What is the transformer principle?
Alternating current in one winding creates a changing magnetic flux that induces a voltage in another — power moves across with no electrical connection.
The transformer principle is electromagnetic induction. Alternating current in the primary winding produces a changing magnetic flux in the core. That flux links the secondary winding and induces a voltage in it, so energy passes from one side to the other through the magnetic field alone.
The turns ratio sets the voltage: more secondary turns raise it, fewer lower it, and the current changes in the opposite sense so that power in stays roughly equal to power out. Because the windings are only magnetically linked, the secondary is electrically isolated from the primary — the idea behind the isolation transformers that feed fixtures in an AGL series circuit. There the CCR forces a constant current through the primary, so the secondary current follows it through the current ratio and the secondary voltage is whatever the lamp load needs.
- Energise the primary. AC flows through its turns and sets up a changing flux.
- Carry the flux. The magnetic core guides almost all of it through the secondary winding.
- Induce the output. The secondary voltage follows the turns ratio and drives the load.
Frequently Asked Questions
Q1What is the transformer principle?
Alternating current in the primary winding creates a changing magnetic flux in the core. That changing flux induces a voltage in the secondary winding. This is electromagnetic induction, also called mutual induction.
Q2Why does a transformer only work on AC?
Induction needs a changing flux. Steady DC produces a constant flux and induces no voltage in the secondary. The CCR therefore supplies alternating current to the series loop.
Q3What decides the output voltage?
The turns ratio. The voltage ratio equals the ratio of turns on the primary to turns on the secondary, so more secondary turns give a higher voltage and fewer give a lower one.
Q4What happens to the current?
It changes in the opposite sense to the voltage. A step-down transformer gives a higher current on the secondary, a step-up gives a lower one, and the power in is approximately the power out apart from small losses.
Q5Are the primary and secondary connected electrically?
No. They are linked only by the magnetic flux in the core, so the secondary is electrically separated from the primary. This separation keeps the fixture isolated from the high voltage of the series circuit. It does not make the secondary safe to touch while the circuit is live.
Q6How does this apply to an AGL isolation transformer?
The series loop current flows through the primary, and the secondary feeds the lamp or LED fixture. Series isolation transformers commonly have a 1:1 current ratio, so the fixture receives the loop current while staying electrically isolated from the primary.
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.