Two diodes used with 1 relay

Discussion in 'The Projects Forum' started by nitinsharmaece, Jun 27, 2012.

  1. nitinsharmaece

    Thread Starter New Member

    Jun 27, 2012
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    I want to know why sometimes 2 diodes are used with relay, despite 1 is enough for safety purpose.... Also tell me the applications where 2 diodes are used ??:rolleyes:
     
  2. absf

    Senior Member

    Dec 29, 2010
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    I only know that there is only one diode connected across the relay coil to prevent back emf that would destroy the driving transistor. Where is the second diode connected?

    Allen
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Using two would increase the amperage than can be handled without finding a new part with a higher rating. Just speculating
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Two diodes in series can be placed across the coil for increased reliability. If one diode shorts, the other still works, allowing the relay to continue to function.
     
    strantor likes this.
  5. JohnInTX

    Moderator

    Jun 26, 2012
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    absf likes this.
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  7. wayneh

    Expert

    Sep 9, 2010
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    It's one of those things that seem so obvious once you see it. Cool.
     
  8. Smoke_Maker

    Active Member

    Sep 24, 2007
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    SgtWookie likes this.
  9. ArmbrusterJo

    New Member

    Jun 27, 2012
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    Two diodes in series can be placed across the coil for increased reliability
    [​IMG]
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    John from the Great State of Texas nailed it!

    I have the inside track on that: I just designed the coil suppression network for "a major relay manufacturer" (which may or may not be TE) and this describes it's function exactly. Basically it's just a zener and a diode on a .1x.22" board and two fly leads.

    They put our part inside their relay, actually wrap it over the coil.

    One thing to note is the zener voltage ADDS to the applied voltage during turn off, so don't be stingy with the controlling element's voltage rating.
     
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