Note: Descriptions are shown in the official language in which they were submitted.
-1- 59-259351
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APPARATUS FOR LIMITING ARC DISCHARGE CURRENT
IN INCANDESCENT LAMP
Field of the Invention
The present invention relates to an apparatus to
limit arc discharge current in incandescent lamp.
Description of the Prior Art
When incandescent lamp is dc-illuminated to reduce
flicker, the filament snapping instantly starts arc to flow a
relatively high discharge current through the charged gas.
The arc occurs almost in a short-circuit state
because the circuit resistance during discharge is extremely
low. Upon an actual survey, a discharge current of up to 200
amperes was observed when the filament of 60-watt incandescent
lamp was snapped during illumination with dc 130 volts.
The arc discharge current may damage many of the
circuit parts.
Brief Summary of the Invention
In view of the foregoing, the main object of the
present invention îs to provide a means to limit arc discharge
current in incandescent lamp.
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Still another object of the present invention is to
provide a means to automatically effect such current
limitation.
The invention relates to a device to limit arc
discharge arising in a dc-operated incandescent lamp,
comprising: (a) a rectifier circuit having input terminals
connected with an ac sourc~, and output terminals connected
with the incandescent lamp; (b) first resistance means
connected in series with the incandescent lamp, for limiting
both an initial switch-on surge into the incandescent lamp and
an arc discharge current due to the filament of the
incandescent lamp being snapped; (c) second resistive means
connected in series with the incandescent lamp, for providing
by virtue of a voltage drop across it a signal indicating
occurrence of arc discharge current; (d) switching means
having a trigger ~lectrode and a main current path, the main
current path being connected in parallel with the first
resistive means; and (e) a time constant circuit, having an
output connected with the trigger electrode of the switching
means, to retard, on switch-on, the conduction of the
switching means in accordance with its time constant, and to
suspend the conduction of the switching means when the voltage
across the second resistive means reaches a predetermined
level.
Descri~tion of the Preferred Embodiments
Several embodiments according to the invention will
hereinafter be explained taken in connection with the
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accompanying drawings wherein:
Figure 1 shows the ~asic circuit of the apparatus
according to the invention, particularly the connection
instantly upon turning-on;
Figure 2 shows the basic circuit of the apparatus
according to the invention, particularly the connection
instantly after the filament snapping has generated an arc;
Figure 3 shows a current curve that appears in the
basic circuit as shown in Figure :L;
Figure 4 shows a current curve that appears in the
basic circuit as shown in Figure 2;
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~ 3
FIG.5 shows the circuit of an embodiment according
to the invention;
FIG.6 sh~ws the voltage waveform that appears in the
circuit as shown in FIG.5;
FIG.7 shows the current waveform that appears in the
circuit as shown in FIG.5 upon turning-on;
FIG.8 shows the current waveform that appears in the
circuit as shown in FIG.5 during arc clischarge; and
FIG.9 shows the circuit of an embodiment according
to the invention, wherein a transistor is equipped to the
limiting circuit.
Throughout the accompanying drawings, symbol A
designates rectifier; B, time-constant circuit; C, capaci-
tance; D, diode; L, lamp; R, resistance; S, switch; AC, ac
power source; and SCR, thyristor.
FIGs.l and 2 show the basic circuit of the apparatus
according to the invention.
The circuit as shown in FIG.l is arranged in such a
manner that ac current is supplied from ac power source AC to
capacitance C and lamp L through power switch Sl and the plus
output terminal of full-wave rectifier A. The return circuit
is connected to the minus output terminal of full-wave recti-
fier A through limiting resistance R2.
After a lapse of a prescribed time upon turning-on
of power switch Sl, time-constant circuit B closes breaking
switch S2 to short Limiting resistance R2 connected in paral-
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lel to switch S2. Thus, the full output of recti~ier A issupplied to lamp L.
In the connection as shown in FIG.2, if the filament
of lamp L is snapped when switch S2 is closed, the resultant
arc decreases the circuit resistance to approximately zero to
flow a discharge current of up to 200 amperes. The discharge
current produces a voltage drop across low resistance Rl,
connected in series to the main current circuit, and the
voltage drop operates time-constant circuit B to open breaking
switch S2. Thus, the inflow of the discharge current is
switched to limiting resistance R2 to suppress the circuit
current.
These operation will be further explained with
reference to FIGs.3 and 4.
Instantly after power switch Sl is closed at time
to~ a surge current with a magnitude of 10-fold larger than
that of the rating flows as shown in FIG.3 with the broken
line. If limiting resistance R2 is connected in series with
lamp L, the circuit current is suppressed as shown in FIG.3
with the solid line.
FIG.4 shows the current curve that appears upon
filament snapping. The filament snapping instantly starts arc
at time t2, and the inflow of a discharge current with a
magnitude of 180 amperes continues. If low resistance Rl,
e.g. 2 ohms, is connected in series to lamp ~ as particularly
shown in FIG.2, the discharge current is suppressed to 50
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amperes or lower. Simultaneously, the voltage drop of about
50 volts, produced ~cr~ss resistance Rl by the discharge
current, operates time-constant circuit B to open breaking
switch S2. Thus, the circuit current is suppressed as shown
in FIG.4 with thP solid line.
