FR2780599A1 - Electrical circuit for illumination of instrument panel for use in automobiles - Google Patents

Abstract

The electrical circuit comprises a battery (B), a set of parallel branches (EB), each with light-emitting diode (D) and a resistor (R) in series, and control circuits including a resonant circuit (CO), a filter circuit (CR), a voltage-controlled oscillator (OS), a monostable circuit (M1), an interrupting transistor (TI), and a low-pass filter (R1,C3). In the second embodiment, a set of parallel branches (EB) is with additional transistors (T), each in series with a light-emitting diode (D) and a resistor (R). The low-pass filter (R1,C3), the voltage-controlled oscillator (OS), the monostable circuit (M1), and the interrupting transistor (TI) constitute a negative feedback branch. The voltage controlled oscillator is with a square-pulse output connected to the control input of the monostable circuit. The resistors (R) are of maximum dissipated power 0.1 watts and the transistors (T) are of the complementary metal-oxide-silicon (CMOS) type.

Description

The present invention relates to the field of lighting means for

  indicating devices for motor vehicles, and, more specifically, such means comprising light-emitting diodes. Motor vehicle dashboards are increasingly equipped with such circuits. Depending on the case, the light-emitting diodes may have the role of illuminating a part of the dashboard, or of illuminating a warning light when a particular member of the vehicle is defective or in operation. The invention can also be applied to illuminate an interior dashboard push button

having a translucent part.

  FIG. 1 shows a known circuit comprising light-emitting diodes or LEDs. It consists of a set of parallel branches Si to Sn connected by one of their ends to a terminal

  positive of a battery B and by their second end to a ground.

  Each branch Si to S, comprises, in series, a resistor referenced respectively R1 to Rn, a light-emitting diode referenced respectively Dl to Dn, and a transistor referenced

respectively Tl to Tn.

  Each branch is supplied by the battery output voltage

  B, which is typically about 12 volts.

  Each transistor Tl to T is connected to a control circuit, not shown. k being an integer between 1 and n, when the illumination of a diode Dk is necessary, a control circuit connected to the transistor Tk, which is in series with this diode, provides at its base a saturation current. The transistor Tk becomes conducting, and a current crosses the light-emitting diode Dk as well as the resistor Rk which is connected in series

  with the transistor Tk and the diode Dk.

  Each of the resistors R1 to Rn has the role of limiting the current passing through the light-emitting diode which is in series with it when the corresponding transistor is passing, in order to adapt the characteristics of the battery B to those of the light-emitting diodes.

D1 to Dn.

  Taking into account the characteristics of existing batteries and light-emitting diodes, power resistors are used in the usual way for resistors R. to Rk, capable of dissipating

powers of about 0.5 watts.

  Such resistances are highly dissipative. They therefore cause significant heating of the indicating device as well

  enlightened. In addition, they are expensive and bulky.

  Furthermore, the voltage delivered by the battery B is subject to significant fluctuations. Transistors T1 to T must therefore be dimensioned to withstand high overvoltages between their terminals, as well as high overcurrents. Thus, in the case of a circuit conforming to that of FIG. 1, the transistors must support voltages of 35 volts between their terminals and currents of 100 mA. The cost of the transistors is all the higher as they are dimensioned to withstand high voltages. The main object of the present invention is to provide a motor vehicle lighting circuit comprising light-emitting diodes in which resistors are less expensive, less

  dissipative and less bulky can be adopted.

  A second object of the present invention is to provide a motor vehicle lighting circuit comprising light-emitting diodes in which less expensive semiconductor switches can be chosen to fulfill the role of switch in

  series with light emitting diodes.

  These aims are achieved by means of an electrical lighting circuit of an indicating device, in particular for a motor vehicle, comprising a battery, several parallel branches each comprising, in series, a light-emitting diode and a resistive means, characterized in that it is provided with control and servo means adapted to maintain a voltage between the ends of the branches at a set value which is less than the voltage delivered by the battery and which is compatible with

  a supply voltage for the diodes.

