Patent Publication Number: US-6700327-B2

Title: Device connecting a lamp in a motor vehicle

Description:
FIELD OF THE INVENTION 
     This invention relates to a device for the electrical connection of a filament lamp of a given nominal voltage, in a motor vehicle having an electrical power supply network which is under a network voltage higher than the nominal lamp voltage. 
     BACKGROUND OF THE INVENTION 
     As the number of items of electrical equipment in a motor vehicle becomes higher and higher, an increase in the available electrical power is becoming more and more necessary. This leads to an increase in the network voltage of the vehicle electrical system, in order to avoid the occurrence of excessively high currents in the system. The unidirectional network voltage is usually 12 volts in current automobiles such as family cars or 24 volts in heavy goods vehicles (trucks). Sometimes, this voltage must increase substantially, in particular to reach as high as 42 volts. Technical considerations appropriate to the automotive field, linked to the size of the tungsten filament of the lamp, make it improbable that filament lamps directly supplied at 42 volts can or will be made. Filament lamps arranged to function at a voltage of 12 volts will still be in service even where the network voltage of the vehicle is higher than the nominal lamp voltages. Various solutions have already been proposed to overcome this problem. 
     In this connection, systems are known with a centralized architecture which comprises a DC/DC converter which supplies a reduced unidirectional voltage, for example at 12 volts, from a higher input voltage which may for example be 42 volts. This solution makes it necessary to provide a 12 volt battery as well as the converter. This arrangement results in relatively high cost and a significant space requirement. 
     It is also known to provide apparatus with an independent electrical control unit which enables the unidirectional voltage applied to the lamps to be regulated by a pulse width modulated (PWM) control signal. Such a control unit is remote from the various lamps which are to be supplied, and must be connected to the fittings that carry those lamps, the connection being made by electrical wires. This arrangement has various drawbacks, in particular the size of the control unit to be provided, the power of which must be high enough to supply all of the appropriate lamps in the vehicle. Cabling must be provided between tho control unit and the lamps. In addition, it is difficult with this arrangement to satisfy the requirements for electromagnetic compatibility, due to the emission of parasitic radiation set up by the pulse width modulated control signal. 
     DISCUSSION OF THE INVENTION 
     A primary object of the invention is to provide a device for the connection of a filament lamp, having a nominal voltage lower than that of the power supply network, but which responds better than at present to the various requirements found in practice. In particular, it is desirable that the connecting device enables a double network to be avoided in the cable bundle. It is also desirable that it shall facilitate adaptation to a higher voltage and provide proper screening against electromagnetic radiation. 
     According to the invention, a device for connecting a filament lamp, having a given nominal voltage, in a motor vehicle equipped with a network for power supply at a voltage greater than the nominal voltage of the lamp, is characterised in that the device incorporates an electronic circuit including components for reducing the voltage from the network voltage to a voltage close or equal to the nominal voltage of the lamp, the device also incorporating means for making the electrical connection of the lamp. 
     Preferably, the device further provides mechanical fastening for the lamp. 
     The device may constitute a connector for the lamp, for plug-in connection of terminal tags of the lamp. Alternatively, it may constitute a lamp holder socket. 
     The device preferably includes a casing of plastics material in which the electronic circuit for reducing the voltage is placed, the casing being provided with electrical connecting contacts or sockets for connection with the filament lamp. 
     According to a preferred feature of the invention, the casing incorporates a metallic cooling element for the electronic components of the circuit, the said metallic element also providing screening for purposes of electromagnetic compatibility. 
     Where the device is a connector comprising a casing of plastics material containing the electronic voltage reduction circuit and arranged to receive and provide electrical connection to plug-in tags of the lamp, then, according to a preferred feature of the invention, the casing includes two projecting elements which are open in a face remote from the casing, each said projecting element having an internal metallic tongue, the said tongues being connected to the output terminal of the voltage reduction circuit, the tongues being arranged to receive and mechanically hold the tags of the lamp, besides making the electrical connection thereof. Preferably, the projecting elements completely surround the tongues, whereby to insulate the said tongues electrically. 
     Where the device comprises a lamp holder socket in the form of a sleeve, for example for bayonet or screw-type fastening of the lamp, and has a casing of plastics material containing the electronic voltage reduction circuit, the sleeve is joined to one face of the casing which includes, on another face, a projecting element open at the end thereof remote from the circuit, with two electrical contacts being mounted within the said projecting element. 
     Preferably, the said circuit is arranged to establish pulse width modulation and to provide an output voltage close or equal to the nominal voltage of the lamp. The said circuit preferably then comprises an integrated circuit which is arranged to produce a pulse width modulated signal on its output, together with components for creating a regulated voltage suitable for proper operation of the integrated circuit. 
