• Reverse polarity protection on preexisting equipment

    03/15/2017 at 23:48 0 comments

    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:

    • A diode in series with the power line: it conducts in the "right way", and blocks off any current in the other direction. This is nice and all, but there's the diode drop! It wouldn't have been an issue if the current was low enough, but we're talking about 25A! For a typical 0.6V diode drop, that's 15W of power dissipated by that poor semiconductor.

    • A diode in parallel with the power line. It doesn't conduct in the "right way". If the polarity is reversed, the diode starts conducting, drawing a huge current. As my radios always have an inline fuse, it blows, instead of destroying the equipment. I like this idea better: no power loss, the only downside being you have to replace a fuse everytime you mess your wiring up. As this is a last resort protection measure and you should be careful anyway, this shouldn't happen very often, should it? A simple 5A rectifying diode should do the trick.

    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!