GB2155620A - Photocell and method for calibration thereof - Google Patents

Abstract

A photoelectric control unit including a solid state relay 9. The photoelectric control unit is calibrated electronically by exposing a photocell 15 to a light source of predetermined intensity. A variable resistor 16 is adjusted such that the solid state relay turns on a load, e.g. a lamp 18, at this predetermined ambient light level. A snubber network may be connected across the load 18 to suppress unwanted spikes and thus to prevent false triggering of the relay 9. A zero-voltage detector may be connected, to determine the precise instant during the A.C. power-voltage cycle when the power is switched on to the load. <IMAGE>

Description

SPECIFICATION Photocell and method for calibration thereof The invention relates to the field of illumination. More particularly, the invention relates to lighting control of luminaires. In still greater particularity, the invention relates to a photoelectric control unit and method for calibration thereof. By way of further characterisation, but not by way of limitation thereto, the invention is a photoelectric control unit including a solid state relay which may be calibrated by adjusting a variable resistor.

Existing photoelectric control units are generally of two types; a one part unit and a two part unit. The one part unit type of photoelectric control unit is available as an external or an internal fitting to the luminaire.

The two part unit is generally an internal fitting.

The one part control unit consists of a cadnium sulphide light dependent resistor which is calibrated by a screening technique including masking the surface of the photocell with an opaque material. The switching device is generally a thermal relay. The thermal relay's efficiency is hampered by inherent hysteresis and it is also effected by temperature variations. Because the calibration method is inherently inaccurate, it results in waste of energy resulting in higher costs. Performance of a unit calibrated in this manner can also be hazardous should the luminaire not switch on at the predetermined ambient light level. The photocell may also include an electronically controlled circuit in the form of an electromechanical relay instead of the thermal relay.

While this improves performance somewhat, temperature variations may still affect the unit. Finally, externally mounted control units are more prone to vandalism that internal units.

Mounting the control units internally in the luminaire may result in inaccuracies in that the high level of heat generated by the lamp and the control apparatus subject the control unit to degradation. That is, due to the high internal ambient temperature within the luminaire, the load switching/carrying capacity of the various types of relays (either thermal or electro-mechanical) must be de-rated drastically once the maximum operating load plateau in the current versus temperature curve begins to decrease. Therefore, internally fitted control units are generally not suitable for use irk'high internal ambient temperature luminaires.

In an attempt to solve some of the above limitations, solid state control units have been employed in both one part and two part units for both external and internal fitting. In general, these control units give higher accuracy in switching and better performance in that the moving parts in the thermal or electromechanical relay load switching suffer from contact wear. This wear results in early failure due to the "avalanche" effect caused by the degradation of the contacts creating higher resistance and thus overheating. Solid state control units also generate very high radio frequency interference which is undesirable.

The use of electronic load switching as well as the electronic calibration of the switching function by the use of solid state control units is therefore desirable. However, because the majority of applications of the control units is with high intensity discharge lamps, the inductive load and the heavy starting characteristics of the discharge lamps limit the use of the electronic load switching by solid state control units. Thus, although the solid state control units are more accurate and perform better, high current loading and high ambient temperature may limit their use with high wattage lamps.

The invention is a photoelectric control unit which includes a solid state relay in the control unit. The control unit also includes a variable resistor in the form of a trimming potentiometer. The control unit is calibrated electronically by exposing the photocell light sensor to a stabilised reference light source in a light integrator housing. The light intensity in the housing is set to a predetermined switch-on ambient level requirement. The trimming potentiometer is then adjusted until the lamp is switched on.

Figure 1 is a respective view of the photocell and the photoelectric control unit: Figure 2 is an electronic schematic of the photoelectric control unit; and Figure 3 is a block diagram of the solid state relay.

Referring to Fig. 1, a control unit generally designated 9 includes a base portion 10 and electrical connections 11, 12, 1 3 and 14. A photocell 1 5 is connected to control unit 9 at contacts 1 3 and 14. A variable resistor, which may be a trimming potentiometer 1 6 is connected to the electrical contacts 11 and 1 4.

Contacts 11 and 1 3 are also connected to a power source not shown. Contact 1 2 is connected to a load source or lamp. (Not shown).

A cover 1 7 may be attached to base portion 10.

Referring to Fig. 2, an electronic schematic of the photocell and control unit is shown.

Photocell 1 5 is connected to contact 1 4 on solid state relay 9. Trimming potentiometer 1 6 is also connected to electrical contact 1 4.

A power source 1 7 which may be a conventional 240 volt alternating current source is connected to electrical contacts 11 and 1 3. A load which may be a lamp 1 8 is connected to electrical contact 1 2.

