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Dino Starter Solenoid Flyback Diode

Discussion in 'Technical Q&A' started by racerboy9, Sep 14, 2021.

  1. racerboy9

    racerboy9 Formula 3
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    Nov 3, 2003
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    I am hoping to add a flyback diode to my original Dino starter. I am a bit weak in the electrical department and was wondering if anyone could give me some direction in hooking up the diode correctly. I have included a picture of the terminal end of the solenoid for clarity.
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  3. raemin

    raemin Formula Junior

    Jan 16, 2007
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    Apply 12v to the solenoïd, see if it goes in the appropriate direction, if not you know the voltage was reversed...

    From then on (when the polarity is properly idendified), you just install a diode with the (-) of the diode toward the (+) of the solénoïd and vice versa ( + to -). This way the flyback current from the solenoid winding is captured before it gets back to your electric system.
     
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  4. wmuno

    wmuno Karting
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    I have never heard of this before for a starter application. However, in electrical enginnering it is very common to add a diode to eliminate the voltage spike when a magnetic field is deenergized. The diode is connected to the tab connector on the solenoid and goes to the chassis ground. See https://en.wikipedia.org/wiki/Flyback_diode for the theory behind this application.
     
  5. Ferrari Tech

    Ferrari Tech Formula Junior

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    I am more interested in the why. I understand the why with alternators but a starter? I am ready to learn so new stuff.
     
  6. racerboy9

    racerboy9 Formula 3
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    This guy gives a good explanation:
     
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  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #6 Steve Magnusson, Sep 15, 2021
    Last edited: Sep 15, 2021
    Here is a picture/diagram to put a figure to raemin's words:
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    But I'd also ask "why" with Wade -- there are literally billions of ignition switches and starter solenoids working OK for a long lifetime without a flyback diode. Are you having a specific problem with your ignition switch life or just looking for some "insurance"?
     
  9. raemin

    raemin Formula Junior

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    In the alternator this is another story, diodes are placed by group of three in order to make a rectifier bridge, i.e a device that convert alternative current to continuous.

    As far as the starter is concerned I never had to do it in any of my car, but some other windings have been troublesome in my car (power mirror solenoid being the most nasty one in my case). Maybe @racerboy9 wants to cure such an issue? The concept is quite easy : the rise and the fall of a magnetic field in a winding generate some current. When you switch off the solenoïd, there is a rapid fall of magnetic field, which produces a "negative" current (current spike) that gets back to the electric system. The starter is so close to the battery (unless the spike returns to the neiman first?) that this should not be an issue/ In any case the spike will only occur once (when starting the car), and there is so much noise at this specific moment that you may not notice the interference in your radio...
     
  10. racerboy9

    racerboy9 Formula 3
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    John Corbani (RIP) over in the Dino forum had issues with his ignition switch failing so he put in a new one and it failed far too soon. He took the switch apart and noticed the contacts were burned so he added a flyback diode right at the ignition switch and that seemed to solve the problem for him. I am double checking the starter/solenoid on my Stratos (same starter) and thought as long as I am piddling with it I might as well put a diode back there rather than dig into the dash harness. I didn't think the diode addition would do any harm and might be beneficial. It is about $1 to add one so cheap enough. Steve, thanks for that schematic added to my picture. That is exactly what I was hoping for.
     
  11. lm2504me

    lm2504me Formula Junior
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    If you check some of the electrical schematics for modern cars, you will see a flyback diode for the starter in the schematic. I use a flyback diode in my 308 and 1750 Spider Roundtail to prevent damage to the expensive Bosch contact block in my ignition switch.
    We use diodes in some of our electrical systems at my nuclear plant to protect our electronic assemblies from the coil discharging when continuity is broken.
    I connected my diodes to the ignition switch terminal and ground.
     
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  13. wbt

    wbt Karting

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    Ignition switch to ground would work too, however not really necessary unless driving the starter directly by MOSFET or some other semiconductor sensitive to voltage spikes. No harm done adding it though.
     
  14. Ferrari Tech

    Ferrari Tech Formula Junior

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    Thanks guys. I love learning new stuff and electrical theory is not my strong point. At least I am glad I understand what is going on and why the diode is added now.
     
  15. raemin

    raemin Formula Junior

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    The alternative approach is to capture the spike with a "varistor" instead of a diode. If memory serves me well, there is a slight voltage drop induced by the diode as well as a higher delay.
     
  16. lm2504me

    lm2504me Formula Junior
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    No voltage drop or current flow across a diode when reverse biased by having 12v positive on the cathode. When ignition switch is released, coil field collapses and changes polarity across the coil. The current goes to ground via diode cathode. No arc across opening contacts of ignition switch.
     
  17. racerboy9

    racerboy9 Formula 3
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  18. raemin

    raemin Formula Junior

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    The forward drop of the diode somehow determines the amount of current that will leak, you can reduce this with 0.2v forward drop diodes (a.k.a Schottky) diodes or a varistor. Also the diode will keep the coil energised, so the solenoid will take longer to return to its resting position. Probably not a big issue on a starter (this is not a fast switching device).

    For the starter a Schottky diode is a good idea: these are easy to source and just as easy to install as a regular diode. For other windings that are more frequently used I would try something more elaborate.
     
  19. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Yes, that wiring is correct, but you might want to use a higher-current capable diode (the diode in those pics is a form factor typically limited to 1A max without blowing up).
     
  20. racerboy9

    racerboy9 Formula 3
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    This is the one I installed. Does this seem OK?
    NTE Electronics 1N5400 Silicon Standard Recovery Rectifier Diode, 3 Amp
     
  21. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    That's better. It's not an easy thing to judge because the current rating is for a continuous current and this is a very quick, transient current thing (so going over the current rating for only a brief instant may be more survivable). One thing you could do is measure the conduction of the diode in the direction that it should conduct occasionally after some use just to make sure it is surviving. Otherwise, it's your homework to do some internet searching for what diode current rating is best for a typical starter solenoid application ;).
     
  22. racerboy9

    racerboy9 Formula 3
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    A lot of my flyback diode for a starter solenoid searches ended up in the aviation sector. I bought that particular diode as it was mentioned in several articles. If it creates a problem down the road I can just snip it out. Thanks for your help on this Steve.
     

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