LED Strobe wiring recommendations

[Dean_aeroleds]

Here is some information that should be helpful to builders installing AeroLEDs wingtip lights:

LED strobes operate differently than legacy Xenon strobes.

Legacy Xenon strobes use a flash capacitor that charges up continuously between flashes, pulling a steady amount of current (current is continuously pulsating at the frequency of the charge pump, typically 10's of kilohertz), then dump the charge to the Xenon tube in a single burst.

LED strobes pull their current while the strobes are lit, and pull nearly zero current between flashes.

As a result, the way that the LED strobes are wired will make a huge difference in whether or not audio frequency noise gets into your intercom. Because the current pulses to LED strobes flow in a loop with the outgoing current flowing in the outbound power wire, and the return current flowing in the ground path, there is the potential for the wiring to create time varying magnetic fields that can couple into adjacent wires such as headset jack cables, or even your antenna coax cable.

To prevent this, it is highly recommended that you follow the following wiring recommendations:

1. Use shielded wire, AeroLEDs has 3 conductor 20 gauge shielded wire available for this purpose.

2. Use the shield as the ground return. When the ground current flows immediately adjacent to the power wire, the magnetic field produced by the power wire current is canceled out by the current flowing in the shield. The ground current prefers to stay in the shield rather than flow through structure because generating a magnetic field takes energy, and the current wants to follow the path that takes the least effort because the fields cancel out (called the path of least inductance).

3. Bring the shielded wire run all the way to the panel, where the power wire can go to the switch, and the shield ground can be run to the behind the panel grounding block. If you need to break the wire run at the wing root with a connector or terminal block, that is OK as long as you resume the shielded wire in the fuselage and connect the shield grounds through the interconnect.

4. As much as possible, keep some separation between the strobe wires and sensitive cables such as intercom audio cables, headset jack cables, or antenna coax for the radios.

Note that the above recommendations are primarily intended to prevent audio frequency signals from getting into your intercom. For preventing RFI, you should also follow these recommendations:

1. If you have a mounting bracket that is anodized (silver colored brackets), then you must remove the anodization coating from the screw wells so that the counter-sunk screw heads will make good electrical contact for making the chassis ground connection. You can test the grounding of the wingtip lights by measuring the resistance from the rear set screw head to aircraft structure ground. If your brackets are gold colored then they have a conductive alodine coating and this step is not needed.

2. You must ground at least one of the mounting screws to aircraft struture either directly or via a ground wire.

3. Tie the black ground wire, and the shield braid ground to aircraft structure ground (or wing spar ground in a composite or tube and fabric wing) at or near the spot where the chassis ground for the light is grounded. It is important that the loop formed by the black wire ground from the light, and the chassis ground from the light be kept short. Keeping this loop short is very important as it reduces radio emissions from the wingtip lights by 30db in the communications band wavelengths, both for the position lights and the strobes.

If you have wired your plane, and didn't follow all of the above recommendations and think you have an audio noise problem, we can provide in-line filters that you can put in each wingtip that will help to reduce the edge rate on the current pulses, but ultimately the best way to handle it is to prevent it from occurring in the first place.

Dean Wilkinson

CTO, AeroLEDs LLC

[roee]

Dean,

Pertaining to these issues, I'd like to offer a humble suggestion for your next generation or next design revision of your strobes.

Consider implementing some energy storage right at the light head to smooth out the current draw from the airframe wiring. Ideally, use a constant current regulator to feed a large capacitor, and then power your existing strobe circuit from that capacitor. The constant current regulator should be set to just over the mean current draw of the strobe, i.e. its "on" current X the duty cycle.

This configuration would offer several advantages:

1) Both the voltage and current in the airframe wiring will be constant, not pulsing. This would eliminate these EMI / RFI problems entirely, both magnetic field coupling (an issue with your existing LED strobes) and electric field coupling (an issue with xenon strobes). And it would make the installation insensitive to the configuration of the wiring. No more shielding, careful routing, or careful grounding required.

2) The max current flowing through the airframe wiring will be far smaller than it is currently, equaling only a fraction (the duty cycle fraction) of the "on" current that the wire must carry now. Therefore, smaller gauge wire could be used while maintaining an acceptable voltage drop.

The configuration I'm suggesting would obviously add a few components to the light head, slightly increasing its cost and weight. However, these cost and weight penalties would be easily offset by corresponding cost and weight savings in the wiring (smaller gauge, non-shielded). And of course, simplifying the installation and eliminating the EMI / RFI nuisance should make everyone happy (I'll bet it would cut down on your support calls).

