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Diode on solenoid.


derekliston

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This has been very interesting. I thank everyone for their input, I have certainly learned a bit, not the least being how to test a diode. I found the original diode and tested it this morning and it is definitely stuffed, ie I get a current reading in both directions. I will endeavour to purchase the suggested Jaycar part number diode this morning and see where I go from there. I will let you all know what the outcome is.

 

 

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This has been very interesting. I thank everyone for their input, I have certainly learned a bit, not the least being how to test a diode. I found the original diode and tested it this morning and it is definitely stuffed, ie I get a current reading in both directions. I will endeavour to purchase the suggested Jaycar part number diode this morning and see where I go from there. I will let you all know what the outcome is.

Jaycar do not have a Diode 1N5048 but they did have 1N5408 which, on closer inspection of your photograph, appears to be the correct number so I will give that a try.

 

 

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Jaycar do not have a Diode 1N5048 but they did have 1N5408 which, on closer inspection of your photograph, appears to be the correct number so I will give that a try.

If the starter solenoid has a brand and part number, that may be helpful:

From what you have described, there is something wrong with the way the diode was fitted (as others here have suggested).

 

So, if we can verify what are the various connections on the solenoid, you can then fit it correctly.

 

 

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If the starter solenoid has a brand and part number, that may be helpful:From what you have described, there is something wrong with the way the diode was fitted (as others here have suggested).

So, if we can verify what are the various connections on the solenoid, you can then fit it correctly.

As I have previously said on this thread, it all functioned correctly for 40+ hrs before it malfunctioned. Thanks to the helpful comments I now know how to test a diode, which I did and proved the diode to be faulty (allows current flow in both directions) I have purchased from Jaycar a diode Pt No. 1N5408 which I have terminated, covered with heat shrink and tested to ensure it is working. I will fit it and try it this afternoon and let you all know what happens.

 

 

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Here's a circuit of what I think is wired up in the plane.

 

1873931834_Startswitch.jpg.eba5170a0a8b86ba92d6af92eebffc5a.jpg

 

Or at least it's what I think it should be. Even if the diode shorts out, you would still only have 12V on both sides of the solenoid coil. In that case, the engine might not start at all and you'd likely get a little spark in the start switch every time you tried to start the engine.

 

The back EMF across the coil of the solenoid can become quite high as one end is open circuit when the start switch is released. That's why you should use a diode with a high PIV rating. With the diode in place, that back EMF across the diode should only get up to about 0.7V (the forward voltage drop across a silicon diode), but the coil of the solenoid could generate quite a bit of current through the diode depending on the number of turns and the voltage it tries to generate, so a diode capable of withstanding a high current is best for this application.

 

Even then, being periodically zapped with a high current pulse, and living in a high temperature environment, it's not unreasonable to expect occasional failures of the diode.

 

In a pinch if the diode fails at a remote place where it can't be replaced, it would be OK to just remove it temporarily to get you home. Just don't have any sensitive electronics switched on while starting the engine (if you can avoid it). In fact, this last point is important with or without the diode.

 

 

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Here's a circuit of what I think is wired up in the plane.[ATTACH=full]51932[/ATTACH]

Or at least it's what I think it should be. Even if the diode shorts out, you would still only have 12V on both sides of the solenoid coil. In that case, the engine might not start at all and you'd likely get a little spark in the start switch every time you tried to start the engine.

 

The back EMF across the coil of the solenoid can become quite high as one end is open circuit when the start switch is released. That's why you should use a diode with a high PIV rating. With the diode in place, that back EMF across the diode should only get up to about 0.7V (the forward voltage drop across a silicon diode), but the coil of the solenoid could generate quite a bit of current through the diode depending on the number of turns and the voltage it tries to generate, so a diode capable of withstanding a high current is best for this application.

 

Even then, being periodically zapped with a high current pulse, and living in a high temperature environment, it's not unreasonable to expect occasional failures of the diode.

 

In a pinch if the diode fails at a remote place where it can't be replaced, it would be OK to just remove it temporarily to get you home. Just don't have any sensitive electronics switched on while starting the engine (if you can avoid it). In fact, this last point is important with or without the diode.

Agree entirely that's how it should be...or something along those lines.

 

But if a short circuit diode kept the starter pulled, it can't be wired like that.

 

And furthermore, the diode is probably not doing what we think it should be.

 

Which is why I suggested getting a little more detail on terminals etc.

 

 

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  • 1 month later...

This was a real interesting thread, thanks guys.

 

I don't think there is a diode on my SK Jabiru, but after this I'm sure going to look for it.

 

The thing that bothered me was the possibility of the starter motor staying stuck on due to the contacts in the solenoid welding together. One way of making this safe is to use 2 solenoids in series, but I reckoned this to be too heavy so I have just added a warning light which glows when 12 volts appears at the starter-motor side of the solenoid.( only when start button is pushed normally)

 

I thought this was my own idea, but it was a suggestion in the thread ( thanks Blueadventures), and for the Jabiru, the noise of the running engine could well hide the noise of a stuck starter motor.

 

With the warning light, at least I will know if the welding thing ever happens before taking off.

 

If there is no diode, is the contact welding more likely to occur?

 

 

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This was a real interesting thread, thanks guys.I don't think there is a diode on my SK Jabiru, but after this I'm sure going to look for it.

