Reverse polarity protection

Discussion in 'General Electronics Chat' started by qitara, Feb 15, 2014.

  1. qitara

    Thread Starter Member

    Jan 18, 2013
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    Hi

    What is the best method to protect a device like an inverter from reverse polarity connection ?.

    I am thinking of an diode in series.
     
  2. #12

    Expert

    Nov 30, 2010
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    It depends on how many killowatts the inverter inverts. Sometimes it is just too wasteful to use a diode. A better method might be to use a polarized connector so nobody can plug it in backwards.
     
  3. qitara

    Thread Starter Member

    Jan 18, 2013
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    the inverter was just and example, what i meant was in general.

    Is the diode method not good because it will act like a resistor and lower the voltage ?
     
  4. #12

    Expert

    Nov 30, 2010
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    Sometimes, yes. Sometimes, no. If you're trying to feed a 5 volt regulator for a digital chip with a 9 volt battery, the diode will not make any difference. If you are trying to feed 30 amps DC into the load, the diode will be large and wasteful.
     
  5. qitara

    Thread Starter Member

    Jan 18, 2013
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    So what solution is there in such cases
     
  6. takao21203

    AAC Fanatic!

    Apr 28, 2012
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    Crowbar + fusing
     
  7. #12

    Expert

    Nov 30, 2010
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    Even simpler, a fuse first and a diode that is arranged to short circuit any backward polarity that suddenly arrives at the input terminals. This method eliminates the need to calibrate a crowbar circuit to the expected input voltage.
     
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  8. inwo

    Well-Known Member

    Nov 7, 2013
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    That's the way most commercial products do it.
    And the best way IMO

    Consumers will figure out how to defeat it though.:D
    So use a big enough diode to blow the input pcb trace.

    They will keep putting in bigger fuses until they run out, and then use tin foil.:eek:
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    You can put a bridge rectifier on the DC power input of a device so that it doesn't matter which way the power source is connected.

    You will lose about 1.3V from two diode voltage drop.
     
  10. #12

    Expert

    Nov 30, 2010
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    Full Wave Bridge advantage: Either way you connect the supply, only the right polarity will get to the load and the load will work.
    Disadvantage: TWO diodes dissipating energy during all times of proper operation.

    Single diode in series: proper connection works, improper connection simply does not supply any current, one diode always dissipates energy during proper operation.

    Fuse with shorting diode: proper connection works, improper connection does not supply any current after some small time period, zero diodes dissipating energy during proper operation, improper connection results in a need to replace the fuse.

    There are other ways, like a relay, a PTC thermistor, and mosfet switches. Would you like to discuss those methods?
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Another common approach for higher wattages is a mosfet instead of the diode, this doesn´t waste much power and is usually not too big either.
     
  12. qitara

    Thread Starter Member

    Jan 18, 2013
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    would be interesting :D
     
  13. kubeek

    AAC Fanatic!

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  14. crutschow

    Expert

    Mar 14, 2008
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    To use a MOSFET, it must be a type to fully turn on with the nominal supply voltage. Thus for anything less than a 10V supply, it must be a logic-level type MOSFET.

    Also a Schottky diode will reduce the forward voltage drop as compared to a standard junction diode.
     
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