High-Pressure Discharge Lamp

This invention relates to a high-pressure discharge lamp comprising a discharge vessel provided with two main electrodes between which extends a discharge path and provided with an external auxiliary electrode connected to a starter circuit in the lamp, in which lamp each main electrode is electrically connected to a respective connection terminal of the lamp. The connection terminals are further connected to each other through a voltage division circuit of the starter circuit, wherein the starter circuit also comprises a switching element which partly shunts the voltage division circuit.

A lamp of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 4,447,759. Such a lamp is generally operated with alternating voltage in conjunction with a stabilization ballast. The value of the impedance of the ballast should be chosen in correspondence with the discharge current flowing through the lamp in the operating condition of the lamp. This means that an existing apparatus for operating a high pressure discharge lamp of a given kind cannot be used to operate a lamp of different kind without additional steps being taken e.g., adding an adaptation circuit between the original ballast and the different type of lamp. This is disadvantageous when it is desired to use different high pressure discharge lamps either to obtain a progressively increasing light output or to attain a further saving of energy. The invention has for an object to provide means by which the necessity of taking additional steps is eliminated.

According to the invention, for this purpose a lamp of the kind mentioned in the opening paragraph is characterized in that in series with the discharge path and in parallel with the voltage division circuit there is arranged a semiconductor switch with a control electrode, which control electrode is electrically connected to the switching element.

By the use of a switch connected in series with the discharge path, in the operating condition of the lamp, the current through the lamp is controlled so that it is possible to operate the lamp on equipment provided with a stabilization ballast not specifically adapted to the relevant lamp. When the switch is in the form of a semiconductor switch, the advantage of small dimensions and hence the possibility of incorporation in the lamp is obtained. When the semiconductor switch is arranged in parallel with the voltage division circuit, it is advantageously possible to use for control of the switch the starter circuit present in the lamp.

U.S. Pat. No. 4,342,948 discloses an adaptation circuit for the operation of a high-pressure discharge lamp on equipment provided with a stabilization ballast not adapted to the relevant lamp. In the known adaptation circuit, the switch of the starter circuit is in the form of a controlled semiconductor switch which is connected in series with the discharge path of the lamp connected to the adaptation circuit. The starter circuit is constructed so that ignition pulses in the connected lamp are applied solely to the main electrodes. A transformer winding is included in the series arrangement of switch and discharge path. However, during operation of the lamp the overall lamp current will flow through the transformer winding, which in practice leads to the transformer having comparatively large dimensions so that incorporation in the lamp is hardly possible.

In the known adaptation circuit, further a resistor of comparatively low impedance value is present, which shunts the switch. At the required value of the resistor, this results in that either the lamp current can be controlled to a very small extent by the switch or current flows for a comparatively long time through the resistor, which leads to power dissipation.

In an advantageous embodiment of a lamp according to the invention, the controlled semiconductor switch is shunted by a resistor of at least 1 k.OMEGA.. Thus, an ionization current can be maintained through the lamp for the period in which the semiconductor switch is in the non-conductive state.

In a further embodiment of a lamp according to the invention, the voltage division circuit comprises at least two components which are directly connected to each other and between which the switching element is connected, the two components being shunted by a series arrangement of two Zener diodes of opposite polarities. The Zener diodes ensure that variations in the amplitude of the supply voltage have very little influence on the instant of breakdown of the switching element.

In a further improved embodiment, the series arrangement of discharge path and controlled semiconductor switch is shunted by a series arrangement of a resistor and a capacitor. When the lamp is started, such a series arrangement favors the production of a discharge in the discharge vessel. During operation of the lamp, the added series arrangement leads to the discharge in the discharge vessel being reignited more rapidly.

An embodiment of a lamp according to the invention will now be described more fully with reference to the accompanying drawing. In the drawing:

FIG. 1 shows a lamp partly broken away, and

FIG. 2 shows an electric circuit diagram of the lamp.

In FIG. 1, reference numeral 1 designates an outer bulb of the lamp with a lamp cap 2. The outer bulb encloses a discharge vessel 3 provided with two main electrodes 4,5 between which extends a discharge path 10. The lamp is provided with an external auxiliary electrode 11. The electrode 4 is connected by means of a metal strip 6 to a rigid current conductor 7. The electrode 5 is connected via a metal strip 8 to a rigid current conductor 9. The main electrodes 4,5 are each connected through the rigid current conductors 7,9 to a respective connection terminal of the lamp arranged in the lamp cap 2.

The external auxiliary electrode 11 is connected to a starter circuit in the lamp cap via a current conductor 110. Furthermore, in the outer bulb a heat shield 16 of aluminum is mounted between the discharge vessel 3 and the lamp cap 2. A nickel strip 17 is welded to the rigid current conductor 7 and grips around the heat shield 16, while clampingly surrounding the heat shield 16 and thus positioning it in a simple and adequate manner.

In FIG. 2 the electric circuit diagram of the lamp is represented. Reference numeral 3 denotes the discharge vessel of the lamp provided with the two main electrodes 4,5 between which extends the discharge path 10. Each of the main electrodes 4,5 is electrically connected through a current conductor 7 and 9, respectively, to a respective connection terminal 700,900 of the lamp.

