FR2460088A1 - IGNITION CIRCUIT FOR FLUORESCENT AND SIMILAR TUBES WITH PRELIMINARY HEATING OF FILAMENTS - Google Patents

Abstract

THE SUPPLY CIRCUIT IS USED FOR IGNITION OF FLUORESCENT TUBES, WITH FFT FILAMENTS OF ELECTRODES TO BE HEATED FOR IGNITION; THE HEATING CIRCUIT CARRIES THE FFT FILAMENTS OF THE TF TYPE IN SERIES AND IS DESIGNED TO CONTROL AN IC1 COMPARATOR USING A C6 CAPACITOR CONNECTED TO THE FILAMENTS THROUGH THE D4-D5 DIODES, THIS COMPARATOR IS INTENDED TO OPEN THE CIRCUIT OF HEATING FILAMENTS.

Description

Ignition circuit for fluorescent tubes and the like

  with preliminary heating of the filaments.

  The present invention relates to an ignition circuit having for object the realization of a rational sequence of operations for the ignition of a fluorescent tube. The fundamental characteristic of this circuit is the "soft-start",

  that is to say, the soft ignition of the tube.

  It has been found that the duration of a fluorescent tube depends to a large extent on how its filaments are lit. It usually happens that after a certain

  number of ignitions the tubes darken at their ends

your; this is caused by the evaporation of the metal from the filaments which, when the discharge occurs, undergoes the bombardment of the ions of the gas, which causes the thinning of the filaments. The filament heated by ion bombardment first and by the lamp current then begins to emit electrons, thus ionizing the surrounding space and creating a barrier which intercepts the ions of the discharge,

  preventing them from hitting metal with strong energy.

  Consequently, it is necessary to guarantee the thermionic emission even in the ignition phase and, to obtain this result, it is necessary to cause the filaments to emit

  electrons before ignition, which means that it has to be heated

iron the electrodes before applying sufficient voltage

for striking the arc.

  In addition, the ignition voltage must be applied to the electrodes gradually so that the ignition has

  place at the lowest possible voltage.

  The invention ensures the priming of the tube under conditions

  optimal heating of the filaments of the electrodes of the

  tube, regardless of the external conditions.

  According to the invention, a supply and ignition circuit for the tubes comprises heating control means, which are controlled by the variations in resistance of the filaments and which create the priming conditions as soon as a certain amount is exceeded. temperature, and therefore a resistance

this determined.

In practice, the circuit may include a heating circuit which, carrying the filaments of the tube in series, is designed to control a comparator using a capacitor connected to the filaments through diodes; this comparator is designed to open the filament heating circuit gradually, thus carrying out the "soft-start" This comparator can control a transistor to put it to rest as soon as a preset value of resistance, and therefore of temperature, is reached filaments; this transistor is associated with a diode bridge intended for gradual switching

from the heating position to that of priming and supplying

tion of the tube, with the help of a transformer. The circuit

  also includes a time function

sator, designed to exclude the high supply voltage and to put the control transistor to rest after a

  certain calibration time in case of non-ignition of the tube.

  The attached drawing represents a circuit diagram

according to the invention, reproduced by way of nonlimiting example.

  In this drawing is shown a first block 01 which represents an oscillator and is constituted by the transistors TR1, TR2 and by the diodes D1, D2, by the capacitors 03, 01, by the primaries 1-3 and 2-4 of a transformer T1 - which also includes the reaction winding 5-6 - by a capacitor 02 and by an inductor LT. Block 01 is an oscillator which provides a sinusoidal voltage on secondary 8-10 of transformer T1 and a high voltage,

also sinusoidal, on the strip which runs along the tube.

  Ia high voltage on the secondary 7-9, which is connected to an ST screen placed near the fluorescent tube TF, promotes its ignition. The secondary voltage 8-10 is the one going to the cathodes and it is necessary because the power supply

of a fluorescent tube must be done with alternating current.

the secondary 8-10 is part of a second block, which is further constituted by the fluorescent lamp or tube TF, by a bridge of diodes D3, D6, D7, D8 and by two diodes D4 and D5 acting as rectifiers. This second block is the lamp supply group; it is controlled by a third block in the diagram, which performs functions of

  control and whose fundamental components are a comparison

  HERE, capacitors C7 and C6 and transistors TR3 and TR4. The comparator I01 detects the voltage level on the capacitor C7, which charges via the resistor R7 when the main switch is closed, and on the capacitor 06, which has a voltage proportional to the resistance of the filaments FF? of the tube, in the circuit of which it is inserted together with the diodes D4 and D5. The

  two tensions on 07 and 06 are confronted in the comparison

  tor I01 and depending on whether the voltage on C6 is more or less

  high, output 6 of I01 will be high or low.

