Abstract:
According to one embodiment there is provided a lamp device ( 1 ) having a lighting module ( 3 ) and two pairs of external connection pins ( 5   aa,    5   ab,    5   ba,    5   bb ) for connecting the lamp device to a supply voltage of a lighting fixture. A first terminal of the lighting module is connected to a pin of a first pair of pins by a switch which is closed in response to a voltage difference between both the pins of the first pair of pins and the pins of the second pair of pins exceeding a respective threshold voltage. The lamp is therefore safe to handle even in a condition where one pair of pins is connected and the other is not.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB13/052431, filed on Mar. 27, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/619,518, filed on Apr. 3, 2012. These applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to safety arrangements in lamp devices. More precisely, it relates to a lamp device including circuitry for preventing the occurrence of a potentially harmful voltage at its external connection pins. 
     BACKGROUND OF THE INVENTION 
     Conventional tube lamps (fluorescent lamps) of the type having a two-pin connector at each end of the tube are, in steady-state operation, powered by a DC or AC electric current flowing between the connectors. The connectors are electrically disconnected until an arc has established in the tube. For this purpose, a starting procedure including preheating of the electrodes may be carried out by dedicated starting circuitry, which can be made inherently safe by being enabled only when the lamp is properly inserted into the fixture, so that live electric parts are always protected from touch. The starting circuitry may be of the switch-start/preheat, rapid-start or other type, and is commonly integrated in the lighting fixture. 
     It is desirable to power a tube lamp retrofit in the same manner as the tube lamp it replaces, that is, by a current flowing between its end connectors. Depending on the internal circuitry of the lamp, potentially hazardous conditions may arise when the lamp is partially inserted or partially removed from the fixture. CN 101737664 A proposes a lamp with a protection switch actuated by an external push button located next to a pair of external connection pins. When the push button is depressed, which normally occurs only at proper insertion of the connection pins into a socket, the protection switch connects one of the pins to the electrical components within the lamp. However, such external push button may be easily tampered with. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome this problem, and to provide a lamp device which is safe to handle. In particular it is an object to provide a lamp device reducing the risk of electrical shock during insertion in and removal from a lighting fixture. 
     According to a first aspect of the invention, these and other objects are achieved by a lamp device comprising a lighting module including a first and a second terminal. The lamp device further comprises a first and a second pair of connection pins for connecting the lamp device to a lighting fixture. A first connection pin of the second pair of connection pins is connected to the second terminal. The lamp device further comprises a switch which is operable to connect a first pin of the first pair of connection pins to the first terminal when closed, i.e. in a closed state. The switch is controlled by circuitry of the lamp device. The circuitry is arranged to close the switch in response to a voltage difference between the pins of the first pair of connection pins exceeding a first threshold voltage and a voltage difference between the pins of the second pair of connection pins exceeding a second threshold voltage. It may thus be safe to touch the lamp device even in a condition where one pair of pins is connected and the other is not. More specifically, if only one pair of connection pins is connected to the lighting fixture, e.g. during insertion of the lamp device into a lighting fixture, the switch of the lamp device will be in an open state. A closed current path between the two pairs of connection pins, via the lighting module and its associated terminals, may thereby be avoided. In normal use situations, a non-zero voltage exceeding the first threshold voltage (or the second threshold voltage as the case may be) will only be present when the first (or second) pair of pins is inserted in a supply-voltage socket of a lighting fixture. Hence, the switch will close only when both pairs of pins are inserted into respective sockets at the same time, that is, when neither pair of pins is exposed to touch. Moreover, by controlling the state of the switch on the basis of the voltage difference at both the first and the second pair of connection pins safe handling of the lamp device may be enabled using a single switch. The production costs for the lamp device may thus be limited. 
     According to one embodiment the circuitry is further arranged to open the switch in response to the voltage difference between the pins of any of the first pair and the second pair falling below a respective threshold voltage. Hence, if one pair of the connection pins are disconnected from the lighting fixture (e.g. during removal of the lamp device from a lighting fixture) the voltage difference between the pins of the disconnected pair will disappear wherein the switch may quickly open. This contributes to safe handling of the lamp device. 
