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July 1963 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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Dealing with the problem of
lightning strikes was of concern long before electronic equipment needed to be protected
from its effects. (Before I forget to mention it, Mac introduces Barney to the Zener
diode in this 1963 saga) Fires that were the result of lightning have always been
a problem in nature, but they were really catastrophic to civilization once cities
crowded with close-quartered wooden buildings became the norm. Benjamin Franklin
observed that when the many lightning-induced fires of Philadelphia were sparked
(pun intended), it was almost always the tallest structures in the area that were
hit. Those fire often spread to neighboring buildings and burned down entire city
blocks. It was a devastating and frequency occurrence. Having deduced from his famous
kite flying experiments that lightning was a form of high voltage, high current
electrical discharge*, he came up with the idea of erecting a conducting post to
the highest point on a structure and connecting it to ground. Once Franklin's 'lightning rods' were installed all over the city, fires were rare.
 Out
of an abundance of caution, I have secondary surge arrestors on all my electrical
outlets and a primary whole-house surge arrestor
wired into my circuit breaker panel. Thus far none of them has ever been triggered,
but I never ley my guard down (literally). I also have them on the coaxial cable
and telephone line entry points. To really illustrate the degree of overkill I like
to employ, I installed a surge / brownout protector
on the A/C compressor. Surge protectors are a small price to pay compared to the
potential loss of a computer, television, radio, refrigerator, or other device.
Contrary to the believe of many low-information people, Franklin did not 'invent
electricity.' Also contrary to the belief even of some who know Franklin did not
invent electricity is that a lightning rod's purpose is not to invoke a lightning
strike, but to prevent it by lowering the difference of potential between the cloud
and the rod, thus preventing a corona discharge.
Mac's Radio Service Shop: Taming Transients
 John Frye
Mac and Barney discuss the frequency, amplitude, dangers, and control of random
voltage surges as a storm rages outside.
It was a hot, muggy, July afternoon. Even before lunch Mac and Barney had noticed
characteristic lightning-caused flashing on the screens of the TV sets on which
they were working, and now at one-thirty the sun disappeared behind dark anvil-shaped
clouds towering in the southwest and the earth shook with the growl of distant thunder.
"That does it," Mac said as he pulled the big switch that completely disconnected
all test equipment and service bench outlets from the line. "I know this will probably
break your industrious heart, but we'll take a break until the thunderstorm passes.
Lightning-induced voltage surges are not going to get my service equipment, me,
or my valued assistant if I can help it."
"Gee, thanks!" Barney retorted as he tilted his stool on two legs so that his
back could rest against the wall; "I notice you listed things in the order of their
importance."
"Yes, how about that?" Mac said with a teasing grin. "You know I've always been
a little hipped on the subject of how lightning damage can ruin electronic equipment,
and I'm even more so after reading a short article that appeared in the March, 1963,
issue of 'Newsletter,' published by the Rectifier Components Department of General
Electric."
"You don't need to sell me on the danger," Barney sniffed.
"I've seen too many radio and TV sets on the bench that had been clobbered by
lightning that 'came in' over the owners' light lines. That stuff sure is freakish.
Sometimes it skips all over a set blowing tubes, burning out coils, breaking down
the insulation of transformers, and even fusing the plates of a tuning capacitor
together. Again it only pops the line bypass capacitors, but what a job it does
on those! I've seen dozens of cases in which all that's left of the line bypass
is the two wire leads with little disks on the end that originally connected to
the foil. The capacitor itself is entirely gone except for a few shreds of foil
and paper splattered against the flame-smoked chassis. Those surges must really
pack some voltage."
"That's pretty evident, and I've always wondered just how much voltage, but I
never had equipment suitable for measuring the amplitude and duration of the surges.
The boys at G-E's Advance Technology Laboratories in Schenectady apparently got
to wondering, too, and they had the equipment. Their transient measuring set-up
used a Tektronix automatic oscilloscope and a Beattie & Coleman automatic camera.
Every time a transient came along it was displayed on the scope and its picture
was automatically taken to indicate the peak-to-peak voltage amplitude and the duration.
With this kind of equipment set up in various locations in two different states,
8000 hours of testing time was logged at the time the article was written. The tests
are still going on."
Mac stopped speaking, and the thunderstorm hit with roaring wind and sheets of
rain splashing against the windows. The thunder was almost continuous.
"You say the equipment was set up in 'various locations.' What kind of locations?"
Barney wanted to know.
"All the tests were made across 120-volt lines, but since voltage surges are
produced by a wide variety of causes, varying all the way from bolts of lightning
striking near the lines to different types of electrical apparatus being connected
to and disconnected from the lines, it was decided to monitor a number of locations
in order to photograph a wide range of transients. To this end the equipment was
set up in seven private homes, two hospitals, one hotel, one motel, and one department
store. Results showed the wisdom of this deployment, for some locations had considerably
more and higher transients than did others. Equipment that probably would have operated
without injury on some of the lines would very likely have suffered transient damage
on other lines."
"What was the highest voltage surge measured?"
Before Mac could answer, there was a terrific flash of lightning accompanied
by a sharp snapping sound followed almost immediately by a crash of thunder. The
lights in the shop flickered momentarily.
