AM 5-206
POWER
SUPPLIES / REGULATORS
February 2012
DISTRIBUTION RESTRICTION:
Approved for public release. Distribut ion is unlimited.
DEPARTMENT OF THE ARMY
MILITARY AUXILIARY RADIO SYSTEM
FORT HUACHUCA ARIZONA 85613-7070
AM 5 – 206 Basic Electronics - Power Supplies
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AM 5 – 206 Basic Electronics - Power Supplies
CONTENTS
1
POWER SUPPLIES.................................................................................... 1-1
1.1
2
INTRODUCTION ............................................................................................1-1
A/C TO D/C POWER SUPPLIES..................................................................... 2-3
2.1 INTRODUCTION: ...........................................................................................2-3
2.2 TRANSFORMERS:...........................................................................................2-4
2.2.1 Isolation: .............................................................................................2-4
2.3 RECTIFIERS: ...............................................................................................2-5
2.3.1 Half-Wave Rectifier Circuits: ......................................................................2-6
2.3.2 Full-Wave Rectifier: ................................................................................2-7
2.3.3 Bridge Rectifiers: ...................................................................................2-8
2.3.4 Peak Inverse Voltage (PIV): ........................................................................2-9
3
POWER SUPPLY FILTERS ...........................................................................3-11
3.1 INTRODUCTION: ......................................................................................... 3-11
3.1.1 Pi (71") Filter ...................................................................................... 3-11
4
REGULATORS........................................................................................4-15
4.1 INTRODUCTION: ......................................................................................... 4-15
4.2 BASIC VOLTAGE REGULATOR DESIGNS: .................................................................. 4-15
4.2.1 Introduction:....................................................................................... 4-15
4.2.2 LM317T Variable Voltage Regulator:............................................................ 4-15
4.2.3 LM317T Voltage Regulator with Pass Transistor: .............................................. 4-17
4.2.4 High Current Regulated Supply:................................................................. 4-18
4.2.5 Simple Adjustable Voltage Source: ............................................................. 4-19
4.2.6 2 Cell Lithium Ion Chargers:..................................................................... 4-20
5
HIGH VOLTAGE POWER SUPPLIES ................................................................5-21
5.1
5.2
6
INTRODUCTION .......................................................................................... 5-21
HIGH VOLTAGE POWER SUPPLY DESIGN .................................................................. 5-21
POWER WIRING FOR MOBILE RADIOS ............................................................6-23
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IMPROVEMENTS
(Suggested corrections, or changes to this document, should be submitted through your State
Director to the Regional Director. Any Changes will be made by the National documentation team.
DISTRIBUTION
Distribution is unlimited.
VERSIONS
The Versions are designated in the footer of each page if no version number is designated the
version is considered to be 1.0 or the original issue. Documents may have pages with different
versions designated; if so verify the versions on the “Change Page” at the beginning of each
document.
REFERENCES:
US Army FM/TM Manuals and Handbooks
1. TM 5-811-3 - Electrical Design, Lightning and Static Electricity Protection
2. TM 5-682 - Facilities Engineering Electrical Facilities Safety
3. TM 5-690 - Grounding and Bonding in Command, Control, Communications, Computer,
Intelligence, Surveillance, and Reconnaissance (C4ISR) Facilities
4. TM 11-661 Electrical Fundamentals, Direct Current
5. TM-664 – Basic Theory and Use of Electronic Test Equipment
US Army Handbooks
1. MIL-HDBK 1857 - Grounding, Bonding and Shielding Design Practices
Commercial References
1. Basic Electronics, Components, Devices and Circuits; ISBN 0-02-81860-X, By William P
Hand and Gerald Williams Glencoe/McGraw Hill Publishing Co.
2. Standard Handbook for Electrical Engineers - McGraw Hill Publishing Co.
CONTRIBUTORS
This document has been produced by the Army MARS Technical Writing Team under the authority of
Army MARS HQ, Ft Huachuca, AZ. The following individuals are subject matter experts who made
significant contributions to this document.
