The names on the left are the names of UK photocell companies, past and present, that I have examples of in my collection. Clicking on these links will take you to a page dedicated to that particular company, with a brief company history and examples of photocells made by the company over the years.
PLEASE NOTE THAT I AM NOT A SUPPLIER OF PHOTOCELLS. Please contact the companies listed on the Sales page for new/replacement photocell units. For details of how to wire and install NEMA photocells, please click here to view instructions prepared by RS Components.
Photographs of my photocell testing device can be seen here.
What are Photo Electric Control Units (PECUs)?
PECUs are light operated switches. They switch the supply ON to a load when the light level falls beneath a given value (usually at Dusk), and switch the supply OFF when it rises above another level (usually at Dawn). The ratio between the two light levels is known as the switching ratio.
When the ON level is lower than the OFF level (e.g. if the load switched ON at 70Lux and switched OFF again at 105Lux, then the unit has a ratio of 1:1.5). As can be seen from Table 1, below, positive ratio units have significantly longer annual burning hours than negative ratio units.
When the ON and OFF levels are the same (1:1 ratio).
Most street lighting loads have a warm-up time during which the lamp achieves full brightness. This is typically a number of minutes, and the idea of negative ratio units is that the ON level needs to predict when the lamp will achieve sufficient output, whereas the OFF level is when the light is no longer needed. Typically a 1:0.5 ratio is ideal for this (e.g. turning on at 70Lux and OFF at 35Lux).
Part Night PECUs
Using advanced functionality, PECUs can be designed to predict the time of day by measuring the day length. They are then able to turn off at midnight, and turn back on at say 5:30am. The time is determined from cumulative light measurements at dusk and dawn over a number of nights. The timing is not exact and does not compensate for changes in the time due to daylight saving. Use of such systems can dramatically reduce annual burning hours. For the timing to be maintained it is important that the PECU remains continuously powered. If the power is removed, it will take the PECU some days to re-synchronise and re-establish the correct timing. Examples of such products are the Royce Thompson Night Star and Zodion SS9.
A further variant is a Part Night dimming PECU. These PECUs operate similarly to the above, but instead of extinguishing the light during the early morning hours, the lamp is operated at reduced output. Dimming is achieved by use of suitable control gear in the lantern. Whilst this does not affect burning hours, it reduces the average consumption as the light is operating at reduced consumption for several thousand hours annually. An example of such a product is the Zodion Sontek.
PECUs affect energy consumption in two ways:
The PECU consumes energy. The amount varies according to the technology used. Generally consumption is less than 5 W (approx 44 kWh/year consumption) and many are less than 0.25 W (which equates to 2.2 kWh/year)
Burning hours have the most dramatic effect on consumption. A further 100hrs/year on a typical 100 W load results in an additional 10 kWh/year consumption. Switching level, switching accuracy, and switching ratio most affect burning hours.
A major consideration is how accurately and consistently the load is controlled over a long period of time. PECUs have benefits over time switches since they turn the lights on when the light falls, ensuring that light is provided when there is poor visibility prior to dusk, yet providing optimum control on clear evenings, taking full opportunity of good daylight. The total number of hours that a PECU operates the light for each year is called the annual burning hours. Table 1 below shows a typical example of the burning hours for PECUs with various switching ratios and switching levels. The actual number of hours will vary according to the weather profile of the year, and exact location of the installation.
Table 1: Comparison of typical burning hours
Daylight contains much more than just visible light; however, our eyes are only sensitive to a specific band of wavelengths. The intensity of visible light, corrected for the eyes' varying sensitivity to colour is measured in Lux. Wavelengths of daylight that the eye is not sensitive to do not contribute to a measurement of Lux.
The day-night cycle results from the earth's rotation. A consequence of this is a relatively quick decline of light at dusk. The proportion of visible light to other wavelengths varies not only on a diurnal cycle, but also seasonally and as a result of weather conditions. This means that only direct measurement of visible light level can accurately reflect the level of light that our eyes see as a result of daylight. This poses a problem when sensors used in PECUs have marked sensitivity to wavelengths of daylight outside of that visible to our eyes.
These sensors, although very accurate, may not be accurate at measuring Lux. A particular issue is Infra-Red (IR). Daylight contains significant infra-red; however, IR is strongly affected by atmospheric conditions and can be strongly attenuated in conditions where visible light is much less affected. Accordingly, IR levels can be much lower than those anticipated by the level of visible light. Many semiconductor sensors have marked sensitivity to IR as well as visible light; hence a PECU with significant IR sensitivity could switch on before reaching the target visible light level (in Lux). Sensors with IR sensitivity tend to underestimate equivalent visble light levels in daylight rather than over-estimating them, hence they always tend to operate in a 'safe' manner.
