IE55885B1 - A photoelectric control unit - Google Patents

Description

The invention relates to a controller for a photoelectric control unit and in particular to a controller for a photoelectric control unit for a lamp such as a street lamp.

The controllers for lighting control units which are presently available on the market are either electro-mechanical, electronic or a combination of both. Electro-mechanical units switch powe? to a load by opening or closing a pair of heavy duty electrical contacts. Generally the line voltage has a peak value of over 310 volts which causes arcing across the contacts as they open and close» The arcing in turn causes pitting of the contact surfaces resulting in a short useful life-span for the controller.

A further problem with conventional controllers is that in time, the light-level threshold can drift outside the specified limits due fo drift of the threshold reference value and changes in the characteristics of the light detecting sensor which is normally a light dependent resistor or LDR. Conventional units employ a thermal switching technique where the LDR controls the current through, and thus, the temperature of a bi-metallic strip. The threshold point is determined by the force required from the thermal strip to change over spring loaded electrical contacts. The characteristics of the spring load as well as those of the bi-metallic strip change with time and this causes the accuracy of the unit to drift outside the acceptable limits. Also, the LDR is required to conduct large currents fo heat the bi-metallic strip and the resulting self-heating effect causes the LDR fo change its characteristics over a period. The LDR units commonly used in existing controllers, gradually break down allowing moisture and impurities to effect the selenium cell. This combined with temperature rise in the device contributes to further drift of the light level threshold.

This invention is therefore directed towards providing a controller for a photoelectric control unit which will minimise pitting of the contact surface and hence prolong the useful life of the unit.

The invention provides a controller for a photoelectric control unit, the controller comprising a photoelectric sensor, comparing means for comparing the intensity of sensed light with preset light intensity and switching means operated by the comparing means to switch a lamp, said switching means comprising a switch which operates over a large voltage range.

In one embodiment of this aspect of the Invention the output is a street lamp.

In a particular preferred embodiment of the invention the switch comprises a relay triac electrically connected in parallel. Typically the triac is provided in a triac control circuit which is operated by the comparing means» On switching of the switch, the relay conducts at low voltages and the triac conducts at high voltages.

In one embodiment of the Invention the photoelectric sensor comprises a light sensitive resistor whose resistance increases with the intensity of incident light.

Preferably the controller includes a capacitor which is charged when the triac operates to keep the relay closed for a short period to ensure continuous conduction.

In another preferred embodiment of the invention the photoelectric sensor comprises a phototransistor.

The invention will be more clearly understood from the following description thereof given by way of example only with reference to the accompanying drawings in whichsFigure 1 is an exploded perspective view of a photoelectric control unit according to the invention.

Figure 2 is a perspective view of the photoelectric χ0 control unit of Figure 1? assembled? Figure 3 is a diagrammatic side view illustrating the unit of Figures 1 and 2 in use? Figure 4 is a side, partially cross-sectional view of the control unit of Figures 1 and 2, Figure 5 is a perspective view from above of a portion of the control unit? Figure 6 is a perspective view of a socket housing attachment for use with the control unit of Figures 1 and 2, Figure 7 is a perspective view of a control unit with 5 the housing of Figure 6 attached, Figure 8 is a side cross-sectional view of the control unit with the housing attached, and Figure 9 is a circuit diagram of a controller for a photoelectric control unit.

IO Referring to the drawings and initially to Figures 1 to 5 thereof there is illustrated a photoelectric control unit according to the invention and indicated generally by the reference numeral 1. The control unit 1 comprises control means, in this case mounted on a printed circuit board 2 for switching an output, in this case a street lamp 3. The control means as will be described in more detail below, includes a photoelectric sensor which causes power to the lamp 3 to be switched off when the amount of light falling on the sensor exceeds a pre-set value, in this case 55 lux. The sensor is mounted on the printed circuit board 2 and electrical connection means between the printed circuit board :and lamp 3 is provided by three connector pins 4 which are shaped at their lower ends 5 to engage in a twist-socket provided on the lamp 3. The connector pins 4 are connected at their upper end 6 to connection points (not shown) on the printed circuit board through flexible connecting wires 7. The connections between the connector pins 4 and printed circuit board 2 are capable of withstanding vibrations and/or shudder which would normally be encountered in * use with street lanterns.