•
William P Hand
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1
POWER SUPPLIES
1.1
INTRODUCTION
The term “Power Supply or Power Source” is vague at best. It can encompass anything that will
supply electrical power. This can be anything from a battery to a very sophisticated nuclear
generator. This manual cannot cover all the possible variations but the more common will be
presented. We will briefly cover:
1.
2.
3.
4.
AC to DC power supplies.
Voltage and Current Regulators
Voltage and Current Sources (Standards)
Power Wiring
Power generation methods, including Batteries, are presented in AM 5-210.
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2
A/C TO D/C POWER SUPPLIES
2.1
INTRODUCTION:
Almost all electronic equipment operates on direct current (dc). The required dc is often derived
from an ac power source such as a commercial power line. The ac line voltage must be rectified,
filtered, and often regulated, in order for it to be usable by the equipment. The circuit used to
convert ac into suitable dc is known as a power supply. Figure 2-1 is a simple ac power line to a dc
power supply output.
In most electronic equipment the power supply is one of the major considerations in making
equipment operate properly. Power supply trouble can make the equipment useless or even
dangerous.
Figure 2-1, Simple Power Supply
Figure 2-2 is a block diagram of a power supply that converts ac to dc, and then regulates it to a
specific constant voltage.
The ac line is fed to the transformer, which changes the ac line voltage to the voltage required by
the circuit in question. The rectifier then converts the ac source voltage to a pulsating dc. This
pulsating dc is fed to the filter where the pulsation levels out. At this point the filtered dc can be
used in most types of electronic equipment. When more exact voltage or current is required, a
regulator circuit must be added.
Figure 2-2, Simple Power Supply Block Diagram
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2.2
TRANSFORMERS:
A transformer is an electrical device that utilizes electromagnetic induction to transfer electrical
energy from one electrical circuit to another. Figure 2-3 provides examples of several different
types of transformers.
Transformers are not limited to voltage step-up or step-down applications. Transformers also can be
used as impedance matching devices such as RF / IF transformers and audio output transformers.
2.2.1
Isolation:
Transformers in power supplies also serve the vital safety function of providing electrical isolation
for power-line operated equipment. Some line operated equipment does not use a transformer in
its power supply. This results in a shock hazard for anyone working on the equipment. The
transformer isolates the equipment from the power line earth ground, greatly reducing the danger
of electrocution. There is a special transformer type called an isolation transformer that has the
same input and output voltages. An isolation transformer should be used when servicing any
equipment that is line operated, but uses no power line transformer.
Figure 2-3 Transformer Examples
A power transformer is used with ac voltage only, except in very special cases. No voltage will be
induced in the secondary when a steady dc voltage is applied to the primary. If dc is placed on the
primary of a transformer, there will be a voltage induced in the secondary only for the time that
the magnetic field is building to maximum and the time when the field is falling after the dc is
removed.
There are basically two types of transformers, one with some type of core and one without a core.
An air core transformer is normally a radio frequency transformer, and one with a core is an
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intermediate frequency, audio frequency, or power transformer.
2.3
RECTIFIERS:
A rectifier can be either a tube or a semiconductor device. Its function remains the same; to
convert the ac input to a pulsating dc voltage. All rectifiers are rated according to voltage and
current capacities.
There are many variations of power supply rectifier circuit arrangements, but they can be generally
classified as either: low, medium, or high voltage circuits. Within each classification there are three
basic rectifier circuit designs; half-wave, full-wave, and bridge rectifier circuits.
The diode vacuum tube has a high internal voltage drop due to fairly high plate resistance values,
which limits its use where high values of current are needed. (Reference Figure 2-4.) The internal
resistance can be overcome through the use of solid state diodes. Semiconductors also have
internal resistance, but it is much smaller than that of a vacuum diode.
