What is the secondary winding?
The secondary is the winding that supplies the light fitting. It is electrically separate from the loop, its voltage follows the load, and a break in it puts out only that one fitting.
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Normal. The flux from the primary induces a current in the secondary, which flows through the fitting and back. The secondary current follows the loop current (equal to it on a 6.6/6.6 A unit), while the voltage follows the fitting’s power: about 6.8 V for 45 W at 6.6 A (illustrative).
The secondary winding is the winding of a series isolation transformer that supplies the light fitting. It has no electrical connection to the primary: the magnetic field from the loop current induces a current in it. That current follows the loop current (equal to it on the common 6.6/6.6 A transformer; other ratios, such as 20/6.6 A, are marked on the nameplate), and the voltage across it follows the load, about power ÷ current, so 45 W at 6.6 A is about 6.8 V.
A fault here stays on the fitting side. If the secondary goes open, only that one fitting goes out and the loop keeps running, unlike an open primary, which breaks the whole circuit. With no load, the terminals can rise to a hazardous voltage, far above the normal few volts, so treat an open secondary as live (see Open-Secondary Behaviour and Core Saturation). Match the fitting to the transformer rating on the nameplate, and isolate and prove the circuit dead under site procedures before touching any lead.
Frequently Asked Questions
Q1What is the secondary winding in AGL?
The winding of a series isolation transformer that supplies the light fitting. It has no electrical connection to the primary: the magnetic field from the loop current induces a current in it, and that current flows through the fitting.
Q2What current and voltage does the secondary carry?
It follows the loop current: on the common 6.6/6.6 A transformer the two are equal, and both fall together at lower brightness steps. Other ratios, such as 20/6.6 A, are marked on the nameplate. The voltage is not fixed: it follows the load, roughly the fitting’s power divided by the current. For example, 45 W at 6.6 A is about 6.8 V.
Q3What happens if the secondary goes open-circuit?
Only that one fitting goes out, because the primary still passes the loop current. With no load, the secondary terminals can rise to a hazardous voltage, far above the normal few volts, so an open secondary must be treated as live.
Q4Why must the fitting match the transformer rating?
A fitting that draws more power than the transformer is rated for pushes the winding beyond its design, so it may run hot and fail early. The rating is on the nameplate; check it against the fitting before installing or replacing a unit.
Q5Is the secondary safe to touch because the voltage is low?
No. The normal secondary voltage is low, but an open secondary can rise well above it, and the leads and fitting sit on a circuit whose primary is part of a high-voltage loop. Isolate and prove the circuit dead under site procedures before any work.
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