GB2101820A - Starter circuit for a fluorescent tube lamp - Google Patents

Abstract

A starter circuit for a fluorescent strip lamp having preheated electrodes has an SCR which is fired at progressively later times in successive half cycles of the a.c. supply mains. The SCR is connected across the d.c. diagonal of a rectifier bridge (D1-D4), the a.c. diagonal of which is connected between the tube electrode heaters. The SCR is fired by a pulse from a diac D connected from the junction of a resistor and a capacitor R3, C3 connected in series across the SCR voltage. The charging current for the capacitor is reduced by a transistor T1, in parallel with the capacitor, and the conductivity of the transistor, which is determined by a CR circuit connected to its base, is progressively increased thereby delaying the time of the firing pulse. Ultimately, if the lamp fails to ignite, the transistor draws sufficient current to prevent the triggering of the diac. <IMAGE>

Description

SPECIFICATION Starter circuit for a fluorescent tube lamp This invention relates to a starter circuit for a fluorescent tube lamp having preheated electrodes.

Starter circuits are provided for fluorescent tube lamps in order to enable the electrodes to be preheated and when sufficient preheating has been effected the circuit should enable a high striking voltage to be applied across the lamp to start the discharge. The circuit should then revert to a quiescent state until the lamp is turned off and on again. A particular difficult arises in the operation of 125W 8' fluorescent tubes in that the running voltage of the tube is close to the mains supply voltage, so that many previously proposed starter circuits have failed to distinguish reliably between a lamp that has struck and the one that has not with the result that the starter circuit continues to try and strike the lamp even though it is already struck.

Another problem in the design of starter circuits is that they should preferably be designed for manufacture using components having a relatively wide tolerance and still provide reliable operation.

An object of the present invention is to provide an improved starter circuit for fluorescent tube lamp in which the above desiderata are taken into consideration.

According to the present invention there is provided a starter circuit for a fluorescent tube lamp having preheated electrodes for operation by an alternating current supply through a ballast circuit including an inductor, the starter circuit being connectible between heaters for the electrodes in a series path being arranged after switching on initially to pass an energising current for the heaters and then to open-circuit the connection between the heaters to establish a striking potential between the electrodes, wherein the starter circuit includes a semiconductor control rectifier (SCR) shunted by a capacitor, the conductivity of which SCR determines whether the starter circuit present an open circuit or a closed circuit, a timing circuit connected to apply firing pulses to the gate of SCR through a diac at times in cycles of the supply voltage depending on the timing circuit, and means connected to the timing circuit for progressively delaying the generation of firing pulses after switching on.

The starter circuit may include a full wave bridge rectifier connected between the heaters across one diagonal and havinvg the SCR connected across the other diagonal. The capacitor shunting the SCR may have a resistor connected in series with it to limit the rate of change of current through the SCR.

The timing circuit may include a capacitor charged through a resistor from the voltage across the SCR and the diac being connected from the junction of the capacitor and the resistor to the gate of the SCR to provide the firing pulses. The means for delaying the generation of firing pulses may be a circuit connected to draw a progressively increasing current from the capacitor of the timing circuit and this delaying circuit may include a transistor with an emitter or source resistor connected across the capacitor with a CR time constant circuit connected to the base or gate of the transistor, the time constant of the CR time constant circuit being of the order of 18 seconds.The function of the CR time constant circuit is to provide a voltage which causes the current drawn by the transistor to increase progressively, thus slowing down the rate of charge of the capacitor of the timing circuit and delaying the generation of the firing pulses to the gate of the SCR. After the expiry of about 2 seconds depending on the time constant, the current drawn by the transistor should become so large that the voltage on the capacitor of the timing circuit can no longer rise sufficiently to produce firing pulses through the diac, with the result if the tube is not struck by this time the starter circuit stops the application odf striking potentials to the tube.

The starter is particularly suited for use with 8' fluorescent tubes of 125W or 100W rating. For such a tube the ballast circuit should consist of an inductor and capacitor in series ot provide the necessary striking potential.

