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G3X Power 1 and Power 2 failover question

akschu

Well Known Member
Patron
I have a question about the power input that doesn't seem to be covered in the manual which only states that there are diodes to prevent current from flowing between the two inputs.

If I have 13.5 volts on power 1 and 13.7 volts on power 2, which one will it use?

In other words, what is the failover logic to power 1 and power2? Does it:

If power 1 voltage drops below minimum switch to power 2?

or

Use whichever one has more voltage?

Also, is it possible to define the switch over voltage?

Thanks,
Matt
 
I don't know for sure but my guess is that it uses both with inline diodes to keep the power separated. If that is the case then it would draw current from the higher voltage one (pwr 2 in this case).
 
That's what I was thinking too, but I hope that isn't the case because my secondary power system is only good for 8 amps, and I specifically don't want to have a situation where the current draw pulls down the secondary power system voltage until it swings back to the the primary, to have the secondary come back up, and head back to the secondary power system again.
 
The power distribution system design is in progress. Some of it depends on how the garmin stuff deals with power failover.

Right now I have two completely independent electrical systems. The main system has a standard starter battery and 60 amp alternator. The aux system has a light weight earthX EXT104 battery and a B&C 8 amp alternator.

Because they are separate and have their own alternators, one system might have higher voltage than the other, but I'd rather the garmin gear use the main system even if it has the lower voltage because the system with the higher voltage may entirely depend on the load.

Clear as mudddd?
 
I suspect the power is "diode ORed". That would be the normal way of combining two power inputs. The power inputs are applied to the anode of the two diodes and the cathodes are tied together and provide the output to the unit. The term "diode OR" means that one "OR" the other input can provide the power.

In response to your question the higher voltage will provide the power. The lower voltage diode will not turn on.
 
I suspect the power is "diode ORed". That would be the normal way of combining two power inputs. The power inputs are applied to the anode of the two diodes and the cathodes are tied together and provide the output to the unit. The term "diode OR" means that one "OR" the other input can provide the power.

In response to your question the higher voltage will provide the power. The lower voltage diode will not turn on.

And this is how the GEA24 Keep Alive power works; it provides a higher voltage during start when the main bus drops due to engine cranking. For the Garmin keep alive, I don't think it matters which pin is keep alive and which one is main bus; whatever voltage is higher draws the power.
 
I think I found a solution:

From the BandC regulator documentation (https://bandc.com/wp-content/uploads/2018/05/sd-8_trouble_revA.pdf)
The PMR1 set point is adjusted to 14.4V (28.8V) before it leaves the factory. If you are using a B&C battery, the regulator should not need to be adjusted at installation. Should adjustment be required, a .09" diameter screw provided in one corner of the PMR1 may be used for adjustment. The voltage set point will vary at the rate of approximately 0.16 V (0.32V) per turn and rises with clockwise rotation.

From the ETX104 battery specs (https://earthxbatteries.com/wp-content/uploads/2012/10/ETX104-Product-Spec.pdf)
Standard Charge Voltage 13.9 - 14.6 V

So, if I set the smaller alternator/regulator on my AUX bus to 13.9v and my MAIN bus is running at 14.4v (especially since it makes much more power at lower RPMS) then the load should always be on the MAIN bus until failure.

Here is my current drawing:
 

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G3X will use tge power input offering higher voltage.

I have a question about the power input that doesn't seem to be covered in the manual which only states that there are diodes to prevent current from flowing between the two inputs.

If I have 13.5 volts on power 1 and 13.7 volts on power 2, which one will it use?

In other words, what is the failover logic to power 1 and power2? Does it:

If power 1 voltage drops below minimum switch to power 2?

or

Use whichever one has more voltage?

Also, is it possible to define the switch over voltage?

Thanks,
Matt

Matt- all of the garmin devices with internally dioded P1and P2 inputs will use whichever one offers the higher voltage. I feed the P1 inputs from my primary avionics bus, which is fed by voltage maintained by the alternator, and the P2 inputs from the auxiliary bus, fed from a battery that receives slightly lower voltage through a Schottky diode. It is a great system, and the various devices are happy with either or both sources hot.- Otis
 
Matt- all of the garmin devices with internally dioded P1and P2 inputs will use whichever one offers the higher voltage. I feed the P1 inputs from my primary avionics bus, which is fed by voltage maintained by the alternator, and the P2 inputs from the auxiliary bus, fed from a battery that receives slightly lower voltage through a Schottky diode. It is a great system, and the various devices are happy with either or both sources hot.- Otis

Right, makes sense. So the only real mode of failure on that setup is a short to ground on the aux bus tripping breakers/fuses on the main bus if they blow before the diode. A simple solution is to have a breaker between the diode and main bus so that you could isolate the AUX bus and reset the main should that happen.

I've thought about doing something similar and omitting the little alternator, but I already have it, it's not super heavy, and as the diagram shows, I'm thinking about electronic ignition.

schu
 
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