FIELD OF THE INVENTION

The invention relates to a circuit arrangement for operating a high pressure discharge lamp (further also referred to as lamp) comprising:

input terminals for connection to the poles of a supply voltage source, a first circuit part I coupled to the input terminals for generating a lamp current out of a supply voltage supplied by the supply voltage source, a control circuit coupled to the circuit part I for controlling the amount of power supplied to the high pressure discharge lamp, wherein the control circuit is equipped with a dim terminal for receiving a user adjustable dim signal representing a desired lamp power.

BACKGROUND OF THE INVENTION

Such a circuit arrangement is well known in the art. A user can adjust the dim signal and thereby dim the lamp. Generally speaking the power in the lamp can be increased at any arbitrary rate without causing any problems. However, it has been found that in case the power in the lamp is decreased comparatively fast over a substantial power range, e.g. from 100% to 50% in a couple of seconds, the lamp voltage will temporarily increase very strongly. In case the circuit arrangement periodically changes the polarity of the lamp current, the lamp generally has to be reignited after each polarity change. It has been found that also the reignition voltage increases dramatically when the lamp power is decreased comparatively fast over a substantial power range. These higher lamp voltage and ignition voltage can cause extinguishing of the lamp. Therefore high intensity discharge lamps are usually dimmed comparatively slowly, e.g. from 100% to 50% lamp power in one or several minutes. Such a slow dimming rate can be a disadvantage in scene setting applications. Furthermore, a user cannot perceive such a slow dimming rate and therefore perceives no feedback of his adjustment of the dim signal.

SUMMARY OF THE INVENTION

The invention aims to provide a circuit arrangement, wherein the disadvantages of the prior art have been avoided to a large extent. A circuit arrangement as mentioned in the opening paragraph is therefore in accordance with the invention characterized in that the control circuit is equipped with

circuitry for generating a limit value representing the lowest value whereto the power in the lamp can be decreased from an initial value at a chosen rate, wherein the chosen rate is 10% of initial power per second or higher, and

circuitry for during a first time interval decreasing the power in the high pressure discharge lamp with the chosen rate by at least 3% of the initial lamp power to a power level higher than or equal to the limit value, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is higher than the limit value by at least 3 percent of the initial lamp power and the second value corresponds to a lamp power level that is lower than or equal to the limit value.

It has been found that, for a human observer, a just noticeable decrease in the light output of a lamp is approximately 3%, in case the light output is decreased with a rate of at least 10% of the initial value per second.

Since the light output of the lamp is substantially proportional to the lamp power over a wide range of power values, this requirement can also be formulated as: the lamp power needs to decrease at a rate of at least 10% of the initial lamp power per second and the total decrease needs to be at least 3% of initial lamp power. The initial lamp power is the lamp power corresponding to the first value of the dim signal. When using a circuit arrangement according to the invention, a user thus perceives the dimming of the lamp during the first time interval, in case the power in the lamp corresponding to the first value is higher than the limit value by at least 3% of the initial lamp power. Since the power level in the lamp at the end of the first time interval is higher than or equal to the limit value, the lamp voltage and the reignition voltage do not increase so much that the lamp is thereby extinguished. Preferably, the control circuit is equipped with circuitry for during a first time interval decreasing the power in the high pressure discharge lamp with the chosen rate by at least 10% of the initial lamp power to a power level higher than or equal to the limit value, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is higher than the limit value by at least 10 percent of the initial lamp power and the second value corresponds to a lamp power level that is lower than or equal to the limit value. It has been found that although a user can perceive a change in light output of only 3%, for a user to clearly and effortlessly perceive dimming of a lamp, the lamp light output needs to decrease at a rate of at least 10% of the initial lamp light output per second and the total decrease needs to be at least 10% of initial lamp light output.

Preferably, the control circuit is further equipped with circuitry for during a second time interval, directly following the first time interval, decreasing the power in the lamp with a rate that is lower than the chosen rate, in case the power in the lamp at the end of the first time interval is higher than the lamp power corresponding to the second value of the dim signal. During the second time interval the decrease of the power in the lamp is slower so that the lamp voltage and the reignition voltage do not further increase or even gradually decrease. In practice the decrease of lamp power is chosen much slower than during the first time interval.

