This invention relates to an A.C. power supply controller for a capacitive load, particularly an electroluminescent light source, and a waveform generator suitable for use in such a controller.

Electroluminescent (EL) lights have been used as formation lights for military aircraft. The lights are situated on the aircraft such that they are visible to other aircraft only. The lights must be bright enough for the aircraft's position to be safely identified by other aircraft flying in a formation but not so bright as to be seen by hostile aircraft flying some distance away. The light must therefore be variable to cope with varying formation separation and weather conditions.

The lights present a highly capacitive load to power supply controllers, causing a large difference in the relative phases of the voltage and current in an A.C. supply. Former power supply controllers have taken the form of a

variable transformer supplying a sinewave the amplitude of which can be varied. Variable transformers suffer the disadvantages of large physical size, weight and cost. Further, adjustment is achieved by movement of a wiper over coils which produces electrical noise, upsetting sensitive aircraft avionics. The problem of noise increases with age as dust becomes entrapped within, the transformer.

It is an object of the invention to provide a power supply controller that is inexpensive to manufacture, compact, lightweight and produces a low level of electrical noise.

According to the invention there is provided an A.C. power supply controller for a capacitive load comprising means for producing a pulsed switching waveform centred on the zero-crossing points of an applied alternating voltage waveform, powerswitch means for connecting an applied voltage waveform to the load in response to the pulsed switching waveform, and means for widening the pulse width of the switching waveform symmetrically about the

zero-crossing points to vary the power applied to the load.

According to a second aspect of the invention, there is provided a waveform generator for producing a pulsed waveform centred about the zero-crossing points of an input alternating waveform comprising means to rectify the input waveform and means to determine when the rectified waveform is below a particular value and to generate a non-zero waveform level when the rectified waveform is below said value.

A specific embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 shows a block diagram of a light controller in accordance with the invention;

Figure 2 shows the waveforms used in the light controller shown in Figure 1; and

Figure 3 shows a circuit diagram of the A.C.

power switch shown in Figure 1.

With reference to Figure 1, an array of eight electroluminescence formation lights 1 are controlled by a lighting controller 2 (within the broken lines). A 115V, 400Hz supply is connected to the input of the controller via supply and return ports 3 and 4, the lighting controller 2 and lights 1 being activated and controlled by a cockpit mounted 10K ohm linear potentiometer 5 incorporating a 5 amp switch 6.

The lighting controller 2 comprises a 115V A.C. power rail 7 and a return rail 8 connected to the supply and return parts 3 and 4. A low voltage supply 9 provides a + 15V D.C. supply to the controller components.

A rectifier 10 rectifies the A.C. power waveform to give a full wave rectified output 11 of peak voltage 115V. The zero crossing points of the voltage waveform are automatically derived in this rectification. The rectified output 11 is input to an attenuator 12 where it is reduced in

amplitude before being passed to a comparator 13 where it is compared with a D.C. level set by the linear potentiometer 5.

The linear potentiometer 5 is supplied with a + 5V D.C. supply derived from the + 15V D.C. supply by a POT D.C. supply 14. Thus, by rotation of the potentiometer 5 the D.C. voltage, level on input 15 to the comparator 13 can be varied between 0V and + 5V.

The comparison process will be described later but the result is a pulse width modulated output which is passed via an output driver 16 to a A.C. Power Switch 17. The purpose of the output driver 16 is to isolate the low voltage circuitry from the high voltages present in the power switch 17. It achieves this with an electro-optical coupler in a manner well known.

The power switch 17 switches the formation lights 1 on for the duration of each pulse by completing the connection to the return rail 8. Thus it is the width of each pulse which controls the light

output, the frequency of the pulses being too great for the human eye to detect the on and off periods. The width of the pulses is set by the comparator 13 in the following way.