  Other features, objects and advantages of the present invention

  will appear on reading the description which follows and next to the

  annexed drawings given by way of nonlimiting examples and in which: - FIG. 1 previously described represents an electric circuit of a motor vehicle comprising light-emitting diodes according to the state of the art, - FIG. 2 represents a circuit comprising diodes light emitting, according to the present invention, - Figure 3 shows a circuit comprising light emitting diodes according to a second embodiment of the invention; - Figure 4 is a graph showing values measured over time, on the one hand at the output of a resonant circuit according to the invention, and on the other hand across a set of branches according to this invention; - Figure 5 is a graph showing values taken over time by a supply voltage of branches carrying light emitting diodes and in accordance with the present invention; - Figure 6 is a graph showing values taken over time by an output voltage of a monostable according to the invention, along a time axis of the same origin as in Figure 5; - There Figure 7 is another graph showing values taken over time by a voltage and a supply current of branches carrying light emitting diodes and according to the invention; FIG. 8 is a graph showing values taken over time by a voltage between the drain and the source of a transistor TI, and a current passing through this transistor TI between its drain and its source, in accordance with

The invention.

  - Figure 9 is a graph showing values taken over time by a voltage UN3 at a terminal of an inductance L1 of a circuit according to the invention; - Figure 10 is a graph showing values taken over time by a current passing through an inductance L1 of the same circuit according to the invention; - Figure 11 shows a specific power supply circuit of a dashboard incorporating a power circuit

according to the invention.

  There is shown in Figure 2 an assembly diagram

  corresponding to a circuit according to the present invention.

  The circuit of FIG. 2 comprises, on the right of the figure, a set EB of branches parallel to each other and arranged between a node N1 and a node N2. Each branch of the EB assembly comprises in series a resistor R and a light-emitting diode D mounted

  passing from node N1 to node N2. The node N2 is connected to ground.

  At the far left of Figure 2 is a battery B. A negative terminal of battery B is connected to ground and a positive terminal Npos of the battery is connected to a switching power supply forming a regulator between battery B and l set of EB branches. More specifically, the battery B is connected by its positive terminal Npos to a resonant circuit CO. For the sake of clarity, the CO resonant circuit has been surrounded by

a line of dashes.

  The CO resonant circuit is placed between the positive terminal Npos of the

  battery B and the node N1 of the set EB of parallel branches.

  The circuit CO will now be described, going from the positive terminal Npos of the battery B towards the node N1. It first comprises two parallel branches, one carrying a capacitor C1 and the other a diode D1 mounted passing in the direction going from the node N1 towards the positive terminal Npos of the battery B. The circuit CO then comprises, in series with all of these

  two parallel branches, an inductor L1.

  This set of two parallel branches is connected to the inductance L1 at a node N3 of the circuit. The end of the inductor L1 which is opposite to the set of two parallel branches previously described

  is connected directly to terminal N1.

  Between the terminal N1 and the terminal N2 is placed, in parallel with the branches of the assembly EB, a rectification and filtering circuit CR

  surrounded by a line of dashes for clarity.

  The rectification circuit CR is composed of two parallel branches between the node N1 and the node N2. One of these two branches

  comprises a diode D2 mounted passing from node N2 to node N1.

  The other of these two branches is provided with a capacitor C2.

  The node N1 is connected via a resistor R1 to an input of a square oscillator OS with variable frequency. The input of the square oscillator OS is connected via a capacitor C3 to ground. The square oscillator OS is capable of supplying an oscillating voltage according to a square signal, that is to say a voltage passing from a higher voltage Usup fixed to a lower voltage Uinf fixed, substantially instantaneously. The duration during which the voltage supplied by this square oscillator OS is equal to Usup and the duration during which this supplied voltage is equal to Uinf are, at a given oscillation frequency of this component, both equal. The frequency of the oscillations of the voltage at the output of this oscillator OS, at an instant t, depends on the voltage Um applied at the input of this oscillator. This type of oscillator whose frequency is controlled by a voltage is known according to the terminology

  English under the name of Voltage Controlled Oscillator (VCO).

  The higher this voltage Um transmitted at the input of the oscillator OS, the higher the frequency of the oscillations of the voltage at the output of this oscillator OS. Conversely, the lower the voltage transmitted at the input of the oscillator OS, the more the frequency of the oscillations of the

  voltage at the output of this oscillator is low.

  The output of the OS oscillator is connected to a control input

of a monostable M1.

