GB2132751A - Ambient light control for artificial lighting - Google Patents

Abstract

An ambient-light operated On/Off control unit for a fluorescent lighting load 1 is mains operated and includes a zero-crossing, pulse-triggered triac 16 for controlling power supply to the lighting load. The ambient-light sensor is a photo-transistor 5 having a wide acceptance angle and an optical response which peaks in the infra-red region, the sensor feeding into an operational amplifier 9 with an hysteresis operating cycle, to avoid establishing a hunting condition when the lighting load is switched On. A capacitor 10 provides the ambient-light sensor circuit with a long time-response, to avoid operation due to transient ambient-light changes, and also provides a short lighting-load operating period when the control unit is initially powered to test the fluorescent starter units. At other times when the control unit is powered and the lighting load is not, a LED indicator 25 shows the "power-on" condition. <IMAGE>

Description

SPECIFICATION Improvements in or relating to ambient light control circuit arrangements for artificial lighting Description of the Invention This invention relates to ambient light control circuit arrangements for artificial lighting.

Most buildings rely upon daylight for daytime interior illumination. At times when daylight is low, natural lighting is supplemented or replaced by artificial illumination. A situation commonly arises, in commerical and industrial buildings, that a period of darkness, either early in the day or during a storm, results in artificial lighting being switched on and subsequently left on unnecessarily, with consequent unnecessary lighting costs.

To avoid such haphazard manual control of lighting, ambient light control circuits may be installed for the automatic control of artificial lighting. However, the known automatic control arrangements suffer from a number of disadvantages.

The object of the present invention is to provide improved ambient light control circuit arrangements for electric fluorescent lighting circuits, the form of artificial lighting most commonly used commorcially and industrially.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, the sole figure of which is a schematic circuit diagram.

Referring to the drawing, the control circuit arrangement of the example is connected in series with the lighting load 1 of fluorescent luminaires between the neutral conductor 2 and line 3, which is connected by a switch 3' to the line conductor of the alternating current supply.

A silicon photo-transistor 5 connected in series with a present resistor 6 between line 3 and a baseline 4 monitors ambient light 7 and provides a corresponding signal voltage on line 8. This signal voltage is applied to one input of an operational amplifier integrated circuit (IC) 9, which is configured as a threshold detector with approximately 50% hysteresis in its operating cycle. The second input potential is determined by a resistor potentiometer between lines 3 and 4 and by feedback resistor 9'.

The hysteresis characteristic is provided since otherwise the switching-on of the lighting load 1 would produce ambient light 7 feedback to the photo-transistor 5, thereby setting up a hunting condition.

The photo-transistor type chosen has a spectral response from approximately 600 mm to 1,000 mm, peaking at 850 mm. This preferred sensitivity in the infra-red part of the spectrum permits operational advantage to be taken of the fact that natural light has a greater infra-red content than does fluorescent lighting. Consequently, the photo-transistor 5 is the more responsive to ambient natural light, thereby further avoiding onset of any hunting condition.

The photo-transistor detector 5 also preferably has a wide incident light acceptance angle 100" in the example chosen. This avoids operation of the control circuit to switch off the artificial illumination initiated merely by a local light area, such as a light coloured object passing near to the photo-transistor.

The present resistor 6 provides for different sensitivities of alternative photo-transistor components 5 and provides for the required signal level on line 8.

A capacitor 10 is connected in parallel with resistor 6 and serves two operational purposes. First, when switch 3' is first closed, and line 3 energised, a surge voltage appears on line 8 which switches on the lighting load 1, in this example for some twenty seconds, so permitting an initial check to be made for faulty fluorescent tubes or starters. Second, in continuing operation, the capacitor 10 serves to slow the speed of response of the control circuit. This characteristic ensures that the control unit responds only to long-term ambient lighting changes and does not respond to momentary changes, such as might be produced by passing clouds or temporary obstruction of windows.

The output of operational amplifier IC9 is controlled by the charge on capacitor 10 which in turn is controlled by the incident light 7 on photo-transistor 5. The output of amplifier 9 is fed through a diode 11 to one input of an operational amplifier IC 12, configured as a voltage window comparator. The steady potentials of both inputs of amplifier IC 1 2 are set by resistor potentiometers connected between lines 3 and 4.

The output of operational amplifier IC 1 2 is applied across a resistor 13, capacitor 1 4 combination, the junction of which is connected to the emitter of a unijunction transistor 1 5. Transistor 1 5 generates triac trigger pulses at a rate determined by the RC value of components 1 3 and 14.