Referring to Fig. 3, a low voltage AC circuit 1 9 is connected to contacts 1 3 and 14. A light emitting diode 23 is connected to circuit 19. A photosensitive device 22 which may be a phototransistor or other device, is positioned to receive light for light emitting diode 21. An input to output solution 23 which may be a filter is located between diode 21 and photosensitive device 22. Photosensitive device 22 is connected to an AND Gate 24. AND Gate 24 is connected to a triggering circuit 25 which is in turn connected to an electronic power/line switch 26 which may be a TRIAC.

Switch 26 is connected to contacts 11 and 12 and to a zero voltage detection circuit 27.

A snubber network which may be for example, a resistor 28 and a capacitor 29, is connected across contacts 11 and 1 2 in parallel with switch 26.

Photocell light sensor 1 5 which may include a cadnium sulphide light dependent resistor reacts to a change in the ambient level of iliumination in that its resistance changes with the ambient light intensity faliing upon its surface. That is, if the light level increases, the photocell resistance decreases and if the light level decreases, the photocell resistance increases.

The increase in resistance of the photocell results in a reduced current flow. The reduced current flow at contact 14 commands the solid state relay 9 to trigger such that power will be supplied to the lamp 1 8. However, lamp 1 8 will only be switched on at the zero voltage point during any sinusoidal wave form due to the zero voltage detector circuit 27 which is included in the solid state relay 9.

This zero voltage detector circuitry eliminates radio frequency interference and limits the normally high starting current.

Referring to Fig. 3, solid state relay 9 receives the threshold current at contact 1 3 and 14. Circuit 1 9 receives the current and outputs stabilised low D.C. voltage supply to L. E. D. 21. The light from L.E.D. 21 activates photosensitive device 22. The high input is thus-supplied to AND Gate 24.

Zero voltage detector circuits 27 monitors the AC input voltage and will only turn on when the AC line voltage reaches the zero volt line, that is at the 180 and 360 phase angles. Circuit 27 then provides a high input to AND Gate 24 which, when coupled with the high input from device 22, provides a light input to triggering circuit 25. Triggering circuit 25 then activates electronic power switch 26 to connect power to contacts 11 and 12. Lamp 18 is thus turned on.

Snubber network, which may include resistor 28 and capacitor 29, is connected across the electronic power switch to eliminate false triggering. That is, this network provides the leakage current path to dissipate unwanted spikes or other types of interference thus preventing false triggering of the relay 9 and the unwanted switching on of lamp 1 8.

In order to calibrate the control unit, photocell light sensor 1 5 is exposed to a stabilised reference light source in a light integrator housing. The light in this housing can be set to any predetermined ambient light level. At the predetermined light level, trimming potentiometer 1 6 is adjusted until the lamp 18 is switched on. Thus, the control unit is electronically calibrated eliminating the prior inaccurate calibration methods.

While the invention has been disclosed with respect to a preferred embodiment thereof, it is not to be so limited as changes and modifications may be made which are within the full intended scope of the invention as defined by the appended claims. For example, while a cadnium sulphide photocell has been disclosed, it should be understood that any type of photocell may be employed advantageously with the present invention. In addition, while an EC network has been disclosed as the snubber network, it should be expressly understood that any type of snubber network may be employed. In addition, while the control unit has been described so as to turn on a lamp at low embient light levels, it should be understood that the control unit could also work in an inverse mode. That is, the unit could be used to switch on a load at a high ambient light level and switch off a load at low embient light level. This may be useful in security devices or some types of indicators.

Claims (8)

1. Apparatus for controlling the operation of a luminaire in response to changes in ambient light levels comprising: a photocell light sensor; a solid state relay electrically connected to said photocell light sensor; a power source connected to said relay and said luminaire; and a variable resistor connected to said relay and said power source.

2. Apparatus according to Claim 1 wherein said variable resistor is a trimming potentiometer.

3. Apparatus according to Claim 1 further including a snubber network.

4. Apparatus according to Claim 3 wherein said snubber network is a resistive capacitive network.

5. Apparatus according to Claim 1 further including a zero voltage switching circuit.

6. A method for calibrating a photoelectric control unit for a luminaire such that said luminaire will switch on at predetermined ambient light levels comprising the steps of; exposing a photocell connected to a solid state relay in the control unit to a light source; varying the intensity of the light source to said predetermined ambient light level; adjusting a variable resistor connected to the solid state relay such that said luminaire is switch on.

7. Apparatus substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

8. A method for calibrating a photoelectric control units substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.