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  #11  
Old 02-11-2019, 05:52 PM
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Originally Posted by pilot2512 View Post
easy enough to test the diode. With a voltmeter on diode check, you should get approx .7 volts fwd and open circuit backwards. if you put your - lead on the side with the stripe and + lead on the other, that would be considered fwd.


Jay
It died. Broke in half.
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  #12  
Old 02-11-2019, 06:07 PM
RV7ForMe RV7ForMe is offline
 
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Originally Posted by lr172 View Post
No, I meant the big wire. You could fuse that wire with a 5 amp fuse. The contactor only draws about 1 amp. However, it doesn't produce much of a risk, as the windings in the contactor act like a fuse. It was the reverse polarity diode that gave a direct feed from the battery to your switch, bypassing the contactor. That risk doesn't exist with the diode installed properly.
Larry
Ok thanks

Quote:
Originally Posted by pilot2512 View Post
easy enough to test the diode. With a voltmeter on diode check, you should get approx .7 volts fwd and open circuit backwards. if you put your - lead on the side with the stripe and + lead on the other, that would be considered fwd.
Jay
Ok. Will try that tomorrow.

Quote:
Originally Posted by JonJay View Post
You might want to consider replacing your Master Contactor and switch once you figure out why the diode died and how the smoke got out of the master switch wire. The coil and master switch could be compromised.
Also, it is possible, but extremely unlikely, that the Master Contacor coil has some kind of a short in it. It is the current limiter in the circuit.

Disconnect the the wires to the Maste Contactor and measure the resistance of the coil. It should be low, but not zero.
Ok. Will try that tomorrow. And will also replace but I want to understand the issue here first

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Originally Posted by JonJay View Post
It died. Broke in half.
I don't think I understand?
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  #13  
Old 02-11-2019, 06:28 PM
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[quote=RV7ForMe;1323967
I don't think I understand?[/QUOTE]

A typical “good” diode will have a voltage drop of .7 volts when voltage is applied to it. Yours isn’t good anymore so you can’t test it.

Also, don’t change out your Contactor until your sure the path of the short was through it. The coil is pretty robust, but it is a relatively cheap part.
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  #14  
Old 02-11-2019, 06:34 PM
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n82rb n82rb is offline
 
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Originally Posted by RV7ForMe View Post
Hey,

working on my buddies plane...

Today we made a mistake. And actually set a cable on fire. Well or at least some smoke came out of it. No avioinics were connected so nothing broke but the cable and my PRIDE.

The Battery is connected to the Battery contactor then goes from there to the fuseblock etc. There is a thin wire that goes from the the master switch to the Battery contactor. So far so good.

Based on "aircraft wiring guide" there is a diode accross the coil of the contactor.

"I recommend installing a diode across the coil on each contactor used in the aircraft (master, start- er, landing gear, air conditioning, etc). You can use a common 1N5400 diode available from any electronics supply house."






WHAT HAPPENED:
We accidentally had one wire create a short and when we switched the master on the wire from the master to ground lit up and made some smoke. Obviously that wasn't as intended.

WHAT I KNOW:
The Diode broke!
Everything still works without the diode.

THE QUESTION:
There is no fuse or other security on the cable from the master to the battery contactor and a broken Diode will make smoke in the cabin! While I understand we created a very unlikely scenario but how would I prevent this. I could put a fuse between the master and the contactor but this will create yet another single point of failure that will put out all electrical systems.

Thank you for any help
it did not break it burnt. if it is installed backwards it creates an almost resistance free path to ground which will conduct a very large current and smoke it. the stripe on the diode must go toward the battery end. from the picture it looks like the stripe on the heat shrink was towards the battery so it is possible the line was put on the wrong end when the heat shrink was applied.

here is a great video to explain what the diode is all about.

https://www.youtube.com/watch?v=bvDuyM2e4gw

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  #15  
Old 02-11-2019, 06:44 PM
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Originally Posted by n82rb View Post
it did not break it burnt.
I assumed by his description that the diode cracked in half, which is a typical failure of a diode when it carries a much higher than rated current.
Regardless, “broken” or “burned up” , I assume it is “toast”
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  #16  
Old 02-11-2019, 06:47 PM
RV7ForMe RV7ForMe is offline
 
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Quote:
Originally Posted by JonJay View Post
A typical “good” diode will have a voltage drop of .7 volts when voltage is applied to it. Yours isn’t good anymore so you can’t test it.