The lighting load 1 is directly controlled by a triac 1 6 connected between line 3 and the lighting load 1. Triac 1 6 is zero-crossing triggered by trigger pulses generated by the unijunction transistor 15, shaped and amplified by a transistor 1 7 in a circuit comprising input diode 19, capacitor 20 and series resistor 21, 22 and 23. The junction of resistor 22 and 23 is connected to the gate of triac 16.

As stated above, the intensity of incident light 7 controls the photo-transistor 5, the charge on capacitor 10, the potential of line 8 and the output of IC 9 to diode 11. When the incident light level is above the preset threshold level, the output of IC 9 is high. This condition of IC 9, applied to the described circuitry connected to triac 16, disables the triac 1 6 triggering circuitry and thus supplies no energy to the load 1. Conversely, when the incident light level is below the threshold level, the output of IC 9 goes low, the triac 1 6 triggering circuitry is enabled and the load 1 is supplied.

A series combination of a resistor 24 and a light-emitting diode (LED) 25 is connected between the output of IC9 and line 4. When the output of IC 9 is high, corresponding to the "off" condition of the lighting load, LED 25 is lit, so indicating that power is nevertheless being supplied to the control circuit arrangement. In the "On" condition of the lighting load, the output of IC9 is low and LED 25 is extinguished.

Considering the operation of the circuitry between the output of IC 9 and the gate of triac 1 6 in greater detail, operational amplifier IC 1 2 is configured as a window comparator looking at the voltage across triac 1 6 and the output voltage of operational amplifier IC 9 at diode 11. If either the instantaneous voltage across triac 1 6 exceeds some twenty volts, or if the output of IC 9 is high, then the output of IC 1 2 is low and transistor 1 5 is disabled as a trigger pulse generator. Conversely, if both the voltage across triac 1 6 is less than twenty volts and IC 9 output is low, then the output of IC 1 2 is high and transistor 1 5 is enabled.The pulses generated by transistor 15, after shaping and amplification by the circuit of transistor 17, are applied to the gate of triac 16, at the pulse rate determined by components 1 3 and 14 as stated above, for so long as the level of incident light 7 remains low and during those periods of the supply mains cycle when the potential across triac 1 6 is below twenty volts, or during the triac "On" state when the voltage across the triac is always less than twenty volts.

The use of pulse triggering of the triac provides a sufficient triac gate drive while keeping the power consumption of the control circuit low. The use of zero-crossing triggering prevents large current surges through the triac such as would occur if supplied at mid-cycle of the supply mains, particularly if the fluorescent luminaires were fitted with power-factor correcting capacitors.

There is also connected between line 3 and lines 4 and 2, a network comprising serialconnected capacitor 26, diode 27, capacitor 28 and low value resistor 29, between line 3 and line 2, a diode 30 connected between line 3 and the junction of components 27 and 28, a resistor 31 connected between line 4 and the junction of components 26 and 27 and a Zener diode connected between lines 3 and 4. Capacitor 28 functions as a capacitive voltage dropper for the voltage on line 4.

When power is initially applied to line 3, the voltage across Zener diode 32 takes a certain time to reach the operating voltage for the circuitry. During this build-up time, the outputs of IC9 and IC 1 2 are indeterminate and could result in triac 1 6 being triggered at midcycle of the mains supply. To prevent such a condition arising, a clamp circuit is provided comprising a capacitor 33 and a resistor 34 with shunt diode 35 serially connected between line 4 and line 3, and a series diode 36 from the junction of components 33 and 34 to the emitter of transistor 1 5. The charge voltage across capacitor 33, on initial closure of switch 3', clamps the emitter of transistor 1 5 for a period of approximately one-half second. Transistor 1 5 is thereby disabled and triac 1 6 is not triggered until the power supply between lines 3 and 4 has stabilized.

Claims (6)

1. Ambient-light operated On/Off control means for a fluorescent lighting load, including a photo-electric ambient-light sensor having a maximum optical sensitivity in a preferred spectral region and a zero-crossing, pulse-triggered triac, or like solid-state device, for controlling power supply to the fluorescent lighting load.

2. Ambient-light operated control means as claimed in Claim 1, in which the ambientlight sensor is a photo-transistor connected to feed into an operational amplifier having an hysteresis operating cycle.

3. Ambient-light operated control means as claimed in Claim 2, in which the phototransistor has maximum optical sensitivity in the infra-red region.

4. Ambient-light operated control means as claimed in Claim 2, in which the phototransistor feeds one input of an operational amplifier operating as a threshold detector with some 50% hysteresis in its operating cycle and having a second input supplied from a fixed potential point and from a feedback resistor.

5. Ambient-light operated control means as claimed in any one of Claims 2 to 4, in which the photo-transistor circuit includes a resistor-capacitor combination having a long time constant.

6. Ambient-light operated control means as claimed in Claim 1, constructed substantially as described herein with reference to the accompanying drawing.