Reverse Polarity Protection

As the Yaesu FT-817 doesn’t have any reverse polarity protection or an in-line fuse, I decided to build myself one seeing I do a lot of portable work on summits or in parks.
Diodes work fine but they do incur quite high losses in eventual output power, so with only 5W max available, this isn’t desirable.
The answer lies with the P channel MOSFET (FQP27PO6)
A quick search on line turned up this schematic.

Only one component!

This is quite a simple affair: when the battery is connected up correctly, the MOSFET is turned on allowing current to flow. When connected up incorrectly, it turns off.

It works thus: when the gate to source voltage is around -4V or less, it turns on. So if the battery is a 12V gel cell (as in my case), the voltage through the MOSFET  = 12V – 1V loss (due to the parasitic body diode) which equals 11V with respect to ground.The voltage at the gate is 0V as it is connected to ground. This means that Vgs = 0V – 11V = -11V. This is less than -4V so it turns on.

If the battery is connected up incorrectly, then we have Vgs = 0V –11V = 11V. so the devise turns off.

I mounted the MOSFET on a piece of vero board and used an ice cream stick as strain relief. I also included an in-line quick blow 5A fuse in the positive line.

All the components before final assembly.

Once the heat shrink had been slipped over the component, I heated it up with a hairdryer to achieve a nice, tight finish.

While I was at it I built another one for my Elecraft KX1.

Now it was time to test it.

 

All working as expected with the battery connected up correctly.

Now to switch polarity.

No voltage with reversed polarity.

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