2. Control Basic

The following is a quick review of the types of conventional and electronic controls.

2.1 Thermal Controls

Conventional thermal bimetallic controls are commonly used where initial cost is the primary concern. They are made by several manufactures in the USA, Mexico, South America and elsewhere. Most of the generic controls sold through electrical distributors or home centers are thermal type controls.

The principle of operation is simple. A cadmium sulphide cell is in series with a resistor that is mounted on a bimetallic strip. When light shines on the cell (day) the cell conducts and line voltage flows through the resistor causing it to get hot. This causes the bimetal to bend and toggle a snap blade which in turn opens a set of contacts. This turns the load or luminaire OFF. At night the cell current is low, the bimetal cools and the contacts close.

Because of the thermal mass of the bimetal and resistor, these controls have a long time delay of ½-3 minutes for both turn-ON and OFF. This can be frustrating to utility workers when they want to do a "glove test" on a fixture with a thermal control. It also makes accurate factory calibration very time consuming. Therefore, many manufacturers of thermal controls do calibration to very wide tolerances on several controls at one time. Typical published specifications list the turn-ON levels as a range from 1-3 footcandles (10.8-32.3 lux). One manufacturer lists it as 2-10 footcandles (21.5-107.6 lux).

Turn-OFF levels of thermal controls are between 3-5 times turn-ON levels. This wide ratio of OFF to ON is one reason that thermal controls are not generally used where energy conservation is important. The high turn-OFF wastes morning energy.

Unless the control has well designed ambient compensation, thermal controls can have marginal performance at high or low temperatures.

Because thermal controls generally occupy the lowest price category, quality sometimes comes after economies of production. Although it is possible to use either spark gap or MOV surge protection in these controls, many are sold with no form of surge protection. When these controls find their way to a utility, the result is a very short life and frequent replacement.