GB2319406A - Dimming a medium pressure arc lamp; UV lamp standby mode - Google Patents

Abstract

In a process for dimming a medium pressure arc lamp 31, a dimmer switch 52, or a variable resistor dimming control (104, Fig.8), is operated initially to cut the lamp power to about 50% of full power, and then the lamp power is gradually further reduced to about 20% of full power in response to sensing cooling of the lamp. The lamp 31 is operated by an electronic ballast with a transistor switched by a pulse width modulator 94 in response to lamp current sensed through a resistor 86. Operating the dimmer switch 52 (or resistor 104) changes the effective switching threshold of the pulse width modulator 94 by altering the voltage seen at its pins 93a, 93b. Dimmer switch 51 (or resistor 104) is incorporated in a dimmer feedback circuit 60 which also indirectly senses lamp temperature by means of a resistor 64 which effectively senses lamp current (inversely proportional to temperature). As the lamp cools following the initial power reduction, the increased lamp current causes the voltage at a node 114 to change, and this voltage also influences the effective switching threshold of modulator, thereby giving rise to the additional power reduction as the lamp cools. The lamp 31 may be a UV lamp used for inspection on an assembly line, and may be incorporated into a lamp unit so that the switch 52 is automatically operated to reduce lamp power to a standby state when the lamp is placed in a rest position (Figs.1,3,6).

Description

MEDIUM PRESSURE ARC LAMP AND METHOD OF DIMMING The field of the present invention is medium pressure arc lamps.

Medium pressure arc lamps, most typically generating light in the W range, are used in an assembly line context for inspection, leak detection, curing of materials and the like.

Such lamps most commonly use a mercury vapor arc. They are often hand held with a pistol type grip. These lamps have a housing in which the tube is retained. Shielding is provided by the housing both to protect the tube and direct the W light toward a discrete area.

To start such lamps, a high voltage is required. Once the arc has been struck, voltage may be reduced. As the lamp heats up, a substantial amount of mercury is vaporized, forming a dense plasma field capable of sustaining the arc at reduced voltage. A balance is reached at full power over a period of four to five minutes from a cold start.

Once full power is achieved, it can be sustained for long periods. However, power continues to be expended and heat is generated. Frequently the needs for such lamps do not require continuous cperation. In environments such as an automobile assembly line, actual use of the lamp may be for very short periods with substantially longer periods of nonuse. When a substantial number of such lamps are required for a large assembly process, the power loss, the heating and the simple inconvenience become problematic.

Turning medium pressure arc lamps on and off also creates significant problems. The lamp is adversely affected as expected lamp life is significantly shortened. In the work environment, the characteristics of reignition also create problems. When a medium pressure arc lamp has been turned off, the mercury vapor remains until the lamp is cooled. With the vapor present, striking the arc again requires voltages which are not practical. Consequently, the lamp must cool.

Typically this cooling can require about four minutes before restart is possible. Once restarted, the lamp takes several minutes to reach full power. Potentially a delay in the neighborhood of eight minutes can elapse before the lamp is again ready for use. Typically the difficulties with restriking the arc are such that lamps are frequently left on continuously in spite of the power usage and heating.

Electronic ballasts have become well known for the control of medium pressure arc lamps. These ballasts are able to provide the necessary conditions for both starting and continuous operation. Even so, a lag time of typically eight minutes is required for restriking the arc as described above.

The present invention is directed to method and apparatus for dimming medium pressure arc lamps. Through dimming, substantial energy savings can be realized without compromising the longevity of the lamp and without requiring significant time to reacquire full power.

In a first, separate aspect of the present invention, a process is contemplated for dimming a medium pressure arc lamp.

Power is first cut to about 50% of full power. As the lamp temperature falls, the system senses the current flow which is inversely proportional to lamp temperature. As the temperature drops and less mercury remains in the vapor state, power continues to be cut until it reaches approximately 20 of full power. At this point, the lamp is substantially dimmed but the arc remains. It has been found that the restoration of full power can take approximately 30 seconds with the lamp reaching 75 of full power substantially instantaneously. One means for regulating the power is through pulse width modulation.

In a second, separate aspect of the present invention, a medium pressure arc lamp is contemplated with an electronic ballast coupled to the lamp. A sensor is provided to generate a signal indicative of lamp temperature and a dimmer feedback circuit controls a pulse width modulator responsive to the signal. Power is reduced with decreased lamp temperature. The initial reduction to 50k power may be achieved through a switch. The dimmer feedback circuit then acts to reduce power from 50% to 20% as the lamp cools. Alternatively, a variable potentiometer'may be employed to achieve a range of dimming.

Even so, the feedback circuit with the sensor would be employed.

Accordingly, it is an object of the present invention to provide improved medium pressure arc lamps capable of being dimmed and the process of dimming such lamps.

