Mixed halogen and LED circuits
One loop, one current, two kinds of fixture. They all light, but they do not behave alike, so the mix has to be designed, grouped and checked step by step.
In a series circuit the CCR sends the same current through every primary, and each fixture sits behind its own isolation transformer. That is why halogen and LED fixtures can share one loop and both light. It does not make them behave alike.
Their load, brightness at each step, colour and response differ. A mixed circuit is therefore a design decision with its own checks, and most often a stage on the way to a full conversion. Within any one lighting system the technology must be the same: FAA AC 150/5340-30 (para 1.4) does not allow LED and incandescent fixtures of the same type to be interspersed.
The current through every primary, each fixture behind its own transformer, and the loop staying closed when a lamp or module fails.
The load and its character, the light output at each step, the colour, the response to a step change and the infrared output.
| Aspect | Halogen | LED | In a mixed circuit |
|---|---|---|---|
| Load | Tens of watts, close to a resistive load | A fraction of that, with driver electronics | Total falls; check the CCR range and each transformer’s loading |
| Output by step | Follows the lamp’s own curve | Set by the driver and optic | A match at the full step does not guarantee a match at the low steps |
| Step change | Filament warms and cools slowly | Responds at once | Changes can look different, which matters for flashing and sequenced lights |
| Colour | White light through a filter for coloured lights | Colour made by the LED | Neighbouring lights can look different in tint |
| Infrared | Broad spectrum, including infrared | Little infrared | Vision systems may pick up only the halogen lights |
| Failure | Sudden burn-out | Gradual output loss, or a sudden driver failure | Two kinds of fault to look for and monitor |
What to check
Why it is mixed
Know whether the mix is a stage in a migration or a lasting arrangement, and set an end date or a permanent design to match. Check it against the rule in force at your aerodrome before it is designed.
Grouping
Never alternate the two types along a line or within one lighting system (FAA AC 150/5340-30, para 1.4). Any change of technology falls at the boundary between two systems.
Load and CCR
Add up the mixed load and confirm it sits inside the CCR range. Check each transformer against the load it now carries.
Every step
Check the light output of both types at every brightness step, not only at the full step.
Colour and response
Compare the colour of neighbouring fixtures and how both respond to a step change, flashing or sequencing.
Monitoring
Confirm that lamp-out detection and alarms work for both types, and record the result against the circuit.
Frequently Asked Questions
Q1Can halogen and LED fixtures share one series circuit?
Electrically yes, because the CCR sends the same current through every primary and each fixture sits behind its own isolation transformer. But the two types differ in load, brightness at each step, colour and response, and the visual rules are stricter than the electrical ones. FAA AC 150/5340-30 (para 1.4) does not allow LED fixtures to be interspersed with incandescent fixtures of the same type (for example within one runway edge, centreline, touchdown zone, approach, stop bar or PAPI system, or within one in-pavement bar), and a failed incandescent fixture must not be replaced by an LED one. So a loop can carry both only where each lighting system on it is a single technology, and it needs its own design and checks. Confirm the rule in force at your aerodrome. The explicit ban cited here is the FAA's; ICAO Annex 14 and national regulations such as DGCA CAR may not word it the same way, so check what your regulator requires.
Q2Why can halogen and LED lights look different on the same circuit?
They get the same current, but the light output follows a different curve for each type, so a match at the full step does not guarantee a match at the lower steps. The colour is also made differently, and an LED reacts to a step change at once while a filament warms and cools more slowly.
Q3Why avoid alternating halogen and LED fixtures along a line?
A pilot sees the pattern of the lights as a whole. Alternating the two types puts any difference in brightness or colour into every second light, which makes the line look uneven, and FAA AC 150/5340-30 (para 1.4) does not allow it. Where two technologies share a loop, the change must fall at the boundary between two lighting systems, never inside one system.
Q4How does a mixed circuit affect the CCR?
The CCR load is the sum of all the fixtures, so it is lower than an all-halogen circuit but higher than an all-LED one. The load still has to sit inside the CCR range, and each transformer should be checked against the load it now carries.
Q5Is a mixed circuit meant to be permanent?
Usually not. It most often appears while a site is being migrated in stages, and then only where each lighting system is converted as a whole. If a mix is expected to last, set an end date or design it as a permanent arrangement, with the checks that go with it. A failed halogen fixture is replaced by a halogen fixture until its whole system is converted.
Q6What should be checked on a mixed circuit?
Measure the series current and the load, check the light output and the colour of both types at every brightness step, check how the lights respond to step changes, and check that monitoring and alarms still work. Record the result against the circuit.
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