2.2 AC Relay Controls

The next most common type of conventional control is called the AC relay or electro-mechanical control. It uses a cadmium sulphide cell in series with an AC relay coil powered by the AC line voltage. Like the thermal control, the current through the cell is determined by the amount of daylight. As the current rises with the sun, the magnetic field of the AC relay increases and pulls the armature with increasing force. When this force is stronger than the armature spring force, the relay contacts open and the load or luminaire goes OFF. The opposite occurs at sundown.

Operation of the AC relay control is considered instant for turn-ON and OFF. There will often be buzz and a "slow break" condition during turn-OFF as the slowly rising AC coil current causes
the armature and contacts to chatter or buzz just before opening.

Turn-ON levels of AC relay controls can be set very quickly because of the fast operation of the control. Some manufactures use adjustable internal screens, others use windows with varying transmittance. Turn-OFF levels are usually about 3 times turn-ON. Smaller ratios are possible with hand made controls but, in the reality of a production world, expect a 3:1 average OFF to ON ratio.

In the past, the AC relay control was the most common type used at US utilities. Initial cost was low: about $3.00 to $5.75. The wide ratio and the waste of morning energy weren't important to utilities. Or, perhaps they were important, but no cost effective alternative was known.

AC relay controls are often made without surge protection for OEM and homeowner use. When required for utility use, they are commonly made with air gap arrestors or so called "expulsion" air gap arrestors. AC relay controls can also be used with MOVs when the end user wants superior surge protection. MOVs are rarely used unless specified and the customer is willing to pay a little more.