Depending on the state of this comparator IC1 output,

whether or not the transistor TR3 is in activity, which is in inter-

  face with the transistor TR4 and the pilot. The transistor TR4 acts as a switch of the circuit 2-4 of the FFT filaments, slowed down in its opening by R8 and 08, functioning as a damper, to put the tube, with the cathodes already

hot, under a gradually increasing tension.

  With the closing of the main switch,

  a current flow is established which, crossing the

  filaments 1-2 and diodes D7 and D3, closes on the

  secondary 8-10 passing through filaments 3-4. These fila-

  FFT elements are in fact short-circuited. Current heats the filaments passing through them and changes their resistance, which thus increases with temperature. This variation is detected (through diodes D4 and D5), which

  compares the voltage levels of capacitors C6 and 07.

  Thus, when the temperature and resistance values of the FFT filaments have reached a threshold corresponding to a sufficient temperature of the filaments, the transistor TR4 - via the transistor TR3 and the comparator IC1

controlled by C6 level to be put to rest -

open the bridge D3, D6, D7, D8 in such a way that the electrodes of the tube will have an increasing voltage

Until the point where it will be sufficient to initiate this tube.

  An LED-1 works as an indicator and lights up for

the tube pre-ignition phase.

  The assembly formed by the diode D10, the resistor R7 and

the capacitor 07 constitutes a timer, provided for -

  exclude high voltage and cause the TR4 to be interrupted for a certain time after closing the main switch;

  this also depends on the voltage of the external network.

A D9 diode provides protection against accidental reverse polarity. Indeed, if the polarity were reversed the diode D9 would enter into activity by bringing into play a safety mechanism which could, for example, melt a small section of track, thus disconnecting the remaining part of the circuit. Under these conditions, if he

an external fuse exists, the reversal blows it up.

  Ultimately, one obtains a heating of the filaments of the tube of a value always constant because it is the temperature reached by the filaments - independently of any other external influence - which causes itself

  switching the circuit to the supply position.

It is understood that the drawing represents only an exemplary embodiment, reproduced only as a practical demonstration of the invention, the latter being able to admit variations as regards the form and the arrangements, provided that these variations do not come out not

  of the framework of principles on which this invention is based.

For example, the TR4 organ can be another type of interrup-

  such as a blade relay under a bulb, a conventional relay

than or similar. In this case, it is possible to use

  directly an AC power supply.

Claims (5)

  1. Supply and ignition circuit for fluorescent tubes and the like comprising filaments of the electrodes to be heated for ignition, characterized in that means for controlling the heating of the filaments are controlled by the variations in resistance of the filaments, and thus create the conditions for gradual priming as soon as a certain temperature is exceeded and therefore a determined resistance. 2. Circuit according to claim 1, characterized in that it comprises a heating circuit carrying in series the FFT filaments of the tube, and that it is designed to control a comparator (ICi) using a capacitor (06) connected through diodes (D4-D5) to the filaments, this compa-   rator being provided to open the filament heating circuit.   3. Circuit according to claims 1 and 2, charac-   terrified by the fact that the comparator (HERE) drives a transis-   tor (TR4) to put it to rest as soon as a resistance value (and therefore temperature) of the pre-established filaments (FF!) is reached; and to this transistor (TR4) is associated a diode bridge (D3, D6, D7, D8) provided for switching from the heating position to that of ignition and supply   of the tube, with the help of a transformer (Tl).   4. Circuit according to claims 1, 2 and 3,   characterized by the fact that it comprises a device (Dl0 - R7 - 07) acting as a timer to exclude the high supply voltage and put the transistor to rest   (TR4) command after a certain calibration time.   5. Circuit according to any of the claims   tions above, characterized in that it comprises a device (R8 - 08) acting as a shock absorber for placing the tube, with the cathodes already heated, under a gradually increasing tension.