1. Chapter 8, Harris
CRYSTAL SETS TO SIDEBAND
© Frank W. Harris 2010, REV 12
Chapter 8
POWER SUPPLIES
Once you progress past crystal sets, electronic projects almost always need a power
supply. Low power projects, like a small receiver, can be powered with flashlight batteries or
even a little 9 volt “transistor” battery. Other convenient sources of low power DC are adapter
plugs. These are the black cubes a few inches square that plug into the wall and have a long,
skinny cord that plugs into your recorder or small appliance. They deliver 6, 12, or other DC
voltage at a few watts. Adapter plugs have the safety of a battery with the convenience of wall
power. On the other hand, their voltage may have high AC ripple noise and need to be filtered.
If you plan to power your QRP with household power, you’ll need more than a few watts.
A 5-watt transmitter needs at least 10 watts of power with good voltage regulation. Otherwise
AC ripple on the DC will go right out over the air as a hum or buzz. When you decide to plug a
homebuilt circuit into the wall, you must confront some significant safety issues. Line-powered
power supplies convert 120 volts AC into DC voltage at the required levels of voltage and
current. Actually, a transmitter can also be thought of as an energy conversion device. It
converts direct current into radio frequency current. In this chapter I’ll describe some power
supplies you could use for your QRP transmitter.
Line-powered power supplies
The ideal power supply is called a voltage source. A voltage source is a power supply
that can supply infinite amounts of current without the slightest waver in the voltage. For
example: As every northern resident knows, starting a car can be difficult when it’s below zero.
A cold battery does not supply as much current as a warm battery. So, when you turn the ignition
key on a frigid morning, the battery voltage crashes.
On the other hand, if you had a battery the size of North Dakota, the voltage would not
drop a microvolt when you started the engine. Moreover, you could start all the other cars in
Minnesota simultaneously without voltage drop. Of course, there are also other practical issues
here. For example, your battery would need zero resistance battery cables, zero resistance
2. Chapter 8, Harris
connectors, etc. Well, you get the idea: The ideal voltage source should not lose any voltage, no
matter how much power it supplies. Using ham radio vernacular, a good power supply is a
“stiff” supply.
Lab power supplies
A reliable, line-powered laboratory power supply is useful for checking circuit boards.
No lab should be without one. A big advantage of commercial lab supplies is that the voltage is
adjustable from zero to some high level like 20 volts. Meters show you the current and voltage at
every moment, so you know what is happening. Variable voltage allows you to power up a new
circuit CAREFULLY. You can start with a few tenths of a volt and see what happens. If the
circuit is shorted, you can find out with one volt applied to the circuit board, rather than blasting
it with 12 volts right away. This helps you avoid burning up expensive transistors. Many lab
supplies put out two or even three separate supply voltages at once. Another feature of some lab
power supplies is that they automatically limit the available current to some maximum that you
select.
Bench power supplies are quite generic and there are many modern ones that will serve
you well. Remember, to power your QRP you need about 1.0 ampere at 12 volts DC. A typical
modern, transistor QRP transmitter runs on a 12 volt power supply but its efficiency is only about
50%. Therefore,
10 watts = 12 volts x 800 milliamperes
Homebuilt power supplies for use with rechargeable batteries or line power are described
below. If you aren’t familiar with power supply design, a discussion of the basic principles
follows.
Simple wall-powered supplies for 120 Volts AC
3. Chapter 8, Harris
The diagram above illustrates the simplest, safe, generic, line-powered power supply you
can build. Unfortunately, this supply is too poorly regulated to power a transmitter. However, it
illustrates the minimum safety features and it’s easy to explain.
The following discussion assumes that the reader lives in North America where the
standard household line voltage is 120 volts AC RMS @ 60 Hz. The safety issues explained
here are applicable to other regions of the world. However, voltages, connector types, wire color
codes, and ground configurations are often different. For example, in Europe the standard is 220
volts AC RMS @ 50 Hz.
Power supply safety
Metal enclosures. The supply should be enclosed in a box to insure that children (and
you) won’t get fingers across the 120 volts AC. Ideally the box should be made of metal so that,
in case of a short circuit, a fire is highly unlikely.
Another safety design philosophy is called double insulation. In this scheme the
electronics are housed in a plastic box and extra effort is made to insure that the internal wires
are properly protected so that shorts and loose wires are highly unlikely. A double insulated
plastic box does not necessarily need a ground wire in the line cord. However, in ham work,
metal boxes shield circuitry from stray radio waves and are usually the best choice.
Line cord. The line cord should be the modern, three-wire type with the (green) ground wire
securely connected to the metal box. In case a loose wire in the box causes the hot side of the
AC line to touch the metal box, the ground wire will safely shunt the AC current to ground.
The line cord should pass into the metal box through a rubber grommet so that the metal
edge can’t cut through the insulation on the wire and cause a short circuit. Once inside the box,
the cord should be held captive by a clamp, properly known as a strain relief. The strain relief
insures that if the power supply is ever yanked by its cord, the live wires will not be ripped loose
and short out.
4. Chapter 8, Harris
The wires in a line cord are usually color-coded. The “hot” wire usually has black
insulation while the “neutral” or return wire is white. The third green safety wire is connected to
power line ground. It should be connected directly to the metal chassis. The neutral wire is also
supposed to be connected to the house ground buss out in the circuit breaker box. Looking at a
North American household three-prong socket, the round pin is the “ground” and is connected to
the green wire. The wider, flat pin is the hot side and the narrower flat pin is neutral.
Unfortunately, sometimes wall sockets are wired wrong, so it’s better not to bet your life on the
orientation of the flat pins.
Fuse. The first destination of one of the two power wires, preferably the “hot” black lead, should
be a fuse. As you probably know, fuses are little pieces of solder-like lead mounted in a glass
case. When the current exceeds some calibrated level, like one ampere, the lead melts and the
circuit opens. Fuses, of course, can only be blown once and can’t be reused. A fuse is
represented on the diagram by the squiggle in the diagram labeled “1A,” meaning “one ampere.”
The electrical standards allow a fuse to be considerably larger than necessary, like 5 amperes and
still give adequate protection against shorts. Small circuit breakers serve as reset-able fuses and
are available in low current levels such as 3 or 5 amperes.
