2.3 AC Relay Controls with Time Delay

One of the disadvantages of the conventional AC relay control is that turn-OFF is instantaneous. A flash of lightning or a mis-aimed car headlight may be bright enough to turn the control OFF for a fraction of a second. This creates a problem with the inherent cool down and restrike time of HID lights. For high pressure sodium (HPS), the light may stay OFF for 15-30 seconds; for mercury vapor 2-4 minutes; and for metal halide it may be 10-20 minutes. Also, hot re-strike detracts from the life of HPS lamps. Therefore, it is desirable for photocontrols to have a turn-OFF time delay longer than possible night light flashes.

AC relay controls are available with optional short turn-OFF delay, sort of. A device called a positive temperature coefficient resistor (PTC) across the relay coil will shunt the current away from the coil until the PTC heats up and takes itself out of the circuit. This heat up time depends on ambient temperature but at 25°C it is about 3-6 seconds. It is a simple, inexpensive way to add time delay to an AC relay control. Unfortunately it isn't without some shortcomings.

• It only works once and then has to "rest." If the delay is used, the PTC gets hot and has to cool down before it will operate again. This reset takes about 30-60 seconds at 25°C (77°F). Longer if it is hotter outside.

• Because the PTC is a thermal element, time delay is longer at colder temperatures and shorter at higher ambient temperatures. Above 65°C (which is common on a summer night on top of a energized floodlight or NEMA head fixture) the time delay disappears.

• The temperature coefficient that makes the time delay work also makes the turn-ON level of the control change inversely with temperature. A 1.5 footcandle turn-ON control at 25° C will not be a 1.5 footcandle control at 0°C. If turn-ON levels are important, (as they should be at energy conscious utilities) ask the control manufacturer for a turn-ON versus temperature graph.