2.2 Load Switches, Flash Transfer Relays, and Power Supplies

Load Switches, Flash Transfer Relays, and Power Supplies

Inside the traffic cabinet, the controller acts as the brain, making decisions about which phases should be green, yellow, or red. However, the controller itself operates on low-voltage digital logic and does not have the electrical capacity to directly power the 120-volt incandescent or LED signal heads out on the street. To translate the low-voltage decisions of the controller into the high-voltage power needed for the signal displays, the cabinet relies on a robust system of power distribution and control components. As a Level I Technician, mastering these components—specifically load switches, flash transfer relays, and power supplies—is essential for diagnosing intersection failures safely and effectively.

Power Distribution and Load Switches

The bridge between the controller's logic and the field wiring is the load switch. A load switch is essentially a set of heavy-duty, solid-state relays packaged into a single module.

Operating Principles of Load Switches

In standard NEMA and 170/2070 cabinets, the controller sends a 24VDC (Volts Direct Current) control signal to the input side of the load switch. This 24VDC signal acts as a trigger. When the load switch receives this low-voltage signal, it activates its internal solid-state circuitry (typically using TRIACs—Triodes for Alternating Current) to close the circuit on the load side.

The load side of the switch is connected to the 120VAC (Volts Alternating Current) power bus. When triggered by the 24VDC control signal, the load switch allows 120VAC to flow out to the field wiring and illuminate the signal indication (Red, Yellow, or Green). Therefore, the load switch operates at 24VDC control and 120VAC load. This optical isolation between the low-voltage control circuitry and the high-voltage field wiring protects the sensitive microprocessors in the controller from electrical surges and spikes originating on the street.

Load Switch Configuration

A typical load switch is configured to handle one complete traffic phase, meaning it contains three distinct circuits: one for Red, one for Yellow, and one for Green. When you look at a load switch in a cabinet, you will often see three LED indicators on its faceplate. These LEDs illuminate when the corresponding 24VDC input signal is received from the controller, providing a quick visual confirmation that the controller is calling for a specific display. If the load switch input LED is illuminated, but the field display is dark, the technician immediately knows the problem lies in the load switch itself, the field wiring, or the signal head, rather than the controller.

Flashers and Flash Transfer Relays

When a major malfunction occurs, or during scheduled late-night operations, the intersection must default to a safe, recognizable state: flashing operation. This is accomplished using Flasher units and Flash Transfer Relays (FTRs).

Flasher Units

The Flasher is a solid-state device similar in appearance to a load switch. Its sole function is to take 120VAC continuous power and output it as a pulsing 120VAC signal (typically flashing at a rate of 50 to 60 flashes per minute with a 50% duty cycle). Standard NEMA cabinets usually contain two flasher units to distribute the electrical load of the flashing signal heads.

Flash Transfer Relays (FTR)

While the flasher generates the pulsing power, the Flash Transfer Relay (FTR) is responsible for routing that power to the signal heads. Under normal intersection operation, the FTR is energized by a 24VDC or 120VAC control signal, depending on cabinet design. In this energized state, the FTR connects the field wiring to the outputs of the load switches, allowing the controller to run the intersection normally.

When a critical failure is detected (such as a conflicting green indication), the conflict monitor drops the control voltage to the FTRs. The FTRs de-energize and physically switch their internal electromechanical contacts. This action disconnects the field wiring from the load switches and connects it directly to the pulsing outputs of the Flasher units. Because the FTR relies on gravity and mechanical spring tension to drop into flash when power is lost, it is considered a "fail-safe" mechanical component. Changing the intersection from normal operation to flash mode is the ultimate safety mechanism of the cabinet.

Power Supplies and Battery Backup

The cabinet requires steady, reliable power to operate its electronic components.

Cabinet Power Supplies

The main cabinet power supply converts incoming 120VAC utility power into the stable 24VDC and 12VAC required by the internal logic components, such as the controller, conflict monitor, and BIUs. In a NEMA TS2 cabinet, the power supply is critical; if it fails or experiences a voltage dip, the BIUs will lose communication, and the intersection will immediately drop into flash.

UPS and Battery Backup Systems (BBS)

Power outages pose a significant safety risk at signalized intersections. To mitigate this, many modern cabinets are equipped with an Uninterruptible Power Supply (UPS) or Battery Backup System (BBS).

A typical BBS consists of an inverter/charger unit and a bank of deep-cycle batteries. When utility power is present, the BBS conditions the power going to the cabinet and charges the batteries. If utility power is lost, the BBS instantaneously switches to battery power. The inverter converts the DC voltage from the batteries back into 120VAC to power the cabinet components and the signal heads.

BBS systems can be programmed for different operational modes during an outage. For example, they may provide full normal operation for the first hour of an outage, and then automatically force the intersection into flash mode to conserve battery life for several more hours. As a technician, maintaining the batteries and testing the BBS transfer switch is a critical preventative maintenance task.

Understanding the flow of electricity—from the utility service, through the power supply, into the controller, out to the load switches, through the FTRs, and out to the street—is the foundation of traffic signal troubleshooting.