"Whew! That was a close one!" Mac said as the unmistakable odor of ozone came
in the open door. "I think it was trying to answer your question, for the highest
transient measured on a 120-volt line was one that reached 3740 volts peak-to-peak,
and this occurred in a Florida home during a lightning storm. That's the kind of
surge that pops those line capacitors you were talking about. A voltage of that
order can easily hop across the closely spaced contacts of an open line switch in
a radio or TV receiver; so just turning the set off during a thunderstorm is no
insurance against lightning damage. Pulling the plug during the storm is the only
way to be sure lightning can't get at the set via the power line."
"I'll buy that," Barney agreed emphatically. "I keep all the plugs of my ham
equipment pulled when the station is not in use, especially during the thunderstorm
months. More than one of my ham acquaintances has had his entire station wrecked
by a single bolt of lightning that struck near the power line feeding the station.
While most hams are not stupid enough to operate during a thunderstorm and are careful
to ground the transmitting and receiving antennas, a lot of them leave the back
door unguarded, so to speak, and forget that lightning damage is much more likely
to arrive via the power lines than it is to come in over the antenna. What else
did the G-E boys learn from their surge survey?"
"Well, they found relatively frequent transients occur up to 1600 volts, but
the most common ones fall in the 500-600 volt bracket. Most of the voltage surges
last less than fifty microseconds."
"Offhand I'd say the presence of these voltage surges on the line is a more serious
threat to modern semiconductor apparatus operating from the line than it is to vacuum-tube
apparatus," Barney suggested. "Take for instance two radios that use the same tube
line-up except for the rectifier. One uses a half-wave vacuum-tube rectifier and
the other uses a germanium or silicon diode. Both rectifiers are fed directly from
the line so that any transient appearing on the line appears across the rectifier.
I've got a hunch the semiconductor would be more likely to suffer damage than would
the tube."
"Speaking of ordinary semiconductor rectifiers, I'd go along with your hunch,"
Mac agreed. "The reason semiconductor diodes are sensitive to transient damage is
not too hard to grasp. While a typical 12-ampere silicon rectifier of the conventional
type can momentarily dissipate 1000 watts of heat in the forward conduction direction
of current, it will be permanently damaged by only a few watts of power dissipation
in the reverse or blocking direction."
"How come?"
"In the forward direction the current and its attendant heat losses spread out
uniformly over the entire silicon junction area, allowing the rectifier to take
maximum advantage of its cooling mechanism and heat capacity. However, under the
influence of even a brief voltage transient, the rectifier leakage current driven
by a momentary high blocking voltage peak will find some tiny flaw or weakness in
the junction at which to concentrate. Such weak spots usually occur at the junction
surface where the rectifying junction emerges from the silicon pellet. At these
microscopic spots a fraction of a watt of concentrated heat may be enough to destroy
the blocking properties of the rectifier, no matter how big it is."
"I notice you keep saying 'ordinary semiconductor rectifiers.' Is there another
kind not so easily damaged by voltage surges?"
"Yes. The controlled avalanche silicon rectifier, such as G-E's Model ZJ-218,
can dissipate about as much heat in its reverse as in its forward direction. This
is accomplished by making the high reverse energy dissipation take place in the
avalanche breakdown, or zener, region of the diode characteristic. You know this
zener behavior characteristic of a silicon diode permits it to be used as a voltage
regulator. As long as you stay within the thermal limitations of a zener diode,
it will maintain virtually constant voltage across itself regardless of the avalanche
current through it. No damage will result from true avalanche action to a diode
with a uniform junction.
"A rectifier diode with uniform avalanche breakdown taking place at a voltage
below that at which local dielectric surface breakdown occurs can dissipate hundreds
of times more reverse energy caused by transient high voltage than one in which
the surface breakdown point is reached before avalanche current starts. Once the
avalanche current begins, it holds the voltage down so the level where surface damage
to the rectifier might occur is never reached. Such a device has its own 'built-in'
transient voltage suppressor."
"How do you go about making a controlled avalanche silicon rectifier?"
"There are two important steps: 1.
You control the geometry of the junction surface very precisely to reduce the
voltage gradient at that surface and make it capable of supporting high voltage.
2. You carefully control the impurity concentration determining voltage at which
avalanche occurs so that avalanche always begins below the voltage where surface
damage might be encountered.
"Is such a rectifier similar in other ways to an ordinary silicon diode?"
"Generally speaking, yes. The CASR's big advantage lies in its ability to cope
with high reverse currents. Its forward current handling ability is very similar
to that of a conventional diode. However, the high degree of surface stability should
payoff in increased reliability. Also CASR's can be operated in long series strings
at very high voltage without the use of voltage equalizing resistors, since each
cell can be operated in its avalanche region without damage. Voltage safety factors
can be greatly reduced. Instead of the usual 2:1 or 3:1 safety factor between rectifier
peak reverse voltage and steady-state line peaks, CASR's can often be applied with
little or no safety factor. A 1200-volt p.r.v. CASR can be used instead of a 2000-3000
p.r.v. conventional rectifier."
"Sure sounds like they'd be a natural for use in the high-voltage power supplies
for table-top kilowatt linear amplifiers now becoming so popular with SSB hams,"
Barney said, cocking one ear to the sound of the storm. "Sounds like it's coming
back," he observed, settling comfortably back against the wall.
"I just was thinking we could be working on that stack of transistor radios with
the v.o.m.'s without any danger of the big, bad lightning getting to us," Mac said,
standing up. "On your feet, Buster!"
"Your trouble is you think too much,"
Barney answered disgustedly.
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
stories was the brainchild of none other than John T. Frye, creator of the
Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
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