Figure 2-4, A Tube Has Resistance.
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2.3.1
Half-Wave Rectifier Circuits:
Figure 2-5 shows two of half-wave rectifier circuit configurations. Part A is a simple diode tube
rectifier, and part B is a comparable solid state rectifier circuit.
In the tube type half-wave rectifier shown in part A, when the positive alternation of the input is
present at the place of VI, it makes the plate positive with respect to the cathode. Thus a pulse of
current will flow as shown by the arrows. On the negative alternation of the input waveshape, the
plate of the tube is negative. There will be no current flowing, and thus no output voltage, as
shown in the output waveshape. As long as there is an input voltage, the tube will continue to
conduct on alternate halves of the input cycle. Because current flows only when the plate is
positive with respect to the cathode, only one-half of the input voltage is delivered to the output;
thus the name half-wave rectifier.
Figure 2-5, Half-wave rectifier.
The solid state version of a half-wave rectifier, shown in part B of Figure 2-5, will operate exactly
as does the tube version. The diode will conduct only when the anode of Diode is positive with
respect to the cathode
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In our study of ac theory we learn how to determine the peak, output an approximate average
voltages. In order find the peak dc output voltage in a half wave rectifier circuit. Consider the dc
our] pulse as follows:
Ep = 1.414 X Erms
The voltage is referenced to the bottom of the secondary winding of the transformer.
2.3.2
Full-Wave Rectifier:
The full-wave rectifier is simply two halfwave rectifiers that function on opposite halves of the
input cycle.
Figure 2-6 is the schematic diagram of a full-wave rectifier with a center-tapped transformer using
solid-state diodes. When the anode of Dl is positive with respect to its cathode, it will conduct. This
condition exists on the first cycle shown in the waveform in the figure. When conduction occurs,
current will flow through Dl and the load. Because of the negative voltage on the anode of D2 while
Dl is conducting, D2 is cut off (not conducting). However, when the second half of the input cycle
occurs, Dl will be cut off and D2 will conduct. It can be seen that there is conduction through the
load during the full cycle of the input waveshape. With the full-wave rectifier circuit using a tapped
transformer, we get an output voltage of only half the full secondary voltage. Halfwave circuits
provide the total transformer secondary voltage. At first it looks as if there is no advantage to the
full-wave rectifier because for the same total secondary transformer voltage, the output voltage is
approximately the same as it is in the halfwave rectifier. The major advantage of the full-wave
circuit is that the output frequency for the full-wave rectifier is twice that of the half-wave. With
this higher frequency the pulses can be smoothed out (filtered) with much smaller filter capacitors,
and at a much lower cost.
Figure 2-6
Full Wave Rectifier
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2.3.3
Bridge Rectifiers:
Another full-wave rectifier circuit, called a bridge rectifier, uses a total of four rectifier diodes,
working two in opposite directions, one on each half cycle of the input waveform. With four diodes
connected as shown in Figure 2-7, we have the other popular type of full-wave rectifier. This type is
known as a. bridge rectifier.
Looking at the wave forms of Figure 2-7 we can see that as the upper end of the transformer goes
positive with the ac input line, diodes Dl and D3 are both biased into conduction and current will
flow (as shown by the arrows) through the diodes and the load. As the ac line changes polarity, D2
and D4 are biased into conduction with D1 and D3 cut off. Thus current continues to flow through
the load, but on this half of the cycle, through different diodes.
Figure 2-7 Full-Wave Bridge Rectifiers.
It is evident that current flows through the load throughout the full ac line input cycle, but in the
same direction through the load. Thus the name full-wave. The bridge full-wave rectifier has the
same output waveform as that of the center-tap full-wave circuit. Both have a ripple (unsmoothed
dc pulses) with a ripple frequency of twice the applied frequency.
