The electronic ballast for a fluorescent lamp available on the market consists of a filter, a rectifier, a frequency converter and a general output circuit 1, as shown in Fig. 1. The output circuit 1, in addition to tubes 11, includes a resonance circuit composed of ca- pacitors Cl, C2 and an inductor L, a pair of lamp tubes which serve as a preheater and a starter and are connected in parallel to a thermistor RT and a capacitor C2. The reso- nance circuit provides an operation voltage approximate to sine wave for the lamp tube, and the inductor L in this circuit also serves as a ballast. Two filaments or electrodes rl and r2 of the tube 11 are made of twin-screw tungsten wire, the middle portion of the lamp tube is coated with phosphor powder G, both ends of the lamp tube are supported and fixed by side bars M, as shown in Fig. 2 (Fig. 2 is an amplified diagram for the structure of one of the two electrodes). Tungsten filament H has a resistance property and the phosphor powder G is capable of transmitting electrons under certain tempera- ture.

When the above output circuit is working, transient high frequency current il, after passing through capacitor Cl and when entering electrode rl of the lamp tube, is di- vided into three paths of current i2, i3 and i4, and then is converged to pass a circuit formed by inductor L. Current i2 flows through lamp tube 11 and is the maximum sub- current necessary for keeping the lamp tube 11 to discharge ; current i3 flows through capacitor C2, its function is to constantly heat electrodes rl and r2 auxiliarily; current i4 has a relatively small current value at normal lighting and generally can be ignored for analysis. The lamp tube 11, when lighted, will generate a voltage drop at both ends since the current flows through electrodes rl and r2, at the point E of the tungsten fila- ment H, there exists a maximum positive potential or negative potential, point E is likely to be bombed by both positive and negative ions existing quantitatively in the discharge space, thus forming a substantially fixed heat point ; point F is at the edge of phosphor powder G and closest to point E. Thus, in the entire coating layer of phosphor powder, point F has a highest temperature, thus forming an electronic transmission point. When the heat of the heat point (point E) raises the temperature of point F high enough to make the electrodes of the lamp tube capable of transmitting, the space in vicinity of the electrode transmitting point of normal activity will form an electron cloud layer. At this time, the electrode has the lowest potential drop, if the temperature is not high enough, there is no electronic cloud layer, and the electrode potential drop rises, making the bombard energy of the ions grater, thus resulting in the temperature rise of the heat point until the transmitting point forms an electronic cloud layer, this is a process of automatic balancing. After the transmitting point forms an electronic cloud and the heat point stabilizes, if the capacitance of capacitor C2 is relatively large, the current flowing through the filament is also large, thus making the temperature of the transmitting point rising too high. It is shown by experiments that the current i3 flow- ing through capacitor C2 is an important reason to accelerate the consumption of phos- phor powder G. When the tube is lit, current i3 forms a voltage drop on electrodes rl and r2, making to form a potential difference between the transmitting point and the heat point. Along with the removal of the transmitting point, this potential difference becomes greater, so does the bombard energy of the ions, thus making the temperature of the filament rising and hence accelerating the evaporation and sputtering consump- tion of the phosphor powder. In addition, unnecessary power consumption is also in- creased and the emitting efficiency of the lamp is lowered. What is more serious is that current i3 is a multi-peak wave-form, as shown in Fig. 3. When this current flows through electrodes rl and r2, it brings about a multi-peak voltage fluctuation, thus making the transmitting point bombarded more strongly by the ions, the phosphor pow- der seriously sputtering and further accelerating the consumption of the phosphor pow- der, and hence shortens the life time of the lamp tube.

The phosphor powder is continuously consumed because the powder G at the transmit- ting point of the filament is frequently sputtered and evaporated due to bombardment when the lamp tube is lit, the position of the transmitting point also moves gradually away from the heat point along with the consumption of the phosphor powder. When the phosphor powder on the filament is exhausted, the lamp tube will stop working and its life terminates. So, the consumption speed of the phosphor powder is directly related to the life time and emitting efficiency of the lamp tube.

An object of the present invention is to overcome the above defects and to provide an output circuit of the electronic ballast for a fluorescent lamp for controlling the con- sumption speed of the phosphor powder and for increasing the life time and emitting efficiency of the lamp tube.

To realize the above object, the output circuit of the electronic ballast for a fluorescent lamp according to this invention comprises lamp tubes, a capacitor, inductor and a thermistor, two leads of said capacitor and inductor are respectively connected to the two output terminals of the frequency converter of the electronic ballast and one of the pins of the two electrodes at one side of the lamp tube, two leads of said thermistor are connected to pins of two electrodes at the other side of the tube and in that said output circuit further comprising a second capacitor, its two leads are each connected to the two electrode pins of the tube in a manner of connection to capacitor and inductor.

