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Diode for starter, master and aux ground power solenoids/relay

jeffsvan

Well Known Member
Could someone recommend a Diode (part#) to put across the posts on the relay/solenoids that I can easily purchase. Want to reduce spark jumping back at the switches.

Can I use the same diode part# for all three (trying to keep it simple)
 
Diode

A 1N4001 (or higher voltage 1N4004) will work in all three locations....

Could someone recommend a Diode (part#) to put across the posts on the relay/solenoids that I can easily purchase. Want to reduce spark jumping back at the switches.

Can I use the same diode part# for all three (trying to keep it simple)
 
Those diodes are significantly underrated for flyback application. I would look for something in the 3A If and 1000V Vr range, like the RGP30M-E3.
 
Those diodes are significantly underrated for flyback application. I would look for something in the 3A If and 1000V Vr range, like the RGP30M-E3.

Meh...

Even the 1N4001 (1A 50V) is adequate, if you look at *peak* ratings. But the 3A versions are physically more robust (fatter body and heavier leads). And the higher voltage ratings don't cost that much more. When I ran an electronics repair company, we just purchased the higher rated devices to minimize our inventory.

Another number for the 3 amp 1000 volt devices is 1N5408.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2060353.m570.l1313.TR2.TRC0.A0.H0.TRS0&_nkw=1n5408&_sacat=0

The same device can be used on all the relay coils you're likely to use in an airplane.
 
I found some 1N5406 in my parts inventory.
Would that work?
I'm not super keen on electronics specs.
 
Even the 1N4001 (1A 50V) is adequate

I've seen 430V spikes from my intermittent duty starter solenoids. In fact, one such spike blew out my 400V rated flyback diode, shorted it to ground, and disabled my landing gear.

...I might be a little bit passionate about this!
 
The 1N5406 diode should work fine. When the controlling switch is opened, the polarity of the coil induced voltage is such that the diode will be forward biased and conduct any induced current. The induced voltage will be limited to the forward voltage drop of the diode which is about 1 volt. The 1N5406 diode is rated at 3 amps average and 200 amps peak for 8 milliseconds. Be sure to connect the banded end of the diode to the positive side of the coil and the other end of the diode to the negative side of the coil. Like Charlie said, any diode will work. The important thing is for the diode to be mechanically strong enough not to break from vibration.
 
I've seen 430V spikes from my intermittent duty starter solenoids. In fact, one such spike blew out my 400V rated flyback diode, shorted it to ground, and disabled my landing gear.

...I might be a little bit passionate about this!

For total clarity, are we talking about diodes in the same location? I was addressing a suppressor across *the coil* of the contactor, to suppress arcing across the controlling switch. If the diode was across the *contacts* of the contactor, to suppress flyback from the starter windings, the equation would change quite a bit.
 
The diode that failed was across the contactor's coil (protecting the controlling switch).

I had assumed that as long as the flyback diode's Vr rating was sufficient for bus/coil voltage, and it had a reasonably high If, it would work for flyback purposes. Based on the failure though, I think the Vr has to exceed whatever inductive spike you'd see...even if it's in the forward direction of the diode.

Edit: I'm going to get the scope out and play around with this today. I thought I had a handle on what was happening, but the more I think about it the more questions I have...
 
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Not looking to start a laboratory study:

Just to clarify I was looking to just have one part# diode to make life simple and satisfy these items:

Diode for starter,
master,
and aux ground power solenoids/relays.

This 1N5406 seems to have specs that are sufficient. It is a larger diode with thicker leads.

Can I get some agreement?
 
So, I'm not much closer to understanding the failure...but I have turned up some interesting info in the meantime. All of this is for the four-post starter solenoid (ACS 11-03162):

Without flyback diode: -510V peak pulse
Current through flyback diode: 6.3A decaying over about 40 ms
Coil inrush current: 7.3A
Coil steady state current: 3.6A​

40 ms is an eternity in the world of diodes--a reasonable forward current limit is definitely something to consider.

Interestingly, I have come across datasheets listing different limiting Vr values for peak reverse vs. RMS. Maybe there is something relating Vr with forward voltage (not drop)?

To get back on topic, I'm liking the looks of the 1N5408.
 
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Regarding the op's Q, as previously stated , (almost) anything with 2 wires and a part number that starts with 1N*** will work to snub a relay coil in small a/c. I agree that a 3 or 5 amp rated unit with stout leads would be a good choice. In any case, the relay coil energy itself is not likely to damage a small diode like a 1N1*** series.

HOWEVER------

There IS one badass element in most aircraft that is often not snubbed. It is capable of generating a spike of hundreds of volts and hundreds of ampws which translates into thousands of Joules. It has caused electrical mayhem for years. The starter. This starter spike is negative polarity and if not effectively snubbed could easily take out the typical small diode across the starter or master solenoid and possibly other electronic devices $$ as well.