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2.3.4
Peak Inverse Voltage (PIV):
The major consideration in selecting rectifier diodes is the peak inverse voltage (PIV). The PIV
rating of a diode is the voltage that it must be capable of standing when it is not in the conduction
condition. In the half-wave rectifier a capacitor is placed across the output to filter (smooth out)
the pulsating dc. The effect is shown in Figure 2-8. For very high voltages, two or more diodes can
be placed in series to get a high enough PIV. The total PIV is the sum of the individual diode peak
inverse voltage ratings. Diode PIV must be 2.8 X the ac rms value.
Figure 2-8
Half-Wave Rectifier with Series Diodes for Higher PIV
In the full-wave bridge circuit, the output voltage is approximately 1.41 times Erms. The PIV and the
voltage applied to the load are the full secondary peak voltage. This PIV can be found by the
equation,
Epiv = 1.414 Erms
A very important thing to note is that the PIV for a bridge is equal to the peak secondary voltage
and not twice the secondary voltage as in the half-wave rectifier.
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3
POWER SUPPLY FILTERS
3.1
INTRODUCTION:
Power supply filters fall into two classes, depending upon which component is the input element. If
the first filter component is a capacitor, the filter is a capacitive-input filter. And conversely, if the
first component is an inductor, the filter is an inductive-input filter. Capacitive-input filters have a
high output voltage with respect to the transformer voltage, but poor voltage regulation. In
contrast, choke (inductor) input filters provide better voltage regulation, but have a lower output
voltage than a capacitive filter would have from the same transformer.
Regulation is a measure of how constant the output voltage remains in the face of changes in
current demanded by the load. The load is whatever device, circuit, or system the power supply is
intended to operate. Very few real loads draw a constant amount of current. Some loads can
tolerate varying output voltage (poor regulation), while other loads require a very constant output
voltage (good regulation).
Choke (inductor) input filters are not very common today because of the high cost, large physical
size, and weight of the choke. Filter circuits using filter chokes are fairly rare because of the
availability of inexpensive semiconductor regulator circuits that do a far better job of regulating
than a choke. Figure 3-1 is a graph showing the variation of output voltage with load for a choke
and a capacitor input filter. This curve shows the effect of a varying load on a power supply output
voltage.
Figure 3-1
Comparisons of Filters Showing Voltage Versus Current
Percent of ripple
=
Erms X 100
EDC
The frequency and magnitude of the rectified pulse voltage are the major factors in determining
just how much filtering is needed. The question is how much ripple the equipment being powered
can tolerate. The following equation shows how to determine the percentage of ripple
3.1.1
Pi (71") Filter
Because the pi (π") filter is the most common of the three, we will cover it more completely than
the other types. Figure 3-2 is the diagram of π" filters showing the waveforms of the input and the
voltages across C1 and C2. The voltage waveshape across C2 is also the output voltage of the filter.
We have used a half-wave rectifier to provide the input voltage for the filter circuit in Figure 9-13.
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Figure 3-2
Illustrates the Three Major Types of Power Supply Filters
When the first half-wave pulse appears across capacitor C1, current will be flowing and capacitor C
will begin to charge. C1 will charge to the peak voltage of the input pulse during the time the pulse
is present. During the time that the half-wave rectifier delivers no output voltage, C1 discharges
into the load through L. C1 will recharge to peak voltage, and again when the rectifier produces no
output voltage, C1 again will discharge into the load. If C1 is made large enough, it will store
enough electrons during the half-wave rectifier's output pulse to provide current to the load during
the rectifier's off time without too great a discharge. The discharging of C1 will produce an output
waveform similar to that shown in the figure.
The inductor L functions to retard any change in current flowing through it. Thus, when the
rectified current pulse is applied to the choke L, it tends to retard the change in current through it.
This retarding of the current has an effect similar to C1 but acts on current variations. Because of
the action of C1 and L, the current applied to C2 is flowing much more steadily than at the input to
the filter. Capacitor C2 charges and discharges like Cl, providing additional filtering to provide
nearly pure dc for powering electronic equipment.
