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Polarity Protection

Discussion in 'General Electronics Discussion' started by duke37, Apr 24, 2014.

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  1. duke37

    duke37

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    Jan 9, 2011
    Polarity Protection
    I was thumbing through my notebook and came across this circuit drawn about 25 years ago.
    I have not used it.

    The circuit uses a MOSFET in a reverse connection, it should need no current to drive and give a very low voltage drop. The diode I assume is the diode inherent in the MOSFET.

    Is it likely to work and will there be any disadvantages
     

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    Last edited: Apr 24, 2014
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes I think that will work. This is how I understand it.

    Yes, I think the diode shown is the internal body diode of the MOSFET.

    When voltage is applied the right way round, the diode conducts and enables current to flow in the load. It also keeps the MOSFET's source voltage close to the negative rail, so the positive voltage on the gate will bias the MOSFET ON and its low ON-resistance will carry the current, so the voltage drop caused by the diode will disappear.

    If voltage is applied with the wrong polarity, the body diode won't conduct and the source and gate will both have the same voltage on them (they'll both be equal to the top rail). So the MOSFET won't be forward-biased, and the body diode won't conduct, so no current will flow.

    I would add some gate protection for the MOSFET - a series resistor between the positive rail and the gate, and a 12~15V zener diode with cathode to the gate and anode to the source.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The other thing to remember is that the mosfet doesn't act as a diode. This configuration will allow current to flow both ways. This isn't normally an issue, but you can't use the circuit where you want isolation of 2 power supplies for example.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I don't understand, Steve. The MOSFET won't conduct when the input supply polarity is reversed. What do you mean?
     
  5. kpatz

    kpatz

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    Feb 24, 2014
    I think Steve is referring to a situation where the "load" in the circuit supplies power somehow, such as if it's a 2nd power supply or a battery in a charging circuit or something, the MOSFET won't stop current flow in that direction.

    But for most circuits where the "load" consumes power but doesn't supply it, this circuit works fine. If you need to isolate two sources of power you'll need two FETs or a diode.

    This circuit is intended to protect the load if the supply power is connected backwards, such as installing the batteries the wrong way or connecting a power supply the wrong way, without imposing the voltage drop of a series diode.
     
  6. duke37

    duke37

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    Jan 9, 2011
    I was thinking of using this on an electric fencer. These are used with a variety of batteries at various voltages. The most common battery is a 10V lead acid battery i.e. a tractor battery with a shorted cell. It is easy to clip the leads on the wrong way. My brother puts a red disc over the positive terminal and a black one over the negative but if things go wrong, they will.

    Voltage drop and power loss is critical.
    Next is making an efficient switch mode high voltage (400V) generator, other things are taking precedence at the present.

    The old fencers with a mechanical chopper didn't mind the polarity but needed the right voltage.
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Ah, right. If the load is powered from another source, the MOSFET will conduct and feed that source back into the input power supply. Good point Steve.
     
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