Providing 12V Power to RV-Based Hardware?

jp93023 asks: "I am putting together a mobile computer lab in a converted RV. RVs have a 12v DC system for running most interior lights, built-in appliances, fans and so forth. They have a parallel 120v AC system for providing normal outlets, which is great when you are plugged in. It strikes me that when I am running from battery power (which will be most of the time) I will be expending precious watts converting the 12v from the marine batteries up to 120v, only to convert it right back down to 5, 9, 12, and 20v DC for all the PCs, laptops, etc. The equipment package will include desktops, laptops, digital tape decks, etc., so I'd be planning on bypassing the built-in transformers for everything but the laptops. Have any slashdotters put together a unified low voltage DC power distribution system for such a situation? Would the power savings be worth it? Any pointers to products or plans would be great!"

Comments

Appliances are the problem

[Jon Peterson]

Hi,

If the RV situation is similar to the boat situation, then the existing DC system is fine. The issue is getting appliances that expect DC input from the wall rather than from their own little transformer. I would be wary of taking 'normal' appliances and chopping the transformer off and sticking the wires into your DC system.

You can buy power tools, radios, even microwaves designed specifically for use with boat (and, I assume, RV) DC power systems. I'm sure the range is restrictive and maybe they are expensive, but they work.

However, an RV has a big old engine, right? So you aren't likely to be short of power, really, right? So why not just use the AC system and forget about the inefficiency :-)?

Re:Appliances are the problem

[flonker]

As far as computers are concerned:

Laptops generally have a direct DC in, but that probably isn't relevant in this case.

Google returns MPBS1 - ATX DC-DC 12 Volt Car Power Supply, but not much else. (at least for the minimal searches I've done.)

Helpful information on inverters & laptop adap

[pmyre]

I hope this link helps.

http://www.techtv.com/screensavers/wirelessandmo bi letips/story/0,24330,3393834,00.html

Peter Norton showed a few laptop adapters (to charge bateries on 12v) and some inverters (12v to 110v).

For my personal usage (RV & travel) I will select the inverter, I just think it's more versatile. You can plug a boombox, recharge your GPS, recharge your cell phone, your PDA,... You just don't need to purchase a bunch of adapters.

Without an inverter I would be very afraid of the 'quality' of the electricity.

If you build your own solution, please let us know!