Figure 2a shows the rectified and attenuated voltage waveform 18 generated by the rectifier 10 and attenuator 12. P _a. is the DC voltage level set by the linear potentiometer 5 on input 15 to the comparator 13. The comparator 13 determines where these voltages are equal that is to say where the waveforms p and 18 intersect. The points of intersection are marked as i to i _β on Figure 2a. When the comparator 13 detects intersection i^, it produces + 15V on its output to the output driver 16 (see Figure 2B) . This level is maintained until i ₂ is detected and the level is switched to Ov producing a pulse pi symmetrical about zero- crossing point z . The level is switched to a + 15V at the next intersection i ₃ and switched to Ov at i ₄ to produce pulse p ₂ symmetrical about zero- crossing point ~___ . Similarly, intersections i ₅ and i ₆ produce p ₃ symmetrical about zero crossing point z ₃ .

The broken lines in Figures 2a and 2b show the pulses generated by the intersection of a higher DC level P ₂ set by the linear potentiometer 5. The width of these pulses is wider than that set at the lower level p _x giving a longer period for the lights 1 and hence a higher light output. It should be noted the brilliance is thus increased by widening the pulse from the zero-crossing points of the voltage curve.

Figure 2c shows the voltage at the load, that is to say, the lights 1. Considering this curve from pulse _x , the supply 115V A.C. is connected to the load during the width of p _x , points a and b on the V load curve. At b the connection is broken but because of the high capacitance of the load, the voltage remains almost constant decaying only slightly. Between c and d, corresponding to pulse p ₂ , the 115V A.C. is again connected, this time on its positive cycle. Again during the interpulse period, the load is disconnected and the voltage remains almost constant. Thus the voltage at the load follows the full 115V A.C. supply only for

the duration of the pulses as shown in Figure 2c. As the pulse width is increased, more of the full 115V A.C. supply is applied to the load until full power is applied.

By symmetrically "widening" the pulses about the zero-crossing points of the voltage waveform, the current flow as shown by the solid line in Figure 2d conforms to the supply current shown in broken outline.

The relationship between the rate of widening of the pulses and the rate of operation of the potentiometer 5 is approximately logarithmic, as determined by the characteristic of the rectified a.c. waveform. Thus the requirement for an expensive logarithmic potentiometer to match the logarithmic response of the eye to the light output, is avoided.

The power switch 17 is shown in greater detail in Figure 3 and it comprises in the main two power transistors T and T _z for switching alternate cycles of the voltage waveform. Diodes D _x and D ₂

ensure a unidirectional current flow through T _x and T ₂ respectively. R _x ensures the correct bias for transistor T _x while R ₂ performs the same function for T ₂ .

In response to an output pulse from output driver 16, transistors T _x and T ₂ will be turned on. If the voltage waveform is in its positive cycle current will flow from the 115 A.C. line 7 through the load via diode D _x , T and R ₃ to the neutral line. Diode D ₂ will prevent a current flow through T ₂ during the positive cycle.

During the negative half of the cycle, current will flow from the neutral line via T ₂ , R ₄ and D ₂ through the load to the 115V A.C. line 7. Diode D prevents current flow through T during this part of the cycle.

Transistor T ₃ , resistors R ₅ and R ₃ perform a current limiting function on T _x . As the current increases through R ₃ , the voltage drop across the base b and emitter e of T ₃ increases until T ₃ is switched on. The pulse is then shorted to neutral

and T _x switched off. Transistor T ₄ , resistors R _e and R ₄ perform the same function on T ₂ .

Zener diodes Z _x and Z ₂ provide reverse power protection to T ₃ and T ₄ respectively. In the event of an applied reverse voltage exceeding the breakdown voltage of the zener diodes, Z _x and Z ₂ , these diodes conduct. Zener diodes Z _x and Z ₂ are 18V diodes.

In alternative embodiments of the invention, the rectifier, attenuator and comparator may be replaced by other means to generate a waveform symmetrical about the zero-crossing points of the A.C. voltage to be controlled. It may also be possible to produce pulses from a halfwave rectified A.C. waveform in a similar manner to the way in which pulses were produced from a full wave rectified A.C. waveform as earlier described.