FR2786652A1 - Electronic circuit for supplying power to light-emitting diode from grid, comprising rectifier, inductor, diode, transistor as commutator and control circuit - Google Patents

Abstract

The proposed circuit is for supplying power to an electroluminescent or light-emitting diode from a.c. power grid, and comprises a rectifier circuit (D2) in series with a resistor (R2), an inductor (L1) connected in series with the light-emitting diode (D1), a diode (D3), a transistor (T1), a resistor (R1), and a control circuit (26) connected to the gate of transistor. The control circuit (26) comprises a bistable circuit (28) with analog inputs (THR,TRG), direct digital output (q) and analog output (DIS), a Zener diode (Z1) in parallel with a capacitor (C2), a diode (D4) in series with a resistor (R4) connected to node A, a capacitor (C1), and a resistor (R3) between node A and analog output (DIS). The bistable circuit (28), commercially available as TS555, contains a chain of resistors for voltage division to two operational amplifiers with outputs connected to inputs of a balanced circuit, and a bipolar transistor in the line of inverted output. The transistor (T1) is of MOS (metal-oxide-semiconductor) type and acts as a commutator at given frequency and in cyclic mode, as controlled by the bistable circuit (28). The bistable circuit (28) is in active state in connection with first reference voltage, and the voltage on capacitor (C1) via the resistor (R3); it is in passive state in connection with second reference voltage, and the voltage on capacitor via the resistor (R4). The circuit comprises additional components to ensure that a current flows through the resistor (R4) only when the bistable circuit (28) is in active state. The direct output (q) of the bistable circuit (28) is connected to a blocking diode (D4) in series with the resistor (R4) to force a current to the capacitor (C1).

Description

POWER SUPPLY OF A LIGHT EMITTING DIODE

  The present invention relates to the field of circuits

  supply of an indicator light with an alternating voltage.

  The invention relates more particularly to a power supply circuit.

  tion of an indicator light consisting of a light emitting diode-

cente, operating on the sector.

  Figure 1 shows a classic example of a

  stage intended to supply a load 18 with an alternating voltage

  tive Vin by associating the powering of the load 18 with the lighting of an indicator light 12. The indicator light 12 is part of a supply circuit 10 of the type to which the present invention applies. The assembly receives the alternating voltage Vin between two supply terminals 14 and 16. Terminal 14 is connected, by means of a switch 22 controlling the supply of load 18, between a first terminal 19 of load 18 and

terminal 16.

  The circuit 10 includes a voltage lowering resistor 20 in series with the indicator light 12, and, in this example, this association in series is connected in parallel with the load 18. By way of example, the load 18 can be an electrical appliance, the switch 22 a switch for putting this appliance into operation and the indicator light 12 a warning light

Operating.

  Generally, the indicator light 12 is a small neon tube, which conventionally sounds like 1 mA at a voltage of 70 V. If the voltage Vin is the sector of 220 V / 50 Hz, we

  may use a resistance 20 of the order of 150 k.Q.

  There are several drawbacks to using a neon tube. First of all, the internal elements of a neon tube undergo, in operation, erosion due to the gaseous plasma, and the tube consequently has a rather short lifespan. In addition, a neon tube, however small, is relatively bulky and it is sometimes difficult to integrate into a small device. In addition, the light emitted by a neon tube is not naturally directive, and it may be necessary to add a reflector to the

  tube, which increases its size.

  To avoid these drawbacks, consideration has been given to replacing the neon tube with a light-emitting diode, which has a

  extended service life, takes up little space, and produces

directive.

  However, a light-emitting diode requires, to operate, at least 10 mA at a voltage of the order of 1 V (or lower). Thus, if in Figure 1 the indicator light 12

  is a light emitting diode, a resistor must be used

  dissipative tance 20 of the order of 20 kD which will dissipate a power approximately ten times higher than in the case of the tube

  neon. Such power dissipation can affect the proper functioning of

  operation of the indicator light supply circuit,

  especially when confined in a small space.

  An object of the present invention is to provide a

  power supply circuit for a light-emitting diode

  nant on the sector, of small size and dissipating a reduced power. To achieve this object, the present invention provides a circuit for supplying an indicator light with an alternating voltage, in which the indicator light consists of a light-emitting diode, and in which the circuit comprises means for supplying the light-emitting diode to go

  of voltage pulses taken from the alternating voltage.

