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Diode Reverse Voltage Leak or Invalid Measurement Setup?

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valin(at)starflight.aero
Guest

Posted: Sat Jul 04, 2015 11:04 am Post subject: Diode Reverse Voltage Leak or Invalid Measurement Setup?

Hello,

 

We’re getting set to do power on testing of our electrical system and avionics.  We’re using the (Bob’s) Z-12 power grid at 28 VDC with a 12 VDC bus and 12VDC hot battery bus.

 

We have schottkey diodes isolating the various buses.

 

With the positive battery terminal connected and the 12 VDC hot battery lead connected, but NOT the battery ground to the single point ground,  I checked the voltage at various points and saw the 12+ VDC where it should be but was surprised to measure about 3.5 VDC on the Main Bus and even the Ground Bus.

 

The meter set up was with the VOM meter ground lead to the battery ground (not connected to the airplane’s ground bus) and then the positive prove to the various measurement locations.

 

At first I thought we must have a pin out wrong in a connector somewhere – and maybe we do.  After some trouble shooting, I was able to determine that the voltage seems to be leaking in reverse past the diode from the 12 VDC Hot Batt Bus to the 12VDC Bus and then through the 24 to 12 VDC converter to the Main Bus.  But somehow, when I put the meter’s positive probe on the ground bus, it also shows 3.5 VDC.  So not even sure this is a valid way to test that I’m using…

 

Here’s our Power Grid.  Sorry Bob that it is in my pictogram format…  The anode side of Diode B is where I’m seeing 3.5 VDC with 12VDC on the cathode side, as expected, coming from the hot battery bus.  

 

I did a test of a spare diode of the same model on the bench with a single 12 VDC battery and measure 4.9 VDC on the anode side of the diode with the cathode connected to the battery’s positive terminal.  See photo below.

 

My questions.  

 

<![if !supportLists]>1.      <![endif]>Is this even a valid voltage measurement setup?  Is this real?  If both the positive and negative side of the electrical system are showing 3.5 VDC relative to the unattached battery ground, then there is no electrical potential difference/voltage between them…<![if !supportLineBreakNewLine]><![endif]>

<![if !supportLists]>2.      <![endif]>Are these the wrong diodes for this application?  (spec sheet attached)<![if !supportLineBreakNewLine]><![endif]>

<![if !supportLists]>3.      <![endif]>What should I do?<![if !supportLineBreakNewLine]><![endif]>

 

I’d sure appreciate some help with this…  Oh, and Happy Independence Day!

 

Thanks!

 

Valin

(Confused)

 

[img]cid:image004.jpg(at)01D0B659.C30873C0[/img]

 

 

[img]cid:image006.jpg(at)01D0B659.C30873C0[/img]

 

[img]cid:image008.jpg(at)01D0B659.C30873C0[/img]

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jluckey(at)pacbell.net
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Posted: Sat Jul 04, 2015 3:51 pm Post subject: Diode Reverse Voltage Leak or Invalid Measurement Setup?

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