Location of diodes in the Alternator Restart system?

[edasmus]

My experience with this is limited but here it is. My plane has had overvoltage situations occur over the years and the system worked as designed in my case. I wrote a long post on this years ago and I'm sure it's somewhere on this site.

It has long since been resolved and the condition does not occur anymore but it took awhile to find the culprit of what was causing the overvoltage condition. It was a short in the circuit breaker box to the left of the pilot's knee. The main large thick wire that provides power to the the buss bar vibrated and wore a small hole through the plastic insulator box and allowed intermittent contact with the skin of the aircraft and caused arcing. It actually burned a small hole in the skin of the aircraft.

Anyway, voltage would exceed the limit (which I think is about 32.5 volts if memory serves) and the overvoltage sensor would trip the alternators offline. Turning everything off and then turning the master switch off for a few seconds and then turning the master back on would reset the overvoltage sensor. Turning the alternators back on and then the remainder of the electrical system and I'd be back in business until the next time it happened (which might be months later).

Anyway, occasionally turning the alternator switches back on after resetting the master switch would not bring the alternators back on line. A press of the alternator restart button would solve that.

Anyway, my alternator restart batteries get replaced at every annual inspection and realistically I test each system about once, maybe twice a year. I figure that's good enough for me. If my alternators quit for any reason, (which they never have once this aforementioned problem was resolved), I'll attempt to get them back online. If that fails, I'm unloading the electrical system, putting the gear down, and then deciding how long I care to fly without the electrical system. I fly day VFR so as long as the gear is down, not having the electrical system isn't a serious problem. You serious instrument pilot's obviously have more at stake.

It's kind of pain to test the overvoltage system which is why I do not do it all that often. If one is to test it, make it the very first thing you do after starting the engines and bringing the alternators on line. I would strongly suggest keeping all avionics out of the equation. They shouldn't "feel" anything which is the point of the entire system but no sense injuring your avionics while finding out the over voltage protection didn't work as designed. I share Rick's sentiment as I test this on the ground.

Further opinions appreciated. Interesting thread. Thanks for starting it Rick.

BTW Rick, I've been on the waiting list at Bowman KLOU for about 5 months now. Hoping for a hangar there by next year sometime.

[Learjetter]

There is more info on this topic in other threads, and even more in the old CPA forums, but most of it boils down to this: Tom Carr recommended removing the alt restart battery pack and placard in the system inop with a logbook entry. His theory is that pressing that button releases unregulated current from the alternators into the system and if a diode is failed, could start an electrical fire or explode the batteries. My airplane was missing the battery pack on delivery, and I’ve chosen to just placard it inop and move on.

I’ve run the overvolt test and it went fine, and twice I’ve turned off an ALT and back on with no issues (all as ground tests). I mitigate the risk of not having the battery pack by limiting my flight in Hard IFR.

-LJ

[Rick Erwin]

Hi Ed, I have had a couple of High Volt events also, the checklist restored alternator power … for a while. The fix was two new voltage regulators and replacing all the diodes on the electrical panel circuit board on the front firewall.

I took me probably six to eight months for my name to come up on the wait list for a hangar. Since I needed one of the 48 ft. hangars, it took a little longer for one of those to come open. I expect you’re in that same boat. Once they let you know your name has come up and you get moved, it’ll be great having you here!

Rick