120 V plate transformer from the 240 V line to avoid light
blinking. T1 is a 2:1 step-down transformer.
to the earth ground and create a potentially
lethal shock hazard. A
ground fault circuit
interrupter
(GFCI or GFI) is also desirable
for safety reasons, and should be a part of the
shack’s electrical power wiring.
resistors and voltage dividers. Also include
filament power if the transformer is supply-
ing vacuum tube filaments. The National
Electrical Code (NEC) also specifies maxi-
mum fuse ratings based on the wire sizes used
in the transformer and connections.
After multiplying the various voltages and
currents, add the individual products. This is
the total power drawn from the line by the
supply. Then divide this power by the line
voltage and add 10 to 30% to account for the
inefficiency of the power supply itself. Use a
fuse or circuit breaker with the nearest larger
current rating. Remember that the charging
of filter capacitors can create large surges of
current when the supply is turned on. If fuse
blowing or breaker tripping at turn on is a
problem, use slow-blow fuses, which allow
for high initial surge currents.
For low-power semiconductor circuits,
use fast-blow fuses. As the name implies,
such fuses open very quickly once the current
exceeds the fuse rating by more than 10%.
7.2.1 Fuses and Circuit
Breakers
All transformer primary circuits should be
fused properly and multiple secondary outputs
should also be individually fused. To deter-
mine the approximate current rating of the fuse
or circuit breaker on the line side of a power
supply it is necessary to determine the total
load power. This can be done by multiplying
each current (in amperes) being drawn by the
load or appliance, by the voltage at which the
current is being drawn. In the case of linear
regulated power supplies, this voltage has to
be the voltage appearing at the output of the
rectifiers before being applied to the regulator
stage. Include the current drawn by bleeder
7.2
Chapter 7
7.3 Power Transformers
Numerous factors are considered to match
a transformer to its intended use. Some of
these parameters are:
1. Output voltage and current (volt-ampere
rating).
2. Power source voltage and frequency.
3. Ambient temperature.
4. Duty cycle and temperature rise of the
transformer at rated load.
5. Mechanical considerations like weight,
shape and mounting.
At a given voltage, 50 Hz ac creates more
flux in an inductor or transformer core because
the longer time period per half-cycle results
in more flux and higher magnetizing current
than the same transformer when excited by
same 60-Hz voltage. For this reason, trans-
formers and other electromagnetic equipment
designed for 60-Hz systems must not be used
on 50-Hz power systems unless specifically
designed to handle the lower line frequency.
transformer primary winding. These varia-
tions, coupled to the secondary windings,
produce the desired output voltage. Since
the transformer appears to the source as an
inductance in parallel with the (equivalent)
load, the primary will appear as a short circuit
if dc is applied to it. The unloaded inductance
of the primary (also known as the
magnetizing
inductance
) must be high enough so as not
to draw an excess amount of input current at
the design line frequency (normally 60 Hz
in the US). This is achieved by providing a
combination of sufficient turns on the primary
and enough magnetic core material so that the
core does not saturate during each half-cycle.
The voltage across a winding is directly
related to the time rate of change of magnetic
flux in the core. This relationship is expressed
mathematically by V = N dF/dt as described
in the section on Inductance in the
Electrical
Fundamentals
chapter. The total flux in turn
is expressed by F = A
e
B, where A
e
is the
cross-sectional area of the core and B is the
flux density.
The maximum value for
flux density
(the
magnetic field strength produced in the core)
is limited to some percentage (< 80% for ex-
ample) of the maximum flux density that the
core material can stand without saturating,
since in saturation the core becomes ineffec-
tive and causes the inductance of the primary
to plummet to a very low level and input cur-
rent to rise rapidly. Saturation causes high
primary currents and extreme heating in the
primary windings.
7.3.3 How to Evaluate
an Unmarked Power
Transformer
Hams who regularly visit hamfests fre-
quently develop a junk box filled with used
and unmarked transformers. Over time, trans-
former labels or markings on the coil wrap-
pings may come off or be obscured. There
is a good possibility that the transformer is
still useable, but the problem is to determine
what voltages and currents the transformer
can supply. First consider the possibility that
you may have an audio transformer or other
impedance-matching device rather than a
power transformer. If you aren’t sure, don’t
connect it to ac power!
If the transformer has color-coded leads,
you are in luck. There is a standard for trans-
former lead color-coding, as is given in the
Component Data and References
chapter.
Where two colors are listed, the first one is
the main color of the insulation; the second
is the color of the stripe.
7.3.1 Volt-Ampere Rating
In alternating-current equipment, the term
volt-ampere
(
VA
) is often used rather than the
term watt. This is because ac components must
handle reactive power as well as real power.
If this is confusing, consider a capacitor con-
nected directly across the secondary of a trans-
former. The capacitor appears as a reactance
that permits current to flow, just as if the load
were a resistor. The current is at a 90º phase
angle, however. If we assume a perfect capaci-
tor, there will be no heating of the capacitor,
so no real power (watts) will be delivered by
the transformer. The transformer must still be
capable of supplying the voltage, and be able
to handle the current required by the reactive
load. The current in the transformer windings
will heat the windings as a result of the I
2
R
losses in the winding resistances. The product
of the voltage and current in the winding is
referred to as “volt-amperes,” since “watts” is
reserved for the real, or dissipated, power in
the load. The volt-ampere rating will always
be equal to, or greater than, the power actually
being drawn by the load.
