What is a constant-current circuit?
A circuit whose current is held fixed by the CCR — the voltage adjusts, so every lamp on the loop gets the same current.
A constant-current circuit is one where the supply holds the current at a fixed value whatever the load. In AGL the supply is the CCR: it measures the loop current, compares it with the value set for the chosen brightness step, and adjusts its output voltage to keep the current there.
This suits lighting because brightness follows current. With the same current through every lamp, they all shine equally, regardless of position, cable length or the voltage dropped along the way. Each brightness step is a different fixed current. The CCR can only do this within its rating: an overloaded or open loop defeats it. The voltage it uses is covered under series circuit voltage.
- Set. The chosen brightness step sets the target current.
- Sense. The CCR measures the actual loop current.
- Compare. The measured current is checked against the target.
- Adjust. Output voltage is raised or lowered to correct any difference.
- Hold. The current stays at the set value as the load or cable changes.
- Protect. The CCR shuts down if it cannot hold the current, as on an open loop.
Frequently Asked Questions
Q1What is a constant-current circuit?
A circuit in which the supply holds the current at a fixed value whatever the load, by changing its output voltage as needed. In AGL the supply is the constant-current regulator, or CCR, feeding a series loop.
Q2Why do AGL systems use constant current rather than constant voltage?
The brightness of a lamp depends on its current. With the same current through every lamp on the loop, they all shine equally, whatever their position, the cable length or the voltage dropped along the way.
Q3How does the CCR hold the current constant?
It measures the output current, compares it with the value set for the chosen brightness step, and raises or lowers its output voltage to bring the current back to that value. The current stays fixed while the voltage varies.
Q4How is the brightness changed?
Each brightness step is a different fixed current. A higher step sets a higher current and a brighter light, up to the maximum, commonly 6.6 A. The CCR then holds that new current.
Q5Does the CCR hold the current whatever happens?
Only within its rating. If the load is too large, it cannot reach the set current on the higher steps and the lamps run dim, or its overload protection trips. If the loop opens, it cannot drive current at all and its protection normally shuts it down.
Q6How does a constant-current circuit differ from a series circuit?
A series circuit describes the layout: one loop with every load in it. Constant current describes how it is powered. AGL uses the two together, so the same current flows through every lamp.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.