diode polarity

Discussion in 'General Electronics Chat' started by rfhelp, Mar 31, 2009.

  1. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    I did a diode test on a 1n914. I got 675 with the neg probe on the black band end, and the pos probe on the other end. Reversed them and got no reading.

    Does this mean that the band is the neg end? cuz I had it toward the pos end on my rfid system and now it is stuck on. Don't need the fob anymore to activate it. I am told putting the protection diode in backwards can short the module and fry the output keeping it on.
     
  2. t_n_k

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    Mar 6, 2009
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    In every case, the band on the diode package marks the cathode. I think we are beyond the days when Radio Shack sold unmarked and unrated diodes.

    There are exceptions, of course, but for really big diodes like the 1N1189. Stud mount and the press fit packages usually have some indicator, but not a band.
     
  4. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    You know what guys, I just don' t have the vocabulary. I read this whole thing and still don't know which direction (+) or (-) to point the black band on this 914 diode.
    Forward bias and reverse bias is great but doesn't tell me anything. One guy tells me I must have had it in wrong but the black band was on the (+) side of the power supply, yet, it is now toast. I know its toast cuz It smells like it.
     
  5. loosewire

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    Apr 25, 2008
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    If you are not able to post diagram,it harder to understand.You can lose
    these guys they think fast,some times you lose track.Its best never
    to power something up until you are postive you have it right.
     
  6. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    I guess what I thought to be a simple question was not. I thought all the bands on the resisters would be on the same side as a standard thing. I have been trying much of the day on several forums to get an answer to which side the band is on in relation to the positive and neg leads coming from the power source.

    I admit I am way over my head on this but can be a pretty quick study in most cases. I build custom bikes, rebuild engines, been a professional stuntman/coordinator for over 20 years but wiring has always been a real struggle for me. I thought I would dive head first into this anyway.
     
  7. Feign

    Active Member

    Mar 30, 2009
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    I haven't read All your threads and post, but I think you are in the same boat as me.
    Have a look at figure A at this link
    http://www.hobbyprojects.com/the_diode/diode_protection_circuits.html

    it goes in backwards, backwards being that it conducts the current left in the coil when the switch is turned off.

    Are you still unclear which lead is cathode and which anode?
     
  8. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    I understand the Cathode is banded side and the anode is the other side. I am unclear as to which way the current flows. from cathode to anode or the other way.
    If the cathose end (with the band) is soldered to the positive lead and the anode to the neg lead where does the spike current go when the coil collapses?
     
  9. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    Which is it, a lot of difference,are you working with auto coil. Do you know where the part was when you removed
    the part or is the part still the curcuit.Do you want an easy explaination
    how the auto coil worked.
     
    Last edited: Mar 31, 2009
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If it's a protection diode (for protection against reverse EMF, for example, across the coil of a relay or a motor's terminals) the cathode (end with the band) goes towards the more positive side.

    But if the RFID module smells burnt, then it needs repair or replacement.

    Current flows from negative to positive. During normal operation, when the relay coil is energized, the diode is not conducting. When the relay coil is de-energized, the magnetic field around the coil collapses. This causes the current throught the coil to try to keep flowing, and as a result the polarity of the voltage across the coil swaps ends. When this happens, the diode starts conducting that current, keeping the reverse polarity voltage low.

    If your diode measured OK when you removed it, and it was originally connected properly, there was something else that caused the problem.
     
    Last edited: Mar 31, 2009
  11. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    I am using it as a protection diode. I def smelled some electrical odor when I took the box apart. Not much but enough I noticed it.

    So what does the diode actually do when placed across the neg/pos leads. I would think the spike would just follow the leads? What does the diode to and how does the current flow? ( cathode to anode or the other way. does the spike travel through the positive lead backwords or through the neg lead?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    I added more to my prior post, but I'll re-post it to try to keep continuity going.

    Current flows from negative to positive (electron flow). During normal operation, when the relay coil is energized, the diode is not conducting. When the relay coil is de-energized, the magnetic field around the coil collapses. This causes the current through the coil to try to keep flowing, and as a result the polarity of the voltage across the coil swaps ends. When this happens, the diode starts conducting that current, keeping the reverse polarity voltage low.

    Without the reverse-EMF protection diode, the reverse-EMF pulse can be very high voltage (even hundreds of volts), and will destroy nearby semiconductor components.

    If your diode measured OK when you removed it, and it was originally connected properly, there was something else that caused the problem.
     
  13. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    Thanks Sarg
     
  14. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    with the symbol for diode is the line ubove the point of the triangle representing the band? and the point being the cathode end?
     
  15. loosewire

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    Apr 25, 2008
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    From his first post,sounds like 665 or what ever was a ditital
    meter instead of diode checker or multimeter.
     
    Last edited: Mar 31, 2009
  16. studiot

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    Nov 9, 2007
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    Did your meter come with any instructions and did you read them?

    Be aware that the 'diode test' function on modern meters is not a resistance test.
    The test displays the voltage drop across the diode (in millivolts)
    Thus the drop was 675 millivolts, showing conduction in the forward direction.

    Older analogue meters apply +ve to the black lead (yes black) and negative to the red to get the readout polarity correct.

    but again I say read the manual

    This means that when the red lead is connected to the anode (and the black to the cathode) the diode will show between 300 and 800 millivolts.

    A good diode will read OL when connected the other way round.
     
    Last edited: Mar 31, 2009
  17. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    No, He, being me, used the diode checker on my multimeter
     
  18. rfhelp

    Thread Starter Active Member

    Mar 23, 2009
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    I just checked again. With the pos red lead on the cathode the meter reads 1 or no reading for this one. with the neg black lead on the cathode I got 655
     
  19. studiot

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    I apologise I need some of my own advice.

    On older analogue multimeters the +ve terminal (should be red lead) of the meter is connected to the -ve terminal of the internal battery on resistance and diode ranges.

    On modern digital meters the +ve terminal of the meter is connected to the positive terminal of the internal battery.

    This means you diode is OK if it reads OL or high in the other direction. A shorted diode can read 6xx in both directions.

    I have corrected my original post.
     
  20. studiot

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    The markings on the diode can also be misleading.

    The convention is to mark the cathode with a band around the body of the diode, leaving the anode unmarked.

    However I have some Russian 1N4007 diodes with no band, but a red blob on the anode end.
     
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