CONTINUOUSWAVE

Hi to all, the first time to post, have a little bit of nervous that I can write topic as well as completely, the situation is following: 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 http://www.kynix.com/Detail/669784/DC.html
. 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-vo ... 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?
If you have any suggrstions, thanks for your help!

The electronics on a boat are usually wired to an isolated battery, called the HOUSE battery. The HOUSE battery is completely separate from the engine cranking battery, and its voltage output will not sag during engine cranking. Nor will there be any voltage transients from the heavy load of the engine starter motor coming on-line or off-line--that load is not on the HOUSE battery or any of the HOUSE battery distributed power.

As long as the electronic devices that presently are powered from 120-VAC through transformers are really 12-Volt DC devices, there should not be any problem running them from an isolated 12-Volt HOUSE battery.

A typical modern VHF Marine Band 25-Watt radio can be run from the HOUSE battery. A radio only draws about 7-Amperes during transmit and much less during receive. The radio should not cause any pollution of the 12-Volt DC power, as long as the radio's antenna is not right next to the radio; if the antenna is properly installed, and is 10 to 15 feet away (or more), is properly matched to the transmission line, and is elevated above the radio itself and other electronics, the transmitted signal should not interfere with other devices connected to the same 12-Volt DC power

The devices you have linked to are in two categories:

--uninterruptible power supplies

--power filters

There should not be any interruptions in the power from a 12-Volt DC battery, unless there is a wiring fault. The nature of an uninterruptible power supply (UPS) is the use of batteries to fill in for AC mains power. I don't see how having a UPS on a 12-Volt battery power source is going to work. You'd have one battery backing up another.

Generally there is no need to filter the power from a 12-Volt DC battery as it is completely pure power. The problem with noise on the power from a 12-Volt battery occurs when a charging source is attached. Engine alternators deliver a pulsating DC power to the battery, and this can apply a noise signal onto the 12-Volt power.

You can measure the ripple voltage on a 12-Volt battery being charged by using the AC Volt function of a DMM. If you find you have several Volts of AC ripple on your 12-Volt battery, then you have a noisy power source.

I just measured the AC ripple voltage on my bench 12-Volt battery while it is being charged by a precision float charger. The ripple voltage is 0.003-Volt AC RMS. Since the battery is at 12.65-volts, a ripple of 0.003 represents a noise on the power that is down 72.5 dB, or about 0.02-percent noise and distortion.

Of course, the charging current is very low, probably less than 1-Ampere. If the battery were being charged at 40-Amperes, the AC ripple would be much higher.

If a filter network is installed between the battery being charged and any loads connected to the battery; the filter will reduce and AC noise being delivered to the loads. The filter works typically as a low-pass filter.

A low-pass filter lets signals below its cut-off frequency pass through the filter without attenuation; signals above the cut-off frequency are attenuated. For a low-pass filter to effectively work on a DC battery charging circuit, the filter must have DC continuity.

The most basic design is an L-network filter. The filter has two elements: one element in series with the source current, and a second element shunted across the load element. In an L-network low-pass power filter the shunt element is a capacitor and the series element is an inductor. You can think of the filter working like this: the series inductor tends to block any AC signals; the shunt capacitor tends to shunt any remaining AC signals to ground.

For the filter to be effective at removing pulsating AC voltages riding on the 12-Volt DC power, the filter must suppress any AC signals that are above a rather low frequency. To do that requires large inductance and capacitance. And the inductor must be able to handle the full load current of the device or devices connected downstead.