     Preferably, the frequency of the pulse width modulated signals is chosen to be large enough to prevent any perception by the human eye of any variation in light intensity from the lamp, having regard to retinal persistence. In particular, the frequency is in the range between 60 Hz and 500 Hz. 
     The connecting device according to the invention enables additional connections to be avoided with respect to a current 12 volt or 24 volt network or a future 42 volt network. In this connection, the network voltage is 42 volts, while the nominal voltage of the lamp is 12 or 24 volts. 
     Preferably, the said components of the circuit are soldered on a printed circuit, as are the output terminals and the input wires. 
     Electromagnetic compatibility is facilitated by the fact that the lamp filament is very close to the control means which reduce the voltage to the lamp input value. 
    
    
     Further arrangements, objects and advantages of the invention will appear more clearly on a reading of the following detailed description of some preferred embodiments of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic perspective view of a device according to the inventions consisting of a plug-in connector. 
     FIG. 2 is a perspective view of another version of the connecting device in the form of a lamp socket. 
     FIG. 3 is a diagram on a larger scale of the electronic voltage reduction circuit incorporated in the casing of the device. 
     FIG. 4 is a diagram showing the supply pulses for a lamp. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Reference is first made to FIG. 1, which shows a device B for the connection of a filament lamp L having a given nominal voltage V 0 , which is for example equal to 12 volts, for a motor vehicle having an electrical supply network which operates at a unidirectional voltage V 1  greater than V 0 . The voltage V 1  may for example be equal to 42 volts. 
     The device B comprises a casing  1  which in particular is in the form of a rectangular parallelepiped, and which is preferably made of plastics material. The casing  1  includes an internal circuit  2  having electronic components  2   a,    2   b,    2   c  for reducing the network voltage V 1  to a voltage which is close to, or equal to, the nominal voltage V 0  of the lamp. The device B is a connector C for the lamp L, which has plug-in tags E. 
     The casing  1  has on one of its major faces two projecting elements  3  and  4 , each in the form of a hollow rectangular parallelepiped. The generatrices of the latter lie at right angles to the major face of the casing. The elements  3  and  4  are open on the side remote from the casing  1 , and they include internal metal tongues  5   a  and  5   b.  These tongues are connected to the output terminals of the voltage reduction circuit  2 , in such a way that the voltage V 0  is applied to them. The tongues  5   a  and  5   b  are arranged to receive, and to retain mechanically, the tags E of the lamp L, while also making the appropriate electrical connections. 
     The projecting elements  3  and  4  completely surround the tongues  5   a  and  5   b  so as to insulate the tongues electrically. The tongues do of course act as connecting terminals. 
     The electronic circuit  2  is connected to two wires  6   a  and  6   b  which extend through the wall of the casing and connect the device B to the power supply network of the vehicle at the voltage V 1 . The components of the circuit  2 , indicated at  2   a,    2   b,    2   c  are soldered on a printed circuit  2   d,  as are the output terminals  5   a  and  5   b  and the input wires  6   a  and  6   b.  The casing  1  is compact, and is rigid enough mechanically to protect the components inside it. The casing also prevents any access to the working parts. The casing  1  may in fact also incorporate metallic cooling portions (not shown) which serve as radiators to dissipate electrical energy and provide screening against electromagnetic radiation. 
     The connector C, consisting of the casing  1  with the voltage reduction circuit  2 , is of sufficiently reduced size to be able to be installed in place of an ordinary connector of a kind that does not provide a voltage reduction facility. 
     Reference is now made to FIG. 2, which shows another version in which the connecting device B has a lampholder socket  7  joined to one face of a casing  11  of plastics material which, like the casing  1  in FIG. 1, contains an electronic circuit  2  with components for reducing the voltage from the voltage V 1  of the electrical network of the vehicle to the nominal voltage of the lamp. The electrical contacts (not visible in FIG. 2) of the socket  7  are connected to the output of the circuit  2 . The socket  7  shown in FIG. 2 is of the type in which the lamp is fixed by a bayonet fitting, but it could be of a different type, for example the screw type. 
     On another face, generally at right angles to the one that carries the socket  7 , the casing  11  has a projecting element  8  in the form of a parallelopiped which is open at its end remote from the circuit  2 . Two contact elements  9  and  10  are mounted in the projecting element B. The contact elements  9  and  10  are arranged to engage in a connector which is connected to wires that are themselves connected to the power supply network at 42 volts. Here again, the socket  7  and casing  11  form a unit which is compact enough to replace an ordinary lamp holder. 
     Reference is now made to FIG. 3, which is one example of a suitable voltage reduction circuit  2 , shown in circuit diagram form. The circuit  2  is arranged to provide an output voltage V 0  close to 12 V, by pulse width modulation (commonly referred to as a PWM signal). This circuit will now be described. 