     The lamp device is suitable for use in a lighting fixture providing a supply voltage between a pin of the first pair and a pin of a second pair, and a voltage between the pins of each of the first pair and the second pair of pins. One type of lighting fixture with such a configuration is a lighting fixture for a fluorescent lamp, i.e. fluorescent tube. The lamp device is particularly suitable for use in a fluorescent tube lighting fixture including a start circuit of a rapid-start type. Such a start circuit may be arranged to provide a voltage for heating filaments at each side of a fluorescent tube. Each of the first pair of pins and the second pair of pins of the lamp device may accordingly be arranged to receive a respective filament heating voltage. The state of the switch of the lamp device (and accordingly the flow of the main current between a pin of the first pair of pins and a pin of the second pair of pins) may thus be controlled on the basis of the filament heating voltages. The lamp device may thus retrofit an existing lighting fixture for a fluorescent lamp, i.e. fluorescent tube. 
     According to one embodiment the circuitry of the lamp device controlling the switch comprises a control circuit and a circuit element. The circuit element is arranged to provide a control signal to the control circuit in response to the voltage difference between the pins of the second pair exceeding the second threshold voltage. The control circuit is in turn arranged to close the switch in response to the voltage difference between the pins of the first pair exceeding the first threshold voltage at a same time as or while the control signal is received from the circuit element. The control signal may thus be referred to as an enabling signal which together with the voltage difference between the pins of the first pair influences the state of the switch. 
     The switch and the associated control circuit may be provided in the form of a relay. For safety purposes and ease of implementation the relay may advantageously be a normally-open relay. In other words the relay may be arranged to assume an open state when not energized. 
     The circuit element providing the control signal referred to above may include a first part arranged to receive a voltage from the second pair of pins and a second part galvanically insulated from the first part and being arranged to provide the control signal to the control circuit. Thereby a flow of charge carriers from the portion of the lamp device including the second pair of pins to the control circuit and the portion of the lamp device including the first pair of pins may be avoided. This may improve the safety of the lamp device. 
     To enable reliable operation of the lamp device in combination with an AC voltage the lamp device may advantageously include a first rectifier arranged to receive an AC voltage from the first pair of pins and provide a rectified AC voltage to the control circuit. To enable reliable operation of the lamp device for a large range of voltage differences occurring between the pins of the first pair when connected to a lighting fixture the rectifier may provide a rectified AC voltage to the control circuit via a power converter. The power converter may be arranged to receive a first voltage from the outputs of the rectifier and provide a second voltage, which is larger than the first voltage, to the control circuit. Advantageously, the power converter may be arranged to provide the second voltage in response to any first voltage falling within a voltage range. 
     The lamp device may also comprise a second rectifier arranged to receive an AC voltage from the second pair of pins and provide a rectified AC voltage to the circuit element. In case the circuit element includes the above discussed first and second part, the second rectifier may be arranged to provide the rectified AC voltage to the first part of the circuit element. 
     The circuit element may for example be an optocoupler. An optocoupler provides a galvanic isolation between the primary side and the secondary side (i.e. the light source and the photodetector). A voltage difference between the pins of the second pair may thus cause the primary side to induce a current forming at the secondary side, the induced current forming a control signal which is provided to the control circuit. An optocoupler may provide a compact implementation of the circuit element. 
     Alternatively, the circuit element may be a current transformer. A current transformer also provides a galvanic isolation between the primary side (i.e. the primary winding) and the secondary side (i.e. the secondary winding). A current transformer may be a cost-effective implementation of the circuit element. In an embodiment including a current transformer, the lamp device may include a second rectifier arranged to receive an AC voltage from the secondary winding of the current transformer and provide a rectified AC voltage to the control circuit operating the switch. This may simplify implementation of the control circuit since it needs only be arranged to handle a rectified control signal, i.e. a control signal having a single polarity. 
