Keep the following in mind when working with FETs:
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FETs are off when the Gate and Source are the same voltage (or close)
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N Channel FETs turn on when the Gate is higher than the Source
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P Channel FETs turn on when the Gate is lower than the Source
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N-CHannel FETs have a Body Diode that conducts from Source to Drain (Source usually is connected to ground)
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P-CHannel FETs have a Body Diode that conducts from Drain to Source (Source usually is connected to power)
This is not all-encompasing, but it will help avoid the most basic errors. Many FETs these days are "Logic Level" meaning they will be fully on when the G-S voltage is around 3V or more. Some FETs, especially older parts, require more voltage to turn on.
Steve