In order that the invention may be fully understood and readily carried into effect it will now be described with reference to the accompanying drawings, of which Figure 1 is a block diagram showing the connection of a starter circuit according to the present invention for striking a 125W 8' fluorescent tube; Figure 2 is a detailed circuit diagram of one example of a starter circuit according to the present invention intended for firing a 125W8'tube; Figures 3A, B, C and D show respectively the voltage waveforms at the points A, B and C with reference to the neutral terminal in Figure 1 and the current through the switch circuit;; Figure 4 shows the variation of the off-state voltage across the starter circuit plotted against the phase of the cycles of the mains voltage at which the SCR turns on, the upper trace showing a phase of about 100" and the lower trace a phase of 125 ; Figure 5shows a plot of the voltage across the terminals of the switch circuit showing the increase of voltage occurring following the switch-on of the mains supply and the fall when the tube strikes; Figure 6 shows the current waveform through the switch circuit plotted relative to the mains voltage waveform when a maximum current is flowing through the switch circuit; and Figure 7 illustrates the relationship between the holding current iH and the storage time tq of the SCR for reliable operation.

Figure 1 shows an alternating current mains supply connected to line and neutral terminals Land N of the fluorescent tube circuit. The tube T has heaters F1 and F2 for its filaments which are connected in series through terminals T1 and T2 of starter switch S. The heater F2 is connected to the terminal N and the heater F1 is connected to the terminal L through ballast inductors LB and ballast capacitor CB in series. For a 125W 8' tube the ballast inductors LB should have an inductance of 0.38 Henrys and the capacitor CB should have a capacitance of 7.2 uF.

Figure 2 shows the circuit of the starter switch S of Figure 1 and shows the terminals T1 and T2 connected across one diagonal of a diode rectifier bridge D1, D2, D3 and D4. A semiconductor control rectifier SCR is connected across the other diagonal of the diode bridge and is shunted by resistor R1 and capacitor C1 in series, resistor R2 and capacitor C2 in series. The junction of resistor R2 and capacitor C2 is connected via a diac D to the gate of the SCR which is connected through a resistor R5 to the cathode of the SCR. The junction of resistor R2 and capacitor C2 is also connected to the cathode of the SCR through the collector-emiter path of a transistor T1 in series with a resistor R4. The junction of the resistor R3 and the capacitor C2 is connected to the base of the transistor T1.The values of the components shown in Figure 2 for a starter circuit for a 125W 8' tube are as follows: D1, D2, D3, D4 1N4007 R1 1/4 W 5% 68R R2 1/4 W 5% 2.2M R3 1/4 W 5% 820k R4 1/4 W 5% 18k R5 1/4 W 5% 100k C2 4.7nF glow starter capacitor C2 10nF10%50V C3 22jlF 20% 16V electrolytic TI BC184 hFE > 100 Diac VBO = 32V + 4V iBO < 100 'iA SCR 800V, selected TIC106 TIC108 TIC116 Basically the starter shown in Figure 2 consists of a semiconductor controlled rectifier switch SCR connected across the recitified voltage terminals of a diode bridge D1, D2, D3 D4. The SCR turns on once every mains half cycle because of the full wave rectification.When the SCR is off a high voltage is developed across the starter terminals T1, T2 and it is this voltage which is used to strike the tube. When the SCR is conducting, the impedance between the starter switch terminals T1, T2 is low allowing 'pre-heat' current to flow through the tube filaments F1, F2 (Figure 1). The phase in the mains cycle at which the SCR switches on is progressively delayed relative to the last zero crossing from mains switch on with the result that the peak voltage developed across the starter terminals T1, T2 between successive switching cycles of the SCR increases. The phase at which the SCR triggers initially is chosen such that a sufficient striking voltage for the tube is not developed until the tube filaments F1, F2 have received a suitable pre-heat. The following describes the operation of the starter switch circuit in more detail.Reference will be made to Figure 3 for the ballast circuitry node voltages and current waveforms during one switching cycle of the starter.

