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Thursday, November 3, 2016

Reverse Polarity Protection

When I opened the package from India and saw Farhan's s beautiful board with all those little SMD parts, I immediately worried about frying those parts by accidentally reversing the polarity of the 12 volt DC input.   Believe me, this can happen.  It is especially likely during the early, enthusiastic testing and experimenting that takes place in the days after the arrival of a new rig.  So, my friends:  Save yourselves the agony of fried components!  Don't let your BITX 40 Module go up in smoke!  Install a simple reverse polarity protection circuit BEFORE you start working with your new board. 

Here is what I did:   I just took a diode (a fairly hefty diode) and I soldered it in between the pins of that neat little circular power jack that Farhan sent with the module.  Be sure to solder it in so that it does NOT conduct if you have connected the power correctly.  The arrow should be pointing to positive terminal.  Then put a fuse (3 amp or even a 2 amp) in the line from the connector to the power supply or battery.  If you don't have a holder you can try just soldering the fuse into the line.

With these two little parts, you can save yourself a lot of grief:  If (WHEN!) you connect red to black and black to red, that diode will conduct like crazy and will blow the fuse.  You'll just have to replace the fuse (and not the module).


  1. The only possible problem with this method, is that some solid state devices can be damaged faster than the fuse can blow. I prefer to just add a large diode in series with the power lead. You loose a little voltage to the unit because of the voltage drop across the diode, but it offers better protection. I usually use a schottky diode because of its lower voltage drop.

  2. I have long used the reverse-biased diode across the supply in conjunction with series-wired PTC (self-resetting) fuses. These devices look like a disk ceramic capacitor (usually yellow, can be round or square/rectangular), are available with current ratings from a few 10s of mA to 10s of amps, with available maximum voltage ratings to 60-75 volts - and they are pretty cheap! Best of all once the "oops" is corrected, life goes on without incident.

    If one accidentally connects the power leads backwards (I have!) then *nothing* obvious happens other than the device getting hot (100C or so, maybe) and the current drops off to a "safe" level.

    In some cases - such as where it might be possible that ground currents could flow (say, some mobile installations) then I may even put one in the ground lead.

    If, for some reason, these don't interest you then I might draw your attention to this GQRP article:


    This describes a FET-based "lossless diode" approach that can work in many situations: It will even work with N-channel FETs if the negative power source is isolated from the load.


  3. Isn’t D7 already across the 12 volt power supply?


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