What is a sign circuit?
Airfield signs are powered from a series loop. A regulator pushes a fixed current around it, and each sign takes its share through its own isolating transformer.
Signs, like most airfield lights, are not wired in parallel from a mains supply. They sit on a series circuit, and that choice shapes how signs are powered, dimmed and protected against a failure along the way.
1One loop, one current
A constant-current regulator (CCR) drives a fixed current around a loop of cable. The same current flows through every point, so every sign receives the same drive wherever it stands. Raising or lowering that current in steps changes the brightness of the whole circuit at once.
2An isolating transformer at each sign
Each sign has its own isolating transformer. Its primary winding sits in the series loop and its secondary feeds the sign, so the sign is electrically separated from the loop. The loop stays closed when a sign is disconnected or fails, so one fault does not put out every sign. An open secondary is not harmless, though: it raises the voltage across the primary and heats the transformer, so a dark sign should be repaired promptly. Inside the sign, an LED driver turns the transformer output into the supply the LED module needs.
3Dedicated or shared
Signs may be on a dedicated sign circuit or share a loop with taxiway lighting, depending on the layout and the design. The circuit is sized so the total load of transformers and cable stays within the CCR rating. Transformer, connector and testing have their own entries later.
| Element | Role |
|---|---|
| CCR | Constant-current regulator, driving a fixed current around the loop and setting brightness steps |
| Series loop | Cable that carries the same current through every transformer |
| Isolating transformer | Primary in the loop, secondary feeding one sign, and electrically separating the two |
| LED driver | Converts the transformer output to the supply the LED module needs |
| Sign | LED module and face, powered from the driver |
| Arrangement | Dedicated sign circuit or shared with taxiway lighting, set by the design |
Frequently Asked Questions
Q1What is a sign circuit?
The series loop of cable that powers airfield signs. A regulator called a constant-current regulator (CCR) drives a fixed current around the loop, and each sign draws its power through its own isolating transformer.
Q2Why are signs on a series circuit?
The same current flows through every point in the loop, so every sign receives the same drive wherever it stands on the airfield, and brightness can be changed for the whole circuit from one place.
Q3What does the isolating transformer do?
Its primary winding sits in the series loop and its secondary feeds the sign. It electrically separates the sign from the loop, and the loop stays connected when a sign is disconnected or fails. Transformers have their own entry later.
Q4Do signs have their own circuit?
Either. Signs may be on a dedicated sign circuit, or share a circuit with taxiway lighting, depending on the layout and the design. The choice affects how signs are dimmed and how a fault is isolated.
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