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kirkbauer

Active Member
I have a general question about the various Garmin G3X Touch components that offer dual power inputs. What failure risk is there connecting both of these inputs?

For example, imagine a two-alternator, two-battery split electrical system. Your PFD has one power input connected to alt1+bat1 and the other power input connected to alt2+bat2. It would seem like a very high amount of redundancy, since you'd have to lose both batteries and both alternators before you'd lose power.

But what about failure events? The primary one I can think of is an overvoltage situation. What if one of the alternators sends extremely high voltage to one of the two electrical busses? Could that damage the Garmin components that are connected to it, causing it to fail even though the other electrical bus is just fine? Obviously with extremely high voltage (e.g. a direct lighting strike) it will fail, but is there any reasonable failure where it would be a disadvantage to having two power sources connected to a critical component?
 
kirkbauer said:
I have a general question about the various Garmin G3X Touch components that offer dual power inputs. What failure risk is there connecting both of these inputs?

For example, imagine a two-alternator, two-battery split electrical system. Your PFD has one power input connected to alt1+bat1 and the other power input connected to alt2+bat2. It would seem like a very high amount of redundancy, since you'd have to lose both batteries and both alternators before you'd lose power.

But what about failure events? The primary one I can think of is an overvoltage situation. What if one of the alternators sends extremely high voltage to one of the two electrical busses? Could that damage the Garmin components that are connected to it, causing it to fail even though the other electrical bus is just fine? Obviously with extremely high voltage (e.g. a direct lighting strike) it will fail, but is there any reasonable failure where it would be a disadvantage to having two power sources connected to a critical component?
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My opinion I think KISS is appropriate for the planes we build. Dual busses is over engineering that simply is not advantageous when we have a battery backup in the event of main battery/alternator failure. Why complicate things when the there really is no significant advantage to a more complex system that introduces other possible failures.
 
kirkbauer said:
I have a general question about the various Garmin G3X Touch components that offer dual power inputs. What failure risk is there connecting both of these inputs?

For example, imagine a two-alternator, two-battery split electrical system. Your PFD has one power input connected to alt1+bat1 and the other power input connected to alt2+bat2. It would seem like a very high amount of redundancy, since you'd have to lose both batteries and both alternators before you'd lose power.

But what about failure events? The primary one I can think of is an overvoltage situation. What if one of the alternators sends extremely high voltage to one of the two electrical busses? Could that damage the Garmin components that are connected to it, causing it to fail even though the other electrical bus is just fine? Obviously with extremely high voltage (e.g. a direct lighting strike) it will fail, but is there any reasonable failure where it would be a disadvantage to having two power sources connected to a critical component?
Click to expand...
I have considered the same questions and the disadvantage I am concerned with in my own system design (also dual battery, dual power generation) is that if I have both busses hooked up to both power inputs of multiple avionics and any of those devices start smoking (I know that’s unlikely but a possibility) then I am going to have to shut off both busses and therefore most equipment. I will most likely split devices across the two busses so that if one bus goes down the other bus will have enough equipment to get me out of trouble.
 
It's hard to know without looking at the pinout for whatever box you're talking about, but unless its the brownout protection for the engine monitoring during startup, I think what you're calling dual power inputs really aren't intended to be wired that way.

A D-sub pin is only rated for 5 amps. Something like GTN650 for example can suck up more than that when you hit the push to talk, so Garmin has two power pins in parallel. They both need to be wired to the same source.

Typically your installation manual and pinout diagram will specify if a unit is designed to be pulling power from two different sources for redundancy.
 
Desert Rat said:
It's hard to know without looking at the pinout for whatever box you're talking about, but unless its the brownout protection for the engine monitoring during startup, I think what you're calling dual power inputs really aren't intended to be wired that way.

A D-sub pin is only rated for 5 amps. Something like GTN650 for example can suck up more than that when you hit the push to talk, so Garmin has two power pins in parallel. They both need to be wired to the same source.

Typically your installation manual and pinout diagram will specify if a unit is designed to be pulling power from two different sources for redundancy.
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Well, you may be correct, and if so, this is quite an unnecessary question. For example, the 10" display for G3X Touch (GDU 460) has two power input pins. I was assuming that that was for redundancy, but it sounds like it might actually need to draw from both of those to operate?
 
bonefishdave said:
My opinion I think KISS is appropriate for the planes we build. Dual busses is over engineering that simply is not advantageous when we have a battery backup in the event of main battery/alternator failure. Why complicate things when the there really is no significant advantage to a more complex system that introduces other possible failures.
Click to expand...

For example, I'm considering using the TCW IBBS to keep essential items powered in the event of a failure of my main electrical system. In that case, my primary PFD and my ADAHRS would be connected to the IBBS so that I can safely land the aircraft. My concern was, though, what if the IBBS has some sort of catastrophic failure? Now I lose my PFD even though my primary electrical system is just fine.

So, then I was thinking, on each device, why not hook one power pin to the main bus and the other to the IBBS. I was assuming it could run off of either power input, although according to @Desert Rat it might not work that way anyways.
 
kirkbauer said:
For example, I'm considering using the TCW IBBS to keep essential items powered in the event of a failure of my main electrical system. In that case, my primary PFD and my ADAHRS would be connected to the IBBS so that I can safely land the aircraft. My concern was, though, what if the IBBS has some sort of catastrophic failure? Now I lose my PFD even though my primary electrical system is just fine.

So, then I was thinking, on each device, why not hook one power pin to the main bus and the other to the IBBS. I was assuming it could run off of either power input, although according to @Desert Rat it might not work that way anyways.
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There are specific diagrams for incorporating the IBBS into various Garmin boxes.