To prevent overheating of the connector pins 4 and consequent malfunctioning and possible damage to the electrical components mounted on the printed circuit board 2 the pins 4 are led through a cooling means in the form of a stem which in this case is defined by a cooling chamber 6 where heat generated in the pins 4 is dissipated through the walls of the chamber 8· Referring particularly to Figure 5 the cooling chamber 8 comprises a cylindrical side-wall 10 and a disc-shaped base wall 11 which extends radially outwardly of the side wall 10» A skirt 12 extends axially downwardly from the outer extremity of the base wall 11· The base wall 1.1 is formed with three holes 13 which are equispaced apart at 120” and through which the connector pins 4 are led. As will be apparent particularly from Figure 1 each of the connector pins 4 is cranked at 15 and 16 to define an axially depending leg portion 17, an interconnected radial at base wall portion 18 and an axially extending side wall portion 19. Each of the pins 4 is retained in position in a retaining hole 14 by a self-tapping screw 21 which extends through the«hole 14 to engage in a corresponding hole 22 in the base wall extending portion 18 of each connector pin 4. The connector pins 4 are sized and shaped so that a large surface area of the connector pins 4 is in contact with the side and base walls 10, 11 of the cooling chamber» Consequently heat generated in the connector pins 4 is dissipated through the walls of the chamber 8» To assist heat transfer the cooling chamber 8 is also substantially filled with a heat transfer medium, in this case in the form of a potting compound 24 which conducts heat away from the pins 4 and into the side and base walls 10, 11 of the cooling chamber 8. The potting compound used may be any suitable encapsulating compound having a high thermal conductivity such as a Humiseal 2B74.

A sealing ring gasket 27 which is usually of neoprene material is provided on the base wall 11 of the cooling chamber 8 to prevent ingress of moisture and dirt through the joint between the cooling chamber skirt 12 and the socket provided on the lantern 3.

It will be appreciated that the cooling chamber defines a stem which both raises the assembly from the hot parts of the lantern and minimises the heat carried by the connectors to the electronic components mounted on lo the printed circuit board* In tests, in which the control unit was mounted on a 400 watt lantern at room temperature, the use of the cooling chamber was found to reduce the temperature of the connector pins at the PCB connection points by up to 10° which represents a large improvement in reliability.

The printed circuit board 2 is mounted in a printed circuit board housing 30 which comprises a substantially disc-shaped base 31 and a cylindrical side wall 32 extending axially upwardly from the base 31. Three connector pin receiving slots 33, equi-spaced at 120°, extend through the base 31. Three screw receiving pillars 34 extend upwardly from the base 31 adjacent to side wall 32 and are also equi-spaced at 120° around the housing. Each of the pillars 34 is formed with a hole 35 for reception of a self-tapping screw 36 which extends through corresponding holes 37 in printed circuit board 2 for mounting the board 2 to the pillars 34. Reinforcing webs 38 extend between each pillar 34 and the side wall 32.

The upper rim of the circuit board housing side wall 32 is formed with a step portion 39 for reception of a complementary step portion 40 which extends around the lower periphery of a translucent conical cover 41 through which light passes to the photoelectric sensor on the printed circuit board 2, The canopy or cover 41 is of conical shape to prevent accumulation of dirt and also discourage birds from perching on the cover. The cover is typically of a plastics material which is treated to protect against ultra-violet light. For fitting the printed circuit board housing 30 and cooling chamber 8 together the housing 30 is formed with a socket which in this case is defined by a skirt 45 which extends axially downwardly approximately mid-way across the base for reception of a spigot defined by the upper edge of the side wall 10 of the cooling chamber 6.