The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which: Figure 1 is a side view of a first embodiment of a lamp with a dimmer switch.

Figure 2 is a side view of a second embodiment of a lamp with an adjustable dimmer.

Figure 3 is a side view of a third embodiment of a lamp with a dimmer switch.

Figure 4 is a side view of a fourth embodiment of a lamp with an adjustable dimmer.

Figure 5 is a side view of a fifth embodiment of a lamp without an electronic ballast.

Figure 6 is a perspective view of an electronic ballast for the lamp of Figure 5 with an end cap of the lamp illustrated in association with the ballast.

Figure 7 is a cross-sectional side view of a lamp.

Figure 8 is a schematic of a lamp and electronic ballast with an adjustable dimmer.

Figure 9 is a schematic of a lamp and electronic ballast with a dimmer switch.

Figure 10 is a comparative set of curves indicative of time versus power for medium pressure arc lamps.

Turning in detail to the drawings, the embodiments of Figures 1 through 7 exhibit major similarities and are here disclosed with reference first to the universal components and then separately to individual features. A lamp housing 20 is shown to be conventional in form. It includes a rear end cap 22, a main body 24 and a lamp cavity 26. A hood 28 contains a filter to eliminate visible light, thereby passing W light.

As seen from Figure 7, a lamp 31 includes an arc tube 30 associated with a lens 32 and a reflector 33 in conventional arrangement. Further, the housing conveniently includes a handle 34. A ballast housing 36 receives the power cable 38 and supports the ON/OFF power switch 40. An electronic ballast 42 is located within the housing 36.

The ballast housing 36 is found to take different forms in the several embodiments. Further, the ballast housing 36 is associated with the lamp housing 20 in different ways among the several embodiments. In Figures 1 and 2, the ballast housing 36 is fixed to the bottom of the handle 34 and forms a base having nonskid feet 44. The size and position of the ballast housing 36 in Figures 1 and 2 provide for convenient resting of the entire lamp assembly on the base when not being used.

In Figures 3 and 4, the ballast housing 36 is simply mounted as a component upon the main body 24 and rear end cap 22 of the lamp housing 20. In this instance, the power cable 38 first enters the overall lamp through the handle 34. A ballast cable 46 accesses the ballast.

In Figures 5 and 6, the ballast housing 36 is separately mounted and contained from the lamp housing which is coupled by the ballast cable 46. A hook 48 is associated with the lamp housing 20 and specifically the rear end cap 22. A receiver 50 mates with the hook 48 to hang the lamp housing 20 when not in use. In Figure 6, only the rear end cap 22 of the lamp housing 20 is illustrated, simply for convenience.

Two modes of operation are contemplated in the preferred embodiments as most completely illustrated in Figures 8 and 9.

In a first mode, Figure 9, a nonadjustable dimmer switch is contemplated. This mode of operation is illustrated in the embodiments of Figures 1, 3 and 6 as including a two-position dimmer switch 52. In Figure 1, the switch 52 is located on the bottom of the ballast housing 36, extending below the feet 44.

When the lamp is placed on a flat surface, the switch 52 is actuated to dim the lamp. For use, the lamp must be lifted from the supporting surface. In Figure 3, the dimmer switch 52 is shown extending from the hood 28 of the lamp housing 20.

This is arranged so that the lamp may be supported by the hood when not in use. Again, when placed on a flat surface, the dimmer switch is actuated to reduce lamp power. In Figures 5 and 6, the dimmer switch 52 is located with the receiver 50 on the ballast housing 36. The switch is activated by placing the hook 48 into the receiver 50 to again reduce the power to the lamp. Alternatively, the embodiment of Figures 2 and 4 illustrate a dimmer knob 54 positioned for convenient manual actuation, Figure 8.

Turning to Figure 10, comparative curves are presented showing start and restart of a conventional medium pressure arc lamp (curve A) and that of the preferred embodiment either having the dimmer switch 52 or operating the dimmer knob 54 so as to approximate the dimmer switch 52 (curve B). In the conventional arrangement as shown in the upper graph of Figure 10, a cold start is shown to take five minutes to reach 100%.

When turned off, the arc is instantly shut off. With no power passing through the lamp, the lamp begins to cool. Without allowing the mercury vapor to cool and plate out on the lamp, the amount of voltage required to strike the arc is very high and beyond practical capabilities for such a lamp.

Consequently, the lamp must be allowed to cool, typically for about four minutes, before restart. As the lamp need not completely cool, the warm-up curve does not have to start from zero. Typically four minutes is required between restart and full power.

