Electrical surge suppressor and dual indicator apparatus

Surge suppression is provided by varistors connected between, respectively, hot and neutral conductors and hot and ground conductors, each through a common fuse that avoids hazardous overvoltages on the varistors. A first indicator circuit portion provides an indication that the power is on and surge protection is present and includes a first indicator device such as a green light-emitting diode that is energized when the fuse is intact, and a second indicator circuit portion is provided for indicating power is on and surge protection is lacking that includes a second indicator device such as a red light-emitting diode that is energized by the line conductors when the fuse has been interrupted. The surge suppressor and dual indicator apparatus can be contained within a wiring device such as a receptacle. A single varistor unit containing two varistor disks with three leads, including a center tap, allows three effective surge suppressor circuit branches in a compact configuration.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to apparatus for the protection of loads connected 
with an electrical distribution system and for indication of the status 
and capability of the surge suppressor system. The invention relates 
particularly to such apparatus suitable for incorporation within household 
and commercial wiring devices. 
It is beneficial to have a surge suppressor to protect load against some 
voltage transients that do not trip the circuit breaker of a distribution 
circuit. This is particularly so in the case of relatively delicate 
electronic equipment such as video recorders, personal computers, and 
other valuable household and commercial appliances. It is also desirable 
to have an indication that the surge suppressor is in good working order. 
It is recognized that considerable prior art relates to the protection of 
equipment against overvoltages with an indication of the occurrence of an 
overvoltage. Many protection systems are fairly complex, expensive and 
bulky. The present invention is primarily directed to a system that is 
simple, inexpensive and compact and is used, normally, for local load 
protection and is in addition to conventional distribution circuit 
protection devices such as circuit breakers. 
In accordance with a prior art system for applications addressed by the 
present invention, a surge suppressor unit is provided in a receptacle 
into which the line cord for the load is plugged. It includes a suppressor 
circuit branch, for connection between the hot and neutral conductors of 
the distribution system, that includes a fuse and a metal oxide varistor 
in series. Such a surge suppressor circuit branch is connected between the 
hot and neutral conductors of a distribution system; other conductor pairs 
of a three wire system may be protected as well. The fuse is selected to 
have an interruption, for fire safety, if a failure of the varistor 
occurs, such as shorting due to an overvoltage. In such an arrangement it 
is known to utilize an indicator device, such as a green light-emitting 
diode, that is powered only if the fuse is intact. This light will light 
and therefore indicate the power is on and the fuse is present. If the 
indicator light is not on, this indicates the power is not on or the fuse 
has blown. It would be desirable to know which of these two events is the 
case. 
Systems are known in which the one light used which is energized when 
everything is correct is arranged to blink if surge protection is lost. 
This requires an extra timing circuit portion and its attendant cost and 
complexity. 
In accordance with another version of such equipment, there is provision to 
provide a buzzer alarm when protection is lost and the power is still on. 
This, too, incurs additional cost and complexity. 
It is desirable to provide an alternative system that gives positive 
indication of whether the fuse is or is not operable by two distinct 
indicator devices in a relatively simple and inexpensive arrangement. 
Therefore, in accordance with one aspect of the present invention, there is 
provided an electrical surge suppressor and indicator system that 
includes, at least, a first surge suppressor circuit branch for connection 
between two line conductors and including a fuse and a first varistor in 
series. A first indicator circuit portion indicates power is on and surge 
protection is present and includes a first indicator device energized by 
the line conductors if the fuse is intact. A second indicator circuit 
portion indicates power is on and surge protection is lacking and includes 
a second indicator device energized by the line conductors if the fuse has 
been interrupted. 
In certain embodiments of the invention the first indicator device is a 
green light-emitting diode, and the second indicator device is a red 
light-emitting diode. Each light-emitting diode is in series with 
current-limiting resistance. In the case of the green light-emitting 
diode, which is energized when the fuse is intact, it is in series with a 
resistor and the red light-emitting diode is in series with that same 
resistance, plus an additional resistance. The first and second indicator 
circuit portions may both include the first resistor. The second 
indication circuit would then include a separate, second resistor that is 
not in the first indicator circuit portion. 
