Protective apparatus for encapsulating electrical circuits

An encapsulating apparatus for protecting an electrical circuit, the electrical circuit comprising a plurality of discrete electrical components supported on a circuit board and connected electrically to each other and to external electrical lead lines. The encapsulating apparatus comprises an imperforately walled hollow container defining oppositely first and second openings through the wall thereof. The container further comprises internal support means for receiving and positioning the electrical components in a predetermined relationship to the container and to each other with the lead lines extending outwardly through the first opening. The second opening is completely closed by a cap having a portion corresponding in size and configuration to said second opening. The cap further comprises at least one irregularly shaped arm projecting into the container when the cap is in closing relationship to the second opening, positively fixedly positioning the components in their predetermined relationship. The cap is further retained in closed relationship with respect to the second opening by an epoxy or other suitable means. The container is then substantially filled by a potting compound which is applied as a fluid and hardens rigidly. When the liquid potting compound solidifies, the electrical components are permanently sealed within the container.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of and protective apparatus for 
insulatively encapsulating electrical circuits. 
2. Description of the Prior Art 
Protective housings for electrical circuits have been employed in the past. 
For example, U.S. Pat. No. 4,345,290 to Johnson shows a plastic housing 
for electrical elements disposed and potted therein to provide both 
electrical insulation and mechanical protection from the environment. The 
housing is a hollow enclosure defining first and second openings through 
its walls, the first opening having internal threads. The electrical 
elements are inserted into the housing through the first opening. A 
cylindrical cap member has an externally threaded first end of a diameter 
dimensioned to fit within said housing first opening, to engage the 
threads therein. When threaded into the first opening, the cap member 
effectively retains the electrical components within the housing. However, 
Johnson does not provide a means for fixing the position of the circuit 
elements within the housing in a predetermined relationship. Because the 
circuit elements may be displaced and subsequently damaged when the 
insulative potting compound is injected into the capped housing, there is 
a need to find an encapsulating apparatus which effectively fixes the 
position of the circuit elements within the housing prior to introduction 
of the potting compound. 
SUMMARY OF THE INVENTION 
The present invention is a new type of protective electrical circuit 
encapsulating apparatus which fixes the position of the circuit components 
within the container prior to the introduction of potting compound, and a 
method for encapsulating the electrical circuit. 
The encapsulating apparatus of the present invention comprises an 
imperforately walled hollow container or housing defining oppositely 
disposed first and second openings. An electrical circuit comprising a 
plurality of discrete electrical circuit components connected electrically 
to each other and to external electrical lead lines, is directed into the 
container through the second opening. An internal support mechanism 
receives and positions the electrical components in a predetermined 
relationship to the container and each other with the lead lines extending 
outwardly through the container first opening to connect the encapsulated 
circuit to other circuits or apparatus. 
After the electrical circuit components have been inserted into the 
container, the second opening is completely closed by a cap having a 
portion of corresponding size and configuration. This cap further 
comprises at least one irregularly shaped arm projecting into the 
container when the cap is in closing relationship to the second opening. 
The arm positively fixes the position of the components in their 
predetermined relationship. 
An insulative potting compound, preferably a liquid which hardens into a 
solid, is introduced into the container through the first opening until 
the container is substantially filled. The insulative potting compound 
encompasses the arm and components, thereafter hardening around said arm 
and components to permanently fix and seal the circuit components within 
the container. 
The internal support and cap arm of the present invention stabilize the 
circuit components in place prior to the introduction of the potting 
compound into the container. This prevents damage to the circuit 
components due to displacement of the components by the force of the 
injected potting compound. 
The present invention also includes a method for assembling the 
encapsulating apparatus. An imperforately walled hollow container is 
formed defining first and second oppositely disposed openings through its 
walls. An internal support mechanism in the container receives and 
positions the electrical components and lead lines in a predetermined 
relationship to the container and to each other, with the lead lines 
extending outwardly through the second opening. A cap member is attached 
to the housing to completely close the second opening of the container. A 
potting compound is then injected into the container to substantially fill 
the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the encapsulating apparatus of the present 
invention is generally shown at 10 in FIG. 1. The encapsulating apparatus 
protects an electrical circuit which comprises a plurality of discrete 
electrical components connected electrically to each other and to external 
electrical lead lines 46 (which are typically metal wires with plastic 
insulator coatings or sleeves). The encapsulating apparatus includes a 
walled hollow container or housing 12 defining a first opening 22 in a 
first wall and an oppositely disposed second opening 24 in a second wall. 
