Protect your sensitive electronic equipment from
lightning damage by building your own ...
By TJ Byers
"Snap, crackle, pop" may be familiar breakfast sounds to
some people, but for many others, such noises signal
disaster ...lightning disaster.
Each year, the damage done by lightning runs into the
millions of dollars, and the human casualties number in the
thousands. As a matter of fact, more people are killed each
year by this form of electric discharge than by all other
natural disasters combined.
Lightning wreaks the most financial havoc,
however, when it damages electronic equipment. Stereos,
televisions, and home computers succumb to the effects of
this force daily. And the most perplexing part of the
problem is that lightning doesn't even have to strike an
appliance directly to inflict damage. Flashes many miles
away can turn hundreds or thousands of dollars' worth of
circuitry into high-tech junk.
The problem comes from the utility-fed electricity used to
run your equipment. The power delivered to your home has
probably traveled through hundreds of miles of wire and
several substations before it reaches you.
When lightning strikes, it does so with the force of
millions of volts. You can actually hear lightning
discharge several miles away (and I don't mean the thunder)
if you listen to an AM radio during a thunderstorm. The
highvoltage discharge generates radio waves that are picked
up by your radio's antenna.
In a similar fashion, the sprawling utility grid can pick
up static from lightning. When a strike comes close to a
power line, the wires act as a large antenna and absorb
part of the energy. A power surge as high as 2,500 volts
can be injected into the grid in this manner. These voltage
surges travel down the wires into your home and right into
appliances, where they can destroy sensitive electronic
parts ...even though the equipment is turned off.
Fortunately, you can protect your costly possessions from
this threat without having to unplug them every time the
sky clouds over. There is an electronic device called a
metaloxide varistor (MOV for short) that can serve as a
watchdog on the voltage of your AC line.
As long as your household voltage remains within normal
limits, the MOV does nothing. Let the voltage suddenly
surge to 130 or higher, though, and the MOV swings into
action. What it does is absorb the extra voltage created by
the spike and dissipate it as heat. When the line voltage
returns to normal, the MOV goes back on standby. The entire
sequence happens in about onemillionth of a second.
Metal-oxide varistors are inexpensive and usually are
readily available (see the sidebar for access information).
The amount of protection you need, however, will depend on
the type of equipment you have. Television sets are more
sensitive than radios, and computers are extremely
vulnerable. Therefore, I'm providing you with three types
of MOV systems to choose from, according to the degree of
protection you need.
The simplest protective device requires no construction at
all on your part. It consists of a single "MOVed" plug that
is inserted into an outlet and has the appliance cord
plugged into it. This product, which is solo' by Radio
Shack under the name Voltage Spike Protector (part number
61-2790), contains a single MOV device connected across the
hot and the neutral power lines. Any transient exceeding
130 volts is readily suppressed by the MOV and is thus
prevented from . r, tering the equipment.
Unfortunately, this simple device doe protect against
all forms of power surge. 1; the lightning strike
is such that voltage spike, of equal proportions are
induced in both the hot and neutral wires, the MOV
won't sent the change
To remedy this shortcoming, I've \e con structed a surge
protector that contains three MOV devices. One is wired
across the ho and neutral lines, just as is the case with
the Voltage Spike Protector. To keep the spike that are
common to both lines from creeping into the system, though,
I've added two other MOV's-one from each line to the ground
wire. With this arrangement, the output t ol: age
can never be more than 130 volts iii direction.
You can duplicate this surge protect a matter of minutes,
once you've collected t t proper parts and tools. First,
you'll need ,; sixoutlet expansion plug. Radio Shack',
61-2622 is one example. Remove the card board back from the
outlet by unscrewing the four coarsely threaded rivets on
the back w the device.
When you have the back cover off, your, faced with six
copper strips that are used to expand a two-outlet socket
into a six-outlri one. If you have the misfortune, as I
did, o: having the metal strips tumble into hands as you
remove the cardboard, don't panic. Simply shove them back
into their slots with your thumb.
The next step is to solder the MOV devices into place.
