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On my boat I have a number of electronics devices that run on 12V DC and normally get their power from an AC transformer. They include a media player, a network router, and a portable hard drive. Since the boat is wired for 12V DC I would like to set these up to run directly off of the boat's DC. This will free up a lot of power strip space, and eliminate the inefficiency and need to run them through the boat's DC to AC inverter when not connected to shore power.

I've read that this is dangerous territory since the boat's DC system is subject to surges and noise, for example when the starter or the electric windlass are engaged or the VHF radio is in use, and that this can damage electronics directly tied to the boat's DC system. I've also read that while the marine 12V system's voltage can vary from 12 to 14.4v (depending on whether the alternator and charger are engaged), these devices are not tolerant of these voltage fluctuations and are designed to expect a constant regulated voltage from their AC-DC transformer (wall wart). There are marine surge and noise suppression products available ranging from $50 - $200 to address this concern.

The marine market is renowned for its high markup. For safety and navigation applications where help is far away this is merited, but honestly if any of this equipment I am connecting failed nobody would get hurt or lost. The price of the commercial products seems high and I suspect the circuit should be relatively simple. The loads are all less than 5 amps.

I am interested in implementing a custom circuit to accomplish this protection.

Where should I start?

Are there reference designs for such a thing? What are the critical components: voltage regulation, surge suppression, noise suppression?

I've done a bunch or Arduino projects and am comfortable soldering, breadboarding, and putting together a custom through-hole project, but I lack a formal EE education and background. I've also done quite a bit of re-wiring on the boat so am pretty comfortable with the marine DC distribution and circuit protection and the ABYC standards.

A couple examples of commercially available products:

http://www.sto-p.com/pfp/online.htm

http://shop.sailnet.com/charles-1224-volt-suppressor-p-38606.html

edit: is something like this a good starting point: http://www.ti.com/product/lm5118 and if so what else should I add? Ferrite chokes?

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  • \$\begingroup\$ Not experienced in boating parts, but car 12V systems have the same 12~14~18V surges with the alternator and such. Most car 12v adaptors are built to deal with this. Don't see why a boat 12v setup would be any different. Have you looked into car 12v adaptors, like usb chargers or anything? \$\endgroup\$ – Passerby Jun 5 '13 at 15:49
  • \$\begingroup\$ @Passerby, sure I could try something like that, but I was looking for a small electronics project. Also I suspect I should be able to put a solution in place that is of higher quality and lower cost than most of the car adapters. \$\endgroup\$ – Pat James Jun 5 '13 at 16:13
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A simple buck/boost isn't going to help when you get a load dump on the power system. It's likely to fry. You need to specify the system to survive 100 Volt inputs.

I'd try with a 10A fuse followed by a 5 kW 14V TVS diode (unidirectional) followed by a small electrolytic rated for 100V or more (10 uF or so) followed by a high-inductance inductor rated for the total current of your draw (you can get 220 uH 6A inductors pretty easily) followed by a big electrolytic (1000 uF) rated for 50V followed by another 14V 5 kW TVS diode followed by an actual boost/buck regulator to regulate it to 12V no matter the over/under-voltage. Getting a 12V boost/buck regulator with many amps of capacity can be challenging -- a few amps is easy to find on eBay or pololu or wherever.

The TVS-es are about $2 each, the inductor is about $5, the capacitors maybe $1 total and the regulator perhaps $10-$30 depending on the source. (Or more, if you need high current capacity)

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The damaging event that you are concerned with is called a Load Dump. It can expose electronics to greater than 100 volts for hundreds of milliseconds. They happen in automotive environments, and I assume in boats too.

Most semiconductors are not designed to handle 100+ volts in, which is why a "simple" DC/DC switching converter (a.k.a. Boost/Buck converter) will not survive in this environment.

Some people will claim that using something like a TVS (Transient Voltage Supresser) diode will protect you, and it might. It also might blow up. You need something to help limit the current too. I would start with an LC filter followed by a TVS. But, and this is huge, you must simulate the circuit to make sure it will handle those voltages correctly-- without damaging any electronics you have connected but also to make sure you don't damage the protection devices themselves.

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A custom circuit for this task shouldn't be hard to come up with by any means. I've never had experience with marine systems, but no doubt marine electricity is like all electricity, right?

You say your system provides 12V with some ripple, sometimes reaching as high as 14.4V, and all you want it to do is provide a constant 12V?

Enter the Buck/Boost Regulator IC

No really, shouldn't it be that simple? It looks like this IC has high output accuracy, and 84% efficiency. It has a 2% ripple, and can put out 1.5A. Earlier I posted a link to a simple 12V regulator TO-220, but that kind of thing probably wouldn't work because they need an input of higher that 12V due to losses. If your devices need more than 1.5A, I'm sure there are similar higher powered solutions also. I couldn't find any places that sell this particular one, but to be honest, your LM5118 looks even better.

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protected by W5VO Jun 5 '13 at 18:33

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