[SOLVED] using 2x 1n4004 diodes for back emf suppression with 120volt mains

Discussion in 'General Electronics Chat' started by kingArgon, Dec 24, 2014.

  1. kingArgon

    Thread Starter Member

    Mar 20, 2011
    33
    1
    I want to control an US outlet with a microcontroller. I have a relay that can handle 10A @ 125VAC. I need a back emf protection diode by i only have 1n4004 diodes. Can i connect multiple 4004 diodes in series to create something equivalent to a 1n4007?
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    1,679
    How do you plan to use diodes on a AC circuit for back EMF protection? :confused:
     
  3. kingArgon

    Thread Starter Member

    Mar 20, 2011
    33
    1
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    The back EMF diodes are to protect the device driving the coil of the relay, not to protect the contacts. To protect AC contacts, it is not unusual to use a capacitor/resistor series unit. Networks such as that are called snubbers.
     
  5. kingArgon

    Thread Starter Member

    Mar 20, 2011
    33
    1
    Thats what i want to do; is protect the microcontroller thats enabling the relay.
     
  6. Brevor

    Active Member

    Apr 9, 2011
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    1N4004 diodes are fine to use across the coil, even 1N4001 diodes are OK.
     
  7. jjw

    Member

    Dec 24, 2013
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    The voltage and and the current needed to drive the coil are important, not what happens in the contacts.
    If the coil voltage is below 200V, 1N4004 is OK.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    The bare minimum back emf diode clamps the voltage spike - that means that current flows but decays in the coil, it causes slow contact release with excessive arcing on the contacts.

    Tyco released an appnote covering this - last time I looked it was still on the Element14 website.
     
  9. jjw

    Member

    Dec 24, 2013
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    Ok.
    I just answered to OP:s question if he needs two diodes in series ( maybe to increase max. value for the reverse voltage )
     
  10. kingArgon

    Thread Starter Member

    Mar 20, 2011
    33
    1
    correct! i want to increase the reverse voltage. I can use just one 1n4004 with MAINS 120v and this will be adequate to protect against reverse voltage?
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    The diode is forward biased during the emf spike and clamps it close to 0.7V - its reverse biased when you energise the relay, and only needs a reverse voltage equal to the relay coil operating voltage.
     
  12. kingArgon

    Thread Starter Member

    Mar 20, 2011
    33
    1
    That answers my question perfectly! Thankyou!
     
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