Safety enhanced transformer circuit

Details Safety enhanced transformer circuit Safety enhanced transformer circuit

2000-07-11

2003-01-07

Jackson, Stephen W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Transformer protection

C361S042000, C361S045000

Reexamination Certificate

active

06504691

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a safety-enhanced protective circuit for a neon transformer which is used to energize a neon tube or an argon tube for illumination.
BACKGROUND OF THE INVENTION
FIG. 1
shows a conventional circuit of the kind which detects a ground fault of a neon transformer. A leakage transformer (non transformer)
11
includes a primary winding
12
having one connected through a switch
13
to an input terminal
14
and the other end connected to an input terminal
15
. The transformer also includes a pair of secondary windings
16
,
17
having their starting ends connected together at a junction
20
which is connected to a ground terminal
18
of a transformer casing
36
, and which thus is connected to the casing
36
. The ground terminal
18
is connected to the ground and the both terminals ends of the secondary windings
16
,
17
are connected to output terminals
21
,
22
, across which sign lamps
23
such as neon tubes or argon tubes are connected. An alternating current power or commercial power is input across the input terminals
14
,
15
and is boosted by the transformer
11
to be applied across the sign lamps
23
for lighting them.
A protective circuit
10
is provided to detect any ground fault, namely, a contact of the sign lamps
23
or their wirings with the casing
36
or a tower on which the sign lamps
23
are mounted and to interrupt the input a.c. power in such event. Specifically, tertiary windings
25
,
26
are provided in the vicinity of the secondary windings
16
,
17
and are magnetically coupled therewith , and function as part of the protective circuit
10
. Usually the tertiary windings
16
,
17
are disposed so as to be interposed between the core on which the secondary windings
16
,
17
are disposed and the lowermost layer of the secondary windings
16
,
17
with a layer of an insulation material having a high withstand voltage capability on the order of 6000 to 7000 V interposed between the secondary windings
16
and
17
and the tertiary windings
25
,
26
to provided an enhanced electrical insulation therebetween while assuring a satisfactory magnetic coupling between the secondary windings
16
,
17
and the tertiary windings
25
,
26
.
At one end, each of the tertiary windings
25
,
26
is connected together in phase opposition such that their induced voltages cancel each other while the other end of respective tertiary winding
25
,
26
is connected to an input of a rectifying and smoothing circuit
27
, an output of which is connected through a Zener diode
28
across a parallel circuit including a resitor
31
and a cpacitor
32
. A triac
33
has its gate and cathode connected across the parallel circuit. The triac
33
is connected in series with a relay drive coil
34
across the input terminals
14
,
15
and the switch
13
comprises a relay contact which is controlled in accordance with the energization of the relay drive coil
34
.
Under a normal condition, voltages induced across the teritary windings
25
,
26
are substantially equal in magnitude to each other, but are opposite in phase, whereby an input voltage to the recitifying and smoothing circuit
27
is nearly zero. However, upon a ground fault of the sign lamps
23
or the wiring thereof, one of the secondary windings which is associated with the ground fault will be short-circuited, causing a substantial decrease in the induced voltage in the teritiary winding which is coupled with this secondary winding to allow the full induced voltage in the other teritary winding to be applied to the rectifying and smoothing circuit
27
. This voltage is rectified and smoothed, and an increase in the rectified and smoothed output voltage turns Zener diode
28
on, with consequence that the triac
33
is rendered on to energize the relay drive coil
34
to open the switch
13
, thus interrupting the supply of the input a.c. power to the transformer
11
. The switch
13
comprising the relay contact is thrown to the normally open position NO, whereby the holding current to the relay drive coil
34
flows therethrough.
It will be noted that in the described conventional circuit, the pair of tertiary windings are used and disposed below (or inside) the lowermost layer of the pair of secondary windings with a high withstand voltage insulation. The provision of the tertiary windings requires time and labor, reducing the production efficiency of the neon transformer.
Protection against a secondary ground fault of such a neon transformer is also disclosed in FIG. 3 of U.S. Pat. No. 5,847,909 issued Dec. 8, 1998, where the protective circuit does not employ a tertiary winding, but uses an increased number of parts and results in a complicated arrangement, which renders it difficult to utilize a conventional box for containing a neon transformer.
For a neon transformer, it is mandated by legal regulation that the ground terminal
18
be always connected to the ground for use in views of the safety consideration. However, there is a likelihood that a dealer who undertakes constructing a neon tower which uses neon lamps may forget the work of connecting the ground terminal
18
to the ground. A no-ground connection protective circuit which detects such condition during use to interrupt the supply of the a.c. power is proposed and shown in FIG. 2 of the U.S. Patent cited above. However, this no-ground connectin protective circuit again requires an increased number of parts and results in a complicated arrangement, rendering it difficult to utilize a conventional box for containing a neon transformer.
The ground fault protective circuit shown in
FIG. 1
detects any ground fault which occurs on the secondary side of the neon transformer immediately to interrupt the supply of the a.c. power to the transformer, and thus is free from any likelihood of causing a fire. However, it is necessary to repair a location where the ground fault has occurred . It will be understood that finding the location of the ground fault is an awful burden for a neon tower of an increased size, for example. If a ground fault causes a ground current to flow to produce sparks, the location of the ground fault may be discovered in a relatively simple manner by relying on light produced or a smell of ozone generated by sparks. However, with the ground fault protective circuit shown in
FIG. 1
, when a power switch, not shown, is turned off to interrupt the supply of the a.c. power to the input terminals
14
,
15
and thus to interrupt the self-holding current to the relay drive coil
34
, and the switch is restored to its normally closed position NC before reclosing the power switch, the ground current again flows through the location of the ground fault, which is immediately detected by the ground fault protective circuit
10
to throw the switch
13
to the normally open position NO, interrupting the supply of the a.c. power for the second time. Accordingly, the reclosing of the power switch fails to produce a ground fault condition in order to discover the location of the ground fault. This takes time to identify the location of the ground fault, preventing a repair from being completed rapidly. The same is true with the ground fault protective circuit disclosed in the cited Patent.
It is an object of the invention to provide a no-ground connection protective circuit for a neon transformer which employs a reduced number of parts and a simple arrangement to be capable of detecting a secondary ground fault to interrupt the supply of the a.c. power.
It is another object of the invention to provide a ground fault protective circuit for an neon transformer which temorarily overrides the function of the ground fault protective circuit to enable the discovery of the location of a ground fault to be facilitated.
It is a further object of the invention to provide a no-ground connection protective circuit which employs a reduced number of parts and a simple arrangement to provide a protection against no ground connection of a ground terminal.
SUMMARY OF THE INVENTION
A

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