Reverse polarity protection on preexisting equipment

M.daSilvaM.daSilva wrote 03/15/2017 at 23:48 • 3 min read • Like

Yes, I know, this is electronics 101, this was written for personal reference.

I'm using radio transceivers in all kind of conditions, including portable, where you sometimes have to wire the power supply in a hurry. I never got it in reverse, but I fear this may happen one day. As this kind of equipment can be quite costly, such a mistake can have quite catastrophic consequences.

I'll take the example of one of my radios, the Yaesu FT-857. Very nice piece of kit, although the power amp transistors are directly connected to the power rail. One mistake, and they're blown (along with other, costly components). There are a few ways to avoid this, two of them being extremely simple:

We can add overvoltage protection with a carefully chosen Zener diode wired in parallel. As this radio shouldn't take more than 13.8V+10%, 15.18V, I'll pick a 15V 5W Zener, the 1N5352B. If the voltage goes over that value, once again, the diode conducts, drawing a high current, and (hopefully) blowing the fuse.

I don't have access to this kind if diodes right now, but here's an example of such a mod by Alpha-Telecom (brazilian YT channel, only thing he does is fix HF transceivers. I highly recommend it if you speak Portuguese) here. The diodes are directly soldered to the power supply cables.

This is an extremely simple and cheap mod, which can be done for all sorts of equipments and can save their lives!