It is also preferred that the container 12 further comprise an outwardly 
projecting, externally screw threaded neck 20 coincident with the first 
opening 22. 
Container 12 further includes an internal support mechanism 26 for 
receiving and positioning the electrical components in a predetermined 
relationship to the container 12 and to each other, with the lead lines 
extending outwardly through the first opening 22. 
The electrical circuit components 33 are preferably connected electrically 
by a circuit board, as shown by 36 in FIG. 3, or by other suitable means. 
As shown in FIGS. 2 and 3, internal support 26 preferably extends 
outwardly with respect to the inner walls of container 12. To properly 
support circuit board 36, support 26 includes two U-shaped substantially 
parallel flange members 14A and 14B integrally molded as part of the inner 
wall of container 12. Circuit board 36 is inserted through second opening 
24 and between flanges 14A and 14B. Flanges 14A and 14B are further 
tapered such that a gap 15 created between flanges 14A and 14B narrows 
from second opening 24 toward the first opening 22 as shown in FIG. 4. The 
gap 15 at the narrowest end adjacent first opening 22 is slightly less 
than the height of circuit board 36. Thus, it is desirable for flanges 14A 
and 14B to spread slightly as circuit board 36 is inserted into support 
26. The inherent memory of flanges 14A and 14B cause them to tightly grasp 
circuit board 36 once it is in place. It is also preferred that support 26 
is substantially axially aligned with first opening 22 and second opening 
24, and tangential to the periphery of the neck 20, as shown in FIG. 2. 
Lead lines 46 connected to circuit board 36 are directed through neck 20 
and serve to connect the electrical circuit carried by circuit board 36 to 
other electrical circuits and equipment. 
After circuit board 36 is positioned on the support 26, second opening 24 
is completely closed by a cap 18 which has a first end portion 28 
corresponding in size and configuration to said second opening 24. Cap 18 
comprises at least one and preferably a pair of arms, such as arms 32A and 
32B extending from cap 18 first end portion 28, as shown in FIG. 5. Arms 
32A and 32B project into the container 12 when cap 18 is in closing 
relationship to second opening 24. Arms 32A and 32B positively and fixedly 
position the circuit components 33 in their predetermined relationship 
within the container 12. Specifically, arms 32A and 32B engage an edge 38 
of circuit board 36, as shown in FIG. 3, to prevent circuit board 36 from 
moving along or off support 26 when cap 18 is in closing relationship to 
second opening 24. Arms 32A and 32B are positioned so that regardless of 
the orientation of cap 18 when it is inserted into opening 24, at least 
one of the arms 32A,32B will engage edge 38. Each arm 32A and 32B contains 
an aperture 34, the significance of which will be explained later. Arms 
32A and 32B also include cylindrical reinforcement ribs 35, integrally 
molded with the arms to give arms 32 additional strength. 
A second end portion 30 of cap 18 defines a recessed area 40 as seen in 
FIG. 8. This recessed area 40 can be used to receive identification decals 
(not shown) to identify the components contained within the encapsulating 
apparatus 10, since visual inspection of the components is not possible 
once the apparatus is assembled. 
Cap 18 is bonded in closed relationship with respect to second opening 24 
by a quick drying epoxy or glue 50, by ultrasonic welding, or other 
suitable means. This enhances the speed by which the entire encapsulating 
mechanism can be assembled. 
After cap 18 has been fitted into second opening 24, container 12 is 
oriented with neck 20 upmost, and container 12 and neck 20 are then filled 
with a potting compound 44, preferably a liquid which hardens into a 
solid, such as sold under the trademark Scotchcast 4 by 3M Company, or the 
equivalent. The interior of neck 20 is filled to the approximate top of 
the neck 20. When the potting compound 44 has hardened, the assembly 
operation is complete. 
When the potting compound 44 is injected into the encapsulating container 
12, it encircles arms 32A and 32B and passes through the arm apertures 34 
before it hardens. After potting compound 44 has hardened, cap 18 is 
virtually inextricably locked into place in second opening 24. This 
results in a protective sealed container which is waterproof, dustproof, 
acid proof, and explosive proof. Any attempts to remove cap 18 from 
container 12 requires shearing of the potting compound 44 encircling arms 
32A and 32B and passing through arm apertures 34. 
