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.


Reverse polarity protection

Now that I am a 100% QRP station, I rely more on battery power than ever before. Sure, I do have power supplies but I tend not to use them, preferring to make use of renewable energy whenever possible.

This brought into focus the importance of reverse polarity protection because the last thing I’d want, especially during the hurly-burly of setting up in the field, is to connect up the power the wrong way round and destroy my rig.

The first thought I  had was to insert a diode into the positive line of the battery feed line, but on second thoughts, quickly discarded that idea. You see, suffering a drop of around 0.7V simply because of the diode doesn’t make sense, especially seeing I’d only have around 12V at my disposal.

Better to make use of a Schottky diode, but then again, even though the voltage drop would be far smaller, when you’re dealing with QRP you want to squeeze out as much voltage from your battery and be able to have it at your disposal as you can. OK, Schottky’s have only a 0.3V drop, but I wanted to do still better.

This meant I’d have to think about using a FET.

I asked Jan, VK4EBP, if he had a spare that I could use as I didn’t have any in my junk box. He said he did and supplied me with three FQP27PO6’s.

The idea is to chose a FET with a SDS On Resistance of around 0.07V or better, and a maximum gate-source voltage well above the typical voltage I’d anticipate using. These are rated at around 25V, which would do me fine. Their drain source voltage is also around 60V, so I had a starter.

Next I had to draw a quick circuit diagram so I wouldn’t make a mistake and fry my rig. I had to remember that as I would be using a P-Type FET it would be inserted into the positive line, not the negative.

My quick drawing of what I needed to construct.

My quick drawing of what I needed to construct.

I decided to solder the FET onto a piece of vero board, to which I’d also solder the wires. The idea was to have something rigid so that the contraption wouldn’t fall apart in the field.

Rough and ready, but it'll do the job.

Rough and ready, but it’ll do the job.

I used a rotary tool to grind away the copper trace where necessary. I prepared enough vero board to make two units as well.

Next, it was simple a case of soldering the FET and the wires into place.

Soldering done and waiting for the insulation tape to seal.

Soldering done and waiting for the insulation tape to seal.

I then sealed the board with some insulation tape.

Now came crunch time. I connected the cable up to a power supply in the right order and measured the output voltage at the plug with a DMM. Perfect.

Voltage reading as expected. So far so good.

Voltage reading as expected. So far so good.

Now for the moment of truth. I reversed the leads to the power supply so that the black (negative) lead was connected to the red (positive) terminal on the power supply and the red lead to the negative terminal.

When the DC voltage is reversed, the gate is pulled LOW relative to its source, and the FET turns off.

When the DC voltage is reversed, the gate is pulled LOW relative to its source, and the FET turns off.

All working as intended. I now have a good reverse polarity protection that has a minimal voltage drop across the protection device.