Switch. The power switch can switch just one side of the line, such as the single-pole, singlethrow (SPST) switch shown above. Or, it is even safer to switch both sides of the power line at
once using a double-pole, single-throw switch (DPST). The switch should be rated for at least
125 volts AC and 3 amperes.
The transformer
After the power switch, the line current usually goes to the primary winding of a
transformer. The transformer has two functions: first, it isolates your power supply from the
household supply and from ground. This makes electrocuting yourself much less likely. As
explained earlier, the AC lines supply 120 volts AC referenced to ground. The transformer
secondary delivers AC power that has no relation to ground at all.
For example, I don’t recommend actually trying this, but suppose you were to plug a
well-designed transformer into a wall socket. And suppose that this transformer has high voltage
secondary wires left dangling open circuit: Because of the isolation, you could touch either
secondary wire without being shocked, even if your other hand were hanging onto a grounded
water pipe. Of course if you touch both high voltage wires simultaneously, they will blast you.
A secondary winding is isolated from ground - like a battery floating in mid-air
Think of isolation as a battery hanging from a balloon. Electric circuits require a closed
loop in order for current to flow. For the dangling battery, the current can only flow from one
end of the battery to the other. There is no relationship to ground. If a person standing on the
ground reaches up and touches the battery, no circuit loops are completed, so no current flows.
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As explained in Chapter 2, transformers can change the ratio of current to voltage in
proportion to the number of turns of wire around the core. To convert 120 volts AC to 12 volts
AC, the turns ratio between primary and secondary coils would be 10:1. Or, if you needed to
generate 1200 volts AC for an oscilloscope cathode ray tube (CRT), the turns ratio would be
1:10.
Transformers are not 100 % efficient. They are made from copper wire that has a
significant resistance and iron that dissipates a small amount of energy as heat every time a
magnetic field is generated or its polarity is reversed. In general, the larger the transformer, the
larger the diameter wire used on the windings, the higher the efficiency will be.
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Iron has a definite and abrupt limit on how much magnetic field it can support. Once all
the iron has been magnetized, the iron will contribute no more magnetic field, no matter how
much more current flows through the primary. Obviously, the larger the iron core, the more
energy the iron can pass on to the secondary before the iron saturates. As a general rule, the
larger the transformer, the more power it can pass through its windings.
Rectification
Most electronic devices require DC voltage to work properly. AC is converted to DC by
means of rectifier diodes. Rectifiers are high-power, high-current, high-voltage versions of the
diodes used in crystal sets. Referring to the simple power supply shown earlier, the transformer
is followed by a rectifier diode. The diode only passes positive current flow in the direction of
the arrow. This means that current leaving the diode is in the form of “humps,” or half sine
waves. Only half of the sinewave is passed through, so a single diode rectifier is called a halfwave rectifier. By definition these humps are “DC” since they have only one polarity.
Unfortunately, for most electronics applications bumpy, intermittent sine-wave halves are grossly
inadequate. For example, if you use them to power a CW transmitter, your signal will sound like
an unpleasant buzz and it will take up 120 Hertz of bandwidth. In the 1920s this was often done
deliberately to modulate Morse code and make it audible in a receiver without a BFO. If you
tried to run a computer microprocessor on these humps without filtering, the processor would
reset 60 times per second.
Peak Volts, RMS volts, and DC volts
Obviously sinewaves don’t stand still at any one voltage, so how are they measured? If
the sinewave reaches peaks of plus 12 volts and minus 12 volts, then most of the time the output
from the rectifier will be much less than 12 volts. Also, the positive voltage seems to cancel the
negative voltage so that the arithmetic average of any sinewave voltage is zero. Obviously that
measurement isn’t useful either.
Rules are needed to name sinewave voltage and current. As you might expect, peak
voltage is the voltage difference between zero and the most positive extreme of the waveform.
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Peak-to-peak or PP voltage means the voltage difference between the most negative peak and
the highest positive peak.
As you may know, the voltage that comes out of North American wall sockets is
officially named 120 volts AC RMS. During my lifetime American line voltage has also been
nominally called “110 volts AC,” “115 volts AC,” “117 volts AC,” and now it is called "120
volts AC." Confusing, no? Anyway, the two power wires coming into your house from the
power pole out in the alley nominally have 240 volts AC across them. Of course it was formerly
called two times 110 volts AC or 220 volts AC. Some people still call it 220 volts. Line voltage
is designed so that each of the two wires from the power pole is at 120 volts AC with respect to
ground.
As explained in chapter 2, inside your circuit breaker box these two wires are connected
to two big metal “buss” bars. There is a third, grounded metal bar that runs down between the
two active buss bars. The individual circuit breakers snap onto these bars like cars on a railroad
track. Heavy power circuits for your electric stove and clothes drier clip clear across the ground
buss to engage both hot lines for a total of 240 volts. Small circuit breakers just clip from one
side to the ground bar to obtain 120 volts for ordinary low-power circuits.
The RMS or Root Mean Square of a sinewave voltage is the peak voltage divided by the
square root of two. RMS voltage can be thought of as "the effective average voltage." It can be
used to calculate AC RMS current, power, and AC resistance and impedance. For example, 120
volts RMS is a sinewave with a peak voltage of 1.414 times the RMS voltage. In other words,
120 V RMS x √2 = 120 x 1.414 = 169.7 Volts Peak. Therefore, ordinary household line
voltage could be expressed as 120 Volts RMS, 170 Volts Peak or 340 Volts Peak-to-Peak.
Filter capacitors - filtering out the ripple
The half sinewave bumps, which are properly known as “ripple,” must be smoothed out
into a continuous DC voltage. This is accomplished by means of a low pass filter. In this case
low pass means that the filter only passes frequencies well below 60 Hz. DC is of course zero
Hertz, which is the lowest frequency possible. The half-wave rectified supply illustrated earlier
is equipped with an L-C “L” filter. L- filters are simply two-element filters that represent the two
legs of the letter “L.”
Capacitors conduct AC and prevent the flow of DC. Inductors conduct DC but resist the
flow of AC current. In an effective DC filter, the component values will be huge, like 10 henries
and 5,000 microfarads. This is because the frequency we want to filter out, 60 Hz, is extremely
low and big components are needed to have an effect on such a slowly changing sinewave.