When light emitting diodes (LEDs) are used as the indicator devices, they 
can be used in different ways to yield different characteristics in the 
information indicated. In one form of connection, the LEDs are like-poled; 
each is in inverse parallel relationship with another, standard, diode 
having a lower forward voltage drop. When the fuse is intact and the green 
LED is energized, there is some current drawn through the second circuit 
portion containing the red LED. With appropriate resistance present (so 
both the red and green are of approximate equal brightness when fully on), 
that current is sufficient to energize the red LED just slightly producing 
a perceptible but dim red glow while the green LED is fully energized and 
considerably brighter. This allows one to readily confirm the operability 
of both LEDs when power is on. 
If one does not wish any perceptible output from the red LED when the green 
is energized, it is merely necessary to change the polarity of one of the 
LEDs, such as the green LED, and its respective secondary diode. 
It is desirable in the systems of interest where there is surge protection 
between the hot and ground conductors to also have surge protection 
between the hot and neutral conductors as well as between the neutral and 
ground conductors. To do this, prior devices have had three separate 
varistor devices in appropriate circuit branches between the indicated 
conductors. They include a single fuse with respective varistors in 
branches between the hot and neutral conductors and between the hot and 
ground conductors. A third varistor, not in series with a fuse, is located 
between the neutral and ground conductors; if it fails its failure is not 
indicated externally. 
A substantial degree of protection between the ground conductor and the 
neutral conductor is provided by the series connection of the first two 
varistor devices. In accordance with a further aspect of the invention, 
two similar varistor metal oxide disks are packaged as a unit within a 
single insulating casing and are provided with three terminals--one on 
each side of the outside surfaces, and a third centertap between the two 
disks. Therefore, with minimal bulk this provides effectively three 
varistors for protection between each pair of conductors. Any varistor 
failure, in contrast to the prior art, is indicated externally, such as by 
the dual indicator system described above. Such a compact, center-tapped, 
arrangement of two varistor disks is also believed novel and useful in 
other applications. 
The compactness and economy of apparatus in accordance with the present 
invention allows its convenient use within ordinary wiring devices, such 
as receptacles for plugging in various kinds of appliances which are thus 
protected.

PREFERRED EMBODIMENTS 
FIG. 1 is a circuit schematic illustrating a generalized surge suppressor 
showing surge protection provided by the prior art (FIG. 1 is not intended 
to illustrate indication apparatus described as part of the background 
description, above). The circuit of FIG. 1 is for connection on one side 
at supply terminals 10 and 11 to the line conductors of, for example, a 
conventional electrical distribution system such as a 120-volt, 60 hertz 
system in the U.S. In this example, terminal 10 is for connection to a hot 
line conductor or "black" wire, and terminal 11 is for connection with a 
neutral or "white" wire, and the respective conductors of this surge 
suppressor circuit will be so referred to as the hot line conductor 12 and 
the neutral line conductor 14. 
On the other side of the surge suppressor circuit are terminals for 
connection with a load that is to be protected by the surge suppressor. 
Terminal 14 is on the hot line conductor 12, and terminal 15 is on the 
neutral line conductor 13. 
In addition, the circuit of the surge suppressor includes a conductor for 
connection with the ground conductor of the electrical distribution system 
at a terminal 16 where the ground or "green" wire 17 is connected. 
Conductor 17 may have another terminal 18 for connection with the load. 
Three surge suppressor circuit branches are illustrated in FIG. 1. A first 
surge suppressor circuit branch includes a fuse 20 and a first varistor 22 
connected between the line conductors 12 and 13. A second surge suppressor 
circuit branch includes the fuse 20 in series with a second varistor 24 
connected between the hot line conductor 12 and the ground conductor 17. 
In addition, a third surge suppressor circuit branch including third 
varistor 26 is connected between the neutral line 13 and ground 17. 