     According to one embodiment the lamp device further comprises a housing, wherein the lighting module is arranged in the housing and the pins of the first and second pairs of pins extends through the housing. The housing may be electrically isolating. At least a portion of the housing may be light transparent. The electrically active parts of the lamp device may thus be arranged inside the housing, where they are protected from touch. The lamp device may thus be arranged such that the only conductive parts of the lamp device which are accessible from the outside are the first and second pairs of pins. As described above, the design of the lamp device makes also the first and second pairs of pins safe to touch. 
     In lighting fixtures it may e.g. due to wear and tear occur that one of the contacts in the socket of the lighting fixture into which the lamp device is inserted becomes loose. This may for example happen in older lighting fixtures. As a result a pin of either pair of pins may lose electrical contact with the fixture. Consequently there may be no well-defined voltage difference between the pins of the respective pair. The lamp device may then not operate correctly. 
     According to one embodiment a lost electrical contact may be handled by including in the lamp device a safety circuit associated with the first or the second rectifier and arranged to close a current path between the outputs of the associated rectifier in response to a voltage difference between the pair of outputs exceeding a set threshold voltage. The safety circuit may thereby provide a bypass path for a main current when a pin looses electrical contact. The safety circuit may clamp the voltage difference between a pair of outputs of the associated rectifier in response to a voltage difference between the pair of outputs exceeding the set threshold voltage. If the lamp device includes a power converter the safety circuit may be arranged to receive a voltage from the outputs of the first rectifier and close a current path between the inputs of the power converter. The closed current path may provide a voltage drop such that the output voltage from the safety circuit (or from the power converter if present) is sufficient for closing the switch. Thereby, the lamp device may produce light even if a pin loses electrical contact with the lighting fixture. 
     According to another embodiment a lost electrical contact may be handled by a safety circuit associated with the first or second rectifier and arranged to disconnect an output of a pair of outputs of the associated rectifier in response to a voltage difference between the pair of outputs exceeding a set threshold voltage. If the safety circuit is associated with the first rectifier the control circuit may thereby be disconnected from the output of the first rectifier. Analogously if the safety circuit is arranged between the above-mentioned second rectifier and optocoupler, the optocoupler may be disconnected from the output of the second rectifier. Advantageously a safety circuit in accordance with either of the two previous embodiments may be associated with each of the first and the second rectifier. 
     According to a second aspect of the invention there is provided a method of operating a lamp device comprising a lighting module including a first and a second terminal and a first and a second pair of connection pins for connecting the lamp device to a lighting fixture, wherein a first connection pin of the second pair of pins is connected to the second terminal. According to the method a first voltage difference between the pins of the first pair is monitored. Also a second voltage difference between the pins of the second pair is monitored. In response to the first voltage difference exceeding a first threshold voltage at a same time as the second voltage difference exceeding a second threshold voltage, a first pin of the first pair of pins is connected to the first terminal. The method may further comprise disconnecting the first pin of the first pair of pins from the first terminal in response to a zero voltage at any one of the first and the second pair of pins. The details and advantages discussed in relation to the lamp device of the first aspect apply correspondingly to the method of the second aspect. 
     Further possible combinations of features are recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention. 
         FIG. 1  schematically illustrates a lamp device in accordance with a first embodiment. 
         FIG. 2  illustrates a circuit implementation for the first embodiment. 
         FIG. 3  illustrates an alternative circuit implementation for the first embodiment. 
         FIG. 4  illustrates an optional safety circuit of a first design. 
         FIG. 5  illustrates an optional safety circuit of a second design. 
         FIG. 6  schematically illustrates a lamp device in accordance with a second embodiment. 
     
    
    
     Unless otherwise indicated, like reference numerals refer to like elements throughout. 
     DETAILED DESCRIPTION 
     Detailed embodiments will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the detailed embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. 