  According to an embodiment of the present invention,

  the voltage pulses have a frequency and a cyclic ratio

  than fixed. According to an embodiment of the present invention, the power supply circuit of a light indicator comprises means for rectifying the alternating voltage intended to supply the

light emitting diode.

  According to an embodiment of the present invention, the light emitting diode is connected in series with an inductor and a switch, a freewheeling diode being connected in parallel with the light emitting diode and the inductor. According to one embodiment of the present invention, the power supply circuit of a light indicator comprises means for opening and closing the conimmutator at said frequency and

with said duty cycle.

  According to an embodiment of the present invention, the means for controlling the switch comprise a bistable connected to close and open the switch when it is

activated and inactive respectively.

  According to an embodiment of the present invention, the bistable is activated by a first circuit comparing a first reference voltage and the charging voltage of a capacitor through a first resistor, and the bistable is activated by a second circuit comparing a second reference voltage and the charging voltage of the capacitor across

a second resistance.

  According to an embodiment of the present invention, the power supply circuit of a light indicator comprises

  means for a current to pass through the second resistor only-

  ment when the bistable is activated.

  According to an embodiment of the present invention, the capacitor is connected to the direct output of the bistable by the association in series of the second resistor and a blocking diode connected so as to force a current in the second

  resistance of the direct output from the bistable to the capacitor.

  These and other objects, features and advantages of the present invention will be discussed in detail in

  the following description of particular embodiments

  made without limitation in relation to the attached figures, in which: FIG. 1, previously described, represents a simplified diagram of an assembly provided with a power supply circuit of an indicator light according to the state of the art ; FIG. 2 represents a simplified diagram of an assembly provided with a power supply circuit of an indicator light according to an embodiment of the present invention;

  FIG. 3 shows in more detail a diagram of the circuit

  baked power supply of an indicator light of Figure 2; Figure 4 shows a diagram of the control means of Figure 3; and FIG. 5 illustrates, in the form of chronograms, the

  operation of the circuit of figure 3.

  The same references designate the same elements in

different figures.

  A feature of the present invention is to provide

  lie a light emitting diode constituting an indicator light, from voltage pulses taken from a

  alternating voltage, for example the mains.

  Preferably, the light-emitting diode is supplied by means of a current produced by an inductor subjected to these voltage pulses so as to avoid flickering of the

diode.

  FIG. 2 represents an assembly 11 comprising, as in FIG. 1, a load 18 receiving via a

  switch 22 a voltage Vin applied to supply terminals

  tion 14 and 16. A circuit 24 according to the invention for supplying an indicator light Dl is, for example, connected to terminals 19 and 16 of the load 18. The supply circuit 24 comprises a

  light emitting diode Dl connected in series with an induc-

  tance L1 and a switch Tl, between a terminal 25 and the supply terminal 16, which serves as a reference terminal. A rectifier circuit D2 is connected between terminals 19 and 25 to supply, between terminals 25 and 16, a rectified voltage Vr. The voltage Vr corresponds to the voltage across the load 18, rectified. A freewheeling diode D3 is connected in parallel with the diode Dl and the inductor L1. The circuit 24 further comprises a control means 26, the low and high supply terminals of which are connected, respectively, to the terminal 16 and, by means of a voltage lowering resistor R1, to the terminal 25 An output of the control means 26 is connected to a control terminal of the switch Tl, here the grid of a

MOS type transistor.

  When powered, that is, when the commu-

  guard 22 is closed, the control means 26 controls the

Tl pulse switch.

  Preferably, these pulses are of fixed frequency and duty cycle so as to simplify the constitution of the

circuit 24.

  When the switch T1 is closed, the freewheeling diode D3 is blocked and the inductance L1 is subjected to the voltage Vr. When the switch T1 is open, the freewheeling diode D3 conducts and the inductance Ll acts as a current generator. The diode Dl is supplied by the uninterrupted current which flows through

  inductance Ll. The frequency and form factor of the pulses

  control elements of the switch Tl determine the power transmitted to the inductor L1, and the minimum and maximum values

  of the current flowing through the light-emitting diode D1.

  Figure 3 shows in more detail the supply circuit.

  ment 24 of FIG. 2 according to an embodiment of the present invention. A low R2 protective resistance

  value (for example of the order of less than 1 k & 2) is, preferably

  rence, placed in series with the rectifying circuit D2.