Just my 2 cents.

Cheers,
-Roee

[Dean_aeroleds]

Roee,

The lights do have capacitors in them. In order to do what you are suggesting, I would need to add a capacitor that is physically as large, or larger than the lights themselves.

Best Regards,

Dean Wilkinson

Originally Posted by roee

Dean,

Pertaining to these issues, I'd like to offer a humble suggestion for your next generation or next design revision of your strobes.

Consider implementing some energy storage right at the light head to smooth out the current draw from the airframe wiring. Ideally, use a constant current regulator to feed a large capacitor, and then power your existing strobe circuit from that capacitor. The constant current regulator should be set to just over the mean current draw of the strobe, i.e. its "on" current X the duty cycle.

This configuration would offer several advantages:

1) Both the voltage and current in the airframe wiring will be constant, not pulsing. This would eliminate these EMI / RFI problems entirely, both magnetic field coupling (an issue with your existing LED strobes) and electric field coupling (an issue with xenon strobes). And it would make the installation insensitive to the configuration of the wiring. No more shielding, careful routing, or careful grounding required.

2) The max current flowing through the airframe wiring will be far smaller than it is currently, equaling only a fraction (the duty cycle fraction) of the "on" current that the wire must carry now. Therefore, smaller gauge wire could be used while maintaining an acceptable voltage drop.

The configuration I'm suggesting would obviously add a few components to the light head, slightly increasing its cost and weight. However, these cost and weight penalties would be easily offset by corresponding cost and weight savings in the wiring (smaller gauge, non-shielded). And of course, simplifying the installation and eliminating the EMI / RFI nuisance should make everyone happy (I'll bet it would cut down on your support calls).

Just my 2 cents.

Cheers,
-Roee

[Dean_aeroleds]

Just to reinforce the importance of following our installation instructions and properly grounding the body of the light, here is an email I just received from someone who had noise and was able to solve it by grounding the mount for the wingtip light:

"I just want to send a thank you I have been trying to get the strobe noise out of my headset for over a year and your tech support had me clean the anodizing and ground the mount to light ground. I can?t believe that that worked so well!! I flew for 30 minites after fix and the quite was so loud it made my ears shout for joy!!!"

Here is a photo of what the mounting bracket ground should look like (note that it shares the same pin in the connector as the black wire, so that the grounds are common):



Also, on the other side of the connector, it is important that the shield ground (which is what you should use to ground the mating pin on the aircraft side of the connector) be connected to structure with a very short ground wire. In the picture below, this is accomplished by using a Raychem solder shrink (an environmentally sealed heat shrink that includes solder to solder the ground wire to the shield directly when you use hot air to shrink it on):



The solder shrinks can be purchased here:
http://www.newark.com/te-connectivit...lue/dp/22T9900

These solder shrinks are very cool and you may find them useful for other wiring on your plane as well.

Also ground the shield at the grounding block behind the panel when you run the power wires to the switches.

Another very important item is to not run the wire for the lights in direct contact with coax for antennas or intercom headset jack cables, as magnetic fields can couple noise into those coaxes. A few inches of separation is all that is needed to prevent magnetic (transformer like) coupling into the very sensitive radio and intercom coax cables.

The above photos are from a rag and tube plane that initially had noise breaking squelch in their Garmin touch screen radio (auto squelch), but after I worked with them on making the chassis ground, shortening the shield ground wire to the spar (was 10 inches long initially), and had them separate the wire from the antenna coax by about 5 inches (was tied to the coax initially), there was no longer any noise on any frequency, and the Garmin radio was nice and quiet unless a transmission was being received.

Avoiding noise in your radio and intercom with our lights is really not hard, but it requires taking care when you install the wiring to follow the above guidelines for grounding and wire routing.

On a final note, we have started coating our mounting brackets with gold colored alodine so that they are conductive and the step for removing the anodization from the bracket where the screw heads make contact won't be necessary with the gold alodined brackets. If you have a silver colored bracket, then it has the clear anodized surface that is non-conducting and the screw-well must be sanded to remove the anodization in order to allow the mounting screw to make the ground connection.

Best Regards,

Dean Wilkinson
CTO, AeroLEDs LLC

[Glas467]

Great post Dean, thank you!