What is the series-circuit principle?
One idea behind airfield lighting circuits: connect the lights end to end on one loop, and the same current flows through every one of them. So every light can be given the same output.
Think of water in a closed ring pipe: whatever flows past one point must flow past every other. Electric current in a single loop behaves the same way. That is the series-circuit principle, and it is why an airfield can have hundreds of lights on one circuit and still have them match.
1Same current, same output
A light’s output depends on the current through it. On one loop, each light carries the same current, so each gives the same output, however far along the loop it sits or however long its cable. A parallel circuit would not do this, because cable voltage drop would leave the far lights dimmer.
2Voltage is shared
The loop’s supply voltage divides between the lights and the cable. The voltage drops add up to the total, so adding lights raises the voltage the supply must give. A constant-current regulator adjusts its voltage to keep the current at the chosen brightness step, and isolation transformers let each light share the loop and keep it complete.
3The weak point is a break
Because there is only one path, an open circuit stops the current and every light on the loop goes out. So the loop must stay unbroken, and faults are found by testing continuity and insulation along it. Both are covered in later entries.
| Term | Meaning |
|---|---|
| Series circuit | Parts connected end to end in one loop |
| Current (I) | The same at every point on the loop |
| Voltage (V) | Shared between the lights; the drops add up |
| Constant-current regulator | Holds the loop current steady |
| Isolation transformer | Lets each light share the loop and keeps it complete |
| Open circuit | Breaks the loop and every light goes out |
| Parallel circuit | Same voltage on each branch; each takes its own current |
Frequently Asked Questions
Q1What is the series-circuit principle?
When parts are connected end to end in one loop, the same current flows through every one of them. The supply voltage is shared between them, and the voltages across each part add up to the total.
Q2Why do airfield lights use series circuits?
Light output depends on current. If every light carries the same current, every light gives the same output, whatever its position or the length of cable to it. A constant-current regulator holds that current steady.
Q3How is a series circuit different from a parallel circuit?
In a parallel circuit each branch has the same voltage and takes its own current. In a series circuit each part has the same current and takes its own share of the voltage.
Q4What happens if a series loop is broken?
With the loop open, no current flows, so every light on it goes out. That is why the loop must stay complete, and why open circuits are found by testing continuity along the loop.
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