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Endurance Bus Diode & Voltage leak

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sjhdcl(at)kingston.net
Guest

Posted: Fri Mar 31, 2006 7:40 am Post subject: Endurance Bus Diode & Voltage leak

Bob,



Is it normal for a diode to leak voltage back onto the MAIN bus?



With the EBus X Feed Switch on (no MAIN bus) it get 12.8V at the Endurance

bus and about 6.85 V at the MAIN bus terminal. With the MASTER Off.



I understand the forward voltage drop when the MAIN bus is powered and I

measured it to be 0.6V. But I can't find any reference to this backwards

'leak' of voltage by the diode.



The diode is the standard 1" by 1" block (D-25) from B&C.



Another question: Some diagrams show only attaching 2 of the tabs on the

diode. One to Endurance and one to MAIN.



Other diagram shows joining the two terminal on the diode and then going

to the Endurance Bus.  Is this for redundancy since these 2 terminals are

electrically the same?



Steve

RV7A

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mprather(at)spro.net
Guest

Posted: Fri Mar 31, 2006 8:18 am Post subject: Endurance Bus Diode & Voltage leak

Reverse biased diodes _do_ leak (current).  Maybe a few microamps.



The voltage behind a reverse biased diode will depend on what it's

connected to.  Presumably, the bus that your diode is connected to has all

of the switches turned off - very light loading with which to discharge

the leakage current through the diode.  I would guess that if you

connected a 1kohm resistor between the bus and ground and remeasured the

voltage, it would be very low (V = IR = 0.000005A * 1000ohm = 0.005V).



Matt-

 	  Quote: 	
		   



 Bob,



 Is it normal for a diode to leak voltage back onto the MAIN bus?



 With the EBus X Feed Switch on (no MAIN bus) it get 12.8V at the

 Endurance bus and about 6.85 V at the MAIN bus terminal. With the MASTER

 Off.



 I understand the forward voltage drop when the MAIN bus is powered and I

 measured it to be 0.6V. But I can't find any reference to this backwards

 'leak' of voltage by the diode.



 The diode is the standard 1" by 1" block (D-25) from B&C.



 Another question: Some diagrams show only attaching 2 of the tabs on the

 diode. One to Endurance and one to MAIN.



 Other diagram shows joining the two terminal on the diode and then going

 to the Endurance Bus.  Is this for redundancy since these 2 terminals

 are electrically the same?



 Steve

 RV7A

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nuckollsr(at)cox.net
Guest

Posted: Fri Mar 31, 2006 8:41 am Post subject: Endurance Bus Diode & Voltage leak

At 10:40 AM 3/31/2006 -0500, you wrote:



 	  Quote: 	
		  



Bob,



Is it normal for a diode to leak voltage back onto the MAIN bus?



With the EBus X Feed Switch on (no MAIN bus) it get 12.8V at the Endurance

bus and about 6.85 V at the MAIN bus terminal. With the MASTER Off.



I understand the forward voltage drop when the MAIN bus is powered and I

measured it to be 0.6V. But I can't find any reference to this backwards

'leak' of voltage by the diode.



The diode is the standard 1" by 1" block (D-25) from B&C.


	


   Diodes are not 'perfect' . . . they have forward voltage drop

   and reverse leakage. Keep in mind that the input impedance of

   a digital voltmeter is on the order of 10-20 MEGohms. To get

   a reading of 6.85 volts due to leakage, you need only

   6.85/10,000,000 or 685 NANOamps. If you turn on any single load

   on the main bus, the reading will drop like a stone.



   I went to the bench and wired a Fluke multimeter, 12v supply

   and several diodes in series with a voltage measurement crafted

   to show the effects of diode reverse leakage. From a selection

   of parts in the bins, I got reverse leakage voltage readings from

   .3 to 7.5 volts on regulator power silicon junction diodes.



   When I measured a Shottky device, the leakage reading was 12V! I put

   a 10,000 ohm load across the voltmeter and the reading dropped to

   .25 volts. This shows that Shottky device leakage is several orders

   of magnitude greater than for silicon junctions.



   Bottom line is that your readings are understandable and not

   significant in the grand scheme of things.

 	  Quote: 	
		  Another question: Some diagrams show only attaching 2 of the tabs on the

diode. One to Endurance and one to MAIN.



Other diagram shows joining the two terminal on the diode and then going

to the Endurance Bus.  Is this for redundancy since these 2 terminals are

electrically the same?


	




    The four diodes contained in a bridge rectifier are identical.

    If you noodle out their position in the device, you can use any

    of 4 combinations of terminals to effect the desired operation.

    But you can't go wrong with wiring per:



http://www.aeroelectric.com/Pictures/Wiring_Technique/diode_wiring.jpg



http://www.aeroelectric.com/Pictures/Misc/s401-25.jpg

        Bob . . .

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

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jebrick(at)comcast.net
Guest

Posted: Fri Mar 31, 2006 9:54 am Post subject: Endurance Bus Diode & Voltage leak

Another source of voltage to the main bus, with the master (and alternator

field) off, is the alternator when the engine is running. There is enough

residual magnetism or something...at least on my L-40 alternator, to produce

about 3.5 volts at high rpm and about half a volt at low rpm. This is with

minimal load on the bus. Significance in my case was that a shunt relay

powered by the main bus would not unlatch in flight when the Bat/Alt switch

was turned off, but it would on the ground at idle.



jb



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