GB2301955A - Pre-heat circuit for a discharge lamp - Google Patents

Abstract

The pre-heat circuit 8 comprises a series circuit of a PTC thermistor and a SIDAC high voltage bilateral trigger device connected in series with the filaments 6, 7 of a fluorescent or other discharge lamp 9. A capacitor C3 may be connected in parallel with the series circuit of the PTC thermistor and the SIDAC. In operation, the voltage across capacitor C3 builds up to the breakover voltage of the SIDAC to provide a current path through the PTC thermistor to heat up the lamp filaments 6, 7. Heating of the PTC thermistor subsequently causes the voltage across the lamp 9 to increase sufficiently to strike the lamp. The voltage across the lamp then falls below the breakover voltage of the SIDAC which therefore blocks to prevent current flow through the thermistor during lamp operation, thus increasing efficiency. Additional components (10, Fig.4) - such as capacitors, resistors and/or inductors - may be connected in series with the SIDAC and the thermistor for ideal conditioning of the pre-heat circuit current and voltage levels. The values of such additional components may be varied according to lamp size/current, required pre-heat time, and the current and voltage ratings of the SIDAC and thermistor.

Description

IMPROVED PRE-HEAT CIRCUIT FOR ELECTRICAL DISCHARGE LAMP FIELD OF THE INVENTION This invention relates to pre-heat circuits for fluorescent lamps and other electrical discharge lamps.

TECHNICAL TERMS It is believed that the technical terms used in the specification will be understood by those to whom the specification is addressed. However, for the avoidance of doubt the following expressions have the following meanings: VMAX - the highest operating voltage that may be continuously applied to the thermistor at ambient temperature (250C).

VN - the rated voltage of the thermistor TRef - the temperature at which the steep rise in resistance starts IN - the rated amperage of the thermistor - ~ the switching current i.e. the current above which the thermistor goes into high resistance mode.

1R - the residual current at the applied voltage of VMAX RN - the rated resistance at ambient temperature.

RMIN - the minimum resistance at the reference temperature TRef TL - the lead solder temperature PTC - Positive Temperature Coefficient.

SUMMARY OF THE PRIOR ART With electronic adaptors and ballasts having rapid start operation the fluorescent tube can become so blackened around the filaments as to reduce the tube life and lumen output. This is due to cold emission from said filaments and to avoid this problem a pre-heat circuit is normally connected in series to the filaments. This circuit heats up the filaments before the high voltage is applied to initiate discharge. Two typical prior art heat circuits which operate similarly are shown in figures 1 and 2. Initially at start up the PTC thermistor has low resistance for about one second. During this period current flow through the tube filaments 1 and 2 heats them up. Meanwhile however the PTC thermistor also heats up due to current flow and increases its resistance.This then causes current flow to fall off and the voltage to increase sufficiently to initiate lamp discharge. While such known arrangement provides a workable solution it has the disadvantage that even during steady state operation of the lamp tube 3 the current flow through the PTC thermistor is never completely cut off. This results in wastage of energy (of the order of 1 to 2 watts), lower efficiency and reduced life of the adaptor circuit 4.

SUMMARY OF THE INVENTION It is therefore an object of this invention to ameliorate the aforementioned disadvantages and accordingly a pre-heat circuit for an electrical discharge lamp such as a fluorescent lamp is disclosed, said circuit comprising a PTC thermistor and SIDAC high voltage bilateral trigger connected in series with the lamp filaments.

BRIEF DESCRIPTION OF THE DRAWINGS The currently preferred form of the invention will now be described with reference to the schematic circuit diagram of figure 3. Figure 4 shows the possible insertion of additional components to achieve conditioning of current and voltage levels in the SIDAC-PTC thermistor branch circuit.

DESCRIPTION OF PREFERRED EMBODIMENT There is an electronic adaptor 5 of any suitable design supplying energy to the lamp tube 9 at an elevated frequency. The output is connected through fluorescent tube filaments 6 and 7 and a pre-heat circuit 8 comprising a serially connected SIDAC high voltage bilateral trigger and PTC thermistor such as those made respectively by Motorola and Siemens Matsushita under code numbers MKP9240 and B59872-C120-A70. Typical characteristics of such components are as shown below.