       FIG. 1  schematically illustrates a lamp device  1  in accordance with a first embodiment. The lamp device  1  comprises a housing  2 . At least a part of the housing  2  is light-transparent. The housing  2  is made of an electrically insulting material. The housing  2  may for example be made of glass or transparent/translucent plastics. The housing  2  is provided with a cylindrical shape. However, other shapes are also possible. For example the housing  2  may present a triangular cross section, a rectangular cross section or some other polygonal cross section. The lamp device  1  comprises a lighting module  3 . The lighting module  3  includes an LED driver  3   a  and a LED module  3   b . The LED driver  3   a  includes circuitry for driving the LED module  3   b  which per se is well-known in the art. The LED module  3   b  includes one or more LEDs. The one or more LEDs may be inorganic LEDs, OLEDs or another type of solid state light source. Although the lighting module  3  of the present embodiment includes an LED module  3   b  the lighting module can also include other types of light sources such as an incandescent lamp, a fluorescent lamp or an HID lamp. 
     The lighting module  3  comprises a first and a second terminal  4   a ,  4   b . The lamp device  1  comprises a first pair of electrical connection pins  5   aa ,  5   ab  and a second pair of electrical connection pins  5   ba ,  5   bb  (commonly referenced ‘pins  5 ’). The first pair  5   aa ,  5   ab  is arranged at a first end of the housing  2 . The second pair  5   ba ,  5   bb  is arranged at a second end of the housing  2 , opposite to the first end of the housing  2 . The pins  5  extend through the respective ends of the housing  2  and are hence accessible external to the housing  2 . The pins  5  hence form external pins  5 . The spatial configuration of the pins  5  is rigid. More specifically the pins  5  on each side maintain a fixed spacing and relative orientation. The pins  5  are arranged to be connected to an electrical socket of a lighting fixture for receiving a supply voltage. The first terminal  4   a  is connected to the pin  5   aa  via a switch  6 . The second terminal  4   b  is connected to the pin  5   ba . As will be explained in more detail below the circuit between the pin  5   aa  and the first terminal  4   a  is open unless the lamp device  1  is correctly inserted to a lighting fixture. The circuit formed between the pin  5   ba  and the second terminal  4   b  is however closed independent of a connection state between the device  1  and a lighting fixture. 
     The switch  6  of the lamp device  1  is operable to connect the pin  5   aa  to the terminal  4   a  of the lighting module  3 . The state of the switch  6  is controlled by a control circuit  7  described in detail below. The control circuit  7  comprises first and second inputs connected to the first pair of pins  5   aa ,  5   ab . The control circuit  7  comprises a third input connected to a circuit element  8 . In the present embodiment the circuit element  8  takes the form of an optocoupler  8 . The primary side, i.e. the light emitter, of the optocoupler  8  is connected to the second pair of pins  5   ba ,  5   bb . The secondary side, i.e. the photosensor, is connected to the third input of the control circuit  7 . The control circuit  7  is connected to the first pair of pins  5   aa ,  5   ab  via a rectifier  9 . Similarly, the optocoupler  8  is connected to the second pair of pins  5   ba ,  5   bb  via a rectifier  10 . 
       FIG. 2  illustrates a first circuit implementation for the lamp device  1 . The control circuit  7  comprises a switch controller  7 ′. The switch controller  7 ′ is arranged to close the switch  6  in response to a voltage difference between its terminals exceeding a threshold voltage. As illustrated, the switch  6  and the switch controller  7 ′ may be included in a same circuit element forming a relay. The relay may be of a type known per se, such as an electromechanical, an electromagnetic (e.g., Reed relay), electro-thermal (e.g., bimetal) or a solid-state type. The actual value of the threshold voltage may vary between different scenarios and for different lighting fixtures. In the present embodiment the threshold voltage is a voltage sufficient for causing the switch controller  7 ′ to close the switch  6 . In some examples the threshold voltage may be zero wherein the switch controller  7 ′ is arranged to close the switch  6  in response to a non-zero voltage difference between its terminals. The switch controller  7 ′ is connected to the rectifier  9  via a transistor Q 1 . The base of the transistor Q 1  is connected to the output of the optocoupler  8 . Although Q 1  is illustrated as a bipolar junction transistor (BJT) other types of transistors may also be used such as metal-oxide-semiconductor field-effect transistors (MOSFETs) or junction gate field-effect transistors (JFETs). 