  The control circuit 26 comprises a bistable circuit 28 comprising an analog activation input THR, an analog inactivation input TRG, a digital Q output (direct output) and an analog output DIS. The low supply terminal Gnd of the bistable circuit 28 is connected to the reference terminal 16. The high supply terminal V + of the bistable circuit 28

  is connected to a terminal 29, connected to terminal 25 by the resistor

  power supply R1. A capacitor C2 is connected in parallel with a Zener diode Zl between terminals 16 and 29. The inputs TRG and THR of circuit 28 are connected together to a first terminal A of a capacitor C1, a second terminal of which is connected to the terminal of reference 16. The terminal A is inter alia connected to the output DIS of the bistable 28 by a resistor R3. The cathode of a diode D4 the anode of which is connected to the output Q of the bistable 28 is connected to the terminal A by means of a resistor R4. The output Q of the bistable 28 is also connected to the gate of the switch T1. The capacitor C2 and the Zener diode Z1 provide, when the circuit 24 is supplied, a voltage

constant at the bistable circuit 28.

  FIG. 4 represents a diagram of an embodiment

  tion of the circuit 28 of FIG. 3 which includes a flip-flop RS 30 whose activation S and inactivation R terminals receive

  the outputs of a first and a second amplifier respectively

  operational AMP1 and AMP2. A chain of resistors R5, R6, R7 connected in series between the high V + and low Gnd supply terminals produce low V1 and high V2 reference voltages respectively taken between the resistor R6 and the resistor R7 and between the resistor R5 and the resistance R6. The voltage Vl is supplied to the positive input of the amplifier

  AMP1 and voltage V2 is supplied to the negative input of the amplifier

  AMP2 cracker. The activation terminal THR is connected to the negative input of the amplifier AMP1 and the inactivation terminal TRG connected to the positive input of the amplifier AMP2. In this embodiment, the flip-flop 30 is supplied between the terminals V + and Gnd. Thus, the high and low logic levels of the outputs Q and Q of the flip-flop 30 are respectively the voltages of the terminals V + and Gnd. Finally, a bipolar transistor T2 of the NPN type, the emitter of which is connected to the terminal Gnd, is connected by its base to the output Q of the flip-flop 30 and by its collector to the terminal DIS of the circuit 28. When the input d activation THR receives a voltage lower than the low reference voltage V1, the amplifier

  AMPi activates the bistable circuit 28. When the inactiva-

  tion TRG receives a voltage higher than the reference voltage

  high V2, the amplifier AMP2 deactivates the bistable circuit 28.

  The resistors R5, R6, R7 are, for example, chosen such that V1 = 1/3 and V2 = 2/3 of the voltage between the terminals V + and Gnd. The output DIS is substantially at the potential of the terminal Gnd when the bistable is inactivated, and it is set to high impedance

when the bistable is activated.

  As a particular embodiment, a circuit

  cooked 28 of the invention may consist of a known circuit

  under the trade name TS555.

  FIG. 5 represents, in the form of timing diagrams, two half-waves of a full-wave rectified voltage Vr and the corresponding development of the voltage VA of terminal A,

  as well as the voltages of the terminals S, R, Q, Q of the flip-flop 30.

  FIG. 5 also illustrates the corresponding development of the voltages VDIS and VL1, respectively, at the terminal DIS

and across the inductor L1.

  The voltage VR is established at a time t1, corresponding

  for example when the switch 22 of FIG. 2 is closed.

  After a short set-up time, the bistable circuit 28 is supplied and, the voltage VA being less than the voltage Vi, the

  flip-flop 30 receives an activation signal S at the output of the amplifier

  AMPi ficitor. The Q output of flip-flop 30 is at a high logic level (bistable activated), that is to say the voltage of terminal 29, and the Q output at a low logic level, that is to say the voltage of reference terminal 16. The transistor T2 is open, no current flows through the resistor R3, and the output terminal DIS of the circuit 28 has a voltage equal to the

  VA voltage. On the other hand, the transistor Tl is closed and the induc-

  tance Li is subjected to the voltage Vr. The terminal Q being at a high voltage, the diode D4 is in conduction and the capacitor Cl is charged through the resistor R4.