  The monostable M1 has an output connected to a gate of a transistor TI placed in parallel with the two branches carrying respectively the diode D1 and the capacitor C1 of the resonant circuit CO. More specifically, the transistor TI is here a power MOS transistor with enriched P channel, of which a drain and a source are respectively connected to the node N3 and to the positive terminal Npos of the battery B.

  The operation of this circuit is as follows.

  The monostable M1 has an equilibrium state in which it provides a constant Ubas voltage at its output. When the voltage which it receives at the input increases suddenly, it adopts, for a fixed period TF specific to this monostable M1, a provisional state in which it delivers a fixed voltage Uhaut greater than the voltage Ubas. This fixed duration TF is much less than an average period of oscillations of the oscillator

square OS.

  When powered by the monostable M1 according to the voltage

  lower Ub.s, the transistor TI is conducting and it is equivalent to a short-circuit.

  When it is supplied with the upper voltage Uhaut, it is blocked and it

equivalent to a circuit breaker.

  The equilibrium state of the monostable M1 therefore corresponds to the on state of the transistor TI, and the provisional state of the monostable to the blocked state of the

transistor TI.

  When transistor TI is blocked for a time interval

  TF, the resonant circuit CO is able to oscillate.

  The diode Dl prevents the capacitor Cl from charging in a direction such that the node N3 is at a potential greater than the node Npos, so that the voltage measured between the node N3 and the ground, referenced UN3 in FIG. 2, describes oscillations according to a function represented by a dashed line referenced 5 in FIG. 4, the values of which are lower than the constant voltage Ubat delivered by the battery B at the node

Npos.

  The rectification and filtering circuit CR performs a smoothing and a rectification of the function 5, so that the voltage Us across the terminals of the set EB follows, during the time interval TF, a function referenced 7 and drawn in line continuous in FIG. 4 which is substantially

  constant around a U 'value lower than the Ubat value.

  When, during a time interval referenced To in FIG. 4, the transistor TI is on, the terminal N3 of the inductance L1 is linked to the terminal Npos of the battery by a short circuit, so that no oscillation occurs. takes place in the circuit. A permanent current tends to settle between the node Npos and the node N1 at the input of the assembly EB, so that the voltage between the terminals the inductance L1 tends towards zero and the voltage US at the terminals of the assembly EB tends towards the value of the voltage Ubat at the terminals of the battery B. Thus therefore, when the switch TI is blocking during a time interval TF, the voltage Us at the terminals of the set of branches EB takes during this interval of time a value U ′ less than the value Ubat, and, when the switch TI is on for a time interval To, the voltage Us across the terminals of the set of branches EB tends

  towards the Ubat value during this time interval.

  The voltage Us therefore tends towards the value Ubat when the transistor TI is on, that is to say when the voltage transmitted to the gate of this transistor by the monostable M1 is equal to Ubas, that is to say when the monostable M1 is in its stable state, and it takes the value U 'when the transistor TI is blocking, that is to say when its gate receives from the monostable M1 a voltage equal to Uhaut, that is to say when the monostable

M1 is in its provisional state.

  The duration TF during which the monostable M1 remains in its provisional state is a fixed duration, specific to the monostable M1. The monostable M1 goes into this provisional state each time the voltage which is assigned to it

  transmitted by the OS oscillator goes from U, f to Uup.

  The average function of the values taken by the voltage Us.

  corresponds to the values taken by the voltage Um across the capacitor C3. Indeed, the resistor R1 and the capacitor C3 constitute a low-pass filter capable of transmitting at the input of the oscillator OS a voltage Um which is substantially continuous, substantially equal to the

average voltage Us.

  An increase in the frequency of the oscillations of the oscillator OS generates a more frequent passage of the monostable M1 in its provisional state, and therefore a more frequent passage of the switch TI in its

blocked state.

  The duration TF during which the monostable M1 remains in its provisional state being fixed, an increase in the frequency of the oscillations of the oscillator OS therefore generates a reduction in the value of the duration To during which the switch TI remains in its stable state passerby. An increase in the frequency of the oscillations of the OS oscillator therefore generates an increase in the TF / To ratio. and therefore a decrease in the

  average value Um of the values taken by the voltage Us.