In case dimming takes place starting at nominal power in the lamp, the chosen rate needs to be 10% of nominal power per second or higher in order for a user to perceive the dimming. Good results have been obtained for embodiments in which the chosen rate is a fixed value. In case this fixed value is chosen at 10% of nominal lamp power per second or higher, the chosen rate is high enough for the dimming to be perceived by a user irrespective of the initial lamp power level, as long as the lamp power decrease in the first time interval is at least 3% of the initial power. Alternatively the chosen rate may for instance be depending on parameters such as the first value of the dim signal and the time lapse during which the lamp has been operated at the initial power level corresponding to this first value.

The decrease in the lamp power during the first time interval may be linear or non-linear. In the latter case the chosen rate is the average rate at which the power in the lamp is decreased during the first time interval.

The first value of the dim signal may correspond to 100% of the nominal lamp power. However, the first value of the dim signal may also correspond to a lower power level in the lamp, such as e.g. 80% of nominal lamp power. The lower limit to which dimming can be effected at the chosen rate without the lamp becoming unstable or extinguishing depends on a number of factors. More in particular it depends on the type, the construction and the filling of the high pressure discharge lamp, on the chosen rate, on the initial power in the lamp being the power in the lamp when the dimming starts, in other words on the first value of the dim signal, and on how long the lamp has been maintained at the power level corresponding to the first value before the dimming starts. To account for the existence of the lower limit the circuit arrangement makes use of a limit value of the lamp power that serves as the boundary value to which the lamp power can be reduced at the chosen rate, without the lamp becoming unstable or extinguishing. The limit value will in practice be chosen somewhat higher than the true lower limit discussed here-above, for instance to account for spread in lamp properties. Generally, the limit value can be chosen closer to the true lower limit in case more parameters are taken into account in the determination of the limit value.

Generally, in case fast dimming at the chosen rate can be effected between a certain initial value of the lamp power and the limit value, this means that fast dimming at the chosen rate to the limit value can also be effected from any lamp power level between the certain initial value and the limit value.

Generally speaking, when the initial value of the power in the lamp decreases, the difference between the initial value and the lower limit to which dimming can be effected at the chosen rate will become smaller. As a consequence, the difference between the initial lamp power level and the limit value of the lamp power also becomes smaller. In case the initial power level equals the limit value, this means that no fast dimming at the chosen rate can take place. For instance in case the limit value is a fixed value (see below) the initial power level can even be lower than the limit value.

In a preferred embodiment, the limit value is a fixed value chosen so that the power in the lamp can be decreased from 100% of nominal power to the fixed value at the chosen rate. In case the dimming starts from a power level that is lower than 100% of nominal power, the same fixed value is used as limit value. This fixed value can be implemented in the circuit arrangement in a very simple way. An important advantage is that, when this preferred embodiment is in operation, there is no need for determining the limit value based on the value of actual lamp operation parameters so that the control circuit can be comparatively simple. However, in this case the real lower limit to which dimming can be done at the chosen rate can be substantially lower than the limit value, more in particular when the power in the lamp at which the dimming starts is substantially lower than 100% of nominal power.

In another preferred embodiment, the control circuit comprises a timer for measuring the time lapse during which the lamp power has been maintained at a value corresponding to the first value and a memory for storing the limit value as a function of the first value of the dim signal and the time lapse during which the lamp power has been maintained at a value corresponding to the first value. In this latter embodiment more parameters are taken into account when the limit value is determined. As a consequence the limit value will be closer to the real lower limit than in an embodiment wherein the limit value is a fixed value, so that dimming at the chosen rate can be done over a wider range. This means that even in case the dimming starts at a lamp power level that is comparatively low, fast dimming at the chosen rate over a lamp power range that is wide enough to be perceived by a user is often possible.

In a further preferred embodiment, the control circuit is equipped for, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is higher than the limit value and the second value corresponds to a lamp power level that is lower than the limit value, during the first time interval decreasing the power in the high pressure discharge lamp from the power

corresponding to the first value to the power corresponding to the limit value with the chosen rate. In this further preferred embodiment the lamp is always dimmed to the limit value at the chosen rate, in case the second value is lower than the limit value. In other words fast dimming is done to the lowest possible level and only after that the last part of the dimming from the limit value to the second value is done at a slower rate. This way, for given first and second values of the dim signal the dimming takes place within the shortest time. It is noted that the dimming will of course only be perceived by a user if the power range over which the dimming at the chosen rate takes place is big enough.