Bleeder resistor across the capacitor
The purpose of the bleeder resistor across the filter capacitor is to discharge it when the
supply is not in use. Remember that high quality capacitors will hold their charge for many
hours, sometimes days. Bleeders usually aren’t important with a low voltage supply like 12
volts. But if this were a 500 volt supply, a person could get a nasty jolt or burn if they were to
touch the capacitor. This could happen even though the supply is no longer turned on or plugged
in.
If you were to build the half-wave supply shown earlier and put a 10 watt load on it, with
8. Chapter 8, Harris
an excellent transformer and a really huge capacitor, perhaps the DC voltage will at least be
continuous. However, there will still be a 60 Hz sinewave ripple or waves impressed on top of it.
If you used it to power a transmitter, the tone of the Morse code would have a distinct rough
sound as the DC level varies at 60 times per second. When hams gave you a signal report they
would say that your signal tone was a number much less than 9. For example, they might send
“UR RST 593.” (RST means Readability, Strength and Tone. Hams use a scale of 1 to 5 for
readabilitity, and 1 to 9 for both strength and tone.) Because of the harsh note of your tone, you
might only rate a “3.” For this reason, transmitter power supplies always use dual rectifiers to
produce full-wave rectification.
Full-wave rectifiers
Full-wave rectifiers convert both halves of the sinewave into useful DC current. The DC
voltage is now a succession of “humps” with no “off” intervals. With twice as many “humps”
per second, the voltage is much easier to filter. Full-wave rectification is a big step toward
producing a DC source that resembles the smooth continuous voltage available from a battery.
There are two ways to achieve full wave rectification. The circuit above uses two diodes.
What you probably didn’t notice at first is that the secondary of the transformer has TWO 12 volt
RMS AC windings. By having two separate windings, and wiring them in series, one of the
windings can be positive at all times. This allows positive current to flow through one of the two
diodes at all times and greatly decreases the ripple. Notice that, if we wanted, we could reverse
the polarity of the diodes and produce the same waveform with the opposite polarity. That is, if
we wanted a NEGATIVE voltage source referenced to ground, reversing the diodes would do
that.
Bridge rectifiers
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The second way to achieve full wave rectification is to use a bridge rectifier made from
four individual diodes. This configuration allows us to get full wave rectification from a single
secondary winding. The four diodes are soldered in a diamond pattern as shown above. The AC
voltage source is applied across the top and bottom of the diamond. The two diodes on the right
are pointed so that the positive current will always flow to the positive side. The left side is
wired to ground and the diodes point in such a way the negative current is always vectored
toward ground. To say it another way, the positive current always flows “UP” from ground.
Ripple
Because the rectifier is supplying current in the form of “humps,” the voltage output
across the capacitor will also vary up and down. If the capacitance is large the voltage doesn’t
drop to zero during the “valleys,” but it can drop fairly low if the current drain from the supply is
large. The more current that is sucked out of the capacitor, the lower the voltage “valleys.” This
is illustrated in the drawing above. If the choke is large enough, it will work with the capacitor to
smooth out the height of the voltage peaks and raise the level of the valleys dramatically.
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With no load current, output voltage charges to the peak voltage
Notice that, if the power supply is not connected to an external load, the output voltage
will rise up to the maximum voltage that comes through the rectifiers. For a 12 volt RMS
transformer output winding, this is approximately the square root of two (1.414) multiplied times
12 volt RMS volts, or about 18 volts. Referring to the diagram, with no external load the only
load on the capacitor is the tiny one milliampere current passing through the bleeder resistor.
This means that the variation in voltage between the humps will be extremely small. In
summary, if there is no load on the power supply, to a voltmeter it looks like a “regulated” 18
volt power supply with essentially no ripple. If your 12 volt circuit might be damaged by 18
volts, you must not connect a supply like this to your circuit.
When you first turn it on, the voltage will be 18 volts for a moment before the load
current flow is established and the choke and capacitor pull the peak voltage down. RMS (Root
Mean Square) refers to the AVERAGE voltage of the rectified sine wave “humps.” If the output
from the big capacitor were fed directly into your QRP without a regulator, the capacitor would
charge toward the peak voltage of the “humps.” Depending on how much current your QRP was
drawing, 18 volts might destroy some of the components in your QRP. As the load on the power
supply is increased, the output voltage would drop down toward the rated RMS voltage, 12 volts.
As the rated transformer load, say 3 amps, is exceeded, the DC voltage will probably drop below
12 volts.
Power transformer saturation
A second purpose of the choke (series inductor) is to prevent the peak current from the
transformer from exceeding the current rating of the transformer. If the choke were not located
between the rectifier and the capacitor, the current from the “humps” would only flow into the
capacitor when the voltage from the rectifiers is higher than the voltage already stored in the
capacitor. It is like a tidal pool alongside the ocean. The pool can only fill with water when the
tide rises higher than the level of the pool. The result of these pulses of charging current is that
the power transformer must provide much more peak current than it may be rated for. If a
transformer is rated for, say 10 amperes RMS, and these surges of current are drawn in bursts just
a few microseconds long, then the peak current might be 100 amperes. Since the transformer
doesn’t have enough iron for that, the iron saturates and the transformer inductance momentarily
collapses. Suddenly, the transformer will act like a few turns of copper wire shorting out the AC
source. Saturation causes the windings and the transformer to heat rapidly and perform poorly.
Substituting big chokes with small, cheap resistors
In the real world, most low voltage power supplies like this solve the problem with a
cheap resistor instead of a large, expensive iron core inductor. Resistors waste energy, but what
the heck! You can use the resistor with an extra-large filter capacitor that costs less and weighs
less than a choke with equivalent filter value. Or, as we’ll see shortly, a linear voltage regulator
can put a load on the capacitor all the time so that the current flow is not just in short bursts.
Regulators
To provide pure DC at a constant voltage over a wide range of load current, you need a
regulated power supply. The regulator’s first task is to “trim off” unwanted peak voltage and
provide a DC voltage equal to (or similar to) the transformer rated RMS voltage. Regulators
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solve the 18 volt over-voltage problem described above. Its second purpose is to maintain
constant voltage even when the load resistance is changing continuously or during a line voltage
brown out. A regulator circuit is usually added to a power supply like the one above. There are
two basic designs for regulators, linear regulators and switching regulators.