In Figure 9-13B the choke L is replaced by a resistor. The resistor adds a time delay between the
charging and discharging of a single larger capacitor, but not as effectively as does a choke filter.
Choke type filter circuits are being replaced by modern solid state regulators that not only
regulate, but also provide a great deal of electronic ripple reduction (electronic filtering).
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Figure 9-13 Pi ( 11" ) filter circuit and waveshapes.
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4
REGULATORS
4.1
INTRODUCTION:
Voltage regulators are used with power supplies for different reason, depending upon what is
required. Basically most regulators are voltage regulators; however there are regulators that will
supply a set current to a source called current regulators. Within each of these wide categories are
numerous variations that are specifically suited to a specific type of regulation.
4.2
BASIC VOLTAGE REGULATOR DESIGNS:
4.2.1
Introduction:
Generally today most voltage regulators are solid state devices. The basic regulator is a IC chip
designed to provide a specific voltage up to a specified current. Such a regulator IC can be used to
control pass transistors for additional current capability. In this section we will show some sample
designs and not go into the design extensively which is beyond to scope of this guide.
It is highly recommended that a specification sheet be obtained for the specific regulator that is
used, and then filed with the reference documents of the equipment in which it is used. This also
applies to any pass transistors used.
To begin with we will look at the basic regulator then add a simple pass transistor circuit to provide
additional current, and then last we will show a design that will provide a high current output. The
designs can be modified and use additional or different transistors depending on the requirements.
Additionally we will show a “voltage source” that can be used as a reference voltage and last we
will show a simple battery charger.
It must be remembered that in order to regulate high current large heat-sinks will be required and
perhaps even fans. Using high current transistors like the 2N1016D (150 watt) or 2N2125 (250 watt)
as the pass transistor require transistors to drive them so the circuit becomes more complex since a
regulator IC can only drive a maximum amount so additional control transistors are needed.
4.2.2
LM317T Variable Voltage Regulator:
The LM317T is a adjustable 3 terminal positive voltage regulator capable of supplying in excess of
1.5 amps over an output range of 1.25 to 37 volts. This device requires a good heat-sink for safe
operation. The device has built in current limiting and thermal shutdown which makes it
essentially blow-out proof. Reference Figure 4-1
Output voltage is set by two resistors R1 and R2 connected as shown below. The voltage across R1 is
a constant 1.25 volts and the adjustment terminal current is less than 100µA. The output voltage
can be closely approximated from Vout=1.25 * (1+(R2/R1)) which ignores the adjustment terminal
current but will be close if the current through R1 and R2 is many times greater. A minimum load of
about 10mA is required, so the value for R1 can be selected to drop 1.25 volts at 10mA or 120 ohms.
Something less than 120 ohms can be used to insure the minimum current is greater than 10mA. The
example below shows a LM317 used as 13.6 volt regulator. The 988 ohm resistor for R2 can be
obtained with a standard 910 and 75 ohm in series.
When power is shut off to the regulator the output voltage should fall faster than the input. In case
it doesn't, a diode can be connected across the input/output terminals to protect the regulator
from possible reverse voltages. A 1uF tantalum or 25uF electrolytic capacitor across the output
improves transient response and a small 0.1uF tantalum capacitor is recommended across the input
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if the regulator is located an appreciable distance from the power supply filter. The power
transformer should be large enough so that the regulator input voltage remains 3 volts above the
output at full load, or 16.6 volts for a 13.6 volt output.