The reason this type of damage is all too common in aircraft and not autos is because aircraft generally have an electrically actuated battery contactor - the master switch. So, if the master releases the battery during cranking, everything else on the bus is alone in a dark alley with Mr. Starter armed with hundreds of amps and looking for trouble.
 
FWIW I just did relays, 3 ColeHersee units (Z14 setup) and I used 1N5408 for all the diodes. Bought 50 for $5 off amazon. Pretty much cant beat that, and Ive used them elsewhere on the plane as well.......
 
I installed large MOVs across my starter contacts and - I'll probably be soundly flamed for this - I didn't run my starter solenoid in series with the master relay. It's on it's own separate circuit. The risk I'm taking, of course, is that the starter solenoid contacts weld themselves and the starter keeps cranking until the battery is dead. Since I had protected the contacts with the MOVs, I decided to take that risk. Flame away!
 
I installed large MOVs across my starter contacts and - I'll probably be soundly flamed for this - I didn't run my starter solenoid in series with the master relay. It's on it's own separate circuit. The risk I'm taking, of course, is that the starter solenoid contacts weld themselves and the starter keeps cranking until the battery is dead. Since I had protected the contacts with the MOVs, I decided to take that risk. Flame away!

Across the contacts an MOV makes sense to cut out arcing. The diode discusison above is for the coil terminals here to short out the back current from the field collapse.

I'm not great at electronics, and I've not seen MOVs used as you are doing, but I dont see an issue. I also dont see what youre trying to gain since the usual start sequence would be done before any avionics are brought on line....
 
Bob Nuckolls posted, "Some ol' hangar tales just never die. . . . . The 'cranking spike' thingy doesn't exist."
Read his complete post HERE.
 
diodes on relays

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/Transient-voltage-suppression_diode

http://www.periheliondesign.com/suppressors.htm
 
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.:p

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.
 
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?
 
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?
 
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.:p

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
 
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.
 
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.
 
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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?
 
The starter contactor in most airplanes is energized by a switch that gets positive voltage from a fused source. If the wire shorts to ground, the fuse blows and no harm is done. Wiring the starter contactor as described below is dangerous. If the wire shorts to ground, the starter engages.
if the solenoid is powered directly from the bus, the grounding starter switch, on my aircraft at leased, is not fused.
 
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?

Well, a starter motor can be thought of as a constant current device. With a given load, more voltage = faster, less voltage = slower, aprox same current. Stalled starter amps is about the same as turning amps. This relationship will not exist with a fixed resistance load, but does with a DC motor.

Again, the internal resistance of a starter is only milliohms, so it can flow beaucoup amps with only a couple of volts. What limits the current of a starter motor is the generated back emf (voltage), not the motor resistance.

Edit: Yet again, if the starter pulls the battery voltage down to below the dropout voltage of the master solenoid , the master disconnects. It does not depend on the division of current, just too low of a bus voltage produced by starter current and the internal resistance of the weak battery.

Re: fusing a starter solenoid for safety, if I have the picture right, the fuse could protect the starter switch wire or in case of an internal case to coil short of the starter solenoid, but I have not been aware of any such failure. I would think the potential for such a failure is very low. Further, such a failure would likely occur just outside of the hanger door, not a 12k ft. So, maybe keep it simple is better in this case, but unlike Nuckoll I could be wrong.
 
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Yeah, you could. :)

Let's start with why we install protection devices at the supply end of the wire (instead of on the device).

Explain that, and then we'll move forward.
 
Stalled starter current is greater than turning current because there is no back EMF. That is why lights dim when motors start.
A stalled motor is a fixed resistance (but will increase slightly as the wire heats up).
Stalled starter motor current is directly proportional to the voltage.
-
The danger of switching the negative wire of the start contactor is that there is greater chance of a shorted wire energizing the contactor. Then the propeller could move unexpectedly and possibly strike someone. A switched negative wire will be two or three feet long and pass through the firewall. That long negative wire is exposed to danger.
Now compare that to a positive switched wire. It is still exposed to danger for two or three feet. But if it shorts to ground, the propeller will not move because the fuse will blow.
A person might argue that the chances of that wire shorting to ground are slim. True, but if using that reasoning, then no fuses or circuit breakers are necessary for any wires.
Many accidents happen due to a chain of events. That accident chain could be:
An electrical design error.
Vibration that wears wire insulation.
The master switch being left on.
Touching and moving the damaged wire.
A person standing in the propeller arc.
 
Yeah, you could. :)

Let's start with why we install protection devices at the supply end of the wire (instead of on the device).

Explain that, and then we'll move forward.

LOL , Yes the standard aircraft starter ckt is wired through a fuse. Autos and tractors aren't. And, yes, always protect the wire. (Genesis 41:30);)

The issue(side) is: can a starter event damage small electronic devices?

The answer is yes. In the 1960's when marvelous solid state avionics began appearing, starter zaps caused havoc. Avionics/radio bus switches were then installed to isolated these delicate critters. When alternators became ubiquitous the problem faded. Serendipity.

