Avionics Bus Max Voltage and Protection

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dmloftus
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Avionics Bus Max Voltage and Protection

Post by dmloftus »

For engineers and the technically inclined:
Has anyone tried to characterize how clean the electrical buses are (ie Main Bus or Avionics Bus) on the Lycoming DA40-180? I upgraded my cigarette lighter plug to supply up to 7.5A to drive an Inogen One G5 oxygen concentrator. My shop installed a new 10A plug with thicker wiring through a 7.5A breaker connected to the Avionics Bus. I was planning to use Inogen Aviator's DC/DC supply to convert 28VDC ship power to 12VDC to run the G5 with the manufacturer's 12VDC car plug. But that supply unavoidably creates switching noise, thus EMI that can interfere with radios/navigation.
From reading the specs online, I assumed Inogen's AC/DC standard brick also supplied 12V to the device. I was surprised when I received the Inogen G5 device today and found that the standard AC/DC supplies 24VDC @ 5A to the G5. That says to me that the G5 will run anywhere from 12V to 24V, probably just drawing twice the current at 12V.
While I wouldn't want to run a 24V device directly from ship's 28V power, I am thinking it might be good to construct a DC cable that plugs into the lighter plug, then cascades 6-8 10A diodes in series to bring the 28V (my G1000 usually indicates about 28.2VDC) down to 23-24V for the G5. That way I avoid the switching noise generated by Inogen Aviator's DC/DC power converter. The diodes would create some heat, but that should be manageable.

What I'm wondering is the following:

1. Which bus is the G1000 monitoring?
2. How well regulated is the Aviation Bus?
2a. What is the maximum voltage seen on the Aviation Bus?
2b. How much ripple is on the Main or Aviation Bus?
3. Is there already good over-voltage protection, ie to minimize load dump**, on the Aviation Bus?

Automotive power supplies usually need to accommodate load dump, which is a high energy transient (in simplistic terms - think big voltage spike). I'm thinking power on the Avionics Bus must already be protected from load dump to avoid frying a G1000 or electronic subsystems downstream.

**Description from one of my prior companies: In automotive systems, a load-dump transient occurs when an alternator is delivering current to a battery and the battery is abruptly removed. This can happen when a discharged battery loses connectivity while the alternator is generating charging current and other loads are connected to the alternator circuit.

Thanks for any thoughts on my questions and this approach.
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Rich
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Re: Avionics Bus Max Voltage and Protection

Post by Rich »

The system is a nominal 28V, not 24. I've seen some running at 28.8 V, and most (likely all) of us are running a bit above 28V.

There are transient spikes in the system. They are short-duration/low energy spikes that have not been shown to affect anything except the Surefly SIMs. I have plugged several electronic devices directly into my 28V accessory plug (when I had it) without any observable problems.
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Re: Avionics Bus Max Voltage and Protection

Post by dmloftus »

Rich wrote: Fri Aug 05, 2022 11:57 pm The system is a nominal 28V, not 24. I've seen some running at 28.8 V, and most (likely all) of us are running a bit above 28V.

There are transient spikes in the system. They are short-duration/low energy spikes that have not been shown to affect anything except the Surefly SIMs. I have plugged several electronic devices directly into my 28V accessory plug (when I had it) without any observable problems.
Thanks Rich. Understood, that's why I was considering the series diodes to drop the 28V accessory plug to the 24V my device uses.

28.2V - (6 diodes X (0.7V / diode)) = 24V at my device

Maybe I go to 8 diodes and 22.6V. I'm just wondering whether there are voltage spikes above the 28.2V bus large enough to damage the oxygen concentrator.
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Re: Avionics Bus Max Voltage and Protection

Post by Rich »

dmloftus wrote: Sat Aug 06, 2022 12:26 am Maybe I go to 8 diodes and 22.6V. I'm just wondering whether there are voltage spikes above the 28.2V bus large enough to damage the oxygen concentrator.
Hard to say. They didn't actually damage the Surefly, just produced annoying restarts. Their solution consisted of 2 components interposed on the battery feed: a voltage reducer to 16 Volts and a honking electrolytic capacitor large enough be considered a deadly weapon :shock:
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Re: Avionics Bus Max Voltage and Protection

Post by alanhawse »

Perhaps you could use a switching regulator... like this one:


It says that it is 95% efficient... which would be way better than 5A@4v=20watts into those diodes.

Or perhaps build one e.g. a regulator from my previous employer
https://www.infineon.com/cms/en/product ... utomotive/
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Re: Avionics Bus Max Voltage and Protection

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Re: Avionics Bus Max Voltage and Protection

Post by dmloftus »

alanhawse wrote: Mon Aug 08, 2022 6:12 pm Perhaps you could use a switching regulator... like this one:
Thanks Alan, that's the first thing I considered as I worked for switching regulator manufacturers Intersil and Maxim (and Cypress for a short time ;-). I have a commercial switching supply good to 36V. But any kind of switcher (whether I buy or design my own) is going to create EMI, with unpredictable impact on radios and nav. That's probably one of the reasons why Diamond labels the accessory outlet "For Ground Use Only" as they don't want people plugging in devices that will cause interference problems.
Back in the 1980's and early 90's, I designed and flight tested military radars, jamming pods, etc, so I have some experience with EMI effects on the rest of the avionics suite. Switching regulators have switching speeds usually from 200KHz to low MHz range, and they throw off lots of harmonics. I understand the challenge with the diodes dissipating 20W, but they don't create switching noise. I'm thinking it's easier to heatsink the diodes safely than chase EMI gremlins in the avionics or creating mu metal shielding. But I would have to live with Diamond's regulation of the 28V bus. The idea of a couple of extra diodes to drop below my required 24V input to the O2 concentrator is just to provide a bit of headroom for unanticipated spikes on the Avionics bus.
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Re: Avionics Bus Max Voltage and Protection

Post by RookieFlyer »

dmloftus wrote: Mon Aug 08, 2022 8:18 pm ...But I would have to live with Diamond's regulation of the 28V bus. The idea of a couple of extra diodes to drop below my required 24V input to the O2 concentrator is just to provide a bit of headroom for unanticipated spikes on the Avionics bus.
In that case, why not go for a simple circuit using a high current (10A) adjustable three terminal regulator like the LM396? You'd be dissipating the same heat but with the added benefit of excellent ripple suppression and a high tolerance of reactive loads (with suitable capacitors).
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Re: Avionics Bus Max Voltage and Protection

Post by dmloftus »

RookieFlyer wrote: Mon Aug 08, 2022 10:50 pm In that case, why not go for a simple circuit using a high current (10A) adjustable three terminal regulator like the LM396? You'd be dissipating the same heat but with the added benefit of excellent ripple suppression and a high tolerance of reactive loads (with suitable capacitors).
Thanks Ken. I'll take a look at that part. Surprised it didn't come up on a Digikey selector guide. Datasheet says it maxes out at 70W. I need 120W to the load @24V, so even if the bus spiked all the way to 36V, seems it would still only dissipate 60W. And that wouldn't be continuous (God forbid).
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Re: Avionics Bus Max Voltage and Protection

Post by gjh »

O2 concentrators usually contain motors, which means that the supply current to the G5 might not, at all times, be strictly positive. You should consider that in the regulator design or when stepping down with diodes.

There's also the assumption that the device sans power brick will generate less EMI than the device with its power brick, which is far from a given since the brick, in being designed for some kind of commercial application, likely meets some kind of EM compatibility standard.
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