Linear regulators are a sort of automatic variable resistor placed in series with the
output of a simple supply like the ones we have been discussing. The regulator uses feedback
from the load voltage to change the size of the “automatic resistor” and hold the load voltage
constant. For example, in the above power supply the regulator input voltage might vary from
say 15 to 18 volts, but the regulator would change its resistance to hold the output constant at 12
volts DC. A linear regulator not only insures that the load voltage is always the same, it also
“trims” off the ripple.
Switching regulators are more complex circuits that usually involve inductors (or
transformers) and switching transistors. They start with unregulated DC and turn it back into AC
power. This AC power is then passed through a transformer to generate whatever voltage is
needed above or below the original DC voltage. In an equivalent method, the unregulated DC is
pulsed through an inductor to generate higher or lower voltages. Some switching regulators
work directly off the household line. In other words, the 120 volts AC is rectified without a
transformer and results in roughly 120 volts DC that is then converted into AC to drive a small,
high frequency transformer or a step-down inductor. For amateur radio work, switchers usually
make radio noise that you will hear in your receiver. Yes, commercial radio equipment often use
switchers in their designs, but in my experience, getting rid of the switching noise is extremely
difficult.
In contrast to switching regulators, some switchers just boost the voltage, but don’t
regulate the output voltage. These are often called charge pumps. The noise from these
unregulated charge pumps can be insignificant because they only switch at one frequency and
they don’t “dither” back and forth trying to hold the output voltage constant. To summarize,
regulated switchers are noisy and I have pretty well given up trying to use them inside ham
equipment.
Zener diode regulators
The simplest regulator is made from a Zener diode. It is a “linear regulator” because
there are no abrupt pulsed signals involved and it works entirely with DC. It dissipates the
unwanted voltage as heat. A Zener diode is a modified silicon diode subtly different from the
rectifiers used in the power supplies above. Zener diodes are made from silicon that has been
doped (contaminated) with extra ions. The extra ions cause the diode to break down at a specific
lower reverse voltage when a high reverse voltage is applied. Zener diodes are deliberately
manufactured to be inferior rectifiers, but when used as regulators, they are quite useful.
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Why ordinary rectifiers don’t behave like Zeners
Diodes rectify because the reverse voltage is insufficient to force electrons through the Pside of the P-N junction that has no free electrons. However, when enough voltage or “electrical
force” is applied, the P-N barrier breaks down in an avalanche breakdown. When this
breakdown happens at high voltage and high current, the sudden heat generated often ruins the
diode (or bipolar transistor). You may remember the homemade crystal diode discussed in
chapter 4. When this crude diode was reversed biased, it broke down abruptly at about one volt.
At any voltage above 1 volt, it acted like a short circuit and regulated the load voltage to one volt.
At low voltage levels, such a breakdown doesn’t necessarily destroy the diode if the diode
doesn't overheat. This Zener voltage level can also be used as a voltage reference.
Doping silicon diodes makes Zeners
When big, tough, modern 400 volt silicon diodes are “doped” with extra ions mixed into
the semiconductor, the avalanche can happen at lower voltages anywhere below 400 down to as
little as 3 volts. Because the load voltage is low, and the load resistance presumably limits the
current to a safe level, the heating in the silicon is mild enough that the diode survives the
breakdown. In practice, Zener diodes are available from about 3 to 50 volts. 100 volt Zeners
would have to be capable of dissipating a great deal of heat or they would be easily destroyed.
In the circuit above the Zener diode breaks down at 5 volts. If one of these diodes is
placed across a load, the Zener diode will clamp the voltage to 5 volts so the voltage across the
load never rises above 5 volts. Of course the input voltage must always be higher than 5 volts
and the unwanted voltage will be dissipated across the resistor in series with the power supply.
Ohm's law tells us that at least half of the resistor current must pass through the Zener on its way
to ground. So the Zener diode itself is dissipating as much energy as the intended load. As you
can see, Zener diode regulators have poor energy efficiency.
Real versus ideal Zeners
If a Zener diode behaved “perfectly,” the voltage across it would be the Zener voltage, no
matter how many amperes flowed through the diode. This is illustrated by the green curve in the
graph below. Unfortunately, the Zener voltage rises with large currents as shown below on the
red curve. Notice that when the Zener diode is wired backwards, it acts like a normal, forwardbiased silicon diode. Conduction starts at about 0.6 volts. What is called “forward” conduction
for a normal diode is called reverse conduction for a Zener diode.
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Zener diodes are impractical
for heavy current regulation.
They not only dissipate energy in
the series resistor R, they also
burn up energy in the diode.
Moreover, as the diode current
rises, the Zener voltage can be
significantly different from its
nominal value. In practice Zener
regulation is used for light loads
over a narrow range of input
voltage. As you will see later,
Zeners can be used as
VOLTAGE REFERENCES to
run a transistor regulator. In this
way, the regulation can be fairly
good and energy isn’t wasted.
This will be illustrated in an application later in this chapter.
A homebuilt, line-powered power supply for a QRP
Now let’s be practical: The 12 volt supply below worked well for me and can be
assembled entirely from parts at your local Radio Shack.
This 18 watt regulated line-powered supply should be built in a metal chassis. The round
ground pin from the line cord should be connected to the chassis. In that way, if the “hot” black
wire should break and touch the chassis, the metal would not become dangerous to touch. A 2 to
5 ampere fuse in series with the line input protects the supply. The ON/OFF switch should be
rated for 250 volts DC and at least 1 ampere. The line cord should pass through a rubber
grommet on the way into the chassis. Once inside, the line cord should be clamped to the chassis
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so that, if someone picks up the power supply by the cord, the force will be on the clamp and not
on the solder joint where the line cord is fastened to the fuse and switch.
The transformer is rated at 12.6
volts RMS at 3 amperes. I used Radio
Shack part # 273-1511. The rectifier is a
generic silicon bridge rectifier rated for at
least 50 volts @ 3 amperes. Alternatively
you could use 4 individual rectifiers
arranged as shown above. The filter
capacitor following the rectifier just needs
to be large and greater than about 25
working volts. The other three capacitors
serve to stabilize the output and prevent
oscillations and ripple. Adding a 12 volt
dial light so you will know when it is on is
a nice touch.