     A supply wire  6   a  is connected directly through a line  12   a  to one of the output terminals  5   a.  The other power supply wire  6   b  is connected through a line  12   b  to the source electrode of a transistor T 2  of the MOS type. The drain of the transistor T 2  is connected to the other output terminal  6   b.  A bridge is established between the lines  12   a  and  12   b.  This bridge comprises a diode D 1 , the anode of which is connected on the line  12   a.  The cathode of the diode D 1  is connected through a resistor R 1  to the cathode of a Zener diode D 2 , the anode of which is connected to the line  12   b.  The Zener voltage of the diode D 2  constitutes a reference voltage. 
     The base of a transistor D 1 , which is for example of the NPN type, is connected to the cathode of the diode D 2 . The collector of the transistor T 1  is connected to the cathode of the diode D 1 . The emitter of the transistor T 1  is connected to an input e 8  of an integrated circuit I which is arranged to produce a PWM signal on its output e 3 . This circuit I may typically be an integrated circuit of the kind having the reference NE555. An input e 1  of the circuit I is connected to the line  12   b.  Another input e 2  is connected to the anode of a diode D 3 , the cathode of which is connected to an input e 7  through a resistor R 3 . 
     Two capacitors C 1  and C 2  are connected in parallel between an input e 4  and the line  12   b.  The input e 4  is also connected to the input e 8 , and, through a resistor R 2 , to the anode of a diode D 4 . The input e 7  is also connected to the anode of the diode D 4 . The cathode of the diode D 4  is connected through a capacitor C 4  to the line  12   b.  A further input e 6 , together with the input e 2 , are connected to the cathode of the diode D 4 . 
     The output e 3  of the circuit I is connected through a resistor R 4  to the grid of the transistor T 2 . A capacitor C 5  is connected between the grid of the diode T 2  and the line  12   b.    
     The components D 1 , R 1 , T 1 , D 2 , C 1  and C 2  create a regulated voltage of 12 V, corresponding to the Zener voltage of the diode D 2 , this being the voltage required for proper operation of the integrated circuit I. 
     The circuit I generates on its output e 3  the PWM signal which is diagrammatically illustrated in FIG. 4, to which reference is now made. In FIG. 4, voltage is on the ordinate and time on the abscissa. The duration of the pulse Ton (time on circuit) is fixed by the values of R 2  and C 4 , while time out of circuit, Toff, is fixed by the values of R 3  and C 4 . The duration T of one period is equal to Ton+Toff, and is in particular equal to 2 ms, which corresponds to a frequency (f=1/T) of 500 Hz. This frequency is chosen in order to prevent any perception of any variation in the light intensity from the lamp by the human eye, and takes account of retinal persistence. By way of an example, if R 2 =2.37 kΩ, R 3 =22.56 kΩ, and C 4 =100 nF, then Ton=194 μs and Toff−1806 Ωs. 
     The components R 4  and C 5  enable the fronts of the signals applied to the grid of the transistor T 2  to be adjusted so as to conform to the requirements for electromagnetic compatibility (EMC). The transistor T 2  applies the voltage V 1  from the network, that is to say 42 V in the example under consideration, to the terminals  5   a  and  5   b  of the lamp in response to the PWM signal, so as to provide an effective voltage of the same order as the nominal voltage of the lamp which is, for example, 12.8 V. The transistor T 2  may be of the SMART type so as to add regulation and current limitation. One suitable example is that sold under the reference VNP35N07. External regulation of the current may be obtained using a shunt. 
     Upstream of the circuit, filter components of the “selfs” type, and capacitors (not shown in FIG. 3) may be arranged to stop parasitic pulses that may occur on the power supply lines  6   a  and  6   b.    
     A thermal resistance may be created between the terminals  5   a,    5   b  and the circuit  2 , so that heat given off by the lamp L will not be collected in the casing. The casing may also incorporate suitable metallic elements for cooling the transistor T 2 . 
     The invention enables additional connections, as compared with a current 12 V or a future 42 V network, to be avoided. It becomes extremely simple to provide for different models of 12 V and 42 V lamps. 
     The provision of electromagnetic compatibility is considerably facilitated by the proximity of the filament of the lamp L to the control circuit  2  mounted within the connector or in the lamp holder. 
     The use of pulse width modulation with rectangular (square wave) signals creates electromagnetic parasites essentially by the rising fronts of the signals. The magnitude of the radiation resulting from these parasites increases with the length of the antenna. The antenna consists of the connecting wires between the output of the circuit  2  and the connecting terminals of the lamp L. But here, since the voltage reduction circuit is incorporated in the connector or in the lamp holder, the length of the connecting wires, and therefore the length of the antenna, is minimised. Parasitic radiation is therefore considerably reduced. 
     The invention avoids the use of a twin voltage network at 42 V and 12 V in the wiring bundles, which is an important consideration for the manufacturer from the cost point of view. There is no difference between the designs of headlights or indicating devices operating at 12 V, and those operating at 42 V.