The external auxiliary electrode 11 is connected through the current conductor 110 to a starter circuit which comprises the elements 31 to 40 and is composed as follows. The connection terminals 700,900 are interconnected through a voltage division circuit of the starter circuit comprising a series arrangement of a resistor 31, a resistor 32 and a capacitor 33. A switching element 34 is connected to a tapping on the voltage division circuit. The switching element is in the form of an uncontrolled voltage-dependent breakdown element with thyristor characteristics. It is, for example, alternatively possible that the switching element be a controlled semiconductor switch whose control is voltage dependent. The switching element 34 is electrically connected through a primary winding 35a of the transformer 35 to a control electrode of a controlled semiconductor switch 37. The switch 37 is connected in series with the discharge path and in parallel with the voltage division circuit, as a result of which the capacitor 33 of the voltage division circuit is shunted by the switching element 34. Furthermore, the switch 37 is shunted by a resistor 41. A secondary winding 35b of the transformer 35 is connected through a blocking capacitor 36 to the current conductor 110.

A series arrangement, connected with two Zener diodes 39,40 of opposite polarities, forms together with the resistor 31 a connection between the connection terminals 700 and 900.

In a modification of the starter circuit, the position of the switching element 34 and that of the primary winding 35a may be interchanged.

In a further modification of the lamp, the series arrangement of the discharge path 10 and the controlled semiconductor switch 37 in the electric circuit diagram is shunted by a series arrangement of a capacitor 42 and a resistor 43. The series arrangement of the capacitor 42 and the resistor 43 may be provided outside the lamp.

The operation of the electric circuit diagram is as follows. When an alternating voltage is applied as a supply voltage to the connection terminals 700,900 via a stabilization ballast, the capacitor 33 is charged via the resistors 31 and 32. When the voltage at the capacitor 33 reaches the breakdown voltage of the switching element 34, the switching element breaks down and becomes conductive. The capacitor 33 is then discharged abruptly via the primary transformer winding 35a and the semiconductor switch 37. This abrupt discharge produces a voltage pulse in the transformer 35 which is induced in the secondary transformer winding 35b, as a result of which a high instantaneous voltage is applied via the blocking capacitor 36 between the external auxiliary electrode 11 and the main electrodes 4,5 of the discharge vessel 3.

As soon as the current through the switching element 34 falls to zero, the switching element becomes non-conducting again, after which the process described is repeated. The high instantaneous voltage, which due to the process described is applied between the external auxiliary electrode 11 and the main electrodes 4,5, will produce a discharge between the main electrodes via the discharge path 10 and thus cause the lamp to ignite.

The discharge current of the capacitor 33 via the control electrode of the semiconductor switch 37 causes this switch to become conducting. In case the lamp has ignited, a lamp current will flow between the connection terminals 700 and 900 via the main electrodes 4,5 and the discharge path. When the voltage and hence the lamp current fall to zero, the semiconductor switch 37 becomes non-conducting again, whereupon the process described is repeated. During the non-conductive state of the semiconductor switch, a small ionization current can continue to flow through the discharge vessel via the resistor 41. This enhances the reignition of the discharge as soon as the semiconductor switch 37 has been caused to become conducting.

The Zener diodes 39 and 40 ensure that variations in the amplitude of the supply voltage have very little influence on the instant of breakdown of the switching element 34.

In a lamp provided with the series arrangement comprising the capacitor 42 and the resistor 43, the capacitor 42 will be charged during each period of the alternating supply voltage. During starting of the lamp, this results in that the voltage at the connection terminals 700,900 is kept substantially constant immediately after breakdown of the switching element 34, which favors the production of a discharge in the discharge vessel 3. In the operating condition of the lamp, the capacitor 42 will be charged as long as the semiconductor switch 37 is non-conducting. As soon as the semiconductor switch 37 becomes conducting, the capacitor 42 will be discharged via the discharge path. This favors the reignition of the discharge.

In the case of a practical lamp, the latter was operated at an alternating voltage source of 220 V, 50 Hz, and the power consumed by the lamp was 77 W. The lamp was operated in combination with a ballast intended for the operation of a high-pressure mercury vapour discharge lamp of 125 W. The lamp of this example was a high-pressure sodium lamp whose discharge vessel contained 25 mg of an amalgam comprising 18% by weight of Na and 82% by weight of Hg. The discharge vessel further contained xenon at a pressure of about 10 kPa at 300.degree. K. During operation of the lamp, the luminous flux was 6750 lm and the arc voltage between the main electrodes was 120 V. Per half cycle of the alternating voltage, the semiconductor switch 37 was non-conducting for 1.2 ms. The average voltage per half cycle between the connection terminals 700 and 900 was 140 V. The components as shown in the electric circuit diagram of the lamp were dimensioned as follows:

For comparison it should be stated that during operation of a 125 W high-pressure mercury vapor discharge lamp on the stabilization ballast intended therefor, the luminous flux is about 6300 lm. The lamp according to the invention therefore yields during operation an energy saving of about 40% with a comparable luminous flux.