To prevent ingress of dirt and moisture, the component parts of the control unit are securely fixed together and the unit is then coated with a sealing compound to provide additional defensive measures against moisture and dirt ingress.

One advantage of the invention is that the connection means between the control means and the output is arranged to dissipate any heat generated and hence protect the components of the control means. Thus, a more efficient and reliable switch which has a long life in use is provided.

Referring to Figures 6 to 8 there is illustrated a modified photoelectric control unit which is smaller to that described above with reference to Figures 1 to 5 and like parts are assigned the same reference numerals. In this case the unit includes an electrical socket receiving housing SO for housing an output socket 51 which is connected at one end to the connector pins 4 and which is connected through an outlet cable 52 to the output which if is desired to switch such as a street or security lantern. The housing 50 includes an upper radial wall 53 and an axially upwardly extending side wall portion 54 which defines a spigot for insertion in the socket defined by the skirt 12 of the cooling chamber 8. A lower cylindrical side wall portion 55 extends downwardly from the upper wall 53 of the housing 50. The lower side wall portion 53 of the housing 50. The lower side wall portion 55 is turned inwardly to define a base wall 56 having a central opening 57 through which the electrical connection lead 52 is led from the socket 51 for connection fo an output. The socket 51 is carried by a mounting plate 60 which is fixed in position by self-tapping screws 61 which engage in corresponding screw receiving portions 62 which extend downwardly from the upper wall 53. The sealing ring gasket 27 seals the joint between the cooling chamber skirt 12 and the socket receiving housing 50.

Mounting means for mounting the control unit on a fixture such as a wall is in this case provided by a mounting flange 65 which is connected to the socket receiving housing side wall 55 by a connecting web 66. The flange 65 is formed with two axially extending slots 67 for ease of mounting to a fixture. The particular advantage of the control unit for a photoelectric switch according to this embodiment of the invention is that by using the electrical socket receiving housing attachment 50, the control unit may be used in a wide range of applications including security lighting and in other applications where if may be necessary to mount the control unit on a fixture such as a wall rather than directly on a lantern.

Referring te rig. 9 there 1s Illustrated a circuit dieg» pas? ©f s controller for use '-'1'ch a photoelectric switch such es the photoelectric switch described shows with reference to Figs. 1 to 8. The function of the controller is to switch power to a lantern according to the surrounding brightness. The controller comprises a photoelectric sensor, comparing means for compering the intensity pf sensed light with a preset light intensity and switching ^eans operated hy the comparing means to switch an output.

The photoelectric sensor is in this case provided by a light dependent resistor (LDR) which is used to monitor the light level and at specific threshold limits either to close or open the switch- The threshold limits are set to the requirements of the particular user.

The switching means is in this case provided by a relay BUI and a triac TRI1 connected in parallel. When it Is required to switch the load the triac TBI1 is engaged and this is followed a short time later by closure of the relay contacts RU1. sn this way the relay BL1 is not subjee£ The circuit operates as follows. A transformer TRF1 converts Incoming 2gOVrms (nom) to &&Vrms (oom) and this lower AC voltage Is rectified by a bridge consisting of diodes 01 to £><$. Capacitor Cl is smoothing capacitor. 24Vdc is the supply to the remainder of the circuitry.

The LDR resistance is monitored by an operational amplifier Al through a delay circuit consisting of a capac·* itor C2 and a resistor R2. This delay, of approximately 30 secondsP prevents abrupt changes 1« the value of the LOR reaching Al. The operational amplifier 1S used in the comparator mode and It compares the voltage level on the capacitor C2 with a threshold value which is determined by resistors R3„ 89 and Rb. Resistors R3 and R9 fora © potential divider and resistor.R6 provides hys~ teresis, This is to.prevent unwanted oscillation during th<£ switching action. Threshold calibration is possible by selecting valuta for Rsot that will cause the unit to switch at the correct light level. For high reliability th«? voltage across the op-amp must be held at a value less than the maximum rating of the device. The op-arap is rated for 3©Vb but a voltage of 12V which is provided by a transistor Ql Housed· A zener diode CR1 and a resistor R1Q hold the base voltage on the transistor Ql t1 at 12V below rail resulting 1« on emitter voltage which is approximately 11.4V below rail. The transistor Ql also acta as a regulator» further reducing the possibility of oscillatory switching.