Figure 4-1
LM317 Stand Alone Regulator
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4.2.3
LM317T Voltage Regulator with Pass Transistor:
The LM317T output current can be increased by using an additional power transistor to share a
portion of the total current. The amount of current sharing is established with a resistor placed in
series with the 317 input and a resistor placed in series with the emitter of the pass transistor. In
Figure 4-2, the pass transistor will start conducting when the LM317 current reaches about 1 amp,
due to the voltage drop across the 0.7 ohm resistor. Current limiting occurs at about 2 amps for the
LM317 which will drop about 1.4 volts across the 0.7 ohm resistor and produce a 700 millivolt drop
across the 0.3 ohm emitter resistor. Thus the total current is limited to about 2+ (.7/.3) = 4.3 amps.
The input voltage will need to be about 5.5 volts greater than the output at full load and heat
dissipation at full load would be about 23 watts, so a fairly large heat sink may be needed for both
the regulator and pass transistor. The filter capacitor size can be approximated from C=IT/E where I
is the current, T is the half cycle time (8.33 ms at 60 Hertz), and E is the fall in voltage that will
occur during one half cycle. To keep the ripple voltage below 1 volt at 4.3 amps, a 36,000 µF or
greater filter capacitor is needed. The power transformer should be large enough so that the peak
input voltage to the regulator remains 5.5 volts above the output at full load, or 17.5 volts for a 12
volt output. This allows for a 3 volt drop across the regulator, plus a 1.5 volt drop across the series
resistor (0.7 ohm), and 1 volt of ripple produced by the filter capacitor. A larger filter capacitor will
reduce the input requirements, but not much.
Figure 4-2
LM317 with Pass Transistor
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4.2.4
High Current Regulated Supply:
The high current regulator in Figure 4-3 uses an additional winding or a separate transformer to
supply power for the LM317 regulator so that the pass transistors can operate closer to saturation
and improve efficiency. For good efficiency the voltage at the collectors of the two parallel 2N3055
pass transistors should be close to the output voltage. The LM317 requires a couple extra volts on
the input side, plus the emitter/base drop of the 3055s, plus whatever is lost across the (0.1 ohm)
equalizing resistors (1volt at 10 amps), so a separate transformer and rectifier/filter circuit is used
that is a few volts higher than the output voltage. The LM317 will provide over 1 amp of current to
drive the bases of the pass transistors and assuming a gain of 10 the combination should deliver 15
amps or more. The LM317 always operates with a voltage difference of 1.2 between the output
terminal and adjustment terminal and requires a minimum load of 10mA, so a 75 ohm resistor was
chosen which will draw (1.2/75 = 16mA). This same current flows through the emitter resistor of
the 2N3904 which produces about a 1 volt drop across the 62 ohm resistor and 1.7 volts at the base.
The output voltage is set with the voltage divider (1K/560) so that 1.7 volts is applied to the 3904
base when the output is 5 volts.
NOTE
For 13 volt operation, the 1K resistor could be adjusted to around 3.6K and an
appropriate transformer used.
CAUTION
This regulator has no output short circuit protection so the output
should be fused.
Figure 4-3
High Current Regulated Supply
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4.2.5
Simple Adjustable Voltage Source:
A simple but less efficient method of controlling a DC voltage is to use a voltage divider and
transistor emitter follower configuration. Figure 4-4 illustrates using a 1K pot to set the base
voltage of a medium power NPN transistor. The collector of the NPN feeds the base of a larger PNP
power transistor which supplies most of the current to the load. The output voltage will be about
0.7 volts below the voltage of the wiper of the 1K pot so the output can be adjusted from 0 to the
full supply voltage minus 0.7 volts.
Using two transistors provides a current gain of around 1000 or more so that only a couple milliamps
of current is drawn from the voltage divider to supply a couple amps of current at the output. Note
that this circuit is much less efficient than the 555 timer dimmer circuit using a variable duty cycle
switching approach. In the figure below, the 25 watt/ 12 volt lamp draws about 2 amps at 12 volts
and 1 amp at 3 volts so that the power lost when the lamp is dim is around (12-3 volts * 1 amp) = 9
watts. A fairly large heat sink is required to prevent the PNP power transistor from overheating. The
power consumed by the lamp will be only (3 volts * 1 amp) = 3 watts which gives us an efficiency
factor of only 25% when the lamp is dimmed. The advantage of the circuit is simplicity, and also
that it doesn't generate any RF interference as a switching regulator does. The circuit can be used
as a voltage regulator if the input voltage remains constant, but it will not compensate for changes
at the input as the LM317 does.
