GB2098013A - Improvements in or relating to the starting of discharge lamps - Google Patents

Abstract

In a discharge lamp circuit including a low loss ballast arrangement (6) and a starter switch (9) connected across the lamp (1) the inrush current passed during starting is dependent on the random point in the AC supply at which starting is initiated. It is proposed to insert in the circuit a resistance (8) sufficient to reduce the highest possible inrush current sufficiently to prevent damage to the lamp. To prevent this giving excessive losses during running the resistance (8) is connected in series with the starter switch (9) and is therefore not effective when the lamp has started. It may be incorporated in the housing of the switch (9). <IMAGE>

Description

SPECIFICATION Improvements in or relating to the starting of dis charge lamps This invention is related to the starting of dis charge lamps and in particular fluorescent lamps.

A common discharge lamp circuit connects the lamp in series with a ballast across an AC power supply. For starting there is also connected in paral lel with the lamp a starter switch, for example a glow starter. When the power supply is switched on the starter switch closes and current passes through the ballast, one cathode of the lamp, the switch and the other cathode of the lamp in series. The current passing over several cycles of the AC supply heats the cathodes of the lamp, the starter switch then opens and the lamp starts.

It is known, however, that the point in an AC cycle at which a switch, such as the lamp starter switch, closes a series circuit including an AC supply, reac tance and resistance, decides whether the transient current component temporarily augments or diminishes the peaks of the sinusoidal current com ponent.

In some types of discharge lamp circuit the switching point may be fixed and therefore rela tively controllable but in types such as that described hereinbefore it is random. The possibility then arises of damaging the lamp cathodes by a high current which is generally known as "current inrush".

For a simple choke ballast it has been the practice to arrange among other considerations for the value of choke losses together with other resistive ele ments in the circuit to give a short LIR time con stant. Thus the transient decays rapidly and current inrush is limited. However if the choke losses are low and if, a resistor is added in series with the choke to limit inrush (by reducing VR), this may cause unsatisfactorily high losses in the lamp run ning mode.

It is an object of this invention to provide a dis charge lamp circuit allowing low running losses without excessive current inrush at starting.

According to the invention there is provided a dis charge lamp circuit for a discharge lamp having two cathodes, each with two terminals whereby current may be passed through the cathodes for preheating, the circuit including a ballast arrangement arranged to connect an AC power supply to one terminal of each cathode and, connected between the other terminals of the cathodes a starter switch, the bal last arrangement being of sufficiently low loss to allow, at starting, current to pass above a level at which damage may occur to the lamp and the circuit having connected in series with the starter switch between said terminals a resistor of a value chosen to limit the highest possible current at starting to below said level and being disconnected from the circuit when the lamp has started by said switch to reduce losses in the circuit when the lamp is running.

According to another aspect of the invention there is provided a discharge lamp circuit including a low loss ballast arrangement connecting an AC power supply to terminals for the lamp cathodes and a starter switch connected across said terminals, the circuit having a resistor in series with the starter switch across said terminals to limit starting current produced by said ballast which might damage the lamp and to be disconnected from the circuit by said switch when the lamp is running to reduce losses in the circuit.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings of which, Figure 1 shows a simple choke ballast circuit incorporating the invention, Figure 2 shows a series choke-capacitor circuit incorporating the invention, Figure 3 shows a leakage resistance transformer ballast circuit incorporating the invention and Figure 4 shows a leakage resistance transformer ballast with series capacitor also incorporating the invention.

There is shown in Figure 1 a simplified circuit for a discharge lamp 1, the circuit omitting any optional components which are well known and not required for understanding the invention.

Two terminals 3 and 4 are arranged to be connected across an AC power supply 5 for running lamp 1. Between the terminals are connected in series: a choke ballast 6, a first cathode 7 of lamp 1, a resistor 8, a starter switch 9 and a second cathode 10 of lamp 1. The choke ballast 6 is illustrated as comprising an inductive part L and a resistive part rS representing losses. Resistor 8 has a resistance R and the lamp cathodes each have col resistances RH/2, this resistance increasing with temperature.

