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Better reverse polarity protection?

Discussion in 'General Electronics Chat' started by jst3712, Oct 6, 2008.

  1. jst3712

    jst3712 New Member

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    Hi, I can't find much useful info regarding reverse polarity protection without loss of efficiency.
    I don't like the idea of simply using a diode because of the voltage drops involved, but then I am after a circuit that doesn't use too many components, if possible!

    So basically, I was wondering if anyone can assist me with a diagram or any good links showing me how I can do this.

    Thanks.
    Jason
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    A diode in parallel with the unit, and a fuse in series - if you connect the power the wrong way it blows the fuse (and often the diode as well).

    It's the method generally used in car radios - called a 'crowbar' circuit.
     
  3. jst3712

    jst3712 New Member

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    Thanks Nigel, Sounds simple enough - I shall look into that one.
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    There is also a good MOSFET protection circuit, which uses very few parts.
    Perhaps someone else can find a copy of a schematic for this circuit.
    The MOSFET gets put in series with the load, so that if anything is not
    right the MOSFET turns off and no power to load. It works great and is
    very fast, much faster than a fuse too.
     
  6. on1aag

    on1aag New Member

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    Hi Jst3712,

    A circuit like this could do the trick.
    When you look at the graphs you can also
    see the limitations of the circuit.
    It won't work below 5 volt and the load
    current should not exceed 500 mA at 5 volt.
    You could replace the old dog with a more
    expensive logic level mosfet.
    The same goes for a high load current, even
    when the supply voltage is well above 10 volt.
    Because the Rdson of these old fets is far
    higher than those of modern fets the voltage
    drop across the Rdson will become significant.

    on1aag.
     

    Attached Files:

  7. Pommie

    Pommie Well-Known Member Most Helpful Member

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    When I was a teenager (mid 70s), I made lots of beer money replacing the crowbar diodes in CB radios. I got quite a reputation as I only charged £5 to fix this problem. :D

    Mike.
     
  8. on1aag

    on1aag New Member

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    Hi Mike,

    I charged a lot more but I replaced the diode with a
    16 volt / 10 watts zener diode.
    These cost more than a regular diode.
    If you rip someone off you better do it right! :D

    on1aag.
     
    Last edited: Oct 6, 2008
  9. silvarblade

    silvarblade Member

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    yeh ripping with honesty is the key lolz!!
     
  10. tunedwolf

    tunedwolf Well-Known Member

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    CB...hehe I remember those days well...

    I used to replace the crowbar diode & fuse and fit an 8A bridge to the input. I never had one back again for reversed polarity.
    I done a few taxi radios that way as well, there were always a handful of drivers that were colour blind...or had a brain fart every time they had to move their radio from one car to another. Better to have a reputation of being expensive, but good :)

    rgds
     
  11. Dean Huster

    Dean Huster Well-Known Member

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    Use Schottky (hot carrier) rectifiers in series. A single one will only sacrifice 0.1 to 0.2 v. Better yet, stick a whole bridge rectifier circuit in there made up of 4 Schottkys and it won't matter which way you connect the input DC lines!

    Dean
     
  12. Boncuk

    Boncuk New Member

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    How about that?

    I haven't tried it yet (no N-Channel MosFet at hand), but it should work.

    There are two conditions which must apply to have the transistor conduct. There must be a negative or neutral source voltage and a positive gate voltage.

    The transistor has less voltage drop than a Schottky diode.

    Boncuk
     

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  13. jst3712

    jst3712 New Member

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    Thank you

    Hi everyone, thanks for all your replies/suggestions. Easier for me to do one big reply rather than 8, :D

    I noticed a lot of discussion regarding the crowbar diode.... Does that mean the diodes usually/always die should there be a short, even though there is a fuse? Having said this, should the fuse blow, would it be good practice in checking the diode every time after replacing the fuse, or does the diode die only when it wants to kind of thing?

    Thanks to everyone else with their schematics... I will also take them into consideration, but the less components the better.
    The MOSFET idea sounds good though (perhaps for future projects). I would love a schematic though, .... anyone?! :)

    Cheers
    Jason
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Hans,
    Look thru these pdf's for N MOSFET solutions.:)
     

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  15. eblc1388

    eblc1388 Active Member

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    It won't.

    Under reversed supply polarity, the MOSFET is not biased ON but the internal parasitic drain-source diode would conduct and power/destroy the load.

    However, if we swap the drain-source connection, then it will work.
     

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    Last edited: Oct 7, 2008
  16. Hero999

    Hero999 Banned

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    To minimise the risk of harming the diode, use one with a surge rating that exceeds the short circuit current.

    If you use polyfuse then you don't have to bother replacing the fuse everytime.

    Another solution is to use a Schotky diode or a MOSFET as has already been suggested by others.
     
  17. JimB

    JimB Super Moderator Most Helpful Member

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    In my opinion, the simplest and most efficient method of reverse polarity protection is to use a relay, especially if the load current is large.

    Use the contacts of the relay to switch the supply to the load, and put a diode in series with the relay coil so that it is only energised when the supply is the correct polarity.

    JimB
     
  18. Boncuk

    Boncuk New Member

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    Thanks,

    just forgot about the internal diode. ;)
     
  19. Boncuk

    Boncuk New Member

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    Thanks,

    I forgot about the diode. ;)

    Hans
     
  20. jst3712

    jst3712 New Member

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    Ok, well with this idea, I guess I wouldn't have to replace anything (fuse nor diode), and it's only 2 parts... good one... I'll look at this idea as well.

    Cheers
    Jason
     
  21. JOE--B

    JOE--B New Member

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    reversed Polarity P

    hi! I'm looking for a reverse polarity protection circuit diagram can any body help? it needs to be able to handle at leas 25 amps at 12 volts DC. any suggestions please.
     

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