Figure 4-4
Simple Ajustable Voltage Source
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4.2.6
2 Cell Lithium Ion Chargers:
This circuit, SHOWN IN Figure 4-5 was build to charge a couple series Lithium cells (3.6 volts each,
1 Amp Hour capacity) installed in a radio.
The charger operates by supplying a short current pulse through a series resistor and then
monitoring the battery voltage to determine if another pulse is required. The current can be
adjusted by changing the series resistor or adjusting the input voltage. When the battery is low, the
current pulses are spaced close together so that a somewhat constant current is present. As the
batteries reach full charge, the pulses are spaced farther apart and the full charge condition is
indicated by the LED blinking at a slower rate.
A TL431, band gap voltage reference (2.5 volts) is used on pin 6 of the comparator so that the
comparator output will switch low, triggering the 555 timer when the voltage at pin 7 is less than
2.5 volts. The 555 output turns on the 2 transistors and the batteries charge for about 30
milliseconds. When the charge pulse ends, the battery voltage is measured and divided down by the
combination 20K, 8.2K and 620 ohm resistors so that when the battery voltage reaches 8.2 volts,
the input at pin 7 of the comparator will rise slightly above 2.5 volts and the circuit will stop
charging.
The circuit could be used to charge other types of batteries such as Ni-Cad, NiMh or lead acid, but
the shut-off voltage will need to be adjusted by changing the 8.2K and 620 ohm resistors so that the
input to the comparator remains at 2.5 volts when the terminal battery voltage is reached.
For example, to charge a 6 volt lead acid battery to a limit of 7 volts, the current through the 20K
resistor will be (7-2.5)/ 20K = 225 microamps. This means the combination of the other 2 resistors
(8.2K and 620) must be R=E/I = 2.5/ 225 µA = 11,111 ohms. But this is not a standard value, so you
could use a 10K in series with a 1.1K, or some other values that total 11.11K
Be careful not to overcharge the batteries. I would recommend using a large capacitor in place of
the battery to test the circuit and verify it shuts off at the correct voltage.
Figure 4-5
Cell Lithium Ion Charger
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AM 5 – 206 Basic Electronics - Power Supplies
5
HIGH VOLTAGE POWER SUPPLIES
5.1
INTRODUCTION
The high voltage, or HV, power supply is in a class by itself. They have unique problems especially
if they are required to supply very high voltages or much current. Consideration must be made for
the safety of the user since High Voltage can kill, or at the least cause serious injury.
Many High Power RF amplifiers often require 1,500 to 3,000 volts at 2,000 milliamperes or more, if
the final is a tube. Many of the older HV power supplies had Mercury vapor rectifiers, like the 866A
rectifier. They gave off a very pleasant blue glow in the shack when you were making that contact
with that rare DX station on Ascension Island in the South Atlantic late at night. There is something
about that glow of Mercury vapor rectifiers under load in a radio room along with the slight ozone
smell that high voltage can give the air that is exciting.