The invention is intended to allow the use of a low loss choke for which rS is typically 6.3 Q which is a typical value of 3.3 fl for RH would give a total of 9.6 Q for the circuit compared with 23.3 Q for a conven tional choke of typically 20 Q r,.

As mentioned hereinbefore, the inrush current depends in part on the starting point on the supply waveform and partly on the inductance and resistance of the choke ballast. Other relevant factors are the value and sign of residual magnetism, the saturation and hysteresis characteristics of the core material and the source impedance of the AC supply to the circuit.

It has been realised that to reduce the time con stand 7 = L/Rtotai, and thereby to reduce current inrush, an additional resistance R is required. This gives 7 = L/(rs + RH + R) and for said low loss choke of 6.3 fl r a value of R of about 10fl is suitable.

The addition of this extra resistance can negate the value of a low loss choke and this invention avoids unnecessary power dissipation by locating the current inrush limiting resistor8 on the switch side of the lamp 1. In that position it is automatically switched out of the ballast path when the lamp is running and the low loss character of the choke is retained.

It will be appreciated that the invention is applic able to other discharge lamp circuits than the simple one of Figure 1. In Figure 2 there is shown a series choke capacitor ballast of a type which is generally well known but including resistance R at8 to enable choke ballast 6 to be of a low loss type. A capacitor 11 included in this circuit is arranged generally to give Xc z 2XL at mains supply frequency so that the total reactance is capacitive. The circuit is basically a damped series resonant circuit when the switch is closed and the resistor8 has the same effect as in the circuit of Figure 1.

Another typical circuit is the leakage reactance transformer ballast shown in Figure 3. In this circuit, used when the supply voltage is inappropriate for starting and running the lamp 1, a transformer 12, which may be autoconnected, is connected across the power supply and exhibits leakage inductance sufficientto act as a choke ballast. With the exception of the voltage transformation this is similar to the low loss choke ballast of Figure 1 and the resistor 8 achieves the same effect.

A further type of starter circuit, which is essentially a combination of the series choke-capacitor circuit of Figure 2 and the leakage reactance transformer circuit with series capacitor, shown in Figure 4. The considerations are similar and resistor 8 operates in the same manner to reduce current inrush.

Other variations of the starter circuit incorporat- ing the invention will be apparent to those skilled in the art, for example the use of a non-linear resistor which alters resistance with application of voltage, current, temperature or light level. In an alternative form the resistor may be built into the starter switch housing to form an intergral unit

Claims (8)

1. A discharge lamp circuit for a discharge lamp having two cathodes, each with two terminals whereby current may be passed through the cathodes for preheating, the circuit including a ballast arrangement arranged to connect an AC power supply to one terminal of each cathode and, connected between the other terminals of the cathodes a starter switch, the ballast arrangement being of sufficiently low loss to allow, at starting, current to pass above a level at which damage may occur to the lamp and the circuit having connected in series with the starter switch between said terminals a resistor of a value chosen to limit the highest possible current at starting to below said level and being disconnected from the circuit when the lamp has started by said switch to reduce losses in the circuit when the lamp is running.

2. A discharge lamp circuit including a low loss ballast arrangement connecting an AC power supply to terminals for the lamp cathodes and a starter switch connected across said terminals, the circuit having a resistor in series with the starter switch across said terminals to limit starting current produced by said ballast which might damage the lamp and to be disconnected from the circuit by said switch when the lamp is running to reduce losses in the circuit.

3. A circuit according to either of the preceding claims in which the ballast arrangement is a choke ballast.

4. A circuit according to either Claim 1 or Claim 2 in which the ballast arrangement is a leakage resistance transformer ballast.

5. A circuit according to either Claim 3 or Claim 4 including in series with the ballast arrangement a capacitance to give the circuit a capacitative reactance.

6. A circuit according to any of the preceding cliams in which the resistor is a non-linear resistor.

7. A circuit according to any of the preceding claims in which the resistor is incorporated into a housing of the starter switch.

8. AdiScharge lamp circuit substantially as herein described with reference to Figure 1 or Figure 2 or Figure 3 or Figure 4 of the accompanying drawings.