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Clearance Delivery Diode Leakage

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MFleming

Joined: 31 Oct 2017
Posts: 17

Posted: Tue Dec 29, 2020 8:56 pm Post subject: Clearance Delivery Diode Leakage

The wiring has begun for my RV-7. I've decided to use fuse blocks for circuit protection.



The architecture will utilize a clearance delivery (CD) buss. The CD buss has a diode installed per Z101B. When applying DC voltage to the CD bus and measuring the voltage at the main buss, there is a 350 to 390 mv reading.



Is that enough to worry about or do I have a bad diode? This is the B&C Essential Bus Diode w/ 8 watt heatsink PN-221-200.



Heres a photo of the fuse blocks while being tested for leakage. You may be able to make out the 351.8 mv leakage.



PS - the wires depicted was replaced with one with heat shrink...yep, I forgot.



http://myplace.frontier.com/~tokosha/avionics/test_diode.jpg

_________________
Michael Fleming

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Wed Dec 30, 2020 6:15 am Post subject: Re: Clearance Delivery Diode Leakage

A voltage drop across a diode is normal.  A drop of o.4 volts is low.  It will be 

2 or even 3 times that much under heavy load.  Power (heat) equals volts 

times current.  Suppose that the load is 10 amps.  10 amps times 1 volt equal 

10 watts.  That is a lot of heat and the reason for the heat sink.  If you want 

to minimize the amount of power lost, a Schottky diode will waste about half 

as much as a regular diode.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Wed Dec 30, 2020 6:19 am Post subject: Re: Clearance Delivery Diode Leakage

Use heat conductive paste between the diode and the heat sink.

Otherwise the diode could overheat and fail.

Assuming that the heat sink will be mounted to an aluminum surface, also

use heat conductive paste between the heat sink and mounting surface.

_________________
Joe Gores

MFleming

Joined: 31 Oct 2017
Posts: 17

Posted: Wed Dec 30, 2020 11:57 am Post subject: Re: Clearance Delivery Diode Leakage

 	  user9253 wrote: 	
		  A voltage drop across a diode is normal.  A drop of o.4 volts is low.  It will be 

2 or even 3 times that much under heavy load.  Power (heat) equals volts 

times current.  Suppose that the load is 10 amps.  10 amps times 1 volt equal 

10 watts.  That is a lot of heat and the reason for the heat sink.  If you want 

to minimize the amount of power lost, a Schottky diode will waste about half 

as much as a regular diode. 	




So the voltage feedback, leakage is the issue. 



The voltage drop when the diode is supplying current to the CD buss from the main buss is expected. 



When the CD buss is being fed from the battery and the diode is supposed to prevent the main buss from seeing current is when I'm seeing the 350+ millivolts. 



I'm not sure if the feedback through the diode to the main buss is considered OK or not.



 	  user9253 wrote: 	
		  Use heat conductive paste between the diode and the heat sink.

Otherwise the diode could overheat and fail.

Assuming that the heat sink will be mounted to an aluminum surface, also

use heat conductive paste between the heat sink and mounting surface. 	




B&C provided the diode and heat sink as a unit so I'm hopping they assembled it with the proper paste. Would dielectric grease be suitable as a heat sink paste?

_________________
Michael Fleming

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Wed Dec 30, 2020 12:34 pm Post subject: Re: Clearance Delivery Diode Leakage

It is normal to see voltage like you are with no load.  Put a load on the main

bus and that voltage will disappear.  A 12 volt test light will make a good load.

Use a test light that does not have an internal battery.  The test light should

operate off from the aircraft battery. Grease might melt. Home Depot and 

Amazon sells part number 98003/202932736

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Dec 30, 2020 8:31 pm Post subject: Clearance Delivery Diode Leakage

At 09:48 AM 12/30/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Dick Tasker <dick(at)thetaskerfamily.com>



 As far as I can see on your picture, you have no loads anywhere and have the positive supply lead only connected to the CD bus.Â Â  Under those conditions any voltages you can measure are pretty much meaningless.Â  A good DVM has a very high input impedance so puts no load on whatever it is measuring.



 On the other hand, if there is any load on the main bus then you may have a defective diode.Â  To check, just put some sort of load on the main bus (resistor, light bulb, relay coil, etc.).Â  With a load there I doubt you will see anything (unless the diode bridge is actually bad - pretty unlikely). 	


    Right on! I set up an experiment on

    the bench with an exemplar bridge

    rectifier.  A multimeter set to read

    leakage CURRENT indicated less than

    0.1 microamps. Changing the meter to

    read VOLTS gave an indication on the

    order of 400mV.



    What you're seeing is normal and expected.



 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Dec 30, 2020 8:38 pm Post subject: Clearance Delivery Diode Leakage

At 01:57 PM 12/30/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "MFleming" <sagriver(at)icloud.com>



 

 user9253 wrote:

 > A voltage drop across a diode is normal.  A drop of o.4 volts is low.  It will be 

 > 2 or even 3 times that much under heavy load.  Power (heat) equals volts 

 > times current.  Suppose that the load is 10 amps.  10 amps times 1 volt equal 

 > 10 watts.  That is a lot of heat and the reason for the heat sink.  If you want 

 > to minimize the amount of power lost, a Schottky diode will waste about half 

 > as much as a regular diode.



 

 So the voltage feedback, leakage is the issue. 



 The voltage drop when the diode is supplying current to the CD buss from the main buss is expected. 