FIG.5 shows the circuit of an embodiment according
to the invention. This embodiment is arranged so that turn-
ing-on of switch Sl energizes both capacitance C and lamp L.
The return circuit is connected to the minus output terminal
of rectifier A through both limiting resistance R2 and low
resistance Rl, placed within lamp L, in order to limit an
initial surge current.
The ~ilament resistance of lamp L upon turning-on of
power switch Sl is 10 ohms or lower, and the voltage across
capacitance C is, therefore, approximately zero. For this
reason, the surge current upon turning-on of power switch Sl
reaches 20 amperes or higher if limiting resistance R2 and low
resistance Rl are not used. The surge current can be sup-
pressed to a level of one ampere or lower by the insertion of
limiting resistance R2.
After a lapse of the time as determined by ti~e-
constant circuit B consisting of resistance R4 and capacitance
Cl, capacitance Cl charged through resistance R4 energiæes the
gate of reverse-blocking triode thyristor SCR to bring it into
conduction. The conduction of reverse-blocking triode thyris-
tor SCR shorts limiting resistance R2 to supply the full
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output of rectifier A to lamp L.
If the filament of lamp L is snapped during illumi-
nation, an arc occurs and permits a discharge current of up to
200 amperes to flow through the charged gas. Although in
ac-illumination arc occurs similarly but disappears within one
cycle, in dc illumination an arc discharge current continues
to damage the relating circuit parts.
In the circuit according to the invention, a relat-
ively high discharge current produces a voltage drop of up to
100 volts between resistance Rl.
The voltage drop negatively charges capacitance Cl
through diode D to c-ancel the positive voltage from resistance
R4 and also to decrease the gate voltage of reverse-blocking
triode thyristor SCR below its gate-triggering level. Thus,
the conduction of the thyristor is suspended, and the increas-
ing voltage drop between limiting resistance R2 stops the arc
occurred in lamp L.
The current- and voltage-waveforms at this momen~
are as shown in FIGs.6 to 7.
FIG.6 shows the voltage curve that appears between
the plus- and minus-terminals of rectifier A. As is evident
from FIG.6, reverse-blocking triode thyristor SCR requires a
continuous triggering because the voltage curve 7ero-cross-
overs every half-cycles.
If limiting resistance R~ and low resistance Rl are
removed from the circuit as shown in FIG.5, a surge current of
~2 ~
10 amperes or higher flows upon turning-on o power switch S2
at time to as shown in FIG.7 with the broken line. In the
circuit as shown in FIG.5, the circ~lit current is suppressed
by reverse-blocking triode thyristor SCR and limiting resis-
tance R2 as shown in FIG.5 with the solid line, and, after a
lapse of a prescribed time tl, e.g. 10-odd hundredths of one
second, limiting resistance R2 is shorted by reverse-blocking
triode thyristor SCR. Thus, the current across the filament
of lamp L slightly increases at first, but shortly reaches the
stationary level as shown in FIG.7 with the solid line.
As shown in FIG.8 with the broken line, when lamp L
is dc-illuminated in conventional manner, the filament snap-
ping of lamp L produces arc at time t3, and an discharge
current of up to 200 amperes comes into flow. The solid line
as shown in FIG.8 indicates that the discharge current can be
suppressed to 50 amperes by setting low resistance Rl in the
circuit as shown in FIG.5 to 2 ohms. The voltage drop produc-
ed across low resistance Rl by the discharge current nega-
tively energizes the gate of reverse-blocking triode thyristor
SCR to suspend its conduction in the next half-cycle of the
waveform chart as shown in FIG.6.
FIG.9 shows the circuit of another embodiment ac-
cording to the invention. This circuit is arranged by replac-
ing a part of the circuit as shown in FIG.5 with transistor T
so that the voltage drop produced across low resistance Rl
keeps the base voltage at cut-off level. Thus, the operation
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of reverse-blocking triode thyristor SCR is suspended with the
discharge current for a prescribed time.
~ pon turning-on of power switch Sl, transistor T is
conducted by resistances R4 and R6 to charge capacitance C,
and the discharge current from capacitance C conducts reverse-
blocking triode thyristor SCR. The voltage drop produced
across low resistance Rl negative]y charges capacitance C
through diode D to keep transistor T at cut-off state for a
prescribed time so that no triggering voltage is supplied to
reverse-blocking triode thyristor SCR. Thus, the discharge
current occurred in lamp L is broken.
While I have shown and described particular embodi-
ments of my invention, it will be obvious to those skilled in
the art that various changes and modifications may be made
without departing from my invention in its broader aspects and
I, therefore, intend in the appended claims to cover all such
changes and modifications as fall within the true spirit and
scope of my invention.
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