# Airport runway lighting: What is constant current in relation to alternating and direct current?

I’m an apprentice electrician currently replacing runway lights and pulling new wire at a small international airport.

When I asked I was told they are 5kV direct current. Okay… simple enough. What piqued my interest was the label on the lights- they are rated for 50/60 Hz and use a small step down transformer for each light. DC circuit using transformers at 60 Hz? Wait a second. Through some light reading (I’m not that smart) I understand these runway lighting circuits utilize constant current regulators.

The Wikipedia article on constant current defines it as a type of direct current. Is this really the case or is that just a simplified way of looking at it?

I understand the need for constant current when powering lights over long distances but I do not understand what it is in relation to alternating and direct current.

What would this current look like on an oscilloscope? Is it a flat line like DC or does it have a sinusoidal waveform like AC?

Is there any kind of rectification going on in the regulator or is it something completely different?

• Do you have the manufacturers part number for the runway lights? Looking up the datasheet for the lights could give more information. Perhaps there is an AC -> DC power supply inside the lights which then applies a constant DC current to the actual light. Commented Nov 5, 2023 at 18:36
• There is probably a conversion circuit inside the light itself, running 5kVDC all over the place sounds like a total nightmare, but @ChesterGillon said it: the datasheet tells all.
– vir
Commented Nov 5, 2023 at 18:39
• I’ve never encountered an airport light system which uses anything but AC. Most of them in constant current configuration. Commented Nov 5, 2023 at 18:47
• @Chester Gillon I did not try looking up the light! I will get the number tomorrow and see what I can find, I’ll post it here too for anyone curious. Looking at suppliers websites I do believe the entire circuit is constant current which is provided by a large cabinet style regulator, not inside the light… but I am only going off my limited knowledge and research. Commented Nov 5, 2023 at 19:22
• Have a look at electronics.stackexchange.com/questions/470908/… Commented Nov 5, 2023 at 21:55

Most of the time we deal with constant voltage supplies, for example you might have a 12 V supply, and the current in the circuit will depend on the resistance as described by Ohm’s Law I = V/R. So for a 12 $$\\Omega\$$ load the current will be 1 A. If the load changes to 6 $$\\Omega\$$ the current will be 2 A so the voltage eill still be 12 V.
For constant current the current stays constant and the voltage changes to satisfy Ohm’s Law. A 1 A CCS will output 12 V into a 12 $$\\Omega\$$ load, if the load changes to 6 $$\\Omega\$$ the voltage will be 6 V so the current will still be 1 A.