Each of the varistors 22, 24, and 26 is selected to breakover and become 
highly conductive at voltage levels across their terminals sufficiently 
low to provide protection of the load against any likely damage. In 
addition, because economical and available varistors so applied may 
themselves be subject to damage from excess overvoltages, the fuse 20 is 
provided and is selected in order to operate and avoid a fire hazard if 
either varistor 22 or 24 fails and presents a short circuit. When the fuse 
20 operates, both varistors 22 and 24 are disconnected from the hot line 
12. The third varistor 26 still remains in circuit between the neutral 13 
and ground 17. It is desired to provide by the present invention effective 
means for indicating to a user the status of the fuse 20, both to confirm 
it is connected and surge protection is being provided across each pair of 
conductors, or to provide positive indication that it is not connected and 
that the load is energized without protection against surges that may harm 
it. 
Furthermore, a purpose of the invention is to provide simplicity and 
economy so that the protection and indication functions can be performed 
in a package that is economical and convenient for use in general 
household and a commercial applications. 
In FIG. 2 is shown a general schematic of an embodiment of the present 
invention. Elements corresponding to those of FIG. 1 will be found in FIG. 
2 except that the third varistor 26 is not included. Varistor 22a 
provides protection between lines 12 and 13. Varistor 24a provides 
protection between line 12 and ground 17. Varistors 22a and 24a are in a 
single unit 23 with three conductive leads 23a, 23b, and centertop 23c, to 
be described more fully below. 
In accordance with the preferred embodiments of the invention, the 
connection of the varistors 22a and 24a in the manner shown between the 
neutral 13 and the ground 17 conductors provides satisfactory protection. 
As will be subsequently described in more detail, minimization of 
components affords more compact and economical arrangements. All of 
elements within the line 8 may comprise a wiring device such as a 
receptacle, with terminals 10, 11 and 16 wired to supply conductors and 
terminals 14, 15 and 18 being contacts receiving plug blades from a load. 
The circuit of FIG. 2 has a first ("green") indicator circuit portion for 
indicating power is on and surge protection is present. First indicator 
circuit portion 30 includes a first indicator device energized by line 
conductors 12 and 13 when the fuse 20 is intact. Such a green indicator 
circuit portion is substantially in accordance with prior art surge 
suppressors. 
FIG. 2 also includes a second ("red") indicator circuit portion 40 for 
indicating power is on and surge protection is lacking. The second 
indicator circuit portion 40 includes a second indicator device energized 
by the line conductors when the fuse 20 has been interrupted. Impedance 
levels of the indicator circuit portions 30 and 40 are selected so that 
circuit portion 40 is not fully energized when circuit portion 30 is 
energized, thus avoiding any confusion of the indicators' signals. 
The colors, green and red, for circuits 30 and 40, respectively, are 
matters of choice. 
Now referring to FIG. 3, a more specific embodiment of the present 
invention is illustrated by way of example. Here, the elements of the 
general schematic of FIG. 2 are shown with further details of the 
indicator circuit portions 30 and 40. The green circuit portion 30 
includes a green light-emitting diode 31 in series with a first resistor 
32 connected between a first circuit point 23c which is between the fuse 
20 and the first varistor 22a, and a second circuit point 33 that is 
arranged for connection with one of the line conductors, specifically here 
the neutral conductor 13. 
The red indicator circuit portion 40 includes a red LED 41 connected in 
series with a second resistor 42 as well as the first resistor 32 that is 
also included in the green circuit portion 30. The series combination of 
indicator 41 and resistors 42 and 32 is connected between a third circuit 
point 43 for connection with the hot line conductor 12 and with the second 
circuit point previously referred to, 33. Resistor 32 is therefore in both 
circuits 30 and 40. 
By way of further example, the following specific components are identified 
as suitable in a surge suppressor as illustrated in FIG. 3 for use with a 
120-volt, 60 hertz electrical distribution system. 