Transistors Q4 and 05 allow the output of op-amp Al to switch the relay and the triac. A sewer diode CR2 is used to ensure thatthe transistors 04 and 05 are switched off when the op-amp output is 'high*. Th® IH301 output is enable to come within 1.5 volts of the rail value and would not» th@reforcD be able to switch off the transistors 0< and 05. The zener diodes overcomes this.

During darkness the voltage on pin 2 of the op-amp is low and the op-amp output is high. Therefore transistors Qo and 05 ere switched off during the night and on during the day.

The triac TEII1 is switched through transistors Q4, Q3 and Q§» The transistor 08 is disabled when the transistor 04 1$ turned on because its base is held at the emitter voltage. Mhen th® triac TRI1 is to be turned on the transistor 04 ceases to conduct allowing negative going pulses from a mains cycl® crossover detector, consisting of two transistors 06 and 07p to influence the transistor 08. These pulses occur when the mains sine wave is close to ger© volts and cause th® transistor 08 to Quickly charge up capacitor Co through the resis12 1stor Rgg. The voltage et th® base of th® transistor 09 than becomes high enough to switch on th® triac TRI1.

Th® resistor R24 which 1s connected between the gate of the triac TRI1 end th® emitter of the transistor 09 serves to limit th® current through the transistor 09 to a reltable level. Th© resistor R2Z also limits the peak current through the transistor 08 to a reliable level. Mhen the triac TRI1 Is to be switched off the transistor 04 starts conducting thus disabling the transistor 03. The voltage on the base of the transistor 09 now begins to decrease as the energy on the capacitor C6 is discharged through the resistor R23. After a time delay the voltage on the transistor 09 base will be too low to maintain ths triac THX1 in conduction. This time delay is needed for the triec/relay sequencing function and it provides sufficient time to allow the relay to disengage before the load voltage increases. for increased reliability and to reduce radio interference th© triac TRX1 is switched during the mains zero crossover points. These are detected by means of transistors 00 &iad Q7. Th© mains is sampled through a resistor R20 and limited by a xener diode CR3 and a diode 39. The resulting waveform that arrives at the base of the transistor QS 1s approximately square and is further squared by the transistor QS. A differentiator network consisting of $ capacitor £3 end 3 resistor RH provides a negative goli&g pulse time the voltage on the collector of tbe transistor 06 makes the transition from rail to ground. This transition represents the zero cross point when the main cycle Is going from negative to positive. In a similar manner the transistor $?□ capacitor C4 and resistor 816 will generate a pulse during the positive to negative mains cycle transition. Diodes D7 and 08 ensure that only the negative going pulses are transmitted to the base of the transistor 08.

The relay BL1 1s operated through transistors Q5 and Q10.