By comparison, the lamp of the present embodiments exhibits a similar cold start profile. Rather than shutting the lamp off during nonuse, power is reduced to about 50k of full power either through the dimmer switch 52 or the dimmer knob 54. Power is reduced by pulse width modulation. The loss of power causes the lamp to begin to cool. As the lamp cools, current drops. This is sensed by a dimmer feedback circuit which raises the threshold for initiation of the pulse.

Consequently, as the temperature of the lamp drops1 the power directed to the lamp also drops. Minimum power is maintained at about 20% of full power. With a cooled lamp, this power is sufficient to sustain the arc. The time required to reach this sustained reduced power is about three minutes. This reduced level may be sustained for any period of time. When full power is again restored to the lamp, the lamp instantly achieves approximately 75% full power and achieves full power in approximately thirty seconds. Thus, a greatly reduced power mode is available for nonuse periods with a virtually immediate return to full power for use.

Looking to Figures 8 and 9, a lamp 56 is schematically illustrated electrically connected with a conventional electronic ballast 58. A dimmer switch 52 is illustrated in Figure 9 while a variable potentiometer as part of the dimmer knob 54 is employed in the schematic of Figure 8. In each instance, power may be initially reduced to up to about 50% of full power. The lamp will immediately reduce output and begin to cool as well. A dimmer feedback circuit 60 is added to the conventional electronic ballast by tapping into the lamp connector 62 through a sensor indicative of lamp temperature.

This sensor is a resistor 64 to measure current through the lamp. If the voltage applied to the lamp is insufficient in light of the lamp temperature, the lamp system will be unable to sustain the arc. As the lamp temperature decreases, the amount of mercury vapor available also decreases. As a result, resistance increases and current drops. The dimmer feedback circuit 60 then operates to vary the threshold for initiating the pulse.

A more detailed description of the operation of the circuit of FIG. 8 is now discussed by first ignoring the dimmer feedback circuit 60 and then by describing how the dimmer feedback circuit 60 dims the lamp with feedback.

In the ballast circuit 58, a 330 volt difference exists between the upper rail 80 and the lower rail 82. A potential of 330 volts on the upper rail 80 goes to one terminal 61 of the lamp and out of the other terminal 62 of the lamp. The connector 84 connects its input line(s) to its output line. When the lamp is cold, the resistance across the lamp is essentially a short and close to zero ohms.

Consequently, a very large current flows across the resistor 86. The resulting voltage drop and current flow across the light emitting diode 88 are large. The light emitting diode 88 and the n-p-n transistor 90 form an opto-isolator 92 which serves to transmit signals in a system operating under two environments having very different voltage levels; the light emitting diode 88 can operate in an environment having voltages up to 330 volts while the environment of the transistor 90 is 5 volts or less. When the large current flows through the light emitting diode 88, the light emitting diode 88 emits brighter light which is detected by the transistor 90, turns the transistor 90 on, and increases the current flow across the transistor 90.

This increased current flow through the transistor 90 causes the voltage on the line 93 to the threshold (TR) pin 93a and the HR pin 93b of the multivibrator chip 94 to ramp up from a logic zero (about 0 volts) to a logic one (about 5 volts) . The multivibrator 94 acts as an oscillator and an inverter. The five volt input into the TR and HR pins 93a and 93b causes the gate (G) pin 96 to turn off and fall to 0 volts.

The zero volts at the gate pin 96 then turns off the transistor 98. Since the transistor 98 is now off, current cannot flow across the resistor 86, thereby turning off the transistor 90.

Since the transistor 90 is off, the voltage into the TR and HR pins 93a and 93b ramps down from 5 volts to 0 volts. In response to the input voltage to the TR and HR pins 93a and 93b, the multivibrator 94 turns the gate pin 96 on (to 5 volts) which turns the transistor 98 back on and allows the resistor 86 to conduct current again. Thus, this circuit with the multivibrator 94 oscillates so that the voltage signal at the TR and HR pins 93a and 93b resembles a series of triangular peaks, where the voltage signal ramps up from a low voltage to a high voltage and then back down to a low voltage.

The multivibrator 94 changes the state of the gate pin 96 when the voltage on the TR and HR pins 93a and 93b crosses a threshold voltage of 2.5 volts. The dimmer feedback circuit 60 changes the effective threshold voltage. In describing the dimmer feedback circuit 60, the feedback circuit portion comprising resistors 64, 100 and 102 will be first ignored such that it is assumed that the terminal 62 of the lamp is not connected to the resistor 64.

When the dimmer knob 54 is turned to decrease the intensity of the lamp, the resistance of a potentiometer 104 is increased. The increased resistance of the potentiometer 104 decreases the amount of current that flows through the light emitting diode 106 which in turn causes the light emitting diode 106 to emit less light. The light emitting diode 106 and the n-p-n transistor 108 form another opto-isolator 110. The transistor 108 senses the decreased light from the light emitting diode 106 so that less current flows through the transistor 108 (from its collector to its emitter). Since less current flows through the transistor 108, the voltage at node 112 rises.