Fuse 20 is a Littlefuse type 224, 125-volt AC, fuse. Indicators 31 and 41 
are respectively green and red light-emitting diodes type AND124G. The 
green LED has a breakover voltage of about 2.4 V. and the red LED has a 
breakover voltage of about 1.5 V. in accordance with normal commercial 
availability. Each such light-emitting diode 31 and 41 has another, 
secondary, diode 31a, 41a in inverse parallel relation to it to prevent 
reverse voltages of greater than one-half volt from appearing across the 
light-emitting diode in accordance with standard practice. In this 
example, diodes 31a and 41a are Type 4007. The first resistor 32 is a 
20,000 ohm.+-.5 percent, one-half watt, 300-volt resistor. The second 
resistor 42 is a 5,000 ohm, .+-.5 percent, one-half watt, 300-volt 
resistor. The varistors 22a and 24a are each metal oxides varistors of 45 
joules energy storage capacity. They have a breakover voltage of from 
212-214 V. at 1 ma. 
In operation it is seen that whenever terminals 10 and 11 are connected 
with a supply, surge protection for the load is provided in a manner 
described in connection with FIG. 1. While fuse 20 is conducting, the 
green circuit portion 30 is energized, and because it has a lower 
impedance than the second indicator circuit portion 40 as determined by 
the resistors 32 and 42, it will prominently display a green indication 
that power is on and surge protection is present. 
When a surge occurs on any of the lines 12, 13 and 17 that subjects either 
of the varistors 22a and 24a to harm, the fuse 20 will operate and 
effectively take those varistors out of the circuit and also de-energize 
the green circuit portion 20. The green LED 31 thus goes off. The red LED 
41 then sees sufficient voltage to be fully energized and display a 
prominent red indication that the power is on but full surge protection is 
not present and the suppressor unit needs to be replaced or to be 
repaired. 
The circuit of FIG. 3 shows a capacitor 50 connected between the hot and 
neutral conductors 12 and 13. Capacitor 50 is an optional element intended 
for applications where the surge suppressor is used for additional 
filtering of transients that affect the load. 
In a specific example of the present invention, a surge suppressor as shown 
in FIG. 3 is provided with a capacitor 50 that is a 0.001 microf. 
1000-volt AC capacitor. 
An extra useful occurs from the circuit as shown in FIG. 3 with the LEDs 31 
and 41 having the same polarity as shown. The green LED 31 is bright when 
energized and at the same time the red LED 41 is energized just enough to 
glow perceptibly. This permits checking the operability of both under 
normal conditions. 
If for any reason one wishes to avoid the red glow of the LED 41 when the 
green LED 31 is on, it is merely necessary to reverse the polarity of one, 
such as that of green LED 31. In each instance the secondary diodes 31a 
and 41a are retained in their inverse parallel relation to the LEDs 31 and 
41, respectively. 
FIGS. 4A and 4B are plan views of a varistor unit 23 as shown schematically 
in FIG. 2. The difference between the two is that FIG. 4A shows a unit 23 
with three wire leads 23a, 23b, and 23c, and FIG. 4B shows a unit in which 
the center tap conductor 23c' is a flat tab, with an eyelet 25 that 
facilitates connections in certain applications. 
FIG. 5 shows a sectional view of the unit 23 of FIG. 4a. Each varistor 22a 
and 24a is a disk-like element; they are arranged in parallel relation to 
each other with leads 23a and 23b, respectively on each outermost surface 
of the pair of varistors. A third, center-tap 23c, is soldered or 
otherwise bonded between the two varistor disks. The varistors 22a and 24a 
are jointly covered with a single insulating cover or casing 28, formed of 
resinous insulation for example. Unit 23 affords a highly useful element 
itself that is particularly advantageous in the present invention because 
of the compactness and economy thereby achieved. Such a three-lead 
varistor device effectively provides up to three values of non-linear 
resistance in a circuit to which it is applied. 
The invention has been shown in a few forms only but may be otherwise 
formed consistent with the teachings herein.