  When at an instant t2, the voltage VA exceeds the voltage

  sion V1, the amplifier AMP1 ceases to be activated and the activation command S is inactive. The state of flip-flop 30 remains the same. The voltage VA increases until an instant t3 at which it reaches the second voltage V2. When VA reaches the value V2, the amplifier AMP2 activates the inactivation terminal R of the flip-flop 30. The output Q is then inactivated (bistable inactive), which opens the transistor Tl. At the same time, the output Q is activated which closes transistor T2 and brings terminal DIS to

  the voltage of the reference terminal. Diode D4 is then blocked

  quée, and the capacitor Cl is discharged through the resistor R3. The voltage VA thus decreases until it reaches the first voltage V1 at a time t2 'following, which causes a new activation of the flip-flop 30 until a time t3' next, that is to say the start of a new charge cycle and

discharge of the capacitor Ci.

  It will be noted that the charge and discharge period of the capacitor Ci is equal to the sum of the time constants R3C1 and R4C1, the time constant R4C1 essentially fixing the

  charging time and the time constant R3C1 fixing essential-

  the discharge time. Likewise, the duty cycle of the voltage pulses supplied to the inductor L1 is equal to R4 / (R3 + R4). The values of resistors R3 and R4 thus allow

  to adjust the value of the current which will cross the inductance L1.

  The values of the voltages Vl and V2 can also allow

  to act on the frequency of the voltage pulses.

  The current range (between 10 and 25 mA) acceptable for a light-emitting diode is perfectly compatible with the fact of fixing by manufacture the frequency and the duty cycle of the pulses. This simplifies the circuit compared to a current regulating circuit. The desired duty cycle is preferably low due to the differences between the supply voltage of the inductor and the voltage across the light emitting diode. An alternative embodiment, which makes it possible to obtain a duty cycle of the order of one percent, consists in connecting one of the terminals of the resistor R4 to the high supply terminal V +. However, such a solution

  leads to a strong consonation in resistance R4. Therefore

  quent, we prefer the embodiment described in connection with Figure 3, wherein the connection of the resistor R4

  and diode D4 between the terminals DIS and A minimizes the dissipation

  tion in resistance R4. It will also be noted that the low consumption of a circuit 28 of the invention (of the order of 300mA)

  in front of that of the diode Dl (0.3mA) allows the use of a resistor

  tance Ri of high value (close to 200k.) to lower the voltage without generating strong dissipation. It will be noted that FIG. 5 describes an operation with a full-wave rectified voltage, for example obtained with a diode bridge as a rectifying circuit 12, but a person skilled in the art will easily adapt the present invention to a rectified voltage by

  simple alternation. Likewise, the control circuit 26 of the switching

  tor Ti described in relation to Figures 2 to 5 can be

  replaced by a conventional width modulation circuit

  drive (PWM). The duty cycle of the control pulses of the switch Tl will then be adjusted so as to smooth the voltage

supplied to the inductor Ll.

  Finally, the circuit shown in Figure 3 could be

  realized by discrete components, or integrated.

Claims (9)

REVENDICATICOS   1. Circuit (24) for supplying an indicator light with an alternating voltage (Vin) characterized in that the indicator light is constituted by a light-emitting diode (D1), and in that the circuit comprises means (26) to power the light emitting diode from pulses of pre-voltage on the alternating voltage.   2. Circuit according to claim 1, characterized in that the voltage pulses have a frequency and a ratio cyclic fixed.   3. Circuit according to claim 1 or 2, characterized in that it comprises means (D2) for rectifying the voltage   alternative (Wine) intended to supply the light-emitting diode- cente (Dl).   4. Circuit according to claim 3, characterized in that the light-emitting diode is connected in series with an inductor (Li) and a switch (Tl), a freewheeling diode   (D3) being connected in parallel with the light emitting diode- cente and inductance.   5. Circuit according to claim 4, characterized in that it comprises means for opening and closing the switch   (Tl) at said frequency and with said duty cycle.   6. Circuit according to claim 5, characterized in that the means for controlling the switch (T1) comprise a   bistable (28) connected to close and open the switch when   that it is activated and inactive respectively.   7. Circuit according to claim 6, characterized in that the bistable (28) is activated by a first circuit comparing a first reference voltage and the charging voltage of a capacitor (Cl) through a first resistor (R3), and in that the bistable is inactivated by a second circuit comparing a second reference voltage and the charging voltage of the   capacitor through a second resistor (R4).   lZ 8. Circuit according to claim 7, characterized in that it comprises means so that a current crosses the second resistor (R4) only when the bistable (28) is activated. 9. Circuit according to claim 8, characterized in that the capacitor is connected to the direct output of the bistable by the association in series of the second resistor and a diode of   blocking (D4) connected so as to force a current in the second resistor from the direct output of the bistable to the capacitor.