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  #1  
Old 02-11-2019, 04:24 PM
RV7ForMe RV7ForMe is offline
 
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Default Electrical Question?

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
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  #2  
Old 02-11-2019, 04:50 PM
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GalinHdz GalinHdz is online now
 
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Was it a newly installed diode or had it been there and working for a long time? If newly installed, it could have been installed backwards. Just guessing with the information provided.

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  #3  
Old 02-11-2019, 04:56 PM
lr172 lr172 is online now
 
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Quote:
Originally Posted by RV7ForMe View Post
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
Grounding a live wire is not really the fault of the circuit. Not sure why the diode blew. I guess it really depends which wire was shorted to ground. The power may have cycled back through the contactors ground, via the master switch, and then through the diode with too much current.

A fused connection won't help. The fuse would need to be around 300 amps to support the starter and that is more than enough amperage to repeat your recent experiment with electricity, smoke and all.
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Last edited by lr172 : 02-11-2019 at 05:00 PM.
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  #4  
Old 02-11-2019, 04:57 PM
RV7ForMe RV7ForMe is offline
 
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Quote:
Originally Posted by GalinHdz View Post
Was it a newly installed diode or had it been there and working for a long time? If newly installed, it could have been installed backwards. Just guessing with the information provided.

This plane had an accident so we are just putting it back to together. It had been working before. Does my question make sense of am I completely missing some important information here?
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  #5  
Old 02-11-2019, 05:00 PM
RV7ForMe RV7ForMe is offline
 
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Quote:
Originally Posted by lr172 View Post
Grounding a live wire is not really the fault of the circuit. Not sure why the diode blew. I guess it really depends which wire was shorted to ground. The power may have cycled back through the contactors ground, via the master switch, and then through the diode with too much current.

A fused connection won't help. The fuse would need to be around 300 amps to support the starter and that is more than enough amperage to repeat your recent experiment with electricity.
Ok. Are you saying the little wire on the middle of the picture going to the master switch needs to be able to support 300amps? because that is like maybe AWG18 or 16

Also we did try to recreate this experiment in the spirit of trying to understand it. If I reconnect the diode the wire gets hot instantly! It would probably melt in a few seconds.

I understand that we created this issue but it doesn't really give me the warm and fuzzy knowing that a broken diode will cause smoking cables in the cabin. Because it didn't happen by the battery it happened right at the master switch! So in the spirit of experimental aviation I am trying to understand and how to prevent this
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Last edited by RV7ForMe : 02-11-2019 at 05:08 PM.
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  #6  
Old 02-11-2019, 05:28 PM
lr172 lr172 is online now
 
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Quote:
Originally Posted by RV7ForMe View Post

Also we did try to recreate this experiment in the spirit of trying to understand it. If I reconnect the diode the wire gets hot instantly! It would probably melt in a few seconds.
That means the diode is installed backwards/wrong polarity.

Larry
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  #7  
Old 02-11-2019, 05:29 PM
lr172 lr172 is online now
 
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Quote:
Originally Posted by RV7ForMe View Post
Ok. Are you saying the little wire on the middle of the picture going to the master switch needs to be able to support 300amps? because that is like maybe AWG18 or 16
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
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Last edited by lr172 : 02-11-2019 at 05:31 PM.
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  #8  
Old 02-11-2019, 05:29 PM
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Mike S Mike S is offline
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Quote:
Originally Posted by lr172 View Post
That means the diode is installed backwards/wrong polarity.

Larry
Or, maybe the battery is hooked up backwards??
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  #9  
Old 02-11-2019, 05:43 PM
pilot2512 pilot2512 is offline
 
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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.


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  #10  
Old 02-11-2019, 05:51 PM
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JonJay JonJay is offline
 
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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.
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