I have recently decided to upgrade my old van by installing a few 5V/3A sockets using a 100W 12V-to-5V downstep buck that has no reverse-polarity protection. A little bit of searching here and elsewhere has led me to understand that I can facilitate that protection with either a diode, at the cost of a significant voltage drop and risk of reverse leakage, or a MOSFET set before the buck circuit. I've tried looking for appropriate p-FETs but have only found 7812s @1A/1.5A and variable I/O FETs that I understand would require additional components to configure.

Which of these techniques would be more appropriate to use for my purpose? If I use a p-FET, what should I be looking for to find the most appropriate one?

Thank you for your time.

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    \$\begingroup\$ A 7812 is not a FET - its a linear voltage regulator and what is a "variable I/O FET"? I suspect you're looking in the wrong places and using the wrong search terms. \$\endgroup\$ – brhans Jul 14 '16 at 0:27
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    \$\begingroup\$ Are you trying to protect against a reverse 12 volts or a reverse 5 volts? You also have the option of parallel protection with fuses and diodes. Don't make this more complicated than it has to be. \$\endgroup\$ – user105652 Jul 14 '16 at 0:55
  • \$\begingroup\$ Hello, welcome to EE.SE. 7812 p-FET... are you talking about this or something like this? \$\endgroup\$ – rdtsc Jul 14 '16 at 1:29
  • \$\begingroup\$ @brhans I was under the impression the 7812 was a specific type of FET functioning as a regulator. What I mean by "variable I/O FET" is one marketed as having variable input and output voltages. \$\endgroup\$ – S. Gilpin Jul 14 '16 at 2:49
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    \$\begingroup\$ Use polarised connectors to eliminate the risk of polarity inversion. \$\endgroup\$ – Transistor Aug 13 '16 at 8:58

If your protecting the 5 volt 3 amp lines from reverse polarity I would insert an in-line 5 amp fuse for each load. Use a fast blow fuse unless your loads have a high start current, then you would use slow-blow fuses. Install a 6A06 or 6A10 6 amp diode across the 5 volt line on the downstream side of the fuse. The diode has a white line at one end.

The end with the white line goes to the (+) feed from the fuse, the other end goes to the (-) or ground return. Do the same to all loads that need isolated protection. If each load draws less than 3 amps then use a fuse with about 50% greater capacity than the load draws.

The idea here is that if a reverse connection to the 5 volt power is made, the 6 amp diode will clamp the voltage to maybe -.7 volts at most and the high current will blow the fuse. The diode will survive such events as it is rated twice the supply current.

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