Back EMF diode

Discussion in 'The Projects Forum' started by DonRecardo, Jul 14, 2011.

  1. DonRecardo

    Thread Starter New Member

    Dec 10, 2010
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    I am building an 8 channel relay board to be powered from the outputs of an MCU using NPN transistors as drivers. The relays have 5v coils.
    I know I need to put a reversed diode across each relay coil to stop back EMF . I am trying to keep the pcb small so am using smd where I can ,and I have lots of 1n4148s in smd package, would they be suitable ?

    Don
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    How much current do the individual relay coils require, or what is the coil resistance?

    If they are under 100mA, you can probably use the SMD 1N4148's.
    You could also use a small resistor in series with the 1N4148's to take most of the power dissipation instead of the diode.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1,605
    I just got some 1n4xxxx(? 1000V 1A) diodes in a SOD-123 package off Digikey. Small as my 1N4148's.

    4148's are probably OK but make me sleep uneasily. I sleep sounded with a power diode there.
     
  4. DonRecardo

    Thread Starter New Member

    Dec 10, 2010
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    0

    The coils are 70 ohms so about 71mA.
    I think I shall give the 4148 a try

    Thanks for your help

    Don
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    If you used a 62 Ohm resistor in series with the diode, it would cause the coils' magnetic field to collapse very rapidly along with eliminating the HV spike and minimizing the power dissipation in the diode. The resistor would have a peak power dissipation of ~300mW.
     
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