 When the CD buss is being fed from the battery and the diode is supposed to prevent the main buss from seeing current is when I'm seeing the 350+ millivolts. 



 I'm not sure if the feedback through the diode to the main buss is considered OK or not. 	


   Doing the math on your observations:



   Assuming a nominal 10,000,000 ohm input impedance

   of your multimeter in the voltage mode . . .



         E       0.400

   I =  ---  = --------- = 4 NANOAMPS!

          R     10,000,000



   Some multimeters have as much as 20M Ohms input

   impedance which would give you 2 NANOAMPS . . .

   quite normal and insignificant.



   If you put your multimeter into the current

   measuring mode, the impressed current would

   be too small to wiggle the display on the

   most sensitive scale.



 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

MFleming

Joined: 31 Oct 2017
Posts: 17

Posted: Thu Dec 31, 2020 10:12 am Post subject: Re: Clearance Delivery Diode Leakage

[quote="nuckolls.bob(at)aeroelect"]At 09:48 AM 12/30/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Dick Tasker <dick>



 As far as I can see on your picture, you have no loads anywhere and have the positive supply lead only connected to the CD bus.ï¿½ ï¿½  Under those conditions any voltages you can measure are pretty much meaningless.ï¿½  A good DVM has a very high input impedance so puts no load on whatever it is measuring.



 On the other hand, if there is any load on the main bus then you may have a defective diode.ï¿½  To check, just put some sort of load on the main bus (resistor, light bulb, relay coil, etc.).ï¿½  With a load there I doubt you will see anything (unless the diode bridge is actually bad - pretty unlikely). 	




So this is correct, there are no loads during this test. BUT there is not supposed to be a load or current on the main buss in this photo during the test.



The test was supposed to verify that the diode is doing its job of preventing flow from the CD bus back to the main buss when the CD buss is energized and the main buss is deenergized. The test simulate the CD buss being energized while the master contactor is open, ergo - main bus should show no voltage. 



Now if your saying I need a load on the CD buss during this test great...but a load or any current on the main buss would negate the test as far as I can see.



On the other hand it sounds like Bob is saying what I'm seeing is normal...so I think I'll move on to more wiring.

_________________
Michael Fleming

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Thu Dec 31, 2020 4:19 pm Post subject: Re: Clearance Delivery Diode Leakage

Check for electricity on the main bus using a test light.

The light will not illuminate.  That means there is no voltage on the main bus.

Leave the test light connected to the main bus, then measure the voltage with

the digital meter again.  The main bus will now have zero volts.  The diode is OK.

_________________
Joe Gores

MFleming

Joined: 31 Oct 2017
Posts: 17

Posted: Thu Dec 31, 2020 5:10 pm Post subject: Re: Clearance Delivery Diode Leakage

 	  user9253 wrote: 	
		  Check for electricity on the main bus using a test light.

The light will not illuminate.  That means there is no voltage on the main bus.

Leave the test light connected to the main bus, then measure the voltage with

the digital meter again.  The main bus will now have zero volts.  The diode is OK. 	




I see....The test light (load) on the main buss drains the residual voltage and kinda make the diode seal better like a check valve with differential pressure would.

_________________
Michael Fleming

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jan 03, 2021 12:18 pm Post subject: Clearance Delivery Diode Leakage

At 07:10 PM 12/31/2020, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "MFleming" <sagriver(at)icloud.com>



 

 user9253 wrote:

 > Check for electricity on the main bus using a test light.

 > The light will not illuminate.  That means there is no voltage on the main bus.

 > Leave the test light connected to the main bus, then measure the voltage with

 > the digital meter again.  The main bus will now have zero volts.  The diode is OK.



 

 I see....The test light (load) on the main buss drains the residual

 voltage and kinda make the diode seal better like a check valve

 with differential pressure would. 	


    No, leakage through the reverse biased diode is

    relatively constant irrespective of the load. Consider

    the attached drawing. The red boxes represent your

    ship's battery and the diode under study. I've

    depicted a reverse leakage resistor RL2 around the

    diode.



    To read 0.4v across a 10M resistor in series (your

    voltmeter) describes a ratio:



             RL2 + 10M       10M 

            ---------- = ----------

                12V         0.4V



     Solving for RL2

     

         

             RL2 + 10M  = (10M x 12V)/0.4V



             RL2 = [(10M x 12V)/0.4V] - 10M



             RL2 = 290,000,000 ohms which

                   represents a parasitic (leakage)

                   load on the battery of 

                   12/290M ~ 41 NANOAmps



    For this leakage to take say, an 18AH battery

    down to 1/2 capacity would require . . .



    9,000,000,000 nAHr/41nA = 220 Million hours

    or roughly 25,000 years.



    The value for leakage current is generally

    always cited on data sheets for diodes but it's

    so tiny that only in very specialized, small

    signal applications is it significant to the

    design. We CAN measure it, as you have done,

    but in the airframe, its more of a curiosity

    than a concern.



    Also, reverse leakage varies widely with temperature,

    applied voltage and batch-to-batch variations.



    Going back to the drawing I show another 'leakage'

    resistor (RL1) that is internal to the battery.

    All batteries have one of these and depending

    on chemistry and temperature, it too will discharge

    the battery. Most secondary SLVA and Lithium products are

    good on the shelf for 1-2 years.



    I suggest that leakage inside the battery is

    of far greater concern than diode leakage(s) 

    outside the battery.



 

 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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