Normally, a bridge rectifier chip is used to convert AC to a pulsing DC. However, it can also be used to make a DC circuit insensitive to input polarity. This is really handy because I have destroyed more circuits than I like to remember because I accidentally switched the polarity of the power supply.
The downside is that the input voltage has to be about 1.4V higher than the required supply voltage. The power dissipated in the bridge rectifier is 1.4 times the current. This may not be a good solution in low voltage circuits with less head room, but for most normal applications, it is a simple way to protect the circuit from inadvertent polarity changes.
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