C, acts as a snubber network rle capacitor limiting the rate of voltage rise dv/dt across the SCR at turn off. The SCR turns off when the current flowing through it falls below its holding current (iH). The rate of rise of voltage across the SCR is determined by the ringing frequency of C1 with the ballast choke LB. With typical values of C, the ringing is heavily damped by parallel resistances across the switch and disappears rapidly (see Figure 3). The high voltage peals produced by this ringing has an important role in causing the tube to strike. Resistor R1 limits the rate of change of current dl dt produced by the discharge of C1 through the SCR when the SCR turns on.

After the ringing of C1 with the ballast choke LB has died away, the voltage across the switch terminals T1, T2 when the SCR is off is essentially just the sum of the mains voltage and the voltage across the 7.2 uF ballast capacitor CB(the voltage across the inductor LB is approximately zero as I and dlidt are negligibly small). When the voitage across C2, which is charged up by the current flow through R2, is sufficient to break over the diac D, the voltage across the diac D snaps back and C2 partially discharges through the parallel combination of R6 and the SCR gate, thereby triggering the SCR into conduction. Transistor T1 sinks a progressively greater proportion of the current flow through R2 as C2 charges up via R3; the time constant C3R3 is about 18 seconds. Therefore the time taken for C2 to charge up the diac break-over voltage (VBo) increases from one switching cycle to the next. The result of this is that the switch SCR triggers at a progressively later point in the mains cycle with successive switching cycles. The later in the mains cycle that the SCR triggers, the higher is the current flow and the higher is the voltage developed across the ballast circuit capacitor CB. Therefdre the voltage across the starter terminals T1, T2 with the SCR off also increases (see Figure 4). The net result is that the peak voltage across the starter terminals T1, T2 increases cycle by cycle.This feature of the circuit operation ensures that the tube will not have a voltage applied across its terminals sufficient to strike it until the filaments have been adequately preheated.

Base drive for the transistor T1 is supplied through R3. The time integrated current through R3 gradually charges up C2. Therefore the current through the transistor T1 when the SCR is off [i.e. (VC3 - VBE) f R4] steadily increases. Note that when the SCR is conducting C2 decays through R2 and (through the forward biased pn junctions formed by the base-collector and base-emitter junctions of the transistor T1) R2 and R4.

C3. R4 however forms the shortest dominating time constant for the discharge of C3.

If the tube does not strike, as the transistor T1 sinks progressively more of the current through R2, eventually (after, say, 2 seconds) there will be insufficient current flow through the diac D for it to break over.

The SCR will no longer be triggered and the circuit will not operate again until the mains is switched offto allow C3 to discharge. The circuit therefore features a shut-down facility which prevents the starter continually trying to start a faulty tube or re-triggering off the running voltage of a tube connected across its terminals.

The value of the resistor R5 is selected to reduce the storage time (tq) and increase the holding current iH of the SCR to values which ensure that the SCR turns off after the first SCR main terminal current zero-crossing.

However it must remain large enough to ensure that the maximum permissible current through the diac D is not exceeded (otherwise the voltage across the diac D after breaking over becomes excessively low) and that the time constant for the discharge of C2 through R5 and the SCR gate is long enough for the SCR to be triggered.

The function of the diode bridge D1, D2, D3, D4 is not only to provide rectification of the voltage produced across the starter switch terminals T1, T2 but also to enable the starter to produce a striking voltage twice every mains cycle. This feature of the circuit makes the striking of the tube more reliable than it would be with a circuit triggering once every mains cycle.

Figure 5 shows the voltage developed across 125W tube from mains switch on until the tube turns on. The turn on time for the tube is approximately 0.8 seconds. The peak voltage developed across the starter terminals T1,T2 just before tube turn on is ~ 650 V. It is anticipated that an SCR with a forward breakdown voltage (DAM) of 800 V would be sufficient to strike a 125W tube reliably in the specified temperature of operation of the starter (i.e. -5 C to 45"C).

The pre-heat current through the filaments F1, F2 reaches a peak of = 4A (the choke core is heavily saturated in this condition and is responsible for the non-sinusoidal waveform of the current flowing through the circuit, see Figure 6) and an average rms of ~ 1A. A TO-220 packaged SCR proved to have sufficient power dissipation in practice to cope with this amount of current flow until the circuit shut down.

