What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Low Voltage on Essential (avionics) Bus RV7A

Tcheairs

Well Known Member
I've been experiencing momentary loss of power to my autopilot (Dynon A/P 74) and EMS (D-100) when keying the mic on my primary radio (Garmin 430W). The voltage reading on the EMS is 10.8 inflight give or take a point or two. I'm assuming this is the Essential (avionics) bus I am viewing. I'm attaching a link to the electrical schematic for my AC and a photo of my EMS inflight readings.

I finally decided to take the plane to an avionics shop and here are the findings of the technician. I'd really appreciate second opinions on his conclusions.
The voltage you saw on the Dynon was indeed correct. You had 12.5 volts on the main bus but only 10.5 on the avionics (essential) bus when I first checked things. You are dropping an additional 0.5 volts when you key the radio, so the voltage got down to around 10- ish volts which is when the displays blanked during transmit.


AS PER THE TECH:

There is some kind of rectifier/diode in line with the essential bus feed wire that is dropping 1 to 1.5 volts and I have no idea what it is for. That, I believe, is most of your problem.

Additionally, after I wiggled the terminals on the avionics master switch, that mysterious rectifier, and the essential bus bar to check the security and attachment of the wires, I noticed the voltage came up to 11 - 11.5 volts, which brings up another issue I think will continue to plague you. That is the use of push-on terminals on high current carrying circuits.

I highly recommend changing those to ring terminals with screws. If the switch does not have screws then I would install a switch that does. Push-on terminals are notorious for intermittent connections with high current loads (more than 5 amps or so I’d say) – they get hot, begin to build up resistance and drop voltage which makes them get even hotter and compound the problem, eventually failing. The problem is even worse in high humidity climates like ours because corrosion builds on those terminals over time and compounds the problem we’re discussing.

In a 14 volt system, it is essential that you carefully manage any voltage drops, be it from using too-small gauge wires or from poor terminations (bad crimps, poor quality terminals, loose screws on breakers/switches, etc). 28 volt systems give you room to breathe, but 14 does not. Things begin to do strange things below 11 volts so that only leaves you with 3 volts of loss to work with as opposed to 12-13 volts of loss to manage on a 28 volt system. Simply put, 28 volt system is more forgiving than a 14 volt system…..

Hope this helps. Remove or replace that rectifier/diode behind the panel and change the push-on terminals to ring terminals and I think your problems should be solved.



Thanks for comments
 
Last edited:
i don't know exactly what you are asking for, but, you should not see 10.8 volts on any bus while the alternator is on line even with a diode in the system. you should see somewhere around 13.4 on the buss voltage and at least 11.9 or so on a buss fed through a diode. without seeing your system, its hard to tell why the diode is in there. the most obvious answer it to stop the essential bus from back feeding the system with a failure. you will see a .7 to 1.5 volt drop across a diode depending on the type of diode that was used.

bob burns
RV-4 N82RB
 
Voltage drop

There are a lot of different wires behind the panel.
When keying the radio transmitter the antenna wire emits RF.
Systems like EMS are sensitive to RF interference.
This can trigger strange readings that disappear when transmitting stops. Separate wires to reduce problem. Shielded wires can help.
Looking at your diagram the B/C diode works like Bob said. It typically have a voltage drop of >1 V.
You can replace this diode with a schottky diode.
It typically have low voltage drop, less than 0,5 V. It needs to be mounted on a aluminium plate for good cooling.
https://www.mouser.se/datasheet/2/240/DSS25-0025B-1549121.pdf

Your electric shop gave good feedback.

Good luck
 
Last edited:
Some thoughts:
- Jumper around that horrible 1965 diode block and see if the problem is resolved. Side note - if you are using diodes in your power design always use Schottky diodes.
- Use a multimeter to verify buss voltages.
- Decide what you want your system to do. For example if the second battery is to keep the panel up (as in you are eliminating a single point failure risk) then look elsewhere as well, as in the notorious "Avionics Master" switch. There is no need for it with modern avionics - and a thoughtful power design.

I suggest the concept of essential, main and endurance buss design has outlived the transition to LED lights and glass panels. There are better designs, but for you the simple way is to do some measurements to confirm the problem then follow the advice from your local guy.

Carl
 
Just a couple of comments. It doesn't seem that your avionics shop is that far off. However, they may not be that aware of the experimental essential bus configuration that some folks use. There are way too many unknowns to provide an exact answer.

Often for an essential bus there are two paths a separate power switch and a diode path from your main bus. The diode drop can be a volt or so depending on the type of diode that is in place and the current that is being drawn through the diode. For higher currents "better" diodes or diodes with a lower voltage drop at a higher currents are available. You may not have a "good" enough diode for the currents that are required to support transmit on your radio.
 
The rectifier diode prevents backfeeding the main bus via the essential bus in the event of a major short. There is a voltage drop across the rectifier typically 1.2V. Newer schottky's have far less of a drop, 0.2V to 0.4V depending on current.

Anyone that tells you to replace faston connectors/switches with screw terminals is just trying to run up the bill. Fastons are simpler, more robust/reliable.
 
Last edited:
Reply from my Avionics Guy

Problem solved...I'm satisfied that my avionics guy made changes that solved my problem..13.5v on the avionics bus when transmitting..Thanks to all for your comments and suggestions..

Rewired the bus isolation diode per the B & C Specialty drawing provided to divide the current load between two diodes and I replaced the push on terminals with ring terminals where possible (main and essential bus bars & new avionics switch have ring terminals, isolation diode still has push terminals but they were replaced with new terminals). Visually inspected grounding block and I don’t see anything that I felt was improperly done. It does use those push on terminals, which I personally do not like for reasons already stated, but used for grounding doesn’t scare me as much as for current carrying bus wires. Besides, they’re used all the time in other kit planes without issue so it’s hard to justify rewiring an entire airplane based on a personal preference. I would make that part of your annual inspection, though. If you start to see any corrosion buildup on the terminals or any visible signs that any terminal is getting hot, I’d replace that terminal block with one of a different design that didn’t use push-on terminals.
 
Last edited:
Back
Top