Many of the newer solid state amplifiers do not use very high voltages but they do use high current
supplies which will be covered later. Remember Ohms law if the RF power required is 1,500 watts
then the either the voltage or the current must be high. One thousand volts would require at least
1.5 amps. Generally it is easier to have the voltage higher and the current less so to get 1,000
watts you would use 2,000 volts that would make the current requirement 750 milliamperes, a
considerably less amperage requirement than the first statement. It is easier to make higher
voltage than higher current. To go the other way, in order to have 1,500 watts and a voltage input
of 12 volts, then it would require 125 amps of current do be able to get the power required. High
current requires larger wire for example in order to carry that much amperage. 125 amps would
require wire in the order of 00 size. That would be a little less that ¼ inch in diameter, a very
difficult size to use in close quarters. That is the reason that higher voltages are used even with
transistor amplifiers where voltage requirements are often 100 volts or more. At 100 volts the
current required for 1,500 watts would be 15 amperes, which is still a rather high current requiring
a with in the order of a 12 gauge wire. So you can see that there is a trade off when ever higher
power devices are involved
The higher voltages do require better insulation, but that is easier to deal with that the much
larger wire. This is not considering that high current low voltage devices generally require a very
much higher heat sink requirement which will be easier to deal with.
5.2
HIGH VOLTAGE POWER SUPPLY DESIGN
The design of a High Voltage power supply is not complex but there is a very definite requirement
for considering: Safety, voltage insulating requirements and voltage ratings of the various
components. The components are like any basic power supply, as can be seen in the block diagram
Figure 5-1.
The major differences in the power supplies are the PIV for the rectifiers and the voltage ratings
for the other components. The PIV selected should be at least half again the operating voltage.
Thus if the power supply is supplying 2,000 volts then the rectifiers should have a PIV of at least
3,000 volts, a PIV of twice the operating voltage would be safer.
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AM 5 – 206 Basic Electronics - Power Supplies
Figure 5-1, Basic High Voltage Power Supply
The diode must also have a current rating of approximately 50% more that the output current of the
supply and have sufficient heat sink for the current required. It is a good general rule to have all
voltage and current ratings with more that 150% of the operating voltage and current as a
minimum.
The data for PIV, current rating and heat sink required can generally be obtained from the
Specification Sheet for the device.
Figure 5-2 shows a 3,500 volt 400 milliamp supply that the author built using a pair of mercury
vapor rectifiers, or with the Solid State replacement for the tubes the supply could supply 4,100
volts at 650 milliamperes. It was used to supply a class C amplifier using a 450TH triode tube. It
would run cold at 1,000 watts key down and care was needed to keep the power under the legal
maximum.
Figure 5-2
High Voltage Power Supply
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AM 5 – 206 Basic Electronics - Power Supplies
6
POWER WIRING FOR MOBILE RADIOS
Regardless of the type of power supply used, you should design cabling from power supply to radio
to minimize voltage loss. This loss occurs because wire is not a perfect conductor of electricity.
Longer distances between a power supply and a radio, the greater the voltage drop will be. A power
supply should not be more than 25 feet from a radio. (Reference Figure 6-1)
There are two reasons for this: First, voltage drop experienced over 25 feet, even when large
conductor cabling is used, is not acceptable because performance (such as a lowered transmitter
output power) can decline, second, with a longer power cable, more chance of failure because of
any number of unforeseen circumstances.
FIGURE 6-1
DC Power Cable Sizes for a Mobile Transmitter
Electricity in wire behaves somewhat like water in a garden hose. The pressure of the water is
analogous to voltage (electrical "pressure"), internal friction is analogous to wiring resistance and
size of the hose is analogous to size of the wire. When electricity flows from the power supply to
the radio at low current levels, the internal resistance of the wire has little effect. But as current
increases, voltage drop becomes more pronounced. Remember the garden hose? When you shut off
the nozzle, hose pressure is high. Voltage in the wire is also high. But turn on the nozzle, and
pressure along the hose drops; and you can get only so much water through. Now apply this effect
to your power wiring: Turn on the radio a little bit, such as receive mode where little power is
drawn, and things will likely be just fine with #16 or #14 wire at 25 feet. But turn on transmitting
portion and nothing will work correctly because the voltage drops dramatically.
Let's consider wire sizes between power source and radio. At 25 feet, #6 is recommended. At 12
feet, #6, is also recommend and at 2 feet, you could use #8 but #6 would be better.
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