The capacitor C1 must limit the rate of change of voltage dV/dt across the SCR to significantly less than the turn on dV1dt of the SCR under operating conditions. The turn of dV/dt for a typicai SCR with RGK = 100 is = 300V/uS. A value of 4.7nF for C1 limits the dV/dt across the SCR to = 5V/uS. C1 should have a peak voltage rating of at least 1 kV, the maximum possible voltage developed across the starter (i.e. the maximum possible VDRM of the SCR used in the circuit). The resistor R1 must limit the dl/dt through the SCR at turn on to c100A/Lts. If R1 = 68Q dl/dt at turn on ~ 4OA'us.

Dissipation in R1 can be calculated as follows: The dominant dissipation in R1 occurs when the SCR turns on. The current through R1 is then given by

where V is the peak voltage developed across the starter just before the SCR turns on.

Therefore the integrated power developed across R1 in 100 switching cycles (the starter turns on twice in each mains cycle i.e. 100 times per second) is given by

= 50V2C1 if V = 1kV,C1=4.7NF PR, = 0.25W For the circuit to trigger mains volts it must be capable of producing a high enough current flow through R2 to produce a current exceeding the breakover current iBo through the diac D. With the transistor T1 off the maximum current flowing through the diac D is given by: (NB when Vc2 reaches VBo of the diac, dVC2/dt = 0 therefore iC2 = 0).

Vmjn (peak)VBO ioic(max) = V,in(peak)-Vso n2 where Vrnin (peak) is the peak amplitude of the mains voltage with a -6% mains variation. Form (1): iDIAc(max) = 1251lA for R2 = 2.2m + 5%. Therefore, jBO for the diac must be < 125 RA at any operating temperature.

Equation (1) shows that whether or not the starter will trigger off the mains voltage has little dependence on VBO and therefore a tolerance of + 4V on VBO has little significance in this respect. V50 does have an effect on the phase in the mains cycle at which the SCR turns on, however, and therefore will be one of the factors determining the peak voltage developed across the starter terminals just after mains switch on. With V50 in the range 28 to 36V this voltage will be between 400 and 500 volts. This range of starting values for the peak voltage across the tube has an unimportant effect ( 100/) on the pre-heat time.

The resistor R5 must be chosen to give an acceptable combination of storage time tq and holding current 1H for the 5CR as mentioned above. If lH is small enough for the (rectified) current through the SCR to be below WH for a time less than tq, the SCR will not turn off at the first zero crossing of the current. Figure 7 shows the acceptable range of values of tq for values of lah up to 5 mA. The shaded area indicates the effect of the choke LB and 7.2us ballast circuit capacitor CB value tolerances (+ 5% m each case).A selection criterion of H < 5.0 with RGK = 100Q fl 25 C should provide an acceptable combination of iH and tq.

The current drawn by the transistor T1 is approximately given by VC3-VBE = Vc3 -V55 R4 As the voltage across C3 increases, progressively mor e current is drawn through the transistor T1 and the voltage across the switch terminals steadily increases until the tube strikes. Therefore, the rate at which C3 charges up determines the pre-heat time for the tube. C3 continues to charge up after the tube has turned on thereby preventing re-triggering by the tube running \/voltage. If the tube does not strike, C3 still continues to charge up and the starter shuts down after = 2 secs.The dissipation across R3 after shutdown if the tube does not strike is given by:

V2mains(rms) = 0.1W R3 The input impedance of the transistor Tl must be sufficient to ensure that with it switched on, the discharge of C3 through the transistor is insignificant. If the current gain (hFE) of T1 = 100, T1 impedance is = 1.8M. Therefore, for hFE > 100, the discharge of C3 through T1 when T1 is on is negligible compared to the discharge through R4 (when T1 is off).

