What's new
Van's Air Force

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

Buss surge on engine start

petersb

Well Known Member
Have an EXP2BUS running a Dynon Skyview Classic.

EXP2BUS is wired so the Avionics Buss does not shut down during engine start.

Q1: WIll the starter cause a surge back through the Avionics Buss and connected devices ?

Q2: Does the EXP2BUS filter out such surges before they reach the Avionics Buss ?

Q3: Can the attached Dynon filter out such surges without damaging it ?

Q4: The Dynon does shut down during starting, I assume the Avionics Buss is going down to a low voltage that triggers a reboot.

Trying to understand what is happening, we will have a backup battery for the DYNON which hopefully will maintain continuous operation,

Installation includes seperate Master and Starter Solenoids.
 
Starters do not cause voltage surges. They cause cause voltage sags.
Nothing, including Dynon, can filter out nonexistent surges.
You are correct. The aircraft voltage can sag (brownout) to below the minimum required (about 10 volts) to operate the Dynon. Installing the Dynon backup battery will prevent rebooting due to brownouts.

Be sure to install a diode in parallel with each contactor coil with banded end connected to positive. A diode will short out induced voltage, thus preventing voltage spikes which can arc across the contacts of the controlling switch, eventually damaging the contacts.
 
Starters do not cause voltage surges. They cause cause voltage sags.
Nothing, including Dynon, can filter out nonexistent surges.
You are correct. The aircraft voltage can sag (brownout) to below the minimum required (about 10 volts) to operate the Dynon. Installing the Dynon backup battery will prevent rebooting due to brownouts.

Be sure to install a diode in parallel with each contactor coil with banded end connected to positive. A diode will short out induced voltage, thus preventing voltage spikes which can arc across the contacts of the controlling switch, eventually damaging the contacts.

Thanks Joe,

Diodes installed , agreed regarding voltage sag. Like many flying myths have heard for years not to have electronics on when starting engine due to possible voltage spikes etc. while using the starter.
 
Starter SURGES

Starters that use a permanent magnet instead of a wound field most certainly can and will cause voltage surges that will take out avionics. The EXP2 will not filter out surges. Strongly recommend you rewire the EXP2 to have the Avionics shut down on cranking or better yet don't turn the avionics on until engine is started and running. Voltage spikes are of a quick transient nature and can exceed several hundred volts. They can be fast enough that the clamping diode will not completely shunt the spike.

Tyler Bryant
A&P, GROL, Rotax Certified
 
Check out this link on electrical protection

Automotive application notes are a very good reference. Each advancement FWF brings us even closer to parity with the research done for autos. The section 1 of this PDF gives you an idea of what disturbances are there and what causes them.
The good news is modern avionics are well protected but turning them off during the transition from battery to Alternator is still favored by many.

http://www.st.com/resource/en/application_note/cd00181783.pdf
 
I can tell you Bob Nuckolls says the voltage spikes are a myth, I can also tell you Garmin and Grand Rapids advise not to expose their equipment to those same mythical spikes. Who to believe? I think Bob has done his homework on this, but the bottom line is you won?t know on your aircraft unless you put an oscilloscope on it, or just isolate the avionics and move on.
Tim Andres
 
It?s all good until it isn?t. Procedurally, I see little to no advantage and leave avionics off on startup and shut them off before shut down.
If your battery is in good shape, it is the best protection for your system but things can happen.
Why take a chance?
 
I have the Dynon FD180 that comes on with the master (with backup battery). Dynon told me to leave it on during engine start.
 
Quote from the GRT Horizon User?s Guide pg. 1-1 Rev C
Power may be applied before or after the engine is started, although it is preferable to do the latter.
Quote from the Dynon FlightDEK-D180 Pilot?s User Guide Rev H page 2-1
It is acceptable to have the FlightDEK-D180 turned on during engine start.
I am sure that Garmin builds their avionics to the same high standards as the competition. I could not find any reference in Garmin manuals about shutting units off prior to engine start.
Since Bob Nuckolls and avionics manufacturers are all in agreement that it is not necessary to shut off avionics during engine start, believe them all. It will not hurt to shut off avionics if you wish. In fact, switches need exercise just like people do. I prefer to leave all of the avionics turned on while engine starting. None of mine have have failed.
 
Just keep in mind, they are making these statements for ?normal? and properly functioning equipment. I am not advocating anybody change what they are doing, but for my simple VFR flight profile, starting up with avionics on provides no benefit, so why do it?
My electronics tech experience tells me if you are going to have a problem, it often happens when powering up equipment. Powering up while the starter is engaging or opening just doesn?t sit well with me, but I am old school and probably swayed by my training many years ago. Myths are more powerful than truths sometimes.
 
Two Things

First....Garmin has a rather nice video advertising the "new" G3x touch. The video is all about it being installed in a Red Bull racer and it shows the unit powered on when starting.

Second.....I see no reason to have anything powered until after start....UNLESS...you want to monitor your engine's early response after start (like oil pressure, amp draw, etc). If you have an electronic monitor, isn't this the only way to get immediate information?

I had the GRT in the -8 and was informed to leave it powered off until after start...this always bothered me not knowing what the engine was really doing to itself as the GRT units were burping and booting up!
 
I posted one of the above myths on the AeroElectric List. Bob Nuckolls replied that starter voltage spikes do not exist. He is offering $100 to anyone who can prove otherwise. You will have to ask him what proof is required, but I assume he would want oscilloscope screen shots and documentation showing the test setup.
 
Wonder if he would take burnt out parts as proof?

I had a starter still engaged when the engine kicked back just a few months ago. Massive negative spike took out several items that were not protected well enough.
That was after 4 years and hundreds of hours of operation.

What didn't burn out was any of my modern avionics from Gamin, AFS or Dynon.
 
I would ask Bob if his challenge is limited to a standard aircraft starter in and part of the overall electrical system.

Transients are present in any inductive load. We may not see them as the battery is one heck of a capacitor. Find a way to energize the starter without a battery connected. Not sure how to make that happen, but you see the point.

Also, transients occur anytime we open or close a contact. The arc is drawn and a voltage transient will be present throughout the system.

Try this experiment ( preferably on a friend!).
Take a standard 1.5vdc battery. D cell is fine. Take any transformer you can find with a minimum 10-1 ratio. Hold the two wires out of the secondary in your hand. Make and break the 1.5v battery on the primary. See what happens.
Now, 1.5vdc times ten is only 15 volts. Not enough to do anything. However, the arc drawn as you make and break the connection is thousands of volts multiplied by ten. Very low current but it will give you quite a jolt.
It was a fun trick to play on my friends growing up.
Want more fun? Use a 9v battery!
(Don?t do it with someone who has a pacemaker or weak heart please!) I take no responsibility if you hurt yourself or someone else!


We all know motor loads will have transients. I bet Bob has a good explanation of why he feels it is a myth and not a concern in our standard aircraft electrical systems. It isn?t just that, magically, they are immune to normal physics of any other type of motor.
 
Back
Top