The number of volt-amperes delivered by
a transformer depends not only upon the dc
load requirements, but also upon the type
of dc output filter used (capacitor or choke
input), and the type of rectifier used (full-
wave center tap or full-wave bridge). With a
capacitive-input filter, the heating effect in the
secondary is higher because of the high peak-
to-average current ratio. The volt-amperes
handled by the transformer may be several
times the power delivered to the load. The
primary winding volt-amperes will be some-
what higher because of transformer losses.
This point is treated in more detail in the sec-
tion on ac-dc conversion. (See the
Electrical
Fundamentals
chapter for more information
on transformers and reactive power.)
7.3.2 Source Voltage and
Frequency
A transformer operates by producing
a magnetic field in its core and windings.
The intensity of this field varies directly
with the instantaneous voltage applied to the
Fig 7.4 — Use a test ixture like this to test unknown transformers. Don’t omit the isola-
tion transformer, and be sure to insulate all connections before you plug into the ac
mains.
Power Sources
7.3
Connect the test leads to each winding
separately. The filament/heater windings
will cause the bulb to light to full brilliance
because a filament winding has a very low im-
pedance and almost all the input voltage will
be across the series bulb. The high-voltage
winding will cause the bulb to be extremely
dim or to show no light at all because it will
have a very high impedance, and the primary
winding will probably cause a small glow. The
bulb glows even with the secondary windings
open-circuited because of the small magnetiz-
ing current in the transformer primary.
When the isolation transformer output is
connected to what you think is the primary
winding, measure the voltages at the low-
voltage windings with an ac voltmeter. If you
find voltages close to 6 V ac or 5 V ac, you
know that you have identified the primary and
the filament windings. Label the primary and
low voltage windings.
Even with the light bulb, a transformer can
be damaged by connecting ac mains power
to a low-voltage or filament winding. In such
a case the insulation could break down in a
primary or high-voltage winding because of
the high turns ratio stepping up the voltage
well beyond the transformer ratings.
Connect the voltmeter to the high-voltage
windings. Remember that the old TV trans-
formers will typically supply as much as
800 V
pk
or so across the winding, so make
sure that your meter can withstand these
potentials without damage and that you use
the voltmeter safely.
Divide 6.3 (or 5) by the voltage you mea-
sured across the 6.3 V (or 5 V) winding in this
test setup. This gives a multiplier that you can
use to determine the actual no-load voltage
rating of the high-voltage secondary. Simply
multiply the ac voltage measured across the
high-voltage winding by the multiplier.
The current rating of the windings can be
determined by loading each winding with the
primary connected directly (no bulb) to the ac
line. Using power resistors, increase loading
on each winding until its voltage drops by
about 10% from the no-load figure. The cur-
rent drawn by the resistors is the approximate
winding load-current rating.
Check the transformer windings with an
ohmmeter to determine that there are no
shorted (or open) windings. In particular,
check for continuity between any winding
and the core. If you find that a winding has
been shorted to the core, do not use the trans-
former! The primary winding usually has a
resistance higher than a filament winding and
lower than a high-voltage winding.
Fig 7.4
shows that a convenient way to test
the transformer is to rig a pair of test leads
to an electrical plug with a 25 W household
light bulb in series to limit current to safe (for
the transformer) levels. For safety reasons
use an isolation transformer and be sure to
insulate all connections before you plug into
the ac mains. Switch off the power while mak-
ing or changing any connections. You can be
electrocuted if the voltmeter leads or meter
insulation are not rated for the transformer
output voltage! If in doubt, connect the meter
with the circuit turned off, then apply power
while you are not in contact with the circuit.
Be careful! You are dealing with hazardous
voltages!
7.4 AC-DC Power Conversion
One of the most common power supply
functions is the conversion of ac power to dc,
or
rectification
. The output from the rectifier
will be a combination of dc, which is the de-
sired component, and ac
ripple
superimposed
on the dc. This is an undesired but inescapable
component. Since most loads cannot tolerate
more than a small amount of ripple on the
dc voltage, some form of filter is required.
The result is that ac-dc power conversion is
performed with a rectifier-filter combination
as shown in
Fig 7.5
.
As we will see in the rectifier circuit exam-
ples given in the next sections, sometimes the
rectifier and filter functions will be separated
into two distinct parts but very often the two
will be integrated. This is particularly true for
voltage and current multipliers as described in
the sections on multipliers later in the chapter.
Even when it appears that the rectifier and
filter are separate elements, there will still
be a strong interaction where the design and
behavior of each part depends heavily on the
other. For example the current waveforms in
the rectifiers and the input source are func-
tions of the load and filter characteristics. In
turn the voltage waveform applied to the filter
depends on the rectifier circuit and the input
source voltages. To simplify the discussion
Fig 7.5 — Ac-dc power conversion with a rectiier and a ilter.
we will treat the rectifier connections and the
filters separately but always keeping in mind
their interdependence.
The following rectifier-filter examples
assume a conventional 60 Hz ac sine wave
source, but these circuits are frequently
used in switching converters at much higher
frequencies and with square wave or quasi-
square wave voltage and current waveforms.
The component values may be different but
the basic behavior will be very similar.
There are many different rectifier circuits
or “connections” that may be used depending
on the application. The following discussion
provides an overview of some of the more
common ones. The circuit diagrams use the
symbol for a semiconductor diode, but the
same circuits can be used with the older types
of rectifiers that may be encountered in older
equipment.
7.4
Chapter 7
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