What is Ohm’s law in AGL?
V = I × R on a series lighting circuit — the CCR holds the current, and the resistance of the loop decides the voltage it must supply.
Ohm’s law says voltage = current × resistance. On an airfield series circuit the CCR fixes the current for each brightness step, so every fixture on the loop carries the same current. What changes is the resistance: cable plus transformer primaries added up around the loop.
So the voltage follows the resistance. Add load and the CCR raises its output to hold the same current. At its rated output it cannot go higher, and the current can no longer be held. The same relationship is why loop resistance is recorded at commissioning and compared later.
- State it. V = I × R: voltage, current and resistance are tied together.
- Fix I. The CCR sets the current for each brightness step.
- Add up R. Cable and transformer primaries around the whole loop.
- Read V. The CCR output voltage follows the total resistance.
- Know the limit. The CCR rating caps voltage, so current can no longer be held.
- Use it. Compare loop resistance to the baseline and check current per step.
Frequently Asked Questions
Q1What is Ohm's law and how does it apply to AGL?
Ohm's law says voltage equals current times resistance (V = I x R). On an airfield series lighting circuit the CCR fixes the current for each brightness step, so the voltage it must supply follows the total resistance of the loop.
Q2Why does AGL hold current constant rather than voltage?
In a series circuit the same current flows through every fixture, so each lamp receives the same current wherever it sits on the loop. Brightness follows current, so holding current constant keeps the lights matched.
Q3What happens to the voltage when more fixtures are added?
Total resistance rises, so the CCR raises its output voltage to hold the same current. This continues until the CCR reaches its rated output; beyond that it cannot maintain the set current.
Q4What does a change in loop resistance tell you?
A reading that drifts from the commissioning baseline means the circuit has changed. A poor connection or damaged cable can raise resistance, so a change is a prompt to investigate rather than a diagnosis by itself.
Q5Does Ohm's law apply exactly on a running AC circuit?
In simplified form. Cable resistance is measured directly, but transformers and cable add reactance on the running AC circuit, so total impedance (Z) is used. The relationship V = I x Z has the same shape.
Q6Where is Ohm's law used in testing?
In reading loop resistance against the commissioning baseline, and in checking that the CCR output current matches the rated value for each brightness step. The expected values come from the design and the CCR manufacturer.
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.