In the output circuit of the electronic ballast for fluorescent lamp according to this in- vention, said second capacitor has a capacitance of 1000 to 30000 pF.

The output circuit of the electronic ballast for fluorescent lamp according to this inven- tion further comprises a third capacitor, which is connected to the two electrode pins of the lamp tube in a manner of connection in parallel to the thermistor, its capacitance is equal to or less than 2000 pF.

The output circuit of the electronic ballast for fluorescent lamp according to this inven- tion, due to the addition of a second capacitor at the electrode pins on the bases of lamp tube, capacitor, inductor and thermistor, can decrease the current flowing through the filament and eliminate irregular peak voltage wave form generated on the filament, making the phosphor powder consumed more slowly and hence serving to increase the life time and emitting efficiency of the lamp tube.

In order to comprehend the object, features and advantages of this invention more clearly, the preferred embodiments of this invention will be explained in detail in con- junction with the accompanying drawings.

Fig. 1 is a diagram showing the prior art output circuit of an electronic ballast for a fluorescent lamp; Fig. 2 is an amplified view of a structure of an electrode of a lamp tube of one of the filaments; Fig. 3 is a diagram of the current wave-form of the current flowing through a resonant capacitor and the tube when the fluorescent lamp of Fig. 1 is lit ; Fig. 4 is a diagram showing the circuit of a preferred embodiment of this in- vention; Fig. 5 is a diagram showing the circuit of another preferred embodiment of this invention.

Referring to Fig. 3, there is shown an oscillogram of the current passing through the electrodes rl and r2 of the tube 11 of the circuit according to Fig. 1. This current of the tube 11 when lighted shows a multi-peak wave form.

Please refer to Fig. 4, this diagram shows a preferred embodiment of a modified output circuit 2 of the electronic ballast for a fluorescent lamp, comprising lamp tube 21, ca- pacitor Cl, inductor L, thermistor RT and second capacitor C3. Two leads of said ca- pacitor Cl are respectively connected to the frequency converter of the electronic bal- last, i. e., an output end of the frequency converter after the filter and rectifier and pin A of an electrode rl at the side of the lamp tube 21, two leads of said inductor L are re- spectively connected to the other output end of the frequency converter of the electronic ballast and to pin C of an electrode r2 at the side of the lamp tube 21, two leads of said thermistor RT are respectively connected to pins B, D of two electrodes rl, r2 at the other side of the tube 21, said second capacitor C3 has a capacitance of 1000-30000 pF, its two leads are respectively connected to the two electrode pins A, C of the tube in a manner of connection to capacitor Cl and inductor L. The second capacitor C3 is established for forming a resonance circuit, which will avoid producing an irregular voltage wave-form with peaks on electrodes rl and r2 (this is because that the original capacitor C2 is cancelled, so current i3 in the output circuit 1 of the prior art will not exist), at the same time, the voltage drop on rl, r2 of tube 21 will also decrease, thus, the consumption of the phosphor powder will be decreased when the lamp tube 21 is working, and the consumption of energy will also be decreased so that the life time and emitting efficiency will be increased. Capacitor C3 is a polyester medium capacitor which is adopted to withstand 1000 V.

When the above output circuit 2 is used, an energy-saving lamp is taken as the lamp tube 21, after the lamp tube is lit for 2000 hours, observation and test show that the phosphor powder is consumed very little, the transmitting point moves very little, the glass wall of the fluorescent lamp blackens slightly, and the maximum value of the tem- perature rise for the heat point is only about 80 °C.

When output circuit 1 of the prior art as shown in Fig. 1 is used, under the same lamp tube power and lighting condition, observation and test show that the phosphor powder has been consumed seriously, the transmitting point moves greater relatively, the glass wall near the electrode is deposited with the phosphor powder sputtered and blackens seriously, the temperature of the heat point on the cathodes raises by about 200 °C.

According to the observation and test of the above two output circuits and the compari- son of the life time and emitting efficiency of the lamp, the differences lies in that the output circuit 2 of the electronic ballast for the fluorescent lamp according to this in- vention can increase more than 20 % of life time over the output circuit 1 of the prior art, and the emitting efficiency of the lamp tube can increase 5 %.

Please refer to Fig. 5, which shows output circuit 3 of the electronic ballast for the modified fluorescent lamp of another preferred embodiment according to this invention.

In addition to all the modules and connection modes of output circuit 2 for the above embodiment, this output circuit also includes a third capacitor C2\', which is connected to the two cathode pins B, D lamp tube 21 in a manner of connection in parallel to thermistor RT, its capacitance is equal to or less than 2000 pF. The third capacitor C2\' used here is different from that in the output circuit of the prior art, the capacitance of the original capacitor C2 having been greatly decreased, thus, the value of current i3 flowing through the filament does not work, so the irregular peak voltage wave form will not be generated on the filament, thus serving to increase the life time and emitting efficiency of the lamp.