Changes in the voltage at node 112 alter the effective threshold voltage of the multivibrator 94 because the voltage at node 112 contributes to the voltage seen at the TR and HR pins 93a and 93b. In other words, if the voltage at node 114 increases and contributes more to the voltage at node 112, the transistor 90 need not increase the voltage to the TR and HR pins 93a and 93b as much to trigger the multivibrator 94 to toggle. Thus, less voltage from transistor 90 is required to cause the multivibrator 94 to oscillate the gate pin 96 to its next state. Hence, an increased voltage at node 112 increases the effective threshold voltage of the multivibrator 104, thereby decreasing its pulse width.

Now the operation of the dimmer feedback circuit 60 is discussed without ignoring the feedback circuit portion (resistors 64, 100 and 102). Resistors 64, 100 and 102 serve to prevent loss of the arc in the lamp when the lamp is being dimmed. If the lamp is dimmed too quickly, the arc could be lost. Resistor 64 senses the current flow through the lamp and sic th1s current flow is inversely proportional to the temperature of the lamp, resistor 64 essentially senses the temperature of the lamp. Resistors 64 and 100 form a voltage divider. This voltage divider generates a voltage at node 114 which represents the current flow through (and the temperature in) the lamp. The voltage at node 114 is reduced by resistor 102 and arrives at the collector of the transistor 108.

Changes in the voltage at node 114 alter the current flowing through the transistor 108.

As previously described, if the potentiometer 104 is turned to a more dim position, the voltage at node 112 would rise to some desired voltage to reduce the power to the lamp.

With the feedback circuit portion in place, if the potentiometer 104 is turned to a more dim position, the voltage at node 114 controls the amount of current flowing through the transistor 108 and thus, whether the voltage at node 112 actually rises to its desired voltage. If the lamp's temperature were still too hot for the lamp to be dimmed as desired without losing the arc, the voltage at node 114 would prevent the voltage at node 112 from rising to its desired voltage. In essence, the feedback circuit portion including the voltage at node 114 overrides the potentiometer 104.

As the lap cools and greater current flows through the lamp, the voltage at node 114 decreases and permits the voltage at node 112 to rise more. As the voltage at node 114 falls further (i.e., as the lamp cools more), the voltage at node 112 is allowed to rise even more. Since the voltage at node 112 rises as the lamp cools, less power is required to power the lamp while yet maintaining an arc in the lamp.

In the alternate embodiment of FIG. 9, a dimmer switch 52 having two positions is utilized instead of a dimmer knob 5 having many positions. The two positions of the dimmer switch 52 are: the opened position for dimmed light and the closed position for non-dimmed light. When the dimmer switch 52 is closed, the resistor 120 is shorted out. When the dimmer switch 52 is open, the resistor 120 is not shorted out, thereby increasing the effective resistance preceding the light emitting diode 106. The increased resistance decreases the amount of current that flows through the light emitting diode 106 which, as previously discussed, attempts to increase the voltage at node 112. As before, the voltage at node 114 controls whether the voltage at node 112 actually rises to the desired voltage. Thus, as the lamp cools, the voltage at node 114 decreases, the voltage at node 112 rises and the power consumption of the circuit falls.

Thus, a method for dimming a medium pressure arc lamp is disclosed as is the apparatus of such a lamp. While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The invention, therefore is not to be restricted except in the scope of the appended claims.

Claims (10)

1. A process for dimming a medium pressure arc lamp operating at a full power level, comprising cutting power to the lamp to about 50% of the full power level; sensing the temperature of the lamp over time after cutting power to about 50k; gradually further cutting power to the lamp to up to about 20% of the full power level responsive to temperature drop in the lamp.

2. The process of claim 1, cutting power to about 50% and gradually cutting power to about 20% each including pulse width modulation.

3. The process of claim 1 or 2, sensing the lamp temperature by including sensing current through a resistor.

4. A light source comprising a medium pressure arc lamp; an electronic ballast coupled to the lamp; a sensor generating a signal indicative of lamp temperature; a dimmer feedback circuit coupled with the sensor and the electronic ballast to decrease power with decreased lamp temperature.

5. The light source of claim 4, the dimmer feedback circuit including a dimmer switch.

6. The light source of claim 5, the dimmer switch reducing power by about 50%.

7. The light source of claims 4, 5 or 6, the lamp including a first housing with a hook, the electronic ballast including a second housing having a receiver for the hook, the switch being at the receiver and engaged by the hook when the hook is in the receiver.

8. The light source of claim 4, the dimmer feedback circuit including a variable potentiometer.

9. A process for dimming a medium pressure arc lamp, the process being substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

10. A light source constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.