With prior art apparatus the force of the potting compound being injected 
into the housing tends to displace the electrical circuit elements. 
Because of this displacement, circuit wires can be weakened or broken and 
connections pulled apart, thereby damaging the circuit. In an effort to 
limit this damage, attempts have been made to reinforce the circuit wires 
by applying a liquid insulative coating to the circuit wires, which is 
allowed to harden into a solid, prior to introduction of the potting 
compound. With the internal support mechanism 26 and cap 18 of the present 
invention, displacement of the circuit elements is prevented so the 
additional insulative coating is not needed. 
After the encapsulating apparatus 10 is assembled, neck 20 can be extended 
through a hole on any electrical box or bracket, such as one created by 
removal of a knock-out in a circuit box, and fastened thereto by a 
matching lock nut. However, the integrity of the encapsulating apparatus 
10 permits its installation within an electrical circuit box without the 
need to fasten it to the box. This adaptability greatly simplifies the use 
of this apparatus with any existing installation. 
As can be seen in FIGS. 1-8, it is preferred that container 12 be of a 
generally elongated rectangular shape with first opening 22 and second 
opening 24 aligned with its longitudinal axis. Second opening 24 is 
defined as a square opening, for receiving a square first end portion 28 
of cap 18. In this manner, cap 18 having a pair of parallel arms 32A and 
32B defining apertures 34, may close second opening 24 with the arms 32A 
and 32B either horizontally or vertically oriented. 
It is also preferred that the container 12 and cap 18 be composed of 
plastic such as manufactured by Borg Warner under the trademark ABS KGW or 
similar high impact electrical insulating material which is resistant to 
mechanical stress. A color may be added to the plastic material to further 
aid in identifying the circuit components contained therein. 
The method for encapsulating an electrical circuit having a plurality of 
discrete electrical components connected electrically to each other and to 
external electrical lead lines, as disclosed above, comprises of the steps 
of: 
(1) providing an imperforately walled hollow container 12 defining 
oppositely disposed first and second openings 20 and 24 through its wall, 
and having an internal support mechanism 26 for receiving and positioning 
a carrier (circuit board 36) with the electrical components 33 in a 
predetermined relationship to the container 12 and to each other, with the 
lead lines 46 extending outwardly through the first opening; 
(2) guiding the carrier 36 through the housing second opening 24 and into 
engagement with the support mechanism 26 to achieve a predetermined 
relationship of the components 33 with respect to each other and with 
respect to the container 12; 
(3) closing and sealing the second opening 24 with a cap 18 having at least 
one irregularly shaped arm 32A,32B projecting into the container 12 when 
the cap 18 is in closing relationship to the second opening 24, to 
positively fixedly position the carrier 36 and the components 33 in the 
predetermined relationship; and 
(4) filling the container with a fluid applied, rigid hardening potting 
compound 44 for inextricably encompassing the fixedly positioned circuit 
components 33 and the arms 32A,32B to permanently fix and insulatively 
seal the circuit components 33 within the container 12. 
The method described above also preferably includes the step of applying a 
circuit component identifying decal (not shown) to the exterior surface 40 
of the cap 18. 
This apparatus is particularly useful as an encapsulator for heat 
generating electrical components, such as a varistor. A varistor has a 
characteristic of changing its electrical impedance from a relatively high 
to a relatively low value whenever a certain voltage is exceeded and is 
used to protect against the electrical transients which exceed normal 
operating levels. These transients are frequently induced by the 
connection and disconnection of large electrical loads of the user's 
equipment or of the equipment of other users which are coupled into the 
user's electrical circuits by common transformers and primary electrical 
circuits. Elimination of such surges is important because many solid state 
circuit elements are highly voltage sensitive and are easily damaged by 
such surges. The reductions of the physical size of electrical motors by 
the use of thinner insulation on the wire used in field and armature 
windings has decreased proportionately the amount of transient overvoltage 
needed to puncture the insulation, creating a greater need for the use of 
varistors. 
The encapsulating apparatus of the present invention insulates the 
varistors from the environment (both electrically and mechanically) and 
insulates other circuits and components adjacent the encapsulator from the 
heat generated by the varistors when voltage surges pass through the 
encapsulated varistor circuit. 
It will also be apparent to those skilled in the art that a number of other 
modifications and changes can be made without departing from the spirit 
and scope of the present invention. Therefore, it is to be understood that 
the invention is not to be limited except by the claims which follow.