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  #21  
Old 10-18-2017, 11:13 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by lr172 View Post
The diode does not protect the buss, it protects the contacts in the relay/contactor.
A diode on a high power relay/contactor does not protect the contacts in the relay.

It is in place to protect the contacts in the switch that is controlling the relay. In the case of an airplane, the battery master and starter motor switches

Do a google search on relay coil protection diode - there is a lot of info and detailed explanations available.

One simple answer found -

Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors.
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  #22  
Old 10-19-2017, 09:49 AM
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That makes sense Scott. I was also told it keeps the spikes from damaging other equipment in the electrical system. Considering how it is connected, your explanation makes the most sense.
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  #23  
Old 10-19-2017, 09:53 AM
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Quote:
Originally Posted by rvbuilder2002 View Post
A diode... is in place to protect the contacts in the switch that is controlling the relay.
Thank you for pointing that out. In fact, Service Bulletin SB 92-01 was issued on the Aircraft Spruce starter switches to add a diode to keep the contacts from arcing like this (on the left):



My starter solenoid is the "four-lug" version with the "S" and "I" terminals. The "I" terminal is connected with a flexible braid to the contact bar (or disc) that completes the connection between the two large lugs when engaged. I added a large 18V MOV between the "I" terminal and each of the big lugs to (hopefully) protect the main contacts in the starter solenoid.
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  #24  
Old 10-19-2017, 11:43 AM
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Originally Posted by snopercod View Post

My starter solenoid is the "four-lug" version with the "S" and "I" terminals. The "I" terminal is connected with a flexible braid to the contact bar (or disc) that completes the connection between the two large lugs when engaged. I added a large 18V MOV between the "I" terminal and each of the big lugs to (hopefully) protect the main contacts in the starter solenoid.
Interesting. Vans standard solenoid uses a copper strap instead of a flexible braid. I had the strap fail, which doesn't matter unless you are using the "I" Terminal to energize the on board starter solenoid, which was recommended for light weight starters early on. Those instructions now state to do this only if the starter won't disengage.
I like the MOV "Quench Arc" idea. However, those contacts are very robust and the "disc" is designed to rotate making fresh contact points. Mine looked brand new with no pitting after 750 hours. Still not a bad idea.
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  #25  
Old 10-19-2017, 12:50 PM
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The fascinating thing to me (that I still haven't quite wrapped my head around) is that starter solenoids have dual windings (See aeroelectric link above). When the starter switch is first energized, the two windings work in tandem to give the starter solenoid a "kick". When the starter switch is released, the current in the two windings is equal and opposite, thus canceling out any switch contact arcing due to the collapsing solenoid field. That's the theory, anyway. I'm wondering if that arcing seen in my photo above was from inrush, not release. Someone needs to put a digital oscilloscope on a starting system to see what's really happening.
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  #26  
Old 10-19-2017, 11:08 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
A diode on a high power relay/contactor does not protect the contacts in the relay.

It is in place to protect the contacts in the switch that is controlling the relay. In the case of an airplane, the battery master and starter motor switches

Do a google search on relay coil protection diode - there is a lot of info and detailed explanations available.

One simple answer found -

Since an inductor (the relay coil) cannot change it's current instantly, the flyback diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts or possibly destroying switching transistors.
Thanks, I stand corrected here. It is protecting the contacts that are activating the coil, not the contacts driven by the coil. In my background, it was usually another relay switching the coil and didn't think through my explanation.

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  #27  
Old 10-20-2017, 01:55 PM
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Default In the interest of science

In the interest of science, today I sacrificed a spare starter solenoid to see what was really inside it. This was one of the $10 ones that ACS sells Starter Relay and is what I have in my plane. I'm going to have to take back some of the things I posted above. First off, the "I" terminal was connected to the moving disc with a piece of corrugated brass. Secondly, the coil had only a single winding, with one end connected to the frame of the unit and the other end connected to the "S" terminal. The coil resistance was 3.7 ohms. Here are the photos (may it rest in peace):

Flexible brass jumper between "I" terminal and disc:


"I" and "S" terminals:


With the jumper cut, the plunger can be removed:


The plunger:


The coil:


The metal shroud and caps around the coil:


The terminals can't rotate because they are sitting flat on an insulator on top of the coil:
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  #28  
Old 12-29-2019, 08:53 AM
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snopercod snopercod is offline
 
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I don't know where the recent thread went, but in that one I mentioned that I connected two MOVs (ZNRs, Varistors, TVSs) to my starter solenoid to prevent arcing at the main contacts. I used something similar to the Panaxonic ERZV20D270 : Varistor Type: D Series: V between each stud and terminal "I" which is connected to the disc. I also used the required diode between terminal "S" (the coil) and ground to protect the starter switch. Maybe there's a better way to do it, but this is what I did.
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