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  #21  
Old 06-12-2018, 05:57 AM
krhea krhea is offline
 
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Location: Peck, Kansas
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Default http://www.vansairforce.com/community/showthread.php?p=1266331#post1266331

www.PerihelionDesign.com. snap-jacks.

Keith Rhea
RV7
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  #22  
Old 06-12-2018, 06:09 AM
Mich48041 Mich48041 is offline
 
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Bob Nuckolls posted, "Some ol' hangar tales just never die. . . . . The 'cranking spike' thingy doesn't exist."
Read his complete post HERE.
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  #23  
Old 06-12-2018, 07:42 AM
rv8ch rv8ch is offline
 
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Location: LSGG
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Default diodes on relays

Quote:
Originally Posted by Mich48041 View Post
Bob Nuckolls posted, "Some ol' hangar tales just never die. . . . . The 'cranking spike' thingy doesn't exist."
Read his complete post HERE.
Hi Joe, I think these are two different topics - the diodes (or snapjacks) on relays is recommended, but Bob says that "spikes" going back into the electronics is an OWT.

I also went with the Perihelion Design snapjacks (a P6KE18CA Zener) https://en.wikipedia.org/wiki/Transi...pression_diode

http://www.periheliondesign.com/suppressors.htm
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  #24  
Old 06-12-2018, 10:56 PM
Cumulo Cumulo is offline
 
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Location: KHMT
Posts: 13
Default Re:big bang

The diode bridge inside the average alternator is robust enough to snub and dissipate the starter energy provided the alternator "A" lead is intact and on line across the bus.
From my perspective Nuckolls is a new guy. He didn't start monkeying with this stuff 'till Cessna had begun using alternators, so he is not well placed to be the last word on this subject.

Let me describe the sequence involving a starter that has the potential to damage electronic stuff:
1. The Master is turned on, connecting the battery to the bus.
2. The starter button is pushed, energizing the starter solenoid
3. The starter solenoid connects the starter to the bus.
4. several hundred amps flow to the starter and the starter begins to spin the engine.
5. Battery is low, so starter slows and and stalls.
6. the stalled starter with a hundreds of amps demand pulls the bus voltage down to a few volts.
7. Master solenoid drops out , disconnecting battery from bus.
8. the very strong magnetic field of the starter collapses (the starer now is just a massive inductor of copper and iron)
9. A very high energy spike WILL BE produced if there is nothing to snub and dissipate it.

The above is not the normal start sequence. Normally, the STARTER solenoid releases first,turning the starter off and the starter energy is dissipated on the solenoid copper contacts.

BTW, proof by authority is not a good way to argue, but I DO have an A&P, accessories, radio, instrument repairman certs, had a repair station for a decade and stick wise most everything but an ATP. And way back when I worked for a living I was an Electrical Engineer doing mostly military and computer design stuff.

Nuckolls has done some really good stuff and I tip my hat to him, but I can demo blowing a small diode out with a starter spike any old time.

I think that having to start a dark airplane is too restrictive, too old timey. Protection should be standard so strobes, radios, EFIS, EMS audio amps can be on with no fear of damage from any cause. Stay tuned.
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  #25  
Old 06-13-2018, 08:42 AM
Mich48041 Mich48041 is offline
 
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Cumulo, that is a very interesting scenario. But why does the master contactor drop out before the starter contactor? I assumed, maybe incorrectly, that the starter contactor has a stronger spring and would drop out first. If the starter contactor did drop out first, then most of the starter induced energy would be dissipated across the starter contactor contacts.
When you conducted the experiment that demonstrated blowing a small diode that is across the starter contactor coil, did the start contactor coil fuse also blow?
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  #26  
Old 06-13-2018, 08:56 AM
rocketman1988 rocketman1988 is offline
 
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Default Wondering,too.

Yes, I would like to hear the reasoning behind the master dropping off line first, as well.

Though the described scenario may be possible, is it in reality probable?
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  #27  
Old 06-13-2018, 09:03 AM
rv7charlie rv7charlie is online now
 
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Quote:
Originally Posted by Cumulo View Post
From my perspective Nuckolls is a new guy. He didn't start monkeying with this stuff 'till Cessna had begun using alternators, so he is not well placed to be the last word on this subject.
[snipped]
Nuckolls has done some really good stuff and I tip my hat to him, but I can demo blowing a small diode out with a starter spike any old time.
Interesting that working for multiple major players in the aviation field, and working on everything from military hardware to bizjets to single engine GA a/c for around 50 years leaves you 'not well placed to be the last word on this subject'.

Every year or two, somebody pops up on the interwebs claiming they are the new authority on all things electrical, trying to convince us that they are the new messiah and we should no longer bow down to Nuckolls. There's one on the Aeroelectric list right now. Within two or three posts (often in their 1st post), it's obvious that they are just the latest false prophet. (It's really funny that they think that Nuckolls thinks of himself that way....)


I can blow out just about any electronic component, if I misuse it in a circuit.

In that start sequence I snipped from the quote, why would a master contactor, who's coil draws about 3/4 amp max, drop out before the starter contactor, who's coil draws 2-3 amps?

Charlie
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  #28  
Old 06-13-2018, 11:13 AM
Mich48041 Mich48041 is offline
 
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I agree with you, Charlie. I wish that I knew as much as Bob Nuckolls. He is so respected in his field that he has been called to testify in court as an expert witness. He backs up his assertions with lab experiments using sophisticated electronic test equipment.
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  #29  
Old 06-13-2018, 06:10 PM
Cumulo Cumulo is offline
 
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Default

Quote:
Originally Posted by Mich48041 View Post
Cumulo, that is a very interesting scenario. But why does the master contactor drop out before the starter contactor? I assumed, maybe incorrectly, that the starter contactor has a stronger spring and would drop out first. If the starter contactor did drop out first, then most of the starter induced energy would be dissipated across the starter contactor contacts.
When you conducted the experiment that demonstrated blowing a small diode that is across the starter contactor coil, did the start contactor coil fuse also blow?
Well, it is a horse race between the starter solenoid and the master solenoid,

Since the master solenoid is continuous duty, it has a hold coil operating at a much lower magnetic strength that the starter solenoid. So, everything else equal, the master will release first

The very diodes that are the main subject of this thread, do slow the release of both solenoids significantly. However, if a zener is placed in series with the starter solenoid snubber diode, say a30 volt zener (facing opposite to the existing snubber), release time will be shortened considerably. I think some one already mentioned using an all-in-one device that would be equivalent.

Re: fuse in the starter solenoid ckt, if the solenoid is powered directly from the bus, the grounding starter switch, on my aircraft at leased, is not fused. The wire protecting impedance is the coil itself.

Last edited by Cumulo : 06-13-2018 at 06:29 PM.
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  #30  
Old 06-13-2018, 08:07 PM
Mich48041 Mich48041 is offline
 
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If the battery is so weak that it can not hold the master contactor closed (less than 1/2 amp), then how can the battery send hundreds of amps to the starter motor?
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