     If a voltage is applied only at the first pair of pins  5   aa ,  5   ab , the transistor Q 1  will be turned off. There will be no current for driving the primary side of the optocoupler  8  and inducing a current in the secondary side of the optocoupler  8 . Thus no control signal may flow from the optocoupler  8 , through the resistor R 2  and into the base of the transistor Q 1 . The switch controller  7 ′ will hence not receive any current causing the switch  6  to close. No main current may hence flow from the pin  5   aa  to the pin  5   ba . Conversely, if a voltage is applied only at the second pair of pins  5   ba ,  5   bb  the control circuit  7 ′ will not receive a voltage sufficient for causing the switch  6  to close. 
     If a first voltage sufficient for causing the control circuit  7 ′ to close the switch  6  (i.e. a voltage exceeding a first threshold voltage) is applied to the first pair of pins  5   aa ,  5   ab  and also a second voltage sufficient for causing the optocoupler  8  to provide a control signal to the transistor Q 1  (i.e. a voltage exceeding a second threshold voltage) is applied to the second pair of pins  5   ba ,  5   bb  the control signal flowing through the resistor R 2  may turn on the transistor Q 1 . The switch controller  7 ′ may then be powered by the first voltage from the first pair of pins  5   aa ,  5   ab . If any of the first pair of pins  5   aa ,  5   ab  or the second pair of pins  5   ba ,  5   bb  become disconnected from the supply voltage there will not be a sufficient voltage difference for driving the switch controller  7 ′ or the optocoupler  8 , respectively. The switch  6  will then open and interrupt a current flow between the pin  5   aa  and the pin  5   ba . Hence, the control circuit  7  and the optocoupler  8  together form circuitry arranged to close the switch  6  on a condition that a voltage difference between the pins of the first pair  5   aa ,  5   ab  exceeds the first threshold voltage at a same time as a voltage difference between the pins of the second pair  5   ba ,  5   bb  exceeds the second threshold voltage. Correspondingly, this circuitry is arranged to open the switch  6  on a condition that the voltage difference either between the pins of the first pair  5   aa ,  5   ab  or the pins of the second pair  5   ba ,  5   bb  falls below a respective threshold voltage. This implementation hence enables the lamp device  1  to be inserted and removed from a powered lighting fixture with a reduced risk of electrical shock if touching the pins at a free end of the lamp device  1 . 
     As shown in  FIG. 2 , the rectifier  9  includes a capacitor C 1  for averaging the rectified AC voltage. Also the rectifier  10  includes a capacitor for the same purpose. These capacitors provide a simple way to keep the switch from opening between half-cycles of the rectified AC voltage without the need to make specific adaption of the control circuit  7 . 
     The lamp device  1  may be used in different types of lighting fixtures. As one example the lamp device  1  may be used to retrofit an existing lighting fixture for a fluorescent lamp, i.e. fluorescent tube. Such a lighting fixture may, as known per-se, provide a driving voltage between the opposite ends of the fluorescent tube. It may further include a rapid start circuit which applies a heating voltage between the pins of each pair of connection pins of the fluorescent tube. The heating voltage may heat the filaments of the fluorescent tube. When the lamp device  1  is used in combination with such a lighting fixture the respective filament heating voltages are applied to the first pair of pins  5   aa ,  5   ab  and the second pairs of pins  5   ba ,  5   bb . The state of the switch  6  may thus be controlled on the basis of the filament heating voltages. The respective filament heating voltages may be supplied to the lamp device  1  only when all pins  5  are correctly inserted into the lighting fixture. The lighting fixture may be arranged to apply the driving voltage to the pins  5   aa  and  5   ba . A main current between these pins  5   aa  and  5   ba  may thus flow only when lamp device  1  is correctly inserted into the lighting fixture. 