SIDAC HIGH VOLTAGE BILATERAL TRIGGER

RATING SYMBOL MKP9V120 MKP9V240 UNIT MKP9V130 MKP9V260 MKP9V270 Off-Stale Repetitive Voltage VDRM +1- 90 +/-180 Volts OState current RMS (LL=9.5mm.T@=80 C. IT(RMS) 0.9 Arnp 60 Hz Sine Wave. Conduction angle = 1800) On-State Surge Current (Non-Repetitive) @TSM 4 Amps (60 Hz one cycle Sine Wave, Peak Value) Operating Junction Temperature Range TJ 40 to +125 C Storage Temperature Range Tstg -40 to + 150 C Lead Solder Temperature - 230 C (Lead length > 1 .5mm from case.10 a max) THERMAL CHARACTERISTICS

Characteristic Symbol Unit Thermal Resistance, Junction to Lead ROJL 40 C/W (LL=9.5mm) PTC THERMISTOR

Type IN IS IS MAX @4 RN RMIN Ordering Code (V=VMAX) (V=VMAX) mA mA A mA # # VMAX=265V. VN=220V. TRef=120 C C 810 650 1300 10.0 25 26 1.6 B59810-C120-A70 C 830 460 920 7.0 20 3.7 24 B59830-C120-A70 C 840 330 660 4.1 15 6 3.8 B59840-C120-A70 C 850 200 400 2.2 13 10 64 B59850-C120-A70 C-860 140 280 1.5 10 15 9.0 B59860-C120-A70 C 870 100 200 1.0 9 25 15.0 B59870-Cl20-A70 C 872 80 160 1.0 9 35 21.0 B59872-C120A70 C 873 70 140 1.0 9 45 27.0 B59873-C120-A70 C 874 60 125 1.0 9 55 31 B59874-C120-A70 C 875 55 110 1.0 9 65 36 B59875-C120-A70 C 880 55 110 0.4 6 70 39 B59880-C120-A70 C 883 35 70 04 5 120 67 B59883-C120-A70 C 890 30 60 0.2 5 150 84 B59890-C120-A70 VMAX=420V. VN=380V. TRef=120 C C 884 21 39 0.2 3 600 340 B59884-C120-A70 VMAX=550V. VN=500V. TRef=110 C C 885 15 30 0.1 3 1200 675 B59885-C120-A70 C 886 12 24 0.1 2 1500 840 B59886-C120-A70 A capacitor C3 of 3.3 or 6.8 nF or other suitable value may be connected in parallel to the SIDAC-PTC branch. In operation of the circuit the voltage at start up which appears across the capacitor C3 builds up to the breakover voltage of the SIDAC whereby a current path is provided through low resistance PTC thermistor to heat up the tube filaments. During this warm up period which may typically be for 500-2000ms the voltage developed while initially exceeding the breakover voltage for the SIDAC is, due to the low resistance of the PTC thermistor, insufficient to initiate the discharge.As the temperature of the PTC thermistor increases due to current flow however its resistance increases which then reduces the current flow and increases the voltage applied across the tube 9 to values of the order of 300 to 800 volts. This is sufficient to cause the tube 9 with the pre-heated filaments to go into discharge mode. Because of the current flow through the discharging tube the voltage and thus current flow through the SIDAC-PTC thermistor branch then rapidly drops below the breakover voltage and holding current levels with the result that the SIDAC goes into blocking mode and isolates the PTC thermistor. Therefore during subsequent steady state operation of the lamp there is no power dissipated by the pre-heat circuit. This leads to greater efficiency and increased life for the adaptor circuit and tube.

To enable ideal conditioning of the branch current and voltage levels it is envisaged that additional components 10 such as capacitors, resistors and/or inductors may be added in series to the SIDAC and PTC thermistor as shown in figure 4. The specific values of such components would be varied according to lamp size (current), required delay time, and current and voltage ratings of the PTC and SIDAC.

It will thus be appreciated that this invention at least in the form of the embodiment described provides a novel and improved form of pre-heat circuit for electrical discharge lamps. Clearly however the example disclosed is only the currently preferred form of this invention and the specific values of the various components referred to may be varied according to application.

Claims (3)

1. An electrical discharge lamp such as a fluorescent lamp comprising at least one discharge light tube, a pre heat circuit and an electronic adaptor to supply energy to said light tube wherein said pre-heat circuit includes a PTC thermistor and a SIDAC high voltage bilateral trigger connected in series with the discharge light tube filaments and a capacitor C3 connected in parallel with the SIDAC-PTC thermistor branch, whereby in operation of said lamp during start up the voltage across C3 initially builds up to the breakover value of the SIDAC to establish a current flow through the PTC thermistor to heat up said filaments and then subsequently upon heat up and resistance increase of said PTC thermistor the voltage across said tube further increases to cause said tube to discharge and thereafter a resulting fall in the applied voltage across said tube and SIDAC causes said SIDAC to go into blocking mode to block current flow through said PTC thermistor during steady state operation of said lamp.

2. The electrical discharge lamp as claimed in claim 1 wherein said SIDAC-PTC thermistor branch includes additional capacitors, resistors and/or inductors to condition said branch circuit.

3. An electrical discharge lamp having a SIDAC-PTC pre heat branch circuit constructed and arranged substantially as described with reference to figure 3 or figure 4 of the accompanying drawings.