The LM317K programmable voltage regulator
The LM317K regulator chip is packaged in a TO-204 (formerly known as TO-3) metal
case and can deliver 1.5 amperes. To dissipate the heat, the TO-204 case should be bolted to the
metal chassis and insulated by means of a mica washer and silicone grease. Mounting kits for
this purpose are also available at Radio Shack. This regulator chip works by regulating the
voltage between the Vout terminal and the ”adjustment” lead on the regulator. The regulator
regulates this voltage difference to 1.2 volts. This low voltage allows the designer to regulate
voltages equal to or higher than 1.2 volts. In this 12 volt application, the regulator passes current
through a 240 ohm resistor and regulates the voltage across the 240 ohm resistor to 1.2 volts.
This results in a “current source” that drives a constant current through the 2.2K resistor to
ground. This increases the total regulated output voltage from 1.2 up to 12 volts. In other words,
240 ohms is about 10% of the sum of 240 ohms plus 2200 ohms. Because you can “program”
the total regulated voltage, this same regulator can be used to regulate voltages from 1.2 volts up
to about 20 volts.
Another reason you need good voltage regulation is that the tuned stages in your QRP
don’t take kindly to changing the supply voltage. If you tune up the antenna using 12 volts, and
the voltage later goes up or down, the tuning of some stages may change slightly and your signal
might crash in the middle of a QSO (conversation).
The LM317K “clips off” any voltage above 12 volts so your QRP will never see a higher
voltage. Notice that the LM317K is a linear regulator. This means that it turns any input
voltage that is above the regulation voltage into heat. This seems wasteful, but linear regulation
makes no radio noise and produces a flat output voltage with practically no ripple. Notice that
the choke (or a resistor) is not necessary here because the regulator is drawing current from the
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capacitor nearly all the time. This means that the current is flowing from the transformer nearly
all of the time and will not be saturated by high current pulses.
This power supply violates my goal to never use integrated circuits. If there are purists
who share my aversion to integrated circuits, the “low dropout” regulator shown below can be
substituted for the LM317K regulator. A larger power supply than this would be more versatile
and could power a bigger transmitter. Obviously, the higher the power level, the more
cumbersome the regulation becomes. Running a 100 watt transmitter requires a 300 watt
regulated power supply. I sidestepped this project by running my entire ham station on a solarcharged 12 volt battery which is described later in this chapter.
A low dropout voltage regulator for use with a battery
As you will read shortly, two 6-volt lead-acid golf-cart batteries make a fine, high-current
12 volt power supply. Unfortunately, running your transmitter on batteries means that, when the
battery is ready to be recharged, its voltage will drop down to 11 or even 10.5 volts. This means
that (ideally) your QRP must be tuned up to run on 10.5 or 11 volts and then regulated to that
level. Remember that a linear regulator can only deliver LESS than its input voltage. So if
you’re running on a 12 volt battery, you need a regulator that wastes as little voltage as possible.
This waste voltage is called dropout. The regulator shown below receives nominal 12 volts from
a battery. The active regulation element is a big P-channel MOSFET transistor. This transistor
can be arbitrarily large. The bigger it is, the less voltage it will waste across its internal
resistance and the lower the “drop out.”
The P-channel MOSFET
transistor turns on (conducts current)
when its gate voltage is pulled
downward toward ground. So when
the battery voltage drops, the gate
voltage must be pulled down
(toward zero volts) to turn the
MOSFET more on.
When the
battery input voltage rises, the gate
voltage must be raised to turn the
transistor more off and restore the
output to the set voltage.
The gate voltage control is done with an NPN bipolar transistor. The transistor compares
the reference voltage across a 5 volt Zener with a fraction of the output voltage across the sense
resistor pot on the lower right. The slider on the sense resistor contacts the resistance at a level
that produces about 0.6 volts less than 5 volts. As the output voltage rises, the voltage on the
sense resistor rises. This in turn increases the 2N3904 emitter voltage. The difference in voltage
between the base and emitter drops, causing the drive to the NPN transistor to drop. As the NPN
transistor turns more off, the gate voltage on the MOSFET rises toward the battery voltage and
turns the MOSFET more off and lowers the regulated output voltage.
16. Chapter 8, Harris
The purple plot on the left shows the
performance of this simple power supply when
it has been set to 9 volts with a 300 milliampere
load. The red line shows the voltage the load
would receive if there were no regulation. That
is, Vin = Vout. The green line illustrates the
performance of an ideal or perfect linear
regulator. If the transistors had infinite gain and
the Zener diode always produced precisely the
same reference output voltage, then the green
curve is what you would get. As soon as the
regulator is given 9 volts input, it would deliver
exactly nine volts, no matter what the load was.
As you can see, the simple regulator isn’t
radically different from the ideal, but it is far
from perfect.
A precision, temperature-compensated, low-dropout regulator
More nearly perfect regulation can be achieved by replacing the NPN transistor with an
operational amplifier. Operation amplifiers, “op-amps,” are integrated circuits composed of
many or even dozens of transistors. Op-amps perform as though they were nearly perfect
transistors. Similarly the LM336 voltage reference is an integrated circuit made from bunches of
transistors that perform as though they were a nearly “perfect” Zener diode. As the voltage
across the regulator is varied, the voltage across the Zener remains constant within a couple
thousandths of a volt.
17. Chapter 8, Harris
The op-amp is the triangle in the center. This op-amp has 14 pins and the numbers shown
are the pin numbers. This particular chip contains four op-amps. Only one is used in this circuit.
Pins 6 and 5 are the inputs. The input pins function in such a way that the op-amp changes the
output voltage (pin 7) to “try” to keep the two input pins at the same voltage. That is, so long as
the voltage on pin 6 is identical to the voltage on pin 5, the output remains constant. When the
positive pin 5 has a higher voltage than the negative pin 6, the output pin will shoot positive as
high as it can go. When the negative pin 6 is higher than pin 5, the output pin will zoom negative
as low as it can go.
Unlike simple transistors, the op-amp has nearly infinite voltage and current gain. Gains
like 100,000 or a million are common. Also, the inputs draw essentially no current. They have
nearly infinitely high input resistance. This means that op-amps are ideal to “monitor” some
condition, such as the output voltage from the power supply, and then change the OP-AMP
output pin voltage in order to restore the supply to equilibrium. Looking at the diagram, pin 6,
the negative input, always rests at 5.00 volts, so long as the battery voltage input is above 5 volts.
The output voltage pot at the lower right is adjusted to produce the desired output voltage. The
big MOSFET transistor will remain turned on to the exact degree to deliver the voltage you set.