       FIG. 3  illustrates a second circuit implementation for the lamp device  1 . The second implementation is similar to the first implementation but differs in that the control circuit  7  includes a power converter  11 . The power converter  11  is arranged to amplify a voltage received from the rectifier  9  to a level sufficient for closing the switch  6 . Similar to the first circuit implementation the control circuit  7  includes an input for receiving the control signal from the optocoupler  8 . In  FIG. 3  the power converter  11  is an integrated circuit (IC) including an input for receiving the control signal from the optocoupler  8 . The power converter  11  may include circuit logic arranged to supply a voltage to the control circuit  7 ′ for closing the switch  6  on a condition that a voltage difference between the inputs of the power converter  11  exceeds a threshold voltage at a same time as a control signal is received at said input of the power converter  11 . The power converter  11  may provide a stable operating voltage for the switch controller  7 ′ under various operating conditions. This may be especially advantageous when the lamp device  1  is used to retrofit an existing lighting fixture as discussed above since different ballasts of the starting circuitry thereof may result in varying filament heating voltages. The power amplifier  11  may then amplify low heating voltages to a level sufficient for keeping the switch  6  closed. 
     The power converter  11  may be implemented as a boost converter. An example circuit diagram for a boost converter comprising an inductor L 1 , a diode D 2 , a capacitor C 4  and a semiconductor switch in the form of a MOSFET M 1  is shown in  FIG. 4 . The boost converter may be arranged to activate only on a condition that the control signal is received from the optocoupler  8 . The boost converter may for example include circuitry (e.g. in the form of an IC) arranged to receive the control signal from the optocoupler  8  and provide an enabling signal to the gate of the transistor M 1  only on a condition that the IC of the boost circuit receives the control signal from the optocoupler  8 . On a condition that no enabling signal is received by the transistor M 1 , the transistor M 1  assumes an off state wherein the boost converter will be inactive. 
     In lighting fixtures it may occur that one of the contacts in the socket of the lighting fixture into which the lamp device  1  is inserted becomes loose. This may for example happen in older lighting fixtures due to wear and tear. As a result for example the pin  5   aa  or  5   ab  may lose electrical contact with the fixture. The lamp device  1  may then not operate correctly. With reference to  FIG. 3 , if for example the pin  5   aa  loses the electrical contact with the lighting fixture the main current may flow through the pin  5   ab , the first output of the rectifier  9 , the power converter  11 , the switch controller  7 ′ and the switch  6  via the second output of the rectifier  9 . Hence, despite the loose contact at the pin  5   aa  the switch controller  7 ′ is powered wherein the switch  6  may be closed, the closing resulting in a further current increase. In response the voltage across the capacitor C 2  may increase. Eventually the circuit may melt due to over voltage. To deal with this situation the lamp device  1  may be provided with an optional safety circuit. The safety circuit may be arranged between the rectifier  9  and the power converter  11  and be adapted to, in response to a voltage difference between the first and second outputs of the rectifier  9  exceeding a set threshold voltage (i.e. a protection level), clamping the voltage difference. 
       FIG. 4  illustrates an example implementation of such a safety circuit  12  in combination with the second circuit implementation illustrated in  FIG. 3 . It is however to be noted that the circuit  12  may also be used in combination with the first circuit implementation. To simplify understanding of the safety circuit  12  only the switch  6 , the switch controller  7 ′, the rectifier  9  and the power converter  11  are shown. In  FIG. 4  the power converter  11  is implemented as a boost converter. The remaining part of the circuit is identical to that of  FIG. 3 . 
     The safety circuit  12  comprises a Zener diode Z 1 , a resistor R 3 , a capacitor C 3 , a thyristor T 1  and a diode D 1 . In the event of e.g. a loose contact at the pin  5   aa  as described above the voltage over the capacitor C 2  will increase. However, in response to the voltage over the capacitor C 2  exceeding a threshold voltage set by the Zener diode Z 1  the thyristor T 1  will be triggered on. Consequently, the main current will flow through D 1  and T 1  instead of through the power converter  11  and the switch controller  7 ′ and the voltage difference between the outputs of the rectifier  9  may be limited or clamped. 
