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Diode confusion

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airlincoln(at)sbcglobal.n
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Posted: Wed Apr 21, 2010 3:42 pm Post subject: Diode confusion

Getting ready to crimp a Radio Shack 1N5400 diode into a fast-on tab for my SD-8 relay control and thought -- "gee, maybe I better check this diode to make sure it's good before I do this" -- what could be simpler?  Current flows from the non-band side of the diode to the band side, but not the other way (just like the schematic), right?  Get out the multi-meter, set it to measure ohms.  I put the red lead on the non-band side and the black lead on the band side and expect to see zero (or very little) resistance.  Uh oh....infinite resistance...not good...do I have the leads reversed on the meter?  Nope.  I reverse the leads (red on band-side and black on non-band-side...expecting to see infinite resistance) and see...uh oh...3 milliohms.  Now I'm really confused.  I pull another diode out of the packet and repeat the  test....same result.  I check the heat-sinked bridge recitifier for the ebus regular feed...same result.



I believe the chance that three different components turned out defective or mis-labeled are pretty remote and the chance that my knowledge of all things electric is (once again) in need of improvement.  Perhaps I just don't understand how the multimeter works.

My question:

For a functioning diode, shouldn't current flow from the red lead of the multimeter when it's placed on the non-band side of the diode, to the black lead of the multimeter when it's on the band-side of the diode and indicate zero (or very little resistance)?

Thanks to whoever can set me straight on this ridiculously simple concept.

Do Not Archive

Lincoln Keill

RV-7A

 [quote][b]

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harley(at)AgelessWings.co
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Posted: Wed Apr 21, 2010 4:05 pm Post subject: Diode confusion

Your diodes are fine...you hooked the meter up backwards! 

 

 See here for an explanation: http://tinyurl.com/26pwfwg 

 

 Harley

  

 On 4/21/2010 7:41 PM, Lincoln Keill wrote: [quote]                        Getting ready to crimp a Radio Shack 1N5400 diode into a fast-on tab for my SD-8 relay control and thought -- "gee, maybe I better check this diode to make sure it's good before I do this" -- what could be simpler?  Current flows from the non-band side of the diode to the band side, but not the other way (just like the schematic), right?  Get out the multi-meter, set it to measure ohms.  I put the red lead on the non-band side and the black lead on the band side and expect to see zero (or very little) resistance.  Uh oh....infinite resistance...not good...do I have the leads reversed on the meter?  Nope.  I reverse the leads (red on band-side and black on non-band-side...expecting to see infinite resistance) and see...uh oh...3 milliohms.  Now I'm really confused.  I pull another diode out of the packet and repeat the test....same result.  I check the heat-sinked bridge recitifier for the ebus regular feed...same result.         

         

         I believe the chance that three different components turned out defective or mis-labeled are pretty remote and the chance that my knowledge of all things electric is (once again) in need of improvement.  Perhaps I just don't understand how the multimeter works.

         

         

         My question:

         

         

         For a functioning diode, shouldn't current flow from the red lead of the multimeter when it's placed on the non-band side of the diode, to the black lead of the multimeter when it's on the band-side of the diode and indicate zero (or very little resistance)?

         

         

         Thanks to whoever can set me straight on this ridiculously simple concept.

         

         

         Do Not Archive

         

         

         Lincoln Keill

         RV-7A

                            	  Quote: 	
		  



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Checked by AVG - www.avg.com 



  	
 [b]

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endspeed(at)yahoo.com
Guest

Posted: Wed Apr 21, 2010 5:33 pm Post subject: Diode confusion

Check the EAA.ORG website under hints for homebuilders electrical.  They have a nice video with an instructor demonstrating exactly what you are asking about.  It will clear things up for you. 

Bob S.



Sent from my iPhone



On Apr 21, 2010, at 19:41, Lincoln Keill <airlincoln(at)sbcglobal.net (airlincoln(at)sbcglobal.net)> wrote:



[quote]Getting ready to crimp a Radio Shack 1N5400 diode into a fast-on tab for my SD-8 relay control and thought -- "gee, maybe I better check this diode to make sure it's good before I do this" -- what could be simpler?  Current flows from the non-band side of the diode to the band side, but not the other way (just like the  schematic), right?  Get out the multi-meter, set it to measure ohms.  I put the red lead on the non-band side and the black lead on the band side and expect to see zero (or very little) resistance.  Uh oh....infinite resistance...not good...do I have the leads reversed on the meter?  Nope.  I reverse the leads (red on band-side and black on non-band-side...expecting to see infinite resistance) and see...uh oh...3 milliohms.  Now I'm really confused.  I pull another diode out of the packet and repeat the  test....same result.  I check the heat-sinked bridge recitifier for the ebus regular feed...same result.



I believe the chance that three different components turned out defective or mis-labeled are pretty remote and the chance that my knowledge of all things electric is (once again) in need of improvement.  Perhaps I just don't understand how the multimeter works.

My question:

For a functioning diode, shouldn't current flow from the red lead of the multimeter when it's placed on the non-band side of the diode, to the black lead of the multimeter when it's on the band-side of the diode and indicate zero (or very little resistance)?

Thanks to whoever can set me straight on this ridiculously simple concept.

Do Not Archive

Lincoln Keill

RV-7A

  	  Quote: 	
		  

	


[b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Apr 22, 2010 6:15 am Post subject: Diode confusion

 	  Quote: 	
		  My question:



For a functioning diode, shouldn't current flow from the red lead of 

the multimeter when it's placed on the non-band side of the diode, 

to the black lead of the multimeter when it's on the band-side of 

the diode and indicate zero (or very little resistance)?



Thanks to whoever can set me straight on this ridiculously simple concept.


	


   Not all multimeters are the same. I have instruments

   wherein the electron-flows are reversed from each other.

   While the exercise you describe is a learning moment,

   the last time I saw a "bad" diode either in terms of

   markings or electrical condition was many moons ago.

   Just know that there is no "standard" for choosing

   polarity of a multimeter's resistance measurement

   current.



   Bob . . .

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airlincoln(at)sbcglobal.n
Guest

Posted: Thu Apr 22, 2010 9:13 am Post subject: Diode confusion

Bob S. --

Thanks for the great tip -- I haven't been to the EAA site for some time and those instructional videos are fantastic.  I watched all of the ones under the "Electrical" tab plus a few more and learned a bunch...wish I'd done that a lot earlier.  These videos are the perfect complement to the AEC -- being able to actually watch someone demonstrate the correct way to crimp a BNC connector onto a piece of RG-58, solder a wire onto a switch (pre-tinning wire...what a concept!), crimp on a molex connector pin and them remove it from its housing, strip shielded cable, etc., etc. makes reading the AEC a more enjoyable and understandable experience. 

I always knew that alternators were lighter than generators, but never knew that an alternator starts producing voltage around 700 RPM  (compared to a generator's cut-in speed of 1500 RPM) which is why we no longer have to discharge our batteries taxiing around at idle power.  Did you know there are no special tools required to fabricate Aeroquip hoses?..I didn't until today.  When you flare a tube, you should use lubricant identical to the fluid the tubing will carry...never thought about that...makes sense.  All in all, a great resource for those of us still on the steep side of the learning curve.

Thanks again Bob.

Linc





Check the EAA.ORG website under hints for homebuilders electrical.  They have anice video with an instructor demonstrating exactly what you are asking about.It will clear things up for you. Bob S.

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