The "kick" of a released starter is a NEGATIVE spike of whatever amperage the starter was drawing when released. If not snubbed it will generate hundreds of volts and possibly resonate into a positive excursion as well. The alternator diode bridge, if present on the bus, will effectively snub the initial negative excursion since the spike is shorted out by the alternator's 3 sets of 2 diode to ground. this will limit the spike to about 2 or 3 volts negative for about 100 millisec or so.

However, since diodes take a finite amount of time to conduct, some spike could sneak through but will be only microseconds or so. It would take some lab work to know how fast alternator diodes really are.
 
Correlation cannot be assumed to be causation.

Can you cite lab tests demonstrating starter current spikes failing current technology a/c electronics? Be aware that there *were* lab tests demonstrating early solid state a/c electronics failing during the start sequence. But the cause wasn't flyback spikes from the starter. Hint: Silicon isn't the only substance from which you can make a semiconductor, and wasn't what was used in early solid state a/c electronics.
 
Stalled starter current is greater than turning current because there is no back EMF. That is why lights dim when motors start.
A stalled motor is a fixed resistance (but will increase slightly as the wire heats up).
Stalled starter motor current is directly proportional to the voltage.
.

Yep, All true. The sequence usually is that the starter is drawing it's near normal current and due to the weak battery's internal resistance, the bus voltage falls, the starter slows, less back emf - high current, contactor drops out Still lots of amps flowing in the starter. BY the way, if you have ever been zapped by a starter as I have, I doubt you will be anxious for an encore.

And, yes , agreed , a fuse is the safest way to wire a starter switch. However, injecting the fuse issue into this tread is tangential.
 
Correlation cannot be assumed to be causation.

Can you cite lab tests demonstrating starter current spikes failing current technology a/c electronics? Be aware that there *were* lab tests demonstrating early solid state a/c electronics failing during the start sequence. But the cause wasn't flyback spikes from the starter. Hint: Silicon isn't the only substance from which you can make a semiconductor, and wasn't what was used in early solid state a/c electronics.

I still have CK722 transistors and 1N314 diodes. Hoping they make a comeback, like vinyl records. Anyone who understands the last sentence is ooold!
 
Correlation cannot be assumed to be causation.

Can you cite lab tests demonstrating starter current spikes failing current technology a/c electronics? Be aware that there *were* lab tests demonstrating early solid state a/c electronics failing during the start sequence. But the cause wasn't flyback spikes from the starter. Hint: Silicon isn't the only substance from which you can make a semiconductor, and wasn't what was used in early solid state a/c electronics.

Regarding spikes and current techology, as I have said previously, the presence of a passive alternator of common design should provide adequate protection , snubbing any starter energy dumped into the electrical system..

I have an old airplane with a generator. It's a very nice generator that will hold it's own against a modest load just above idle. It is impractical to replace it with an alternator.. so, you can bet I will be augmenting the circuit protection before installing new avionics.

Re: silicon. I go way back. Once upon a time I made replacement solid state vibrators for the Narco junk. I used germanium power transistors. Been there done that, even sold a few.


Referring to the starter release event as a "spike" trivializes it somewhat. There is a massive amount of energy stored in an energized starter which is a ~5 hp DC motor. What happens when a magnetic device such as this is released is well known by anyone competent in this engineering area.
If the device terminals are shorted, the existing current will continue to flow and dissipate in the circuit resistance over L/R time just as a capacitor will discharge over RC time. However, if the load on the starter is a few nick-nacks on the bus, they will receive a 1000 joules in a few milliseconds. it would be a heavy design burden to have every device able to take this full impact. I will have to look at DO-160..whatever and see what the present specs are for energy hits. But, as I said, thank goodness for the alternator and the big diodes inside to provide the bulk of protection.
 
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I still have CK722 transistors and 1N314 diodes. Hoping they make a comeback, like vinyl records. Anyone who understands the last sentence is ooold!

Yes, Vern. I bought A CK722 with paper route money when they first came out. oooooold

It's a positive delight to know there is someone here that knows what a CK772 is, and even has one. Wow.

For the rest of you Boomers and younger, the CK722 was the 1st transistor device a civilian could buy. Made by Raytheon. There were diodes around but a solid state triode. That was magic.
 
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Did you buy those from Poly-Paks?

Hey man, it was the 60's. I don't remember much.

Just to rub it in. I have the January, 1975 edition of Popular Electronics. It's been in a zip lock bag since I read it in 1974 and has my name on the address label. It was a magical feeling when I read it, and somehow I knew it would be a collector's edition.

I also had an original Apple Red Book, but I gave it to my brother for his 60th Birthday. He's an author of several computer books and worked at Apple in the Jobs years.

I also have a hunk of galena for the real DIY crowd.

V
 
LOL! Galena! Now that's hard core, man. Got a cat whisker to go with it? I used to own a collection of Popular Electronics going back to the fifties. My wife tossed them out when we moved in 1986.
 
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