This set point is the place where 5 volts appears on the pot. As soon as the op-amp “sees” that
pins 5 and 6 have the same voltage, the output voltage on pin 7 holds still and stops changing.
A precision Zener diode
The LM336 precision Zener diode is not only extremely accurate, it will maintain its
accuracy over a wide temperature range. Speaking of temperature change, don’t forget to bolt
your MOSFET to a big heat sink. At high input voltages and big load currents, you may expect
the MOSFET to get boiling hot without a heat sink. Keeping it cool keeps its internal resistance
low and improves the regulation. The metal flange on the transistor should be insulated from the
heat sink and metal chassis with a mica insulator and silicone grease.
18. Chapter 8, Harris
As we shall see in chapter 10, building a variable frequency oscillator that reaches
modern standards of frequency drift requires the use of a super-regulated power supply. This in
turn will require you to use regulators made from integrated circuits. Sigh. Of course you could
prove me wrong by building one with discrete parts.
Packaging power supplies
The photo below shows the underside of my all transistor CW transmitter. It is based on
the modules described in chapters 6, 10, 11 and 12. The low dropout 12V QRP power supply is
located just to the left of the center. The circuitry is mounted on a commercial perforated circuit
board. This circuit board is standing on edge but the blue trim pots that adjust the LM336 and
output voltage can be seen. The P-channel power MOSFET that controls the output is just above
the board. The transistor is insulated from the chassis with a plastic screw and a gray silicon pad.
The perf-board circuit at the
upper right with the three
large capacitors is the
precision supply for the VFO.
The two large TO-204
transistors mounted vertically
are the CW keying transistors
for the 100 watt linear
amplifier. The black relay at
upper left is the main DC
relay that engages the 12 volt
battery during transmit.
Each module is mounted to the chassis with Molex or other connectors so that it may be easily
repaired or replaced. Phono plug connectors are used as feed-through connectors wherever
needed.
******************************************************************************
**
BATTERIES AS A POWER SUPPLY
The beauty of batteries
As I mentioned earlier, I run my whole station on solar power stored in a 12 volt battery.
Now I know this sounds like tree-hugging, liberal silliness, but my solar powered station came
about quite logically and has many advantages for a homebuilt station. There are lots of hams
that do this - and no wonder! A lead acid storage battery is a wonderful power supply. It puts
out huge currents whenever you need them and the voltage regulation is excellent. The best
advantages are that batteries make no switching power supply RF noise and there’s no waste heat
from a big linear regulator.
My return to ham radio began in 1997 when I hauled my 1967 homebuilt mobile ham rig
19. Chapter 8, Harris
down from the attic and blew off the dust. Since my old mobile rig runs on 12 volts, I couldn’t
even try out the receiver without a heavy-duty 12 volt power supply. I considered building or
buying a supply but after 10 seconds I decided that was ridiculous. I happened to have some 6
volt golf cart batteries in the garage. I hauled two of them down to the basement and put on an
automotive 12 volt trickle charger. After a day, the batteries recovered and I could fire up the rig.
AM phone was extinct, but there was nothing to stop me from getting on CW, which I did with
good success. Well, that’s not quite true. My adventures included a drifting VFO and a blown
200 watt charge-pump switcher, but those problems had nothing to do with the power source.
My old vacuum tube rig was a power hog. The receiver drew 3.5 amperes at 12 volts.
That’s 42 watts just to listen! The transmitter was much worse, of course, but it wasn’t
dramatically worse than a transistorized rig. After all, a transmitter can’t radiate big power if it
doesn’t draw big power. Moreover, a modern, linear, class A transistor rig can be even more
inefficient than an old Class C vacuum tube transmitter. Either way, my transmitters draw 6 to
20 amperes.
Compared to a golf cart or an automobile starter motor, a ham rig is a low power device.
20 amperes should be plenty of current. A deep-discharge storage battery is best, but there’s no
reason you can’t use an old car battery. Those of us who live in snow country routinely discard
car batteries when they can no longer supply 400 amperes on a frigid January morning. But even
an old car battery will usually supply 20 amperes for five minutes without appreciable voltage
drop. All you need to do is keep a small, one-ampere charger on it continuously. For anyone on
a budget, a free used battery is a darn cheap high current regulated supply.
Requirements of a line-powered 20 ampere, 12 volt supply
I don’t plan to build an equivalent line-powered power supply. But if I did, I wouldn't
build a switching power supply. I’ve had zero luck running ham equipment on switchers. If the
switcher is more than just an asynchronous charge pump and actually regulates the output
voltage, then it will make RF noise which you’ll hear in your receiver.
20. Chapter 8, Harris
A 1967 homebrew all-band transmitter with a switching power supply running on 12 volts DC
The 150 watt switching supply is at the left rear. Originally the supply was a simple
charge pump that used germanium power transistors. It consisted of a free-running multivibrator
(square wave oscillator) circuit that drove a step-up transformer with no feedback. The supply
worked well and didn't produce audible hash in my receiver. Then one day a germanium
transistor blew and I was unable to find a suitable replacement. Germanium transistors seem to
be extinct in the modern world – they can’t really compete with MOSFETs. I rebuilt the supply
using a modern pulse width modulator regulation system and MOSFET power transistors. An
elegant feature of the new switcher was that the output voltage was adjustable. Just by turning a
knob I could vary the transmitter output power from 20 to 80 watts.
Unfortunately, the switcher and the RF amplifier stages “talked to each other.” That is,
when the load increased, the supply increased the pulse width modulation to compensate, but not
without a slight delay. This subtle ripple or “jitter” was hard to see on a scope, but no matter
how much I filtered the DC high voltage, the jitter appeared as a slightly rough note on the CW
signal. In practical terms, I kept getting 598 RST reports.
There was an elaborate 12 volt switcher in QEX magazine a few years ago that solved the
jitter noise problem. I was delighted to see that I wasn’t the only one who noticed “jitter noise.”
The problem is real, but after seeing the complicated Rube Goldberg solutions, my desire to build
one vanished.
If I had to build such a supply again, one method would be to build a charge pump or
other unregulated switched system that boosted (or lowered) the input voltage to just above the
desired voltage. Then I could use a linear voltage regulator to accomplish the regulation function
with a minimum of waste heat generated. This is the low noise approach I used to power my
super-regulated miniature VFO power supplies described in chapter 10.