  Conversely, a decrease in the frequency of the oscillations of the oscillator OS generates a decrease in the TF / To ratio and therefore an increase in the average value Um of the values taken by the voltage Us. The oscillation frequency of the OS oscillator being an increasing function of the Um voltage which is transmitted to its input, a high Um voltage. causes a rapid oscillation of the oscillator OS, and therefore causes a decrease in the average voltage Um of the values taken by the

Us voltage.

  Conversely, a low voltage Um generates a slow oscillation of the oscillator OS, a longer passage of the switch TI in its state

  provisional stable, and therefore an increase in the average value Um.

  The assembly consisting of the resistor R1, the capacitor C3, the oscillator OS, the monostable M1 and the transistor TI therefore constitutes a feedback loop, the effect of which is to stabilize the voltage Um

  mean of Us at a chosen equilibrium or "set point" value.

  This value can be chosen by adapting the values adopted for the capacitance C1 and the inductance L1 of the resonant circuit CO, by adapting the function relating the input voltage of the oscillator OS to the frequency of its oscillations, and by adjusting the fixed duration during which the monostable M1

remains in its provisional state.

  In the embodiment described here, the equilibrium value of the voltage Um is 3 volts while the voltage supplied to the terminal Npos by the battery B is 12 volts. Each branch of the EB assembly therefore receives between its ends

  a voltage Us stabilized around an average of 3 volts.

  Compared with the known circuit of FIG. 1, the voltage between the ends of the branches carrying the light-emitting diodes is greatly reduced. This therefore makes it possible to adopt for the resistors R in series with the light-emitting diodes D lower resistance values. The resistors R adopted here are thus resistors of type 0805 of 50 milliohms dissipating an average power of 0.1 watt, while the resistors placed in series with the light-emitting diodes are usually MELF resistors dissipating an average power of 0.5 watts . The resistors R which can be used thanks to the circuit of FIG. 2 dissipate little energy, are inexpensive and are

space-saving.

  An average current Is equal to the sum of the currents crossing the

  branches of the whole EB here is equal to about 400 mA.

  In the embodiment described here, the branches of the EB assembly do not have any switch, so that the diodes

  can only be powered all at the same time.

  A second embodiment of the invention is shown in Figure 3. The elements of this figure which are similar to those of the figure

  2 use the same references.

  The electrical circuit of FIG. 3 differs from that of FIG. 2 only in that on certain branches of the assembly EB is arranged a transistor T, in series with the resistor R and the corresponding light-emitting diode D. The voltage between the ends of the branches EB being, as described in connection with the circuit of FIG. 2, stabilized around an average value of 3 volts, the transistors T do not have to be dimensioned to support overvoltages as strong as in the case of known circuits. The T transistors are thus standard CMOS (Complementary Metal Oxide Semiconductor) type transistors in volt technology. Such switches are less expensive than the switches used until now in circuits conforming to that of FIG. 1. Other standard switches, or drivers according to English terminology, can also be adopted in place of the transistors, such as integrated circuits, or microcontrollers, thanks to the present invention. These switches are also less bulky than

  switches used in known circuits.

  More generally, the invention allows a reduction of the space occupied on a printed circuit, or Printed Circuit Board according to the

English terminology.

  The assembly formed by the battery B, the resonant circuit CO, the rectification and filtering circuit CR, the square oscillator OS, the monostable M1, the transistor T1, the resistor R1 and the capacitor C3, therefore constitutes a specific high-efficiency supply circuit for the set EB of parallel branches each comprising a light-emitting diode. We have described here such a supply circuit which is of the resonant switching type. This type of circuit makes it possible to obtain a good efficiency, to dissipate little energy, and to limit the electromagnetic radiation emitted. However, other types of circuits, making it possible to supply the whole EB according to a stabilized voltage and relatively

low, can be adopted.

  The supply circuits of Figures 2 and 3 each constitute a cell, which can be integrated into a specific dedicated power circuit

  the general power functions of a dashboard.

  More generally, and as shown in FIG. 9, such a specific circuit, referenced 10 in FIG. 9, has two inputs 20 and 30 per bat which it receives respectively a permanent Uperm voltage, supplied by a battery, and a voltage Ucomm beats, switched, also from a battery. It has three outputs 40, 50 and 60 by which it respectively supplies a permanent UsPerm voltage of 5 volts, a switched Usc mm voltage of 5 volts, and a Us voltage of 3 volts intended to supply light-emitting diodes. Of course, the present invention is not limited to the exemplary embodiments which have just been described, but extends to any variant

conforms to his spirit.

