Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2001-10-31
2004-06-29
Lee, Wilson (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S289000, C315S2090SC
Reexamination Certificate
active
06756744
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a circuit for lighting a discharge lamp. It particularly relates to a method of initiating lighting or re-lighting of a discharge lamp and to an improvement of an igniter used to practice the method. The present invention also relates to a light source device utilizing the circuit and an optical instrument incorporating the light source device.
2. Description of the Related Art
FIG. 9A
illustrates a discharge lamp lighting circuit comprising a ballast
100
for DC lighting and a prior art igniter
220
for application of high voltage pulses. Lighting of discharge lamp
300
is operated generally as follows. In initiating lighting of the discharge lamp
300
(or in initiating re-lighting of the discharge lamp
300
which has been turned off), an initiating voltage is applied across electrodes
300
a
,
300
b
of the discharge lamp
300
. The initiating voltage comprises a voltage (of about 300 V) outputted from the ballast
100
and a thin mustache-like high pulse voltage of several 10 Hz (12.about.25 KV, pulse width=about 0.1 . mu.s, and pulse frequency=several 10 Hz) generated by the igniter
220
and superimposed on the output voltage of the ballast
100
.
FIG. 9C
illustrates a waveform of such a high pulse voltage superimposed voltage applied to the discharge lamp
300
in initiating lighting.
When the high voltage pulses are repeatedly applied across the electrodes
300
a
,
300
b
, dielectric breakdown occurs between the electrodes, with the result that hot electrons are emitted from the negative electrode
300
a
to the positive electrode
300
b
to define a discharge path whereby discharge is initiated. Subsequently, when an appropriate voltage is applied to supply a current, the initial discharge state changes to a transitional discharge state referred to as glow discharge and then to a steady discharge state referred to as arc discharge. When the initial discharge state changes to glow discharge, the voltage across the electrodes
200
a
,
300
b
rapidly drops to about 15V for example. When the discharge state further changes to arc discharge, the voltage increases to for example about 80 V at which the discharge becomes steady.
As described above, a high pulse voltage of about 12 to 25 KV is applied in initiating lighting because, although the lowest voltage for initiating lighting of the discharge lamp
3
is about 500 to 700V, lighting the lamp with such a voltage disadvantageously takes a relatively long time of about 10 to 20 minutes. Particularly where the lamp is used in an optical instrument of the type which requires an initiating time of within one minute for example, a high pulse voltage of 12 KV to 25 KV is inevitably necessary.
Thus, to enhance the efficiency in initiating lighting of the lamp, a high pulse voltage needs to be applied across the electrodes
300
a
,
300
b
. This raises the following problem in lighting of prior art discharge lamp
300
.
Firstly, as shown in
FIG. 9B
, it is required that two power supply leads
340
,
350
of the lamp be spaced from each other by at least 25 mm to prevent short-circuiting during the application of high pulse voltage. Because of this requirement, usable discharge lamps are limited to double-end type lamps only. Further, when a double-end type discharge lamp
300
is used as attached to a reflector
500
, one seal portion
310
of the discharge lamp
300
is fitted in a lamp-receiving portion
510
of the reflector
500
, while one power supply lead
340
is drawn out of a central hole
520
formed centrally of the lamp-receiving portion
510
. Therefore, to keep a necessary distance from the power supply lead
340
, the other power supply lead
350
extending from the other seal portion
320
of the discharge lamp
300
need be extended out toward the back side of the reflector
500
through a through-hole
530
perforating a reflecting surface
520
of the reflector
500
.
The through-hole
530
of the reflective surface
520
is formed by drilling the reflector
500
using a diamond drill for example, which may lead to an increase in cost. Further, the provision of the through-hole
530
may give rise to small cracks in the reflector. Therefore, the reflector
500
may break from the cracked portion due to the thermal cycle resulting from ON-OFF operations of the discharge lamp
300
. Further, when the lamp
300
is broken, the reflector
500
may also break, scattering hot glass pieces. Further, since a high pulse voltage is applied in initiating lighting of the lamp as described above, electric parts having a high withstand voltage need to be used, which increases the cost for making the whole electric circuit.
Moreover, in initiating lighting or re-lighting of the discharge lamp
300
, particularly in initiating re-lighting of the lamp immediately after having been turned off, of which the bulb temperature is high, the internal pressure of the lamp is high so that the insulation resistance between the electrodes
300
a
,
300
b
is also high. Therefore, a high voltage of at least 12 to 25 KV need be applied to the discharge lamp
300
to cause dielectric breakdown between the electrodes
300
a
,
300
b
for staring discharge. Therefore, the prior art igniter
220
uses a second step-up transformer
610
in addition to a first step-up transformer
600
in increasing the voltage to a required value. However, with this method which uses two step-up transformers
600
and
610
, the voltage raising response is poor and the frequency of the high voltage pulses is limited to several 10 Hz. Therefore, there exists a limitation on an improvement in the lighting speed of the discharge lamp
300
.
On the other hand, in some applications of the discharge lamp
300
, the discharge lamp
300
is required to light or re-light at a very high lighting speed (e.g., in one minute when the lamp is used for an optical instrument such as a projector). To fulfill such requirement, a high initiating voltage as described above is inevitably necessary.
It has however pointed out that an application of a high pulse voltage across the electrodes
300
a
,
300
b
in initiating lighting of the lamp may cause sputtering so that electrode materials adhere to the inner surface of an arc tube
300
c
, which accelerates blackening and shortens the lifetime of the lamp.
Further, the prior art igniter
220
uses the two step-up transformers
600
,
610
in two stages. Specifically, since the secondary coil of the second step-up transformer
610
is connected in series to the output side of the ballast
100
, a current capacity which is equal to or more than the output of the ballast
100
is required. For this purpose, it is necessary to use a coil formed of a thick wire on the output side of the second step-up transformer
610
. However, forming a coil of a thick wire increases the size of the second step-up transformer
610
. Further, with such a thick wire, it is difficult to make a coil having a required number of turns for increasing the turn ratio of the transformer.
Therefore, the first step-up transformer
600
is utilized prior to the second step-up transformer
610
to increase the voltage of pulses through two steps, thereby providing pulses of a required high voltage. However, this structure requires an increased parts count and hence hinders the size reduction of the igniter
220
.
In order to reduce the size of the igniter
220
including a large number of parts under such conditions, there is no way but to reduce the spacing between the parts. For this purpose, filler need be provided between adjacent parts to assure insulation therebetween, which causes an increase in weight.
It is, therefore, a first object of the present invention to greatly lower the pulse voltage applied in initiating lighting of a discharge lamp without deteriorating the lighting performance so that the lifetime of the lamp can be significantly improved and limitation on types of usable discharge lamps can be eliminated; in other wor
Lee Wilson
Phoenix Electric Co. Ltd.
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