Zener diodes to protect switches

Discussion in 'General Electronics Chat' started by DaveMusselman, May 9, 2006.

  1. DaveMusselman

    Thread Starter New Member

    May 9, 2006
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    I am wiring a landing lear operating circuit for a 3/4 scale P-51 aircraft. The landing gear is operated by an electrically driven hydraulic pump. The pump has solenoid which powers a valve in parallel with it. The circuit is protected with a 20 amp c.b. I seem to remember using a zener diode to protect the micro switches from fly-back of the inductive loads when power is removed.

    Do I merely place the banded end of the diode towards the powered circuit and the unbanded end at ground, or do I have the orientation backwards. Also do I need to put a resistor in series with the diode?
     
  2. david mendes

    New Member

    Mar 23, 2006
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    It doesn't need to be a zener diode. A rectifier 1n4007 or similar is better.
    Yes, place the banded end of the diode towards the powered (+)circuit and don't add any resistor.
    This way, the diode will be inversely biased until the moment when the power is removed.
    It works ok.
    David Mendes
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    I've not seen a zener used in this application. I have seen ordinary diodes, schottky diodes and fast recovery rectifiers used. One end of the solenoid coils is connected to power or to ground, and the other side is connected to the switch. The orientation of the diode depends on how the coil is connected. What you are trying to do is clamp the end of the coil which is connected to the switch to the positvie supply for a low side switch or to ground for a high side switch.

    Does this answer your question?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    At the company where I am employed we have actually found that the responsiveness of a solenoid can be improved by the use of a zener and a diode in series placed in parallel with the coil.

    If you look at the section on "guidelines for selecting contact protection circuits" (page 6 of the document) in the link below, you will see that among the various protection schemes mentioned is one that uses a zener diode in series with a conventional diode. It notes the advantages of such a configuration.

    Solenoid Flyback protection methods.

    hgmjr
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    Do the zener diodes that you are refering to handle large currents?
     
  6. DaveMusselman

    Thread Starter New Member

    May 9, 2006
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    Thank you. Dave M
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    The zeners were sized to cope with the current that the coil draws during the energized phase. It is that current that gets dumped into the combination diode and zener when the drive to the coil is interrupted during the de-energize phase.

    hgmjr
     
  8. DaveMusselman

    Thread Starter New Member

    May 9, 2006
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    HGMJR, Thanks for the reply. The circuit I am working with has a reversable motor/pump, so power comes at it from two directions. There are also two solid state relays that kill power to the circuit rather than the micro-switches. I have already isolated the sensing micro-switches. The "Gear Up" and "Gear Down" signals from those micro-switches are what activate the two relays. With this in mind, do I even need the protection? Thanks again, Dave .
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    You probably don't need a zener but the use of a convention diode across the relay coil is probably not a bad precaution since contact arcing may occur without it. Arcing across the micro-switch contacts, if it occurs, will damage the contacts over time and may eventually cause the microswitch to fail.

    hgmjr
     
  10. Gorgon

    Senior Member

    Aug 14, 2005
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    Hi,
    If your circuit is reversed you'll need 2 zener diodes in series 'back-to-back' as protection from kickback both ways. The zener voltage must be a bit higher than the highest supply voltage you can get. In addition you'll get 0.6 V from the reversed zener.

    TOK ;)
     
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