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Reverse Polarity Protection (some questions)

Discussion in 'General Electronics Discussion' started by CMA, May 19, 2017.

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  1. CMA

    CMA

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    May 12, 2017
    http://www.reuk.co.uk/wordpress/electric-circuit/reverse-polarity-protection/

    I already read this good articles... and still can't find better one...

    DYI
    http://www.instructables.com/id/Self-Correcting-Polarity-Protection/

    [​IMG]

    1) problem is waste.... and of course oldest method.... right?

    http://www.instructables.com/id/Reverse-polarity-protection-for-your-circuit-with/

    [​IMG]

    2) more advance and less waste but need heatsink?

    3) Is this the image from above very similar to this image below that I found on this forum?

    [​IMG]



    Anyway I thought SDPT relay would be fitting into my application. like this

    [​IMG]

    4) Is this really prefect circuit... no need to improve?
     
  2. duke37

    duke37

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    Jan 9, 2011
    Relays are very thirsty, I would not use them unless you have a powerful supply.
    The MOSFET circuit is very good with very little voltage drop and so very little power loss and so no need for a heat sink.
    A single diode can be used to protect the circiut but will not allow either polarity as a bridge rectifier does. A single diode drops half the voltage of the bridge rectifier.
    If you wish to have a common negative connection, then you can use a P channel MOSFET in the positive line.
    One way of crude protection is to have a fuse in the supply and put a strong diode across the device so that the fuse blows if the polarity is wrong. This limits the wrong polarity to about a volt, the inconvenience means that the operator may be more careful in future.
     
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  3. CMA

    CMA

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    May 12, 2017
    wait you can add diode in the fuse? it sounds better solution. like this

    [​IMG]

    Is that correct path?


    and the battery is automotive 12v and possible max 60 amps as it's for starting system only.... but there is two big fuses as 40 amps and 30 amps AND then reduce to between 10 to 20 amps for different circuits in the car.

    So MOSFET is good enough?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    2 and 3 are equivalent. Neither require a heatsink for substantial loads with the correct mosfet. Large enough forward out reverse polarity will destroy the MOSFET though.

    Your next circuit (the one with the relay) could allow a brief pulse of reverse polarity to be applied to the load. This could be sufficient to destroy semiconductors.

    You don't show the simple series diode or the parallel diode options (although duke37 mentions the former)

    The current capacity of the battery is important for the fuse rating (it must be able to break that current -- and it will likely be higher than 60A) but for other things the actual current drawn by the load is the most important factor.

    Why don't you tell us what the load is?

    Also the likelihood of connecting the power reversed is also a factor in deciding what protection to use. What is the likelihood in this case?
     
  5. CMA

    CMA

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    May 12, 2017
    A) ah okay I think 3) diagram one is more correct

    I am fine if it destroy MOSFET to save important/rare/expensive component part like ECU.

    B) ouch the SPDT realy is still not the completely protection? What about add one more diode before the positive circuit?

    C) If parallel diode on fuse will damage the system so I used series... am I wrong? Maybe I misunderstood about diode will blow up the fuse.


    The load would be everything in the car. important ECU, EFI and Ignition system


    I once damaged my ECU because I jumped the cable in wrong way... It was a long story.... I want to protect the system especially ECU to save my diagnosis time.

    This ECU is what I want to protect... all of circuit.

    [​IMG]

    So that's why I ask which is the more efficient protection.

    Thanks
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Place a diode from rail to rail after the fuse with anode to negative. If you connect the power around the wrong way, the diode will conduct and the fuse will blow. The diode may also fail (short circuit). Such a failure is safe because fuses will continue to blow until the diode is replaced.

    Using a MOSFET, if it fails, it may fail open (safe) or shorted (not safe).

    A series diode is pretty safe in this application because loss of 0.6V is likely to be totally insignificant.

    Even the voltage drop from a bridge rectifier would probably be insignificant but I'd be cautious due to the fact that it would affect the common ground.
     
  7. 73's de Edd

    73's de Edd

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    Last edited by a moderator: May 20, 2017
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  8. CMA

    CMA

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    May 12, 2017
    Exactly I am still confusing about that... I will feel very anxiety if I install all diodes at the each ground points. haha.

    Okay according to de Edd's figure... so I am not able to see how it works... for example.

    Example 1.
    [​IMG]

    As you said diode and fuse will be damaged if reserved polarity.

    BUT what about blue path? will it still damage the load part if it's NOT turning on?

    Example 2.
    [​IMG]

    This is happening if I install diode at wrong place so I have to double where all switch is. BUT what if it happens?

    Will it damage fuse, diode AND that load part if I turn switch on?


    Thank you.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The diode clamps the reverse voltage to about 1V until the heavy current causes the fuse to blow. After that, there is no reverse voltage.

    The failure mode for the diode is a short circuit, so the fuse will continue to blow and no voltage will reach the device until it is replaced.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Oh hang on, I see. Once the connection to the non-ground side of the battery is broken by the fuse blowing, current will stop flowing. Current will not flow unless there is a path to both sides of the battery.
     
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  11. CMA

    CMA

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    May 12, 2017
    BINGO... make a sense now!

    That's very basic circuit that I kept to forget about.

    Simply Reserve polarity; 3 steps
    1. blown diode
    2. blown fuse
    3. Incomplete circuit without hurt the load part.

    GOT IT.

    Thanks!
     
  12. 73's de Edd

    73's de Edd

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    You don't REALLY gots it . . . . IF you put the diode, as per your . . . Schematic 2 . . . where it is on the circuitry PAST the switch.

    On switch turn on, its contacts can spot weld together.

    73's de Edd.
     
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