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    The most simple polarity protection tehnique is to connect a series diode to the power line input. The diode conducts only when the power supply protection is correct. But the incovenient is that at higher current levels, the voltage drops and power loss of the diode affects the power level adversely.

    This polarity protection circuit is dimensioned for 12 V power supplies and avoids the voltage and power loss problem. By correct polarity, the current flowing to the D1 and the relay coil causes the relay contacts to activate. The NO contact closes powering the electronic device. The NC contact opens and the current supplying the relay coil is reduced to a low level just enough to maintain relay activation.

    Polarity protection circuit schematic

    polarity protection circuit schematic

    When reversed polarity occurs, the diode blocks the current and the relay cannot activate. The NO contact remains open and the electronic device is protected from the reversed polarity voltage.

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    5 Responses to "Polarity Protection Circuit"

    1. Extremly informative ,simple and safe’

    2. its .,., powerfull .,., thx.

    3. very usefull

    4. please do not include the so called ‘economy’resistor (120 Ohm) to save some current through the relay-coil; the relay coil do need that amount of milli-amps in order to press the contact together with such a force, to enable the relay to conduct safely the current stated on the relay.

    5. The use of “economy resistors” is a universal practice because the energy required to close the relay is much more than needed to keep the relay closed.

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