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post #1 of 6 Old 01-26-2009 Thread Starter
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Reverse Polarity Protection

Found this interesting circuit that uses a few $us worth of radio shack parts to protect equipment from reverse polarity. Please don't ask why I might have been searching for said circuit (I let the magic smoke out of a power supply today).



Source: Simple Reverse Polarity Protection

Nice circuit, only adds a current draw big enough to power the reed relay's coil, which is next to nothing, and it protects your expensive gear from reverse polarity according to the author. Not bad for a few $us worth of parts and a few minutes time, and if one of the components of the circuit fails while underway you just cut it out of the circuit and continue on. Cheap insurance.

It's for positive earth so I suppose it has to be modified slightly for negative earth.

Edit - No, I guess you don't have to modify it at all.

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Last edited by wind_magic; 01-26-2009 at 01:33 AM. Reason: edit
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post #2 of 6 Old 01-26-2009
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The relay solution is great for high current devices. For lower current devices I prefer a solid state solution with very little voltage drop:

Source RECOM - Reverse Polarity Protection for DC/DC Converters

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Quote:
Originally Posted by TradewindSailing View Post
The relay solution is great for high current devices. For lower current devices I prefer a solid state solution with very little voltage drop:

Source RECOM - Reverse Polarity Protection for DC/DC Converters

Nice one Trade. I like that low voltage drop. I might use one of those diode solutions too for some of the circuits I build myself because I am typically dropping 12vdc down to 5vdc with a voltage regulator, so a 1-2vdc voltage drop over a diode before it hits the regulator is no big deal. Of course for the low power circuits I typically build, I might as well use a cheap rectifier.

I built the circuit from the post I made above. I made it as illustrated, same diodes, even the same radio shack relay, but I took the output from the reed relay and used that to turn on the coil of an even bigger 10amp relay. Works like a charm.

The circuit a pain in the ass to build, with so few components you wouldn't think it would have been much trouble but I have wires all over the place, it looks like a .. I don't know, something really messed up is what it looks like.

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post #4 of 6 Old 01-27-2009
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If your boat is properly wired, why would you need reverse polarity protection on the DC side at all???

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Quote:
Originally Posted by sailingdog View Post
If your boat is properly wired, why would you need reverse polarity protection on the DC side at all???
Well ..

The first time was when I had taken all of the batteries out and started putting them back in and momentarily wired the first battery in backwards. That little mistake required sending the Icom 706 off to the manufacturer to have a diode (I assume) replaced.

The second time was when I purchased a piece of equipment from Europe that was positive earth and I didn't know that, it got wired up wrong, and that sucked current backwards through the 60 watt power supply on a computer. That one was harder to replace because they didn't make it anymore, so I had to modify the way the computer was working to work with a new power supply.

The third was simple slip of a screwdriver while working on something that smoked a capacitor in one of my own boards, that one was easy to fix because I had built the board myself.

The fourth was an automobile amplifier that ... you get the idea ...

I would not want to suffer the current drain of any of these reverse polarity solutions all the time, and so it is not my intention to have them installed all the time. What I am doing is wiring up a new fuse panel and I want to be able to switch the reverse polarity protection in and out on the more expensive equipment whenever I am doing battery maintenance, making modifications to the electrical system, etc, to limit damage that I myself cause. Once the system is stable then I can go back and check everything with a volt meter and take out the protection at the fuse box as part of my final procedures for maintenance.

I doubt that someone who paid other people to do electrical work would need reverse polarity protection, but someone like me who is constantly working with the system makes mistakes because the system changes all the time. I am always making modifications, and accidents just happen. Accidents have happened with me quite a few times now and each time it ends up costing me time and money.

I may end up leaving reverse polarity protection in place on a few things. The Icom radio, for example, is expensive, and it uses so much current when it is on that a little extra current wasted charging a relay coil isn't that big of a deal. Since I really have harmed that radio before, and it is a very important piece of equipment, I might protect it all the time.

That was the long answer.

The short answer is .. "If your boat is properly wired .." is a BIG IF.

Edit - another way to say that is that for most people "electrical system" is a noun, but for me it is more of a verb ...

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Last edited by wind_magic; 01-27-2009 at 03:52 PM.
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post #6 of 6 Old 01-27-2009
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LOL...I understand...As for hooking up the batteries...that was a rookie mistake...and you got what you deserved IMHO, since the battery cables are color coded on most boats. Red is hard to mistake for black or yellow.

If you're working on the electrical system, shutting the proper breakers off to protect the gear would work in 99.999% of the cases, but if you're really paranoid, wire them into the high-end, hard to install/replace stuff.

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