Constant-Current Regulator

Runway lighting systems are supplied from series circuits served by constant-current regulators (CCRs). Each lighting circuit on the airfield has a separate regulator. The CCRs maintain the output current throughout its rated output value, depending on the load. Some of the regulators are equipped with brightness controls. These brightness controls adjust the brightness of the lamps in the lighting system to compensate for visibility conditions.

The CCR uses solid-state devices to maintain a constant-current level in its respective lighting system. The regulators are silicon-controlled rectifiers (SCRs) in a feedback circuit to obtain a constant-current output instead of resonant circuits, moving transformer elements, or saturable reactors. The SCRs are controlled to vary the part of a cycle during which the current is permitted to flow into the load circuit. In the load circuit, the current is maintained constant at any value preset with the brightness control by means of a feedback circuit as the load resistance is varied from maximum to zero. The block diagram (fig. 2-3) shows the elements constituting the regulator. Load current is measured by the current transformer and the Hall unit or multiplier unit, which has an output voltage proportional to the square of the load current. The Hall or multiplier output is filtered and fed into the input of an amplifier and compared with an input from a brightness control potentiometer. The output voltage is a function of the difference between the two inputs. The output voltage is applied to the input of the gate pulse generator that determines the condition angle of the SCRs and changes it to bring the system to equilibrium. Transient overload protection is provided for the semiconductor element of the Hall unit. Open-circuit protection is provided when no current is drawn by the load and the brightness potentiometer output voltage is any value other than zero. Under these conditions, the SCRs will be prevented from

Figure 2-3.\Block diagram of constant-current regulator.

conducting, and the output voltage to the load will be zero.

Remote Control

The airfield lighting systems may be operated completely by the remote control panel assembly. The only operation required at the electrical distribution vault is to ensure that all circuit breakers are engaged, the regulators are set for remote operation, and the load switches are in the ON position. As the electrician, ensure that the unit is installed properly and that the different levels of light intensity desired can be achieved. Figure 2-4 is a typical view of a remote control unit that you may encounter in the installation of a contingency airfield lighting system.

The unit uses 120 volts as the control voltage with low-burden pilot relays to compensate for the voltage drop caused by the long distances usually found between the control tower and the vault. In this type of control system, the switches on the control panel actuate low-burden relays; these, in turn, actuate the power switches, contactors, and the relays controlling the regulators that supply the airfield lighting circuits.

Both the tower and vault control panels are wired into a double-throw "transfer-relay cabinet" located in the vault. This is shown in figure 2-5 with a single line representing the control cable. The transfer-relay can connect either control panel to the pilot-relay cabinet. It can switch the system control from the tower to the vault or from the vault to the tower. The transfer relay has an eight-pole, double-throw, transfer-relay assembly unit. This unit is actuated by a toggle switch.

The low-burden pilot relay is designed to operate at a wide range of voltages lower than the designed 120-volt ac rating. The pilot relay can be actuated at voltages from 50 to 90 volts ac.

The standard control cable is a sevenconductor, 600-volt, Buna-insulated, polychloroprene-sheathed cable. One conductor (black) is a No. 12 American wire gauge (AWG), and the remaining conductors are No. 16 AWG. The No. 12 conductor is the hot lead, and the No. 16s, the "switch legs."