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Power protection – mosfets

Protecting DC powered devices from reverse polarity is good insurance. Diodes can work for this but they have voltage drops of a half a volt or more. Jon Chandler lit an idea with his post about A No-Voltage-Drop Bridge Rectifier. That lead to Mike Szczys’s post about Reverse voltage protection with a P-FET. Other papers on this include a PDF from Infineon on Reverse Battery Protection and Using MOSFET for reversed power polarity protection at MyElectrons. Terry Ritter has a good rundown on the technique in his page on A Modern Breadboarding Technology

The idea is that a mosfet has a very low resistance when turned on and that means low power loss and minimal voltage drop. In order to keep a common ground, a P channel mosfet is used with the Drain towards the battery, the Source towards the load, and the Gate at ground. This depends upon the intrinsic diode between drain and source to provide the voltage between Source and Gate to turn on the device when there is no load. For supplies greater than 5 to 9 volts, a zener and resistor may be used to protect the gate from excess voltage.

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