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Concerns about Z101 Schematic

eric.the.blonde

Active Member
I've long been attracted to the Z12 drawing for my RV-9A project and recently heard of the Z101 drawing having several new features of which the aux/clearance-delivery/pre-flight bus and brownout booster are the most interesting.

I don't like the way the brownout booster is connected to the aux relay in the Z101 drawing. In order to get the benefit of the booster the aux switch must be off. If you switch that off before turning on the main battery master, then you shut down the computer you just loaded your flight plan into. It is not a good idea to have to flip switches in a particular sequence.

Perhaps it's enough to simply move the brownout bus connection to other (energized) terminal of the aux relay. Or perhaps connect the brownout booster directly the pre-flight bus terminal. In either case a diode might be in order although the brownout booster implies that it acts like one with input/output terminals.

Those two thoughts beg another question for more electrically savvy people than me to judge: Why would the current for the brownout booster is be drawn from a circuit (starter engage) that is intended to only energize a relay (starter)? If the AUX switch energizes the pre-flight bus via a relay (S704-1), would it not follow that the brownout bus carrying the same loads also ought to be energized via a relay? Perhaps its load is too transient to matter. I dunno.
 
it is nice to see you analyzing Bob's drawings. They were always meant to be educational, not prescriptive. I think many folks try to blindly follow them without understanding, and that can lead to needlessly complex electrical systems.

Once you understand the intent of everything Bob is describing and then choose your mission, you can pick and choose pieces of his architecture to meet the mission and risk profile.

For example, your example is a dual alternator system with a voltage booster to prevent brownouts. A dual battery system with diode isolation of the aux battery would be an alternative, and doesn't need another piece of complex electronics (that can fail). Keep in mind that many modern EFIS systems have their own backup batteries as well, so are immune from brownouts.

The most troublesome brownout would be for electronic ignitions, and many of them specify the use of aux batteries to prevent this and to provide limp-home capability.

I recently completed a design for a single-battery, single-alternator electrical system. The EFIS has a backup battery, and the P-mags have their internal back-up generators, so even turning off the master in flight allows continued flight. Not comfortable, but possible. I cut out as much complexity as reasonably possible to remove weight, cost and failure modes. It's the opposite of a high $$ IFR panel.

Of course, I'm a hypocrite, so I added back some non-essential stuff (like the OnSpeed advanced angle of attack system, autopilot/autotrim and a 6-pack of huVVer-AVI instruments for the copilot. None of these are essential for flight, so can be turned off to reduce load.

So my recommendation is to have a system design that is immune from brownouts, usually during starting. An aux battery for an electronic ignition or PMAGs, plus a backup battery for the primary EFIS will prevent reboots. That is all you really need unless you are flying IFR.

VV
 
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If the aux switch is turned on to supply power to the aux bus, then it can be expected the the aux bus will lose power when the aux switch is shut off.
The pilot will soon learn not to shut off the aux bus power unless another power source (master switch) is turned on first.
You are right that a diode would be required to prevent the brownout booster from being connected to the battery if your proposed changes were implemented.
Since brownouts only occur when the starter is energized, it is logical to power the brownout booster from the start switch.
Why add another failure point (relay) when the start switch works just fine to power the brownout booster?
The advantage to using one of Bob N's designs is that his architecture has been subject to peer review and bugs have been worked out. Some builders have made changes not realizing the consequences under certain conditions.
 
If a brownout booster is intended to be on continuously, then it would need to be larger than the small one envisioned in Z-101.
If the brownout booster is turned on by the start switch, adding a relay would delay turning on the circuit by several milliseconds, perhaps too late to prevent a brownout.
 
Brown Out Booster

Hello,

I am finalizing my power layout now for the RV-10. I have an electrically dependent engine with electronic ignition and injection. It always needs power. I have chosen a blend between the Z14 dual Battery and the Z101B Dual Alternator.

Basically I have duplicated Bob's Pink section (upper part of the drawing) on the Z101B that provided for a reliable engine bus being fed from a relay , a master solenoid and the alternator with Bobs Z14 with Dual Battery and busses.

I plan on using 2 batteries in the rear that are completely isolated. The Alternators feed a battery Isolation device and then each Battery terminal goes to the A or B battery.

Each battery through their own Engine bus relay or master Solenoid feed each A and B engine Bus and is isolated using Zener Diodes. Expected voltage drop is .2-.3 volts.

Still being reviewed portion: Both batteries also feed the VPX-Pro and a Endurance bus, again via Diode Isolation. The starter is only connected to the A side of the system. Since on start up I will have the busses fed by both A and B batteries, battery B will keep the voltage up while I use Battery A to start the engine.

Mike
 
Thanks for the Input

Thanks for the input. I said that the booster was interesting. Indeed, it is too interesting for my simple mind. I'm growing more comfortable spending megabucks on the designed-for-the-purpose PFD backup battery rather than complicating the electrical system with a gadget of indeterminate pedigree.

I'm still interested in the pre-flight bus in lieu of an endurance bus and still admire the conceptual elegance of Z12. Even if it's expensive to implement.
 
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