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Inductive Voltage Spike in Relay

Discussion in 'General Electronics Discussion' started by andrew.grimm3, Oct 26, 2012.

  1. andrew.grimm3

    andrew.grimm3

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    Dec 7, 2011
    I have this schematic that I'm using to control a 120vAC fan using this relay. I understand the concept of inductive voltage spike that can damage solid state components when controlling the relay with DC. Is there a certain type of diode I should use, or can I just get by with your basic Silicon or Germanium diode in my application?

    The schematic is attached...

    Thanks,
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Just a basic silicon diode. A 1N4001 is almost always sufficient.
     
  3. MrEE

    MrEE

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    Apr 13, 2012
    why do you have 2 transistors? Can you invert the TTL logic? that will save you a transistor.
     
  4. Zork

    Zork

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    Oct 26, 2012
    The best choose is to not use a diode, it reduces the lifetime of the relay, if you need a faster switching time, this isn't the right circuit either.
    relays.te.com/appnotes/app_pdfs/13c3264.pdf

    If the cost has to be low and the coil current isn't so high, the best you can do is use a faster diode, like an 1N4148, the problem of using a slower diode is that on the transition on to off the spike voltage, as you said, will be very high on this time and the diode will not do the switching fast (trr).

    The problem is that on the transistor, there are capacitances like Ccb, that can charge on this 'spike' moment and make an base current to flow, this can possibly cause damage on the load and on the relay contacts.
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    We've been using relays with 1N4148 diodes by the millions for years without problem.
    Where switching times are an issue we've used the zener diode circuit as shown in the article linked by Zork.
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Which is probably why the ULN200X series drivers incorporate a reversed biased diode across the Ce junction as well as a common terminal that places another reverse biased diode across the coil load.

    Chris
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    This would be a close contest against the 1N4001. I don't think I'd hesitate to use either. ;)

    Chris
     
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