Claims (12)

  1. Electric circuit for lighting an indicating device, in particular for a motor vehicle, comprising a battery (B), several parallel branches (EB) each comprising, in series, a light-emitting diode (D) and a resistive means (R), characterized in that there are provided control and servo means (CO, CR, OS, M1, TI, R1, C3) adapted to maintain a tension between the ends of the branches (EB) at a set value which is lower than the voltage delivered by the battery (B) and which   is compatible with a diode supply voltage (D).   2. Electric circuit according to claim 1, characterized in that the control and servo control means (CO, CR, OS, M1, TI, R1, C3) comprise a switching power supply (CO, CR, OS, M1, TIl) adapted to be non-polluting on the electromagnetic level and which comprises a resonant circuit (CO) of which an element (C1) is in parallel with a cut-off switch (TI) able to short-circuit this element (C1), as well as control means (CR, OS, M1, R1, C3) of the cut-off switch (TI) which include a monostable (M1), one output of which is connected to the cut-off switch (TI), an oscillator (OS) controlled by voltage, one output of which is connected to a monostable control input (M1), and one input of which is connected to the output of the resonant circuit, as well as rectifying and filtering means connected to the resonant circuit output.   3. Electrical circuit according to one of claims 1 or 2,   characterized in that the control and servo means comprise a switching power supply adapted to be non-polluting on the electromagnetic plane and which comprises: - a resonant circuit (CO) placed in series between the battery (B) and said parallel branches (EB), the resonant circuit (CO) comprising a cell consisting of a capacitor (C1l) in parallel with a diode (D1), and an inductance (Li) in series of the cell (C1, D1); - a switch (TI) in parallel with said cell (C1, D1) of the resonant circuit (CO); - rectifying and filtering means, consisting of a diode (D2) and a capacitor (C2) in parallel, placed at the output of the resonant circuit; and - switch control means (TI) which comprise: * a low-pass filter (R1, C3) placed at the output of the resonant circuit (CO); * a voltage controlled oscillator (OS) whose input is connected to the output of the low pass filter; and * a monostable (M1) whose input is connected to the output of the voltage controlled oscillator and whose output controls the switch (TI). 4. Electric circuit according to any one of claims 1 to 3, characterized in that some of the branches (EB) also include a controlled switch (T) in series with their light-emitting diode (D) and their resistive means (R).   5. Electrical circuit according to any one of the claims   above, characterized in that the control and servo means (CO, CR, OS, M1, TI, R1, C3) include a   switching power supply (CO, CR, OS, M1, TI).   6. Electric circuit according to any one of claims   above, characterized in that the control and servo means (CO, CR, OS, Ml, TI, Ri, C3) comprise a resonant circuit (CO) of which an element (Cl) is in parallel with a cut-off switch (TI) able to short-circuit this element (Cl), as well as control means (CR, OS, M1, Ri, C3) of the cut-off switch (TI),   connected to one end of the branches (EB).   7. Electric circuit according to the preceding claim, characterized in that the control means (CR, OS, M1, R1, C3) comprise a monostable (Ml), one output of which is connected to the cut-off switch (TI), an oscillator (OS) whose output is connected to a monostable control input (M1), and whose input is connected to a end of branches (EB).   8. Electric circuit according to claim 7, characterized in that   the oscillator (OS) is a square oscillator.   9. Electric circuit according to any one of claims 6 to 8,   characterized in that the control and servo-control means (CO, CR, OS, M1, TI, R1, C3) include rectification means (CR, D2,   C2) of a voltage at the output of the resonant circuit (CO).   10. Electrical circuit according to any one of claims 7 to   9, characterized in that the input of the oscillator (OS) is connected to one end of the branches (EB) by means of a low-pass filter (R1, C3).   11. Electrical circuit according to any one of the claims   above, characterized in that the resistive means (R) are   sized to dissipate each a maximum power of 0.1 watt.   12. Electric circuit according to any one of claims   previous, characterized in that the controlled switch (T) is a   CMOS type semiconductor switch in 5 volt technology.