Preferably, the control circuit is equipped for, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is lower than the limit value and the lamp power corresponding to the second value is lower than that corresponding to the first value, decreasing the power in the high pressure discharge lamp from the power corresponding to the first value to the power corresponding to the second value with a rate lower than the chosen rate. In this case fast dimming at the chosen rate is excluded and dimming can only take place slower than the chosen rate.

Also preferably, the control circuit is equipped for, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is higher than the limit value and the second value corresponds to a lamp power level that is also higher than the limit value but lower than the lamp power

corresponding to the first value, decreasing the power in the high pressure discharge lamp from the power corresponding to the first value to the power corresponding to the second value with the chosen rate. Also in this case this ensures the shortest dimming time. It is once more noted that the dimming will of course only be perceived by a user if the power range over which the dimming at the chosen rate takes place is big enough.

Good results have been obtained for embodiments, wherein the circuit arrangement is equipped with a power control loop comprising circuitry for generating an actual power signal representing the actual power in the lamp, circuitry, comprised in the control circuit for generating a power reference signal representing a desired momentary value of the power in the lamp and circuitry for generating a power control signal out of the actual power signal and the power reference signal and wherein the rate of decrease of the power in the lamp is controlled by the rate at which the power reference signal is changed.

It is advantageous to equip the control circuit with circuitry for inhibiting a decrease in lamp power during a time interval immediately after ignition of the lamp. In this way the lamp can reach stable stationary operating conditions before dimming takes place. This prevents instabilities or extinguishing during the dimming of the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the circuit arrangement according to the invention will be explained making use of a drawing.

In the drawing Fig. 1 shows a schematic representation of an embodiment of a circuit arrangement according to the invention with a high pressure discharge lamp connected to it;

Fig. 2 shows a schematic representation of the dim control as realized by the control circuit in the embodiment shown in Fig. 1, and Fig. 3 shows the power in the high pressure discharge lamp as a function of time during dimming.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In Fig. 1, Kl and K2 are input terminals for connection to the poles of a supply voltage source. I is a first circuit part coupled to the input terminals for generating a lamp current out of a supply voltage supplied by the supply voltage source. CC is a control circuit coupled to the first circuit part I for controlling the amount of power supplied to the high pressure discharge lamp.

The control circuit CC is coupled to the circuit part I and the lamp. These couplings are indicated in Fig. 1 by means of dotted lines.

The control circuit CC is equipped with an input terminal DIM for receiving a user adjustable dim signal representing a desired lamp power. The control circuit is further equipped with circuitry (not shown) for generating a limit value representing the lowest value whereto the power in the lamp can be decreased at a chosen rate. In the embodiment shown in Fig. 1 the chosen rate is a fixed value of 10% of the nominal power per second or higher. The control circuit is further equipped with circuitry for, in case the dim signal is changed from a first value to a second value and the first value corresponds to a lamp power level that is higher than the limit value and the second value corresponds to a lamp power level that is lower than the limit value, during a first time interval decreasing the power in the high pressure discharge lamp from the power corresponding to the first value to the power corresponding to the limit value with the chosen rate and for during a second time interval directly following the first time interval further changing the power in the lamp at a rate slower than the chosen rate until the power in the lamp corresponds to the second value of the dim signal. In case the initial power in the lamp is higher than the limit value by at least 3% of the initial lamp power, a user can perceive the dimming of the lamp. In case the initial power in the lamp is higher than the limit value by more than 10% of the initial lamp power, a user can clearly and effortlessly perceive the dimming of the lamp. Since fast dimming of the lamp at the chosen rate is realized from the initial value to the limit value (the lowest value to which dimming can take place at the chosen rate) the dimming is taking place comparatively fast. The control circuit is also equipped for, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is lower than the limit value and the lamp power corresponding to the second value is lower than that corresponding to the first value, decreasing the power in the high pressure discharge lamp from the power corresponding to the first value to the power corresponding to the second value with a rate lower than the chosen rate. Furthermore, the control circuit is equipped for, in case the dim signal is changed from a first value to a second value, wherein the first value corresponds to a lamp power level that is higher than the limit value and the second value corresponds to a lamp power level that is also higher than the limit value but lower than the lamp power corresponding to the first value, decreasing the power in the high pressure discharge lamp from the power corresponding to the first value to the power corresponding to the second value at the chosen rate.

