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?

  • \$\begingroup\$ 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. \$\endgroup\$ Commented Nov 5, 2023 at 18:36
  • 1
    \$\begingroup\$ 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. \$\endgroup\$
    – vir
    Commented Nov 5, 2023 at 18:39
  • \$\begingroup\$ I’ve never encountered an airport light system which uses anything but AC. Most of them in constant current configuration. \$\endgroup\$
    – winny
    Commented Nov 5, 2023 at 18:47
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Nov 5, 2023 at 19:22
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    \$\begingroup\$ Have a look at electronics.stackexchange.com/questions/470908/… \$\endgroup\$
    – D Duck
    Commented Nov 5, 2023 at 21:55

1 Answer 1


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.

Imagine putting two 120 V lamps in series, with 120 V they would be dim, to get the normal brightness you would need to double the voltage to 240 V. With three in series you would need 360 V. A CCS does this automatically.

The idea of constant current is that you can have long wire runs without the problem of voltage drop, it adjusts the voltage to compensate for any additional resistance.

  • \$\begingroup\$ One big reason to use constant current in lighting is for compensating for the (negative) change in forward voltage with temperature of LED diodes. With CC you turn on a large light at a higher voltage and then as it warms up the driver slowly drops the voltage as the diode forward voltages decrease. Keeps the lights from entering into thermal runaway. \$\endgroup\$ Commented Nov 5, 2023 at 19:06
  • \$\begingroup\$ @GodJihyo Thank you for the in depth explanation! I still have some sleuthing to do as the guys I work with are convinced it’s a DC circuit but evidence is pointing to the opposite. \$\endgroup\$ Commented Nov 5, 2023 at 19:38

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