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Starter Solenoid Switch

Discussion in 'General Electronics Discussion' started by abuhafss, Aug 20, 2014.

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  1. abuhafss

    abuhafss

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    Aug 3, 2010
    Here is a Starter Switch, simply a SPST relay used to power-up the starter solenoid.

    Starter Sw.jpg

    I am just curious why can't a transistor/mosfet be used as proposed below:

    Screenshot 2014-08-20 19.57.59.png
    The lower one is just a schematic of the original one.
     
  2. BobK

    BobK

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    Jan 5, 2010
    Your picture is a switch, not a relay. You cannot have a relay with 2 leads.

    The starter solenoid draws a large current and operates infrequently, so a relay is probably more appropriate than an electronic switch, though there is not reason you could not use an electronic switch as long as it could handle the current.

    Bob
     
  3. KMoffett

    KMoffett

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    Jan 21, 2009
    "Your picture is a switch, not a relay. You cannot have a relay with 2 leads." ???
    There are two wire leads to the coil, and two screw terminals for the high current contacts, like any stater relay.

    Ken
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    It's possible and modern cars may even do it; I don't know. A contactor is more appropriate because of the extremely high currents and the extremely harsh electrical environment (switching noise, spikes caused by inductance, etc). The MOSFET would need a lot of protection components.

    The major benefit of the MOSFET would be the low control current, but as BobK says, that's not significant because (a) the starter isn't used much, and (b) the starter current is huge anyway.

    BTW your MOSFET circuit would not work as shown; you would need a voltage generator to bring the gate voltage 10~15V above the +12V rail.
     
  5. BobK

    BobK

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    Jan 5, 2010
    Duh. Mind fart.

    Bob
     
    Arouse1973 likes this.
  6. abuhafss

    abuhafss

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    Aug 3, 2010
    How about this configuration assuming that the load consumes about 20A? Or does it need further improvements. The diode may be replaced with MOV. Screenshot 2014-08-21 18.39.18.png
     
  7. abuhafss

    abuhafss

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    Aug 3, 2010
    What would be the best transient suppression solution, assuming that the solenoid draws about 35A?
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    For that current and the fact it's a solenoid an MOV is your best bet yes. This will release the solenoid quicker than a diode will because of the higher clamping voltage. Time=Energy/Power
    Note: Observe the VGS max voltage if you do this.
    Adam
     
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