There's plenty of room inside the plastic housing to
accommodate them. Using the "better" model in the photo as
your guide, slip the MOV's into position. Now clip
the wire leads. to length, and solder the MOV's to their
respective copper strips. One MOV goes across the
thin prongs, another connects between one thin prong and
the ground connection (the strip with the round insert),
and the third is soldered between the remaining prong and
the ground. Don't worry about which direction they're wired
in; just be sure that no leads are touching that might
short the circuit.
Replace the cardboard backing, remove the wall plate from
the wall socket you intend to use, and plug in your new
outlet protector. Fasten the adapter in place with the long
center screw that's provided, and you're finished. Any
appliance that's plugged into this outlet is automatically
protected from all forms of voltage surge.
Even the protection offered by the threeMOV outlet is
insufficient in some cases. A personal computer, for
example, not only is allergic to voltage spikes but is also
sensitive to "noise" on the AC line.
Noise can best be described as unwanted voltage excursions
that are under the activation threshold of the MOV
protector. In other words, they are small signals that ride
on top of the standard power voltage. They aren't
particularly harmful in most cases, but they can scramble
the data contained in a computer's memory chips. Words may
come out "mizpeled," and, in some cases, information is
How does this noise get on the line to begin with? It's put
there by other electrical equipment. Refrigerators, air
conditioners, vacuum cleaners, and television sets all
generate electrical pulses that find their way into your
electrical system. In fact, your computer's
printer may be the worst offender. A big reason
for this is that the printer and the computer often share
the same outlet, so the noise has an easy path into the
To remove unwanted noise from an AC line, you need a
filter-which brings us to the third design. In addition to
being MOVprotected, the "best" option incorporates an
effective filter. The entire project can be built inside a
Radio Shack Plug-in Power Strip (part number 61-2620),
though similar units are available from most electronics
and hard ware stores. These power strips feature a
heavy-duty cord that expands to four outlets housed in a
sturdy metal case. As an added feature, the power strips
come equipped with a built-in 15-amp circuit breaker.
To modify one of the devices, you must first split the
aluminum housing in half by removing eight screws, four in
each of the two end plates. However, this is easier said
than done. For some reason, the screws have recessed,
square-drive heads that require a special tool. I didn't
consider the tool to be worth buying, so-after several
false starts-I devised a way to get the screws loose. By
filing slots in the fasteners' heads with a triangular
file, I was able to remove them with an ordinary slotted
screwdriver. After I finally did get the screws out, I
promptly tossed them away (I replaced them with
conventional Phillips-head items).
Once the end plates have been removed, the lower half of
the metal case slides off, exposing the undersides of the
outlets. Begin the modification by removing a short strip
of insulation from each of the three wires connecting the
first two sockets together (looking at it from the
circuit-breaker end). A razor knife works best. Now solder
three MOV devices to these wires, bridging one
across the black and white wires, one across the black and
green, and one across the white and green. After soldering,
bend the leads so the MOV's rest against the backs
of the outlets, as shown in the photograph.
Next, work on the link that connects outlets two and three.
With a pair of wire cutters, completely remove a section of
the black and white wires between the two outlets, leaving
short pigtails connected to the second socket. Strip the
insulation from the pigtails. Do NOT cut the green wire!
This last operation divides the four outlets into two
pairs. In the severed line, you'll insert a filter to
remove noise from the last two outlets. But before you can
do that, you have to cut a small piece of insulation from
the three wires that join sockets three and four, just as
you did when installing the MOV's.
After stripping the wires between three and four, solder an
RF choke between the black pigtail on socket two and the
freshly exposed black wire between three and four. Then do
the same for the white wires. Finally, connect a 0.047
microfarad capacitor across the green and black wires and a
second 0.047 microfarad capacitor across the green and
white leads, laying the capacitors out of the way after
After inspecting your work for shorts, reassemble the case.
Then plug the cord into a convenient outlet, and you're
ready to go. When using the power strip for a computer,
plug the printer into outlets one or two and the computer
itself into three or four.
And there you have it: three different degrees of
protection from snaps, crackles, and pops. Remember,
though, that none of these devices is a lightning
arrester. In the rare instance when lightning
actually strikes your house, damage will ultimately result.
For everyday voltage transients, however, surge protectors
are an effective, inexpensive form of insurance.