     By the switching on of the thyristor T 1  there is also provided a current path between the inputs of the power converter  11 . By choosing the impedances of D 1  and T 1  appropriately the voltage drop across D 1  and T 1  becomes sufficient for driving the power converter  11  and in turn the switch controller  7 ′. Thereby, the lamp device  1  may produce light even if the pin  5   aa  loses electrical contact with the lighting fixture. 
     The safety circuit  12  may also be arranged between the rectifier  10  and the optocoupler  8  wherein the lamp may operate if for example the pin  5   ba  loses electrical contact with the lighting fixture. In that case it may be advantageous to provide additional diodes in series with the diode D 1  to create a sufficient voltage drop for the optocoupler  8  to operate even if the pin  5   ba  looses electrical contact. 
       FIG. 5  illustrates a safety circuit  12 ′ of an alternative design. The safety circuit  12 ′ is similar to the safety circuit  12  but differs in that instead of the diode D 1  and the thyristor T 1 , the safety circuit  12 ′ includes transistors Q 2 , Q 3 , Q 4  and resistors R 4 , R 5 , R 6 . If the pin  5   aa  loses its electrical contact with the lighting fixture, the voltage across C 2  will increase as explained above with reference to  FIG. 4 . When the voltage over C 2  exceeds the threshold voltage set by the Zener diode Z 1 , a current will flow into the base of Q 2  wherein Q 2  will switch on. As a result the base of Q 4  will be pulled to ground wherein Q 4  will switch off Consequently, Q 3  will turn off because the base current to Q 3  will be interrupted by Q 4 . The safety circuit  12 ′ is thus arranged to disconnect an output of the rectifier  9  from the power converter  11  and thus the control circuit in response to the voltage difference exceeding a set threshold voltage. 
       FIG. 6  schematically illustrates a lamp device  1 ′ in accordance with a second embodiment. The lamp device  1 ′ is similar to the lamp device  1  but differs in that instead of the optocoupler  8  the lamp device  1 ′ includes a current transformer  8 ′. The functions of the other parts of the lamp device  1 ′ are analogous to their counterparts in the lamp device  1  as described above. The primary winding of the current transformer  8 ′ is connected to the second pair of pins  5   ba ,  5   bb . The secondary winding is connected to the input of the control circuit  7  via the rectifier  10 . The function of the current transformer  8 ′ is analogous to the function of the optocoupler  8 . An AC voltage applied at the primary winding of the current transformer  8 ′ via the second pair of pins  5   ba ,  5   bb  will induce an AC voltage in the secondary winding of the current transformer  8 ′. The induced AC voltage is rectified by the rectifier  10  and forms a control signal which is received by the control circuit  7 . As described in detail above the control circuit  7  will close the switch  6  on a condition that a voltage difference between the pins of the first pair  5   aa ,  5   ab  exceeds a threshold voltage at a same time as the control signal is received from the current transformer  8 ′. 
     The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the optocoupler  8  and the current transformer  8 ′ both include a primary side and a secondary side which are galvanically insulated from each other. Although this provides added safety other circuit elements which do not include insulation between a primary and secondary side may be used to generate a control signal to the control circuit  7 . A control signal may for example be provided to the control circuit  7  by a simple voltage divider or an operative amplifier connected to the second pair of pins. Moreover, although the illustrated embodiments include certain circuit elements other types of circuit elements may also be used. For example a BJT may be replaced by a MOSFET and vice versa. Many of the circuit elements (such as the Zener diodes and the thyristors) may also be replaced by integrated circuit counterparts including circuit logic providing a similar function as the circuit elements. Furthermore, although the lamp devices  1  includes rectifiers  9 ,  10  they may be omitted in case the lamp device  1  is used in a lighting fixture providing a DC voltage. 
     Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.