Also, don’t change out your Contactor until your sure the path of the short was through it. The coil is pretty robust, but it is a relatively cheap part.
OK Thank you for explaining. Will have to investigate.

Quote:
Originally Posted by n82rb View Post
it did not break it burnt. if it is installed backwards it creates an almost resistance free path to ground which will conduct a very large current and smoke it. the stripe on the diode must go toward the battery end. from the picture it looks like the stripe on the heat shrink was towards the battery so it is possible the line was put on the wrong end when the heat shrink was applied.

here is a great video to explain what the diode is all about.

https://www.youtube.com/watch?v=bvDuyM2e4gw

bob burns
RV-4 n82rb
Really great video explaining it. Thank you.

So IF this is what happened: is it physically impossible for the diode to burn/break under normal circumstances? In other words. Asking the same again. If I had a little breaker in the switch wire to the contactor wouldn't that prevent the wire from getting hot in the first place if this was to magically happen again? Or is this just not something that is smart at all?
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  #17  
Old 02-11-2019, 06:49 PM
RV7ForMe RV7ForMe is offline
 
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Originally Posted by JonJay View Post
I assumed by his description that the diode cracked in half, which is a typical failure of a diode when it carries a much higher than rated current.
Regardless, “broken” or “burned up” , I assume it is “toast”
I haven't opened the heat shrink It is still in the state of the picture. I simply disconnected it. Since it creates a short if connected I assume it now just lets all the current through. What I am getting at, whatever the reason: broken diode=short?

So assume putting a diode on is common practice. When they fail how do they fail? what are the symtoms? I am sure hot wires are not one of the desired outcomes?
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Last edited by RV7ForMe : 02-11-2019 at 06:51 PM.
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  #18  
Old 02-11-2019, 07:03 PM
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Quote:
Originally Posted by RV7ForMe View Post
This plane had an accident so we are just putting it back to together. It had been working before.
Start by looking at things you removed or worked on.

If the battery was removed, make sure it is installed correctly. Reverse hookup will smoke a diode.

Any item that was removed or worked on is suspect.

How bad was the damage, any chance of cutting the insulation of a wire somewhere and causing a short to ground?
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  #19  
Old 02-11-2019, 07:09 PM
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Quote:
Originally Posted by RV7ForMe View Post
I haven't opened the heat shrink It is still in the state of the picture. I simply disconnected it. Since it creates a short if connected I assume it now just lets all the current through. What I am getting at, whatever the reason: broken diode=short?

So assume putting a diode on is common practice. When they fail how do they fail? what are the symtoms? I am sure hot wires are not one of the desired outcomes?
Not sure it is common practice for the Master Contactor. It is for the Starter Contactor, at least per Vans instructions.

They typically fail shorted or open. There is just one junction. About as simple of a solid state device as it gets.
However, I recently had a Rat decide to warm itself in my heat pump. Not much left of him when he got crossed up on the 230v Capacitor. If you have ever wondered, a Rats tail is conductive, which was laying on the control circuit, which fried all of the heat pump and furnace controls. We found some diodes that did fail oddly after they where hit with who knows how much voltage and current as the Rat tail arcing discharged the Cap.
There wasnt much left of the rat. Just a piece of tail and a bit of fur. The rest of him vaporized.
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  #20  
Old 02-11-2019, 07:13 PM
RV7ForMe RV7ForMe is offline
 
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Quote:
Originally Posted by Mike S View Post
Start by looking at things you removed or worked on.

If the battery was removed, make sure it is installed correctly. Reverse hookup will smoke a diode.

Any item that was removed or worked on is suspect.

How bad was the damage, any chance of cutting the insulation of a wire somewhere and causing a short to ground?
Damage was a nose over so new firewall, engine mount, landing gear, canopy... everything is already new except of the canopy that still needs work.

As I said before this was very likely short by wrongfully connecting things. But yes. there were a few wires were insulation was scraped/damaged. I thought I had found and replaced all of them but will look again.

As I said nothing was really connected except for the switches at the point of this wire incidence. Just trying to learn here.

Thank all of you for sharing your knowledge!
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