Reverse polarity protection

Discussion in 'The Projects Forum' started by Jassper, Oct 14, 2008.

  1. Jassper

    Thread Starter Active Member

    Sep 24, 2008
    Whats your suggestion for reverse polarity protection?

    * Relay that only closes when connected properly to provide power to the rest of the circuit?

    * Diode across Hot & Ground threw a fuse that blows when connected in reverse?

    * Zener?

    Other suggestions.
  2. SgtWookie


    Jul 17, 2007
    What's the voltage of the circuit in question?

    If it's 9v-18v, you could use an N-ch or P-ch power MOSFET with the source to one input, and the gate to the other input. Reverse polarity would cause the MOSFET to be turned off. Correct polarity would result in the MOSFET being ON, with a very low resistance connection.

    If it's logic level voltage, you could use a logic-level N-ch MOSFET. They don't make logic level P-ch MOSFETS as far as I know.

    You could use a relay, but that would mean a constant power drain via the coil.
    You could use a diode, but even Shottky diodes have a voltage drop across them, therefore they dissipate power.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    Use a polarized connector so the applied voltage can't be in the wrong polarity.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    I'm fond of the bridge rectifier.
  5. Jassper

    Thread Starter Active Member

    Sep 24, 2008
    The applied voltage is 12 volts amp draw will be about 1 amp, Do you have a simple diagram of the MOSFET?

    In this case, a tiny draw from the relay wouldn't be an issue, but I would rather not use the relay method due to space constraints.

    Ok, now how to eliminate the person making the connection from connecting the wrong wire to the right side of the polarized connector?

    Thought about that, but never tried it or seen it done.

    Thanks everyone for the replies.
  6. davefromnj

    New Member

    Oct 14, 2008
    You can put a PFET in the plus side and an NFET in the minus side in a criss-cross pattern.

    If you are using VERY LARGE CURRENTS, then consider using a diode crowbar, putting a fuse inseries with the + connection, and putting a high current diode after the fuse with its cathode connecting to the rest of your circuit and the fuse, and the anode connected to the - connection.

    I would use the diode crow bar with any lead-acid battery high powered equipment because using reverse protection MOSFET circuits or Diodes would wind up getting very hot with a 10Amp+ current draw.

    0.7Vdrop * 10Amps = 7Watts for a single diode inseries with the + connection.

    10Amps^2 * 0.05ohms Rdson = 5Watts for a MOSFET used in a blocking circuit.
  7. Metalfan1185

    Active Member

    Sep 12, 2008
    Bridge Rectifier X2

    as long as the volt or so difference isn't important. It's nice because no matter what way you put the battery in, it still works!