The thing that bothered me was the possibility of the starter motor staying stuck on due to the contacts in the solenoid welding together. One way of making this safe is to use 2 solenoids in series, but I reckoned this to be too heavy so I have just added a warning light which glows when 12 volts appears at the starter-motor side of the solenoid.( only when start button is pushed normally)

 

I thought this was my own idea, but it was a suggestion in the thread ( thanks Blueadventures), and for the Jabiru, the noise of the running engine could well hide the noise of a stuck starter motor.

 

With the warning light, at least I will know if the welding thing ever happens before taking off.

 

If there is no diode, is the contact welding more likely to occur?

Bruce: The diode is there to protect the start switch from welding rather than the heavy current contacts in the solenoid.

 

 

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This was a real interesting thread, thanks guys.I don't think there is a diode on my SK Jabiru, but after this I'm sure going to look for it.

The thing that bothered me was the possibility of the starter motor staying stuck on due to the contacts in the solenoid welding together. One way of making this safe is to use 2 solenoids in series, but I reckoned this to be too heavy so I have just added a warning light which glows when 12 volts appears at the starter-motor side of the solenoid.( only when start button is pushed normally)

 

I thought this was my own idea, but it was a suggestion in the thread ( thanks Blueadventures), and for the Jabiru, the noise of the running engine could well hide the noise of a stuck starter motor.

 

With the warning light, at least I will know if the welding thing ever happens before taking off.

 

If there is no diode, is the contact welding more likely to occur?

If you have an ammeter, it will show a massive discharge even if you can’t hear that it is stuck.

 

 

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The diode is there to protect the start switch from welding rather than the heavy current contacts in the solenoid

How would the start switch weld itself up? Doesn't that only happen when high amperage is applied? I was under the impression the start switch wiring is very lightly loaded, merely being the initiator of the high amperage solenoid circuit.

Is there some way the start switch wiring can be overloaded with a high amperage feed, or short circuit? Thanks for the very good information from your previous posts.

 

 

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How would the start switch weld itself up? Doesn't that only happen when high amperage is applied? I was under the impression the start switch wiring is very lightly loaded, merely being the initiator of the high amperage solenoid circuit.Is there some way the start switch wiring can be overloaded with a high amperage feed, or short circuit? Thanks for the very good information from your previous posts.

It's in my earlier post with the diagram, but basically, it works like this:

When a coil is energised it builds up a magnetic field around it. That's what pulls the solenoid contacts together. Now, when the current it stopped, the magnetic field collapses and the collapsing field generates a reverse voltage in the coil. If the coil is open circuit (as it is when the start switch is released) the collapsing magnetic field can build up quite a large voltage, large enough to generate a spark between the relatively close contact points of the start switch. This spark will carry a small amount of metal from one contact to the other. If this is repeated enough times, enough material can be built up to cause a short across the contacts. This used to happen quite regularly in cars that had contact points in the distributors and it's why those points had to be regularly changed.

 

 

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  • 4 years later...
On 21/09/2017 at 10:49 AM, derekliston said:

Jaycar do not have a Diode 1N5048 but they did have 1N5408 which, on closer inspection of your photograph, appears to be the correct number so I will give that a try.

 

 

Agree Diode 1N5408 (only 40c each); also I had this hook-up info in my build notes and thought I'd share on this post.  Cheers.

Start solenoid diode hookup.jpg

Edited by Blueadventures
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1N5408 can be a bit slow for this job

I recommend something laster like a 1N4001 (1N4004)  or  much much better an 1N5819.

 

https://www.jaycar.com.au/1n5819-schottky-diode-40v-1a-do41/p/ZR1020

 

1N5408s are very slow and may not offer the protection anticipated.  but they are 100x better than nothing 

 

Glen

electronics engineer

 

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19 minutes ago, RFguy said:

1N5408 can be a bit slow for this job

I recommend something laster like a 1N4001 (1N4004)  or  much much better an 1N5819.

 

https://www.jaycar.com.au/1n5819-schottky-diode-40v-1a-do41/p/ZR1020

 

1N5408s are very slow and may not offer the protection anticipated.  but they are 100x better than nothing 

 

Glen

electronics engineer

 

Thanks Glen  I'll get some and make one up.  (I have made a 1N5408, but not fitted yet.)  

 

I posted the image I had as didn't look like T88  fitted the jumper wire connection.

 

Thanks for your info, much appreciated  Mike

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Mike where does the original info come from to use a 1N54xx series? might just be useful to others to suggest a change of advice.
I'll post  a couple of pictures of the electrical difference between the 5408 and the 5819 when I get a moment 

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On 11/08/2022 at 5:51 AM, RFguy said:

Mike where does the original info come from to use a 1N54xx series? might just be useful to others to suggest a change of advice.
I'll post  a couple of pictures of the electrical difference between the 5408 and the 5819 when I get a moment 

I did out my notes that are not on this lap top as on an external harddrive.  Would be 10 or so years old.  Keen to see the difference.  Cheers.

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  • 4 months later...

just out of interest, it seems that no one actually said or explained what was going on and causing  the starter run on. it looks to me after looking at solenoid connections that it was in a latching mode.   automotive solenoids often have an extra primary terminal as well as the button terminal , the first terminal goes hot + when the solenoid is activated, this provided to bypass the coil ballast resistor only when starting the engine in certain vehicles, since the diode was connected across these two terminals and if diode was connected wrong way around or shorted it would cause a latching event, which would require the battery to be disconnected to un latch the solenoid.

Anyway glad to see it got sorted out

 

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