One idea I’ve had for a high power charge pump would be to use a manually adjusted
pulse width for the AC generation. This way the output voltage could be manually set so that the
linear regulator would waste as little energy as possible. This regulation margin would depend
on how consistent my line voltage was and how stiff the charge pump conversion system was.
For example, starting with 120 volts AC, the charge pump might lower the voltage to say, 18
volts DC. Then the linear could reduce the 18 volts DC down to 12 volts.
Going solar
Because my automotive trickle charger was feeble, I was charging non-stop. Even then, I
was having trouble keeping the battery charged for a couple hours of daily operating. However, I
already owned a 12 watt, 12 volt solar panel which I installed on the roof.
21. Chapter 8, Harris
A simple solar charger circuit
Solar cells are a kind of silicon diode. They are arranged in series so that the forward
voltage drop of each diode adds up to some voltage greater than the voltage of the storage
battery. For example, at 0.6 volts per solar cell, we need at least 20 cells in series to raise the
panel voltage higher than the 12 volt battery. Typically, an open circuited panel puts out 20 volts
in bright sunshine. This extra capability insures that it will continue to charge a 12 volt battery
all day and implies that there are roughly thirty 0.6 volt cells in series. Solar cells are interesting
to play with. I was surprised to discover that if you put your hand over just one of the series
cells, it turns off the whole string, something like Christmas tree lights wired in series. This
means that just one wet leaf stuck on your panel can turn off the whole array.
The panel output current is proportional to the sine of the angle the sunlight makes with
the panel. If your object is to produce the maximum kilowatt-hours during the entire year, then
the angle should equal the latitude. I mounted my panel at 45o, which at 40o latitude, gives me
better performance during the winter. Optimizing for winter is smart because the days are short
and the panel is often covered with snow. Actually 50o may be better here in Colorado. Snow
slides off a steep incline and higher angles are more resistant to hail damage. On the other hand,
mounting them on a steep frame may make them subject to wind damage. There is no way to
win. Many people just mount the panels flat on whatever roof they happen to have. If you wish
to sacrifice some percentage of the output for beauty or mounting convenience, that’s your
decision.
The “lunar discharger”
It’s bad form to connect a solar panel directly to a storage battery. The solar cells are
forward biased diodes with respect to the battery. Therefore, whenever the sunlight quits, the
panel voltage may drop below the battery voltage and a small battery current will reverse and
flow through the solar panel. In other words, at night, the solar charger becomes a “lunar
discharger.” (Never mind. That was a silly joke.) If there are enough extra solar cells, then the
night-time voltage drop may still exceed the battery voltage. In any case, the discharge problem
is typically prevented by a silicon diode in series with the panel to insure that current is never
allowed to flow from the battery back into the panel. Since this check valve diode has a forward
voltage drop that wastes energy, you may as well use a big Schottky diode that will only penalize
22. Chapter 8, Harris
you with 0.2 volts instead of 0.6 volts loss.
Another small issue is lightning protection. My panel is on the roof where, in theory, it
might attract a lightning bolt. I’m not really worried, but I have a connector on the lead from the
panel so I can disconnect it from my shack when I go away on trips or whenever a storm is
particularly frightening. If I lived on a mountaintop or if my house were higher than the other
houses in my neighborhood, I would be more concerned.
Solar panel on the roof
I built my panel frame out of varnished wood. The wooden boards screwed down to the
roof retained water against the shingles and rapidly rotted. I replaced the boards with 2 inch
aluminum angle stock which seems to be a more permanent fix.
Conservation helps
My solar panel charges about as much as the line-powered trickle charger, about 1
ampere. Consequently I still had to use line power to charge occasionally whenever I stayed on
the air too long. A breakthrough came when I built my new transistorized receiver, the one
described in Chapter 13. The new receiver draws 120 mA. That sure beats 3,500 mA. Since
then, I haven’t needed my line-powered charger.
12 Volt power distribution
A storage battery can deliver hundreds of amperes, so it’s wise to isolate the battery from
your rig and solar panel with an appropriate fuse. A 30 ampere fuse should be about right. I
have a master switch to isolate the battery from the ham rig when I’m not using it. A little LED
pilot light tells me when it’s connected and a voltmeter warns me if the battery is not fit for use.
I also have charging and discharging ammeters (ampere meters) so I know the status of my
supply at all times. I find the discharge meter helpful for monitoring my transmitters. You can
even tune the antenna while watching the battery current. The wire in front of the charge meter
goes to my 12 volt desk lamp.
23. Chapter 8, Harris
Battery charge monitor
Maintaining clean battery contacts is critical to keeping the supply voltage constant under
load. Every few months, I clean the contacts using baking powder and water. I know the battery
contacts need cleaning when the pilot light on my transmitter begins to flicker noticeably while
I’m transmitting. I top off the batteries as needed with distilled water to keep plates in the cells
from being exposed to the air. A gel-cell battery shouldn’t have these corrosion and fluid loss
problems. However, if you overcharge a sealed gel-cell, the water cannot be replaced and the
battery will be permanently damaged.
The 12 volt storage battery power system
The storage batteries are shown above. The transistorized inverter to the left of the
batteries converts 12 volts DC to 120 volts AC for those rare times when a storm knocks out our
commercial electricity. On those occasions I have simply strung an extension cord upstairs to
power the TV and few lights. The glass and rubber device is a hydrometer for measuring the
specific density of the battery acid. When the glass float rides high in the green zone, the battery
is fully charged.
Storage battery safety
My power system is small and I rarely have to add distilled water to my batteries. This
tells me that my use of stored energy and the output of my small solar panel are well-matched. If
I were having to add water constantly it would mean that I was wasting energy and converting the
water into gaseous hydrogen and oxygen. This situation could be potentially dangerous. If I had
24. Chapter 8, Harris
a large system powering more of our household needs, the likelihood an explosion would be
serious. Therefore, a large storage battery array should be located out in a well-ventilated place
like a garage. Moreover, the state of the batteries should be continuously monitored with an
electronic regulator to be sure that the batteries are not overcharged and generating significant
amounts of hydrogen. Charge control regulators are mandatory in any large system.
I once read an article written by a fellow who was in charge of maintaining remote
microwave relay stations. Wind turbines powered the stations, but the power systems were not
equipped with any means to turn off the charging current to the batteries during windstorms. The
results were catastrophic explosions inside the stations. Eventually, the phone company realized
the problem and put in charge control regulators.