In a first implementation of this embodiment the limit value is a fixed value chosen so that the power in the lamp can be decreased from 100% of nominal power to the fixed value at the chosen rate without instabilities occurring in the lamp.

The operation of the embodiment shown in Fig. 1 is as follows. In case input terminals Kl and K2 are connected to a supply voltage source, the circuit part I generates a lamp current out of the supply voltage supplied by the supply voltage source and the control circuit controls the amount of power supplied to the lamp.

In case the dim signal is decreased from a first value to a second value, the control circuit determines if the first value is higher than the limit value. If not, and the second value is also lower than the limit value, the control circuit decreases the power in the lamp at a rate that is (substantially) lower than the chosen rate. The user cannot perceive the dimming of the lamp. In case both the first value and the second value are higher than the limit value and the lamp power corresponding to the first value is higher than the lamp power corresponding to the second value, the control circuit decreases the lamp power at the chosen rate from the level corresponding to the first value to the level corresponding to the second value. In case the difference between the power levels corresponding to the first and second value is higher than 3% of the initial lamp power, a user can perceive the dimming of the lamp. In case the power level corresponding to the first value is higher than the limit value and the second value corresponds to a power level in the lamp that is lower than the limit value, the control circuit will decrease the power in the lamp from the level corresponding to the first value to the limit value at the chosen rate and subsequently decreases the power in the lamp from the limit value to the power level corresponding to the second value at a lower dimming rate. When the difference in lamp power between the power level corresponding to the first value and the limit value is higher than 3% of the initial lamp power (the lamp power corresponding to the first value of the dim signal), a user will also in this case perceive the dimming. When the dimming is perceived, the user will generally be satisfied with the result of his adjustment of the dim signal, instead of being frustrated by the impression that the adjustment of the dim signal had no effect at all.

In a second implementation of this embodiment the control circuit comprises a timer for measuring the time lapse during which the lamp power has been maintained at a value corresponding to the first value of the dim signal and a memory for storing the limit value as a function of the first value of the dim signal and the time lapse during which the lamp power has been maintained at a value corresponding to the first value. In this implementation of the embodiment, the limit value will generally be lower than in the first implementation.

By way of example, the limit value in the first implementation can for instance be fixed at 65% of the nominal lamp power. In that case dimming to the limit value can be done at the chosen rate from any initial lamp power between 100% and 65%. Dimming from 65% to 50% of nominal power has to be effected slower than the chosen rate. In practice it will take between 1 and 2 minutes to decrease the power from 65% to 50% of nominal lamp power and it will not be perceived by the user. In case of the second implementation, when the lamp is operated at 65% of its nominal power for about three minutes the control circuit derives from the table stored in it that the limit value is 50% of nominal lamp power. As a consequence dimming from 65% to 50% can be done at the chosen rate so that the dimming time is considerably shorter than in the first implementation and a user can perceive the dimming.

In Fig. 2 "power setting" represents the generation of a dim signal that is supplied to the control circuit via the dim terminal. The control circuit CC generates a power reference signal that is supplied to a power control loop. The control circuit decreases the power reference signal at a certain rate and thereby decreases the power in the lamp. In the control loop an actual power signal is calculated by multiplying lamp voltage and lamp current. A power control signal "control set point" is generated out of the actual power signal and the power reference signal.

Fig. 3 shows the power in a lamp that is dimmed as a function of time. Time in arbitrary units is plotted along the horizontal axis and power in arbitrary units is plotted along the vertical axis. P I is the power in the lamp corresponding to the first value of the dim signal, P2 is the power in the lamp corresponding to the second value of the dim signal and PO is the limit value. It can be seen that the decrease of the lamp power from P I to PO is fast, taking place with the chosen rate during a first interval Δίΐ . The decrease of the power in the lamp from P _D to P2 is taking place much slower during the time interval At2. Although the decrease in lamp power shown in Fig. 3 is linear both during the first and the second time interval, non-linear decrease according to any other shape of the curve is also possible.