Sensors used in PECUs
The most critical part of any PECU is the light sensor. The spectral sensitivity and long-term stability play an important role in providing reliable daylight detection.
Table 2: Comparison of sensors commonly used in PECUs
Cadmium Sulphide (CdS)
CdS sensors operate as light dependant resistors. They have conductivity approximately proportional to the level of light. They were the predominant sensor in the early 1990s and are still frequently used. CdS sensors are becoming less popular due to environmental considerations and they are also subject to some long-term drift.
It is possible to construct a semiconductor diode to produce a current proportional to the incident light level. The currents generate are small and need careful amplification by a circuit that compensates for thermal effects. The fundamental physics of semiconductor junctions means that silicon photodiodes have significant sensitivity to light outside the visible spectrum. Filters can take a number of forms, but are usually bulk-coloured glass slips added to the photodiode assembly during manufacture. Zodion's DyeMatch is a novel approach whereby a special Infra-red blocking due is added to a polymer and special lens caps moulded from this material are fixed over the photodiodes.
A variant of the above is to add a filter that blocks all visible light, so that the photodiode is only sensitive to Infra-red light. This can be incorporated into a PECU that is only sensitive to IR, and virtually insensitive to visible light. This means that the PECU can be used where it is partially illuminated by visible light, often produced by the light that is being controlled by the PECU. Such a product is the Zodion 'Lowlight'. It is typically used in bollards, where the PECU is incorporated into the base compartment and is illuminated by visible light from the lamps within the base reflected from the inside of the cover. The Lowlight is insensitive to this light, and operates by sensing the IR component of daylight transmitted through the cover.
There are three cautionary notes:
Photodiode sensors can also be incorporated into integrated circuits although the functionality may be limited due to the competing requirements of semiconductor processing for optoelectronics and integrated circuits.
PECUs typically use one of three devices to switch the load:
These relays are suited to use with CdS sensors. They have a number of drawbacks, principally power consumption and size. They are increasingly being replaced by products using other load switches. They operate by the action of a heating element on a bi-metal strip, as the strip deflects it causes contacts to make, or break.
These relays are widely used in many applications. They are both small and capable of operation at low power. They are relatively poor at transferring high inrush loads (common to many street lighting applications); this can be mitigated by techniques such as predictive load transfer.
There are a number of semiconductor devices capable of switching street lighting loads. Triacs are the most common; however, Thyristors and MOSFETs have also been used. These devices are reliable, simple to control, and have a good ability to transfer high inrush loads. It is relatively easy to implement zero-cross switching with semiconductor switches.
Table 3: Load Switching Devices
Characteristics of a Street lighting Load
Typical lighting loads have a number of principle characteristics:
Most discharge lighting loads require Power Factor Correction. The most common method of providing this is by the use of PFC capacitors. When connected to the supply it is possible for many hundreds of amps to flow momentarily. The magnitude and duration of the inrush current depends upon the value of capacitance (measured in μF), and the impedance of the supply network. This current is capable of welding together contacts of relays and other switching devices. It is also possible for these currents to fuse semiconductors, and it is reliant on good product design to ensure that the effect of inrush has been taken into account.
Power factor can dramatically change the characteristics of a load. Whilst a load may be 100 W, and draw about 0.4A at unity power factor, it will draw over 1A with a power factor below .035 (common for discharge loads with failed PFCs). Also, various common lamp or ballast faults can give rise to a situation where just the PFC is connected. In this case although no power is consumed the current through the PFC will also be above 1A. Hence the PECU must be capable of operating the load over the range of likely load conditions, including common failure modes of lantern control gear.
It is important to ensure that the maximum ratings are not exceeded, care must be taken to observe both the current rating and also the maximum capacitive load that can be connected.
Formats for Photocells
The 'NEMA' socket is partially defined in BS5972 and has become the de facto connector for most 1-part photocells. The three connections are the incoming live, the switched live out, and a neutral connection (used only in powering the PECU).
This allows for the direct mounting of a photocell onto a Æ20mm thread. Connection is made via wire leads. This arrangement is often used for 'special' photocells where more than three connections are required.
Miniature Photocells are principally used for direct integration within luminaires. As they operate inside the luminaire, specific consideration needs to be given to the operating temperature. It is possible for the internal temperature within a street lighting luminaire to rise to over 100°C, and the photocell needs careful selection to ensure that a reasonable life is achieved.
Two part units are employed where space in the luminaire is at a premium or if the control gear is located in the base of the lighting column. A control unit is also positioned in the base, and a detachable detector is fitted in the luminaire or on another nearby surface that is out of the way of other light sources.
Ingress Protection (IP) Grading
This designation relates to the degree of weatherproofing. IP65 is ideal for use in all applications; however, IP67 may be applicable where the unit is subjected to power washing, or exceptionally forceful jets of water.
© 2002 - English Street Lights Online