The relay 8L1 is of the normally ©n type and when the base voltage of the transistor Q10 Is low the switch will be closed. Mhen the relay Hl 1s to be engaged the transistor Q5 will cease to conduct. The voltage on the capacitor C5 will then begin fo decrease as if is discharged through the resistor 812. Sfter a delay dictated by the capacitor C5 and the resistor 812 the base voltage of the transistor Q10 will go below the minimum holding value for the relay PLI -and the relay contacts close. This delay 1s needed to allow th^ triacL4 TRI! which Is being switched sirnultaneously, tioei to reduce the voltage across the relay contacts to an acceptable level· This is part of the triac/relay sequencing function. When the relay RH Is to be disengaged (at morning) the transistor Q5 again conducts» charging up the capacitor C5 quickly. The relay Hl is then opened quickly before the triac T8I1 Is disengaged· thus protecting th© switch contacts. Diodes D5 and D6 protect the transistor Q10 from the back end that normally accompanies the switching of a relay. A normally 'on' relay Is used here s© that In the event of failure the street lantern being controlled will be switched on permanently. St Is expected that most failures within the controller will result In loss of power to the relay and the controller will thus meet the requirement that th® lantern he switched on. Varistor protection Is employed in th® circuit fo prevent large mains bourne spikes end transitions getting to the circuitry, a varisforB GH1» is placed after the mains transformer TRF1 and takes advantage of the transformer’s secondary resistance. The varis~ tor's resistance decreases significantly as the voltage across if increases and this is used to limit the voltage at the transformer secondary. The transformerP TRF1* because of its inherent winding resistances will not be damaged by the extra load placed on it during line transients. A varistor is used here in preference to a zener diode because of its greater spe&d and current handling capabilities. 2o LOR self heating is reduced by minimising the power dissipation within thg controller through the use of high external resistor values. Power dissipation is approximately 10«w. The use of transistors allows the voltage on the LOR to be measured despite the high source impedance due to th® large resistor values* Any suitable LOR may be used and preferably the LOR should be hermetically sealed to ensure that moisture and impurities cannot enter the LOR and -alter its characfercistlcs. The particular LDR chosien 1$ the; CLH5HGH device which has a stable light to resistance relationship after 100 hours burn-in.

All HPH transistors ere typically type 802370*92 while all PHP transistors are typically type 802128-92. All resistors are usually 5«*5S carbon. The elements designated X Are typically solid oluminium <®0V and those designated · ar© usually long life aluminium electrolytic ELCO series, A conformal coating compound may be applied tc both the board and the components to ensure rigidity of the comp» enents during heavy vibrations or lantern mast shudder» to protect the electronic circuit from impurities from moisture and dirt ingress and to further protect the board and the components from UV radiation through the conical canopy or cover. As ultra-violet tends to break down plasticn both the housing and the conical are doped with a recommended U.v. Inhibitor. The use of a relay assisted trlec overcomes th© problem associated with the switching of hefc'vy loads. Mhen relay contacts open or close arcing will occur between the relay contacts and tfti's wi 11» in time» cause the contacts to become pitted and eventually non-opgrational. The arcing problem could be overcome using a semiconductor switch in the form of a triac or transistor. However» such an arrangement because ©f its saturation voltage dissipates power result» ing in heating. This heating will severely curtail the -16reliability of the device. Thus, the controller according to this aspect of the invention is particularly reliable in use and greatly increases the useful life of a photoelectric control unit. The method of switching using a relay assisted triac has been tested by subjecting a prototype to 40,000 switching operations which is the equivalent of over 100 years operation , and when examined after the test the relay way found to be in excellent condition IO without any signs of pitting.

A phototransistor may be employed as photoelectric sensor instead of the LDR used in the embodiment described above.

Claims (5)

1. a controller for a photoelectric control unit, the controller comprising a photoelectric sensor, comparing means for comparing the intensity of sensed light with preset light intensity and switching means operated by the comparing means to switch a lamp, said switching means comprising a switch which operates over a large voltage range.

2. A controller as claimed in claim 1 wherein the lamp is a street lamp.

3. A controller as claimed in claims 1 or 2 wherein the switch comprises a relay and a triac electrically connected in parallel.

4. A controller as claimed in claim 3 wherein the triac is provided in a triac control circuit which is operated by the comparing means. 5. A controller as claimed in claim 3 or 4 wherein, on switching of the switch, the relay conducts at low voltages and the triac conducts at high voltages. 6. A controller as claimed in any of the claims 3 to 5 including a capacitor which is charged when the triac operates to keep the relay closed for a short period to ensure continuous conduction. 7* A controller as claimed in any of claims 1 to 6 wherein the photoelectric sensor comprises a light sensitive resistor whose resistance increases with the intensity of incident light. -188. A controller as claimed in any of claims 1 to 6 wherein the photoelectric sensor comprises a phototransistor 9. A controller for a photoelectric control unit substantially as hereinbefore described with reference

5. To Figure 9 of the accompanying drawings.