In summary, I get a kick out of operating like a fully solar-powered Field Day station.
Because the whole station runs on 12 volts, in theory I’m always ready for Field Day and there’s
nothing to stop me from going mobile. One of my back-to-nature dreams is that, during a power
failure, I can go right on operating and thumb my nose at the evil corporate power monopoly.
Twice during power failures I have galloped downstairs in the dark to make my first 100% truly
independent radio contact. But before I could even tune the transmitter, the *#@%!$ lights came
back on. Oh, curse those efficient fellows at Xcel Energy Company!
******************************************************************************
**
LED THERE BE LIGHT
Emergency Station Lighting
As explained above, I run my whole station on solar energy stored in a 12 volt battery.
An advantage of battery power is that I operate free from the power grid. This saves very little
money, but it’s quite romantic. Of course at night, even on batteries I need to keep a log and
throw the correct switches. Using candles or a flashlight clenched in my teeth didn’t seem very
sophisticated.
My first idea was to dangle a 12 volt automotive light bulb over my shack. My buddy
Bob, NØRN, uses such a light in his tent on Field Day. This solution is completely practical, but
it seemed old-tech and boring. It also draws about 10 watts of power. If I am going to all the
trouble to build a custom 12 volt light, I figured it should at least be an interesting light.
Modern lighting methods produce more light per watt than tungsten bulbs. Also, energy
conservation is important when your station is powered by a battery charged with a small solar
panel.
25. Chapter 8, Harris
A homebrew LED and flourescent station lamp
White LEDs – a modern marvel.
I happened to notice some “white LEDs” for sale down at Radio Shack. (Part # 276-320)
I thought cynically that they were probably really dim and not really white. I bought one and
hooked it to a variable power supply. I slowly turned up the voltage until I obtained the rated 20
mA DC current. Hey! Not bad! The beam of white light has a slightly bluish cast, but
otherwise, it throws a spot of light out to a couple feet. It’s rated at 1100 mcd. That’s impressive
brightness when you consider its beam angle is 100o. One LED makes a perfect key chain light
and many LED flashlights are now on the market.
I wrote this section some years ago. Since then, screw-in 12 volt light bulbs consisting of
large clusters of white LEDs have become available. I now have one in the shack that draws 3
watts and delivers considerably more light than the two systems below. Even so, there are
lessons to be learned from my experimenting below. The switching power supply described for
the fluorescent light is a design I have used often for several applications.
26. Chapter 8, Harris
I’m told these white LEDs are made from
sophisticated layers consisting of yellow and blue
LEDs and a phosphorous that glows white. As LEDs
go, it has a rather high voltage drop, 3.6 volts. I
figured if I put three in series plus a single dropping
resistor, it could run on 12 volts. The current is
limited with just one 91 ohm resistor. The resulting
triple light consumes just 0.25 watts and is adequate
for keeping a log and operating the station. That’s
amazing when you think about it. We routinely
squander hundreds of watts to illuminate entire rooms
when all we really need is ¼ watt to read our
newspaper or book.
Flourescent lighting
Unfortunately, “adequate” isn’t the same as “comfortably bright.” So my next project
was to build a 12 volt powered compact fluorescent. I fiddled for a couple hours trying to build a
power oscillator capable of producing the required 800 Volts AC needed to ignite the miniature
fluorescent bulb. I had several transformer and switching transistor problems that I was having
trouble solving. I also fried my voltmeter because I forgot to change the range when I put it on
the high voltage. There’s nothing like dead test gear to make you think differently.
I remembered that I still had a 6 volt fluorescent lamp that I used to use for lighting my
pup tent while camping. One day I rolled up the tent while the light was still hanging from the
tent ceiling. Oops! The plastic housing was splintered and the remains of the light assembly
were still in my junk box. I found the parts of the light and resoldered the broken wires. I
connected it to a 6 volt power supply and it still worked fine. Next I built a shiny reflector/
lampshade out of sheet aluminum and it produced loads of light. Unfortunately, I need a 12 volt
light, not a 6 volt light. I didn’t have a second light to put in series with the first, so I used a big
dropping resistor to run the light on 12 volts. That worked OK, but it seemed pretty crude.
Besides, I was already “cheating” because I hadn’t built the fluorescent high voltage supply.
Nifty ”buck-type” switching power supply
I replaced the dropping resistor with a switching power supply to reduce 12 volts DC
down to 6 volts. I got the design from the National Semiconductor data book. Using the
switcher the fluorescent light draws about 2.5 watts at 12 volts. The switcher is about 80%
efficient. That is, it dissipates 10% of the total energy. In contrast a resistor would have
dissipated 50% of my energy. I have used this little supply design for several home projects so
far and found it thoroughly reliable. If you’re a homebuilder, this simple, buck-switcher belongs
in your bag of tricks. If you’re bored with building a 12 volt light, maybe this little switcher will
interest you.
27. Chapter 8, Harris
Switching power supply made from a linear regulator
How can this work?
This switcher uses a LINEAR regulator to generate the pulsewidth modulation needed to
implement a switching power supply. This is essentially the same linear regulator used in the
suggested line-powered QRP power supply. But, when you put a scope probe on the inductor,
sure enough! The linear regulator is switching full on and off in rectangular pulses. Apparently
the 15K Ω resistor and 300 pF capacitor are coupling a feedback pulse from the inductor into the
regulator to cause it to switch full on and full off. The pulse width varies with load and responds
just like a real switcher. Aside from my amazement that linear regulators can work as switchers,
I was also surprised that I hear very little switcher noise in my receiver. The RF noise it produces
is apparently sufficiently isolated from the receiver that I rarely hear hash competing with the
ham signals. On the rare occasions when I do hear it, I simply turn off the fluorescent.
Subjectively, the fluorescent produces roughly twice as much light as the LEDs. In other
words, for ten times as much power the light seems to be twice as bright. On the other hand, it
illuminates a much wider area than the LEDs, so I just run both of them simultaneously. The
LEDs illuminate my log and scratch pad while the fluorescent lights up the station as a whole.
I’ve used this light for hours at a time. It’s bright enough that I’m content with it and I don’t
think about running across the room to turn on “the real lights.” In summary, battery power and
energy conservation are entertaining games. And, if we’re really unlucky, someday our hobbies
might even be useful in a community crisis.