For example, in the G3X installation manual, look at page 24-4, figure 24-1.4 “Power, Backup Power (4 AH), and Antennas Interconnect Drawing.” The TCW backup battery is specially shown with the pinouts. The battery is shown providing backup power to a variety of Garmin units via their power 2 inputs. Ie, this is a supported-by-Garmin installation.
 
kirkbauer said:
For example, I'm considering using the TCW IBBS to keep essential items powered in the event of a failure of my main electrical system. In that case, my primary PFD and my ADAHRS would be connected to the IBBS so that I can safely land the aircraft. My concern was, though, what if the IBBS has some sort of catastrophic failure? Now I lose my PFD even though my primary electrical system is just fine.

So, then I was thinking, on each device, why not hook one power pin to the main bus and the other to the IBBS. I was assuming it could run off of either power input, although according to @Desert Rat it might not work that way anyways.
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Morning Kirk,

The GDU 4XX, GSU 25 and GEA 24 can operate using power from one or both inputs (AIRCRAFT POWER 1 AND AIRCRAFT POWER 2). The pins are internally connected using diodes to prevent current from flowing between the two power inputs. AIRCRAFT POWER 2 is intended to be connected to an alternate power source.

You may be interested in the drawing on page 24-7 of the G3X Installation Manual (rev AU). It provides an example interconnect drawing for the installation you have in mind. That example provides power from the IBBS battery to the PFD, GSU 25 and GEA 24 only. Keep in mind the current limitations of the various models of IBBS batteries when designing your system.

A nice layer of redundancy is added when a G5 is installed with the backup battery installed directly to the unit. It can operate on its own, independent of the electrical system all together for up to 4 hours.

Thanks,

Justin
 
I have a dual power setup as you describe. There are plenty of what-ifs in any scenario, but to address your specific ones: between the alternator voltage regulator, the VPX over voltage protection (bus-1), and the current limiting of the CBs/fuses, I feel pretty good.
 
This thread has drifted from your original question, which I think was a good one. But regarding the backup battery—I’d avoid putting any non vented lithium battery anywhere other than forward of the firewall. The risk of a runaway may be low, but the consequences are potentially deadly. As an alternative to a backup battery, you can add another normal battery for a similar price and have the benefit of extra capacity and the ability to power the entire bus and crank the engine if the other battery goes out.
 
kirkbauer said:
For example, I'm considering using the TCW IBBS to keep essential items powered in the event of a failure of my main electrical system. In that case, my primary PFD and my ADAHRS would be connected to the IBBS so that I can safely land the aircraft. My concern was, though, what if the IBBS has some sort of catastrophic failure? Now I lose my PFD even though my primary electrical system is just fine.

So, then I was thinking, on each device, why not hook one power pin to the main bus and the other to the IBBS. I was assuming it could run off of either power input, although according to @Desert Rat it might not work that way anyways.
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Correct. The two inputs are separated internally with diodes.
 
Thanks for the helpful discussion, but I'm still not understanding the failure modes of the dual power input. Assume I have power input 1 connected to my main bus and the alternator regulator goes bad and puts high voltage onto the main bus. Does that potentially destroy any devices that are connected to it? Or would the devices keep working due to the IBBS connected to power input 2?
 
kirkbauer said:
For example, I'm considering using the TCW IBBS to keep essential items powered in the event of a failure of my main electrical system.
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Is 6AH enough to meet your needs?

If you've not already, you might build a power budget to identify:
-which equipment,
-requires how much current,
-under what conditions,
-for how long.

Knowing power required during normal (engine start, runup, takeoff...) and emergency (alt failure, batt failure, fire...) conditions will help identify equipment that meets your requirements, for an architecture that works as needed.
 
kirkbauer said:
Thanks for the helpful discussion, but I'm still not understanding the failure modes of the dual power input. Assume I have power input 1 connected to my main bus and the alternator regulator goes bad and puts high voltage onto the main bus. Does that potentially destroy any devices that are connected to it? Or would the devices keep working due to the IBBS connected to power input 2?
Click to expand...
I think it's a question for Garmin whether they have overvoltage protection on their devices. Justin?

If not then the device could be damaged by the OV and an additional power source wouldn't matter at that point.
 
esco said:
Is 6AH enough to meet your needs?

If you've not already, you might build a power budget to identify:
-which equipment,
-requires how much current,
-under what conditions,
-for how long.

Knowing power required during normal (engine start, runup, takeoff...) and emergency (alt failure, batt failure, fire...) conditions will help identify equipment that meets your requirements, for an architecture that works as needed.
Click to expand...
I'll definitely be doing this, but I'm not that far along yet. So many choices!
 
Regarding diode-OR inputs, there is a potential failure mode rarely discussed. If one of the diodes fails shorted, then it’s possible for the other working diode to back feed the opposite bus with the shorted diode. That would potentially blow the CB or fuse on one or both sides rendering the dual-fed box useless. Garmin may have used 2 diodes in series on each power input to mitigate that, not sure. I went with a Z-14 architecture and a few items dual-sourced but with some devices such as the displays and ADARHS only on a single bus. That also protects against a runaway bus voltage issue that OVP fails to handle as long as the crossfeed is open.
This post shows the electrical power distribution I’m using:
https://vansairforce.net/threads/show-me-your-wiring-diagrams.203636/post-1707050]https://vansairforce.net/threads/show-me-your-wiring-diagrams.203636/post-1707050[/URL]
Note: the detailed schematics have some pending updates. I’m almost done wiring so I’ll update them when complete.
-Bob
 
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