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FPM5 ~ Reverse Polarity Protection

FPM5 ~ Reverse Polarity Protection


FPM5 ~ Protecting All of the Hard Work

One of the sickest feeling you can have is to plug in a new rig to 12 VDC and watch it go up in smoke as you realize you had the leads to the power supply reversed. yes I have done it and I am sure you have too.
In the SolderSmoke Podcast #191 Bill, N2CQR and I were talking about this subject specifically in regard to the new Farhan, VU2ESE’s rig the Bitx40. Bill having one strongly encouraged the addition of reverse polarity protection using a diode.
Of course as usual any time you make a suggestion you get the naysayers who lurk the various reflectors whose sole goal in life is to spread negativity about anything. Typically these individuals have never built ANYTHING and think they are experts! They are often quick to pan a solution and they quickly jumped on Bill!
I am facing the same situation with my FPM5. Man I have months invested in this rig and the last thing I want to see is it smoking from wrong polarity being applied. But I wanted something more than a diode, otherwise I am sure I would have the same level of negativity applied to me.
So while awaiting for mass to start this morning (instead of reflecting on the holy religious event that was about to start) my mind drifted onto the subject of reverse polarity protection. I have come up with two schemes and these are presented below.
Both schemes involve the use of a diode and a relay. The relay I happen to be using is a very small American Zettler DPST type that is good for 10 amps. These were bought surplus but many of the larger supply houses sell them for about 60 cents and the 1N4007 I bought for 5 cents — so for $0.65 you have protection.
The first scheme is what I call passive in that the relay is only energized when you have the wrong polarity connected to the right terminals. When this happens the relay essentially opens and disconnects the rig from the supply. This scheme is ideal for QRP ops as there is literally no current draw other than leakage current under normal operation. So there should be no complaints about wasting milliamps. You hit the power switch and power is applied to your rig.
The second scheme is more appealing to me in that it is an active approach. You hit the power switch and power is applied to the radio! But if you reverse the leads despite hitting the power switch there will be no juice applied to the radio.
See the schematic below for both schemes.Keep on Soldering!
Pete N6QW