Low voltage diode needed solinoid project

Discussion in 'The Projects Forum' started by Mike N Julie Nyman, Feb 20, 2016.

  1. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    Looking for some advice for the correct diode. I have a solinoid that is controled via a circuit that should apply 12v for 1.5 to 2 seconds to "energize" and then drop to 2v to "hold" this a factory made unit and there are no diagrams avalabe for it. The 12v energize function has failed but the 2v hold function is working. I wish to put a 12v feed to the solinoid via a momentary push button and use a diode to stop the 12v flowing back to the controller board. Im concerned that the volt drop across the diode will make the hold voltage to low.

    Energize volt - 12v to14v dc
    Energize current - 700ma or about 8w
    Hold volt - 2v
    Hold current - 300ma
    I will post a diagram of my plans.

    Any advice appreciated.
    Mike
     
  2. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    Quick Diagram of solenoid issue

    Solinoid Diag01.jpg
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    The 0.7V of a standard silicon diode is insignificant in the greater scheme of things - a Shottky-barrier diode drops only about 0.2V, but you have to watch out for their low reverse breakdown voltage - they generally start at 20V and start getting expensive over 60V. There is the possibility of back emf from the solenoid, so you may have to select the diode PIV accordingly.

    There are various circuits floating about the web for relay economisers. Usually you start with a supply rail that's adequate to hold the relay in but not pull it in. In the resting state; a transistor and a diode network arrange for a boost capacitor to charge from Vcc, The signal to actuate the relay pulses the transistor to shift the capacitor into series with the coil to make sure the relay pulls in. The capacitor is shifted into parallel with a bypass diode, so when its charge decays the coil is still fed by Vcc to hold it in.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I suppose you have tried to fix the 12V energize feature?
     
  5. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    I could live with 0.2v drop I think.
    I was thinking of a:

    IN5822 Schottky Diode 40V 3A
    1N5822 3A Schottky Diode.jpg
     
  6. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    Had A look but there is no diagrams available and it may be a multi layered PCB so a bit beyond me.
     
  7. AnalogKid

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    Aug 1, 2013
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    Good choice.

    ak
     
  8. ian field

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    Oct 27, 2012
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    Mixing Shottky-barrier diodes and inductive loads can be risky - AFAICR; that has a PIV of only 40V.

    A zener in parallel would protect it as long as Vz is just under PIV. The zener has about the same Vf as a regular diode, so the SB would always forward conduct first.
     
  9. AnalogKid

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    Aug 1, 2013
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    Sorry, missed that the new diode would disconnect any protection already in place.

    Mike, you need two of those diodes, one in series with the controller and one in parallel with the solenoid coil. Even with only 2 V across the coil when it is turned off, it can kick up enough reverse EMF to damage something. The controller probably has a diode across its output because of this, but that diode is disconnected by your diode.

    One diode as shown on your drawing, and a second one with its anode to GND and cathode to your first diode's cathode.

    ak
     
  10. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    Thanks for your time guys and excuse my atrocious spelling in y first post as it was 3am in Australia and didn't have spell check on.
    Is this diagram Solenoid Diag02.jpg what your thinking?
     
  11. AnalogKid

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    If you're in Australia, does that mean that both diodes are upside-down?

    ak
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    They use didgerydiodes.
     
  13. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    Yer Mate, always a problem. Electrons all over the floor and lead acid batteries are a real problem.. Didgerydiodes are great but they are a bit noisy.
    Oh, and we use Crocodile clips instead of Alligator clips
     
    Last edited: Feb 21, 2016
  14. Mike N Julie Nyman

    Thread Starter New Member

    Feb 20, 2016
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    So im not just copying your advice, can you give ma a quick explanation on the operation of the diode you suggested I add (Diagram Above)
    Does it let the EMF drain back to ground through the solenoid coil?
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Yup. The same thing that makes your spark plugs spark makes solenoids and relay coils dangerous for electronic components. When energized by DC which is then *rapidly* disconnected - as in electric current stops and the input goes to a very high/infinite impedance, the magnetic field in the coil collapses, inducing a current in the coil that manifests as a terminal voltage that is the opposite polarity of the energizing voltage. And large. It is easy for a 5 V relay coil to kick up hundreds of volts for a microsecond. Because of the polarity change it is easy to suppress with a diode. The diode acts as an automatically switched-in short circuit.

    All of that is because of this: - <-- a minus sign

    One of Maxwell's equations (actually Heavyside's reduction of Maxwell's equations) is a corrected version of Faraday's Law of Induction, and Faraday's Law has a minus sign in it. That minus sign is the reason a generator is harder to turn the greater the electrical load on it is, the reason it takes huge nuclear power plants or structures like Hoover Dam to get generators to turn.

    ak
     
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