Generally the starter will not operate if the mains is turned off and then on again within a period of = 5 secs because of the time constant for the decay of the voltage across C3 (through R4). However, if the mains is turned off and on again very quickly (within 0.25 secs) the tube will in fact re-strike, ensuring that brief interruptions in the mains supply will not turn the tube off.

The operation of the starter is generally satisfactory in practice. Particularly useful features are the quietness in operation, the automatic shutdown if the tube does not strike, the lack of any serious problems related to component tolerances and the consistent and controllable pre-heat time for the tube filaments.

The circuit described is particularly suitable for use with a 125 watt tube and is not so suited to a 100 watt tube. The reason for the lack of 100 W tube compatibility is basically that a 100W tube needs a substantially higher voltage to strike it (i.e. of the order of = 800 V depending on the width of the striking pulse). The circuit described is capable of producing 800V across its terminals but only at the cost of a higher current flow and consequently noisier operation. It is possible to develop a high enough energy striking voltage pulse across the tube without an excessively noisy operation by increasing the period of the ringing (of the snubber network capacitor with the ballast choke) at SCR turn off by increasing C1. However, C, would then need to be increased to at least 0.1 yF, 800V and R1 would have to be a 3W resistor. It may be impossible to fit these components inside a standard starter can.

C1 could be made smaller if the VDRM of the SCR were increased to, say, 1000V but such a device is more expensive than an 800V one.

It has been found that the use of a starter according to the invention can prolong the life of a fluorescent tube lamp by increasing the number of strikes that the lamp can withstand without failure by a factor of between 10 and 30 depending on the type of prior art starters used. In addition a circuit according to the invention imposes a smaller voltage on the ballast than some other types of starter thus enhancing the life of the capacitor.

Claims (9)

1. A starter circuit for a fluorescent tube lamp having peheated electrodes for operation by an alternating current supply through a ballast circuit including an inductor, the starter circuit being connectible between heaters for the electrodes in a series path and being arranged after switching on initially to pass an energising current for the heaters and then to open-circuit the connection between the heaters to establish a striking potential between the electrodes, wherein the starter circuit includes a semiconductor controlled rectifier (SCR) shunted by a capacitor, the conductivity of which SCR determines whether the starter circuit presents an open circuit or a closed circuit, a timing circuit including a diac connected to apply firing pulses to the gate of SCR at times in cycles of the supply voltage, and means connected to the timing circuit for progressively delaying the generation of firing pulses after switching on.

2. A starter circuit according to claim 1 including a full wave bridge rectifier circuit of which the a.c.

diagonal is connectible between the electrode heaters and the d.c. diagonal is connected to the end terminals of the SCR, so that the rectifier circuit is conducting for a.c. when the SCR is conducting.

3. A starter circuit according to claim 2 wherein a resistor is connected in series with the capacitor shunting the SCR.

4. A starter circuit according to claim 3 wherein the timing circuit includes a second capacitor charged through a second resistor from the voltage across the SCR, the diac being connected from the junction of the second capacitor and the second resistor to the gate of the SCR, whereby the diac applies a firing pulse to the gate of the SCR when the voltage across the second capacitor reaches a threshold value.

5. A starter circuit according to claim 4 wherein the second capacitor is substantially discharged through the diac when it applies a firing pulse to the gate of the SCR.

6. A starter circuit according to claim 4 or 5 wherein the delaying means includes a circuit for drawing current from the junction of the second capacitor and the second resistor thereby to retard the rate of charge accumulation in the second capacitor, the current drawing circuit being such that the current drawn from the junction is increased progressively from the time of switching the circuit.

7. A starter circuit according to claim 6 wherein the current drawing circuit includes a transistor of which the controlled current path is connected in parallel with the second capacitor and the control electrode is connected to a third capacitor to respond to the voltage stored in it, there being provided means for feeding current to the third capacitor in response to the voltage across the SCR.

8. A starter circuit according to any preceding claim wherein the delaying means is such that the timing circuit is prevented from applying further firing pulses to the gate of the SCR after a predetermined time interval following switching.

9. A starter circuit for a fluorescent tube lamp having preheated electrodes substantially as described herein with reference to the accompanying drawings.