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Contents
7.1 The Need for 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-Tap 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 Vacuum Tube
7.7.2 Mercury Vapor
7.7.3 Early Solid-State Rectifiers
7.7.4 Semiconductor Diodes
7.7.5 Rectifier Strings or Stacks
7.7.6 Rectifier Ratings versus Operating Stress
7.7.7 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
7.10 “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 Switching 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 Battery Capacity
7.13.2 Chemical Hazards of Batteries
7.13.3 Battery Types
7.13.4 Battery Discharge Planning
7.13.5 Battery Internal Resistance
7.13.6 Battery Charging
7.13.7 DC-AC Inverters
7.14 Glossary of Power Supply Terms
7.15 References and Further Reading
7.16 Power-Supply 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 High-Voltage Power Supply
7.16.5 Micro M+ PV Charge Controller
7.16.6 Automatic Sealed Lead-Acid Battery
7.9.1 Zener Diodes
Charger
7.9.2 Linear Regulators
7.16.7 Overvoltage Protection for AC
7.9.3 Linear Regulator Pass Transistors
Generators
7.9.4 Three-Terminal Voltage Regulators
7.16.8 Overvoltage Crowbar Circuit
Chapter
7
Power Supplies
7.1 The Need for Power Processing
Electronic equipment requires a
source of electrical power. Sometimes
the source can be as simple as a battery,
but in many cases a battery may not
be able to supply sufficient power and/
or the voltages required with adequate
regulation. For example, the voltage of
a vehicle battery may vary from 14 V
when fully charged down to 10 V or less when discharged. The lower end of the voltage range
can compromise the performance of a transceiver, so it may be desirable to insert a device
between the battery and the transceiver to maintain the transceiver input voltage at about
13 V as the battery voltage declines during use.
Another example is operation from commercial utility power, which may vary from 90 to
270 V ac depending on where you are in the world. In most cases the signal generating and
processing equipment we use requires well-regulated dc voltages, sometimes at low voltages
(3-24 V) and at other times high voltages (100-3000 V).
Fig 7.1
illustrates the concept of a power processing unit inserted between the energy
Our transceivers, amplifiers,
accessories, computers and test
equipment all require power to op-
erate. This chapter illustrates the
various techniques, components
and systems used to provide
power at the voltage and current
levels our equipment needs. Top-
ics range from basic transform-
ers, rectifiers and filters to linear
voltage regulation, switchmode
power conversion, high voltage
techniques and batteries. Expand-
ed and updated by Rudy Severns,
N6LF, with support from Chuck
Mullett, KR6R, the material pre-
sented here builds on the earlier
work of Ken Stuart, W3VVN, and
other contributors.
Fig 7.1 — Basic concept of power processing.
Fig 7.2 — Four power processing schemes: ac-ac, dc-dc, ac-dc and dc-ac.
Power Supplies
7.1
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 in-
terchangeably.
The real world is even more arbitrary. 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 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.
In the following discussion it will be as-
sumed that the reader is familiar with the
component characteristics presented in the
Electrical Fundamentals
chapter.
7.2 AC-AC Power Conversion
In most US residences, three wires are brought
in from the outside electrical-service mains to
the house distribution panel. In this three-wire
system, one wire is neutral and should be at
earth ground potential. (See the
Safety
chapter
for information on electrical safety. The ARRL
Technical Information Service (TIS) at
www.
arrl.org/tis
provides references on international
power standards.) 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 potential
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 pos-
sible 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 neu-
tral 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 reli-
ably 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 conduc-
tor. 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 accidental 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
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, 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.
choke or tuned circuit, which could make the
antenna electrically “live” with respect to the
earth ground and create a potentially lethal
shock hazard. A
Ground Fault Circuit Inter-
rupter
(GFCI or GFI) is also desirable for safety
reasons, and should be a part of the shack’s
electrical power wiring.
plied to the regulator stage. Include the current
drawn by bleeder resistors and voltage dividers.
Also include filament power if the transformer is
supplying vacuum tube filaments. The National
Electrical Code (NEC) also specifies maximum
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 sup-
ply is turned on. If fuse blowing or breaker trip-
ping 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 ex-
ceeds 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 determine
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 appli-
ance, by the voltage at which the current is being
drawn. In the case of linear regulated power sup-
plies, this voltage has to be the voltage appearing
at the output of the rectifiers before being ap-
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.
primary windings.
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.
former primary winding. These variations,
coupled to the secondary windings, produce
the desired output voltage. Since the trans-
former appears to the source as an induc-
tance 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 provid-
ing 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 dΦ/dt as described
in the section on Inductance in the
Electrial
Fundamentals
chapter. The total flux in turn
is expressed by Φ = 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
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
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 capaci-
tor connected directly across the secondary
of a transformer. 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 capacitor, 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 transform-
er 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 trans-
Fig 7.4 — Use a test ixture like this to test unknown transformers. Don’t omit the
isolation transformer, and be sure to insulate all connections before you plug into the
ac mains.
Power Supplies
7.3
is the color of the stripe.
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
transformer! 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 house-
hold 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 making 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!
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 pri-
mary winding will probably cause a small
glow. The bulb glows even with the second-
ary windings open-circuited because of the
small magnetizing 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 po-
tentials 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.
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
Fig 7.5 — Ac-dc power conversion with a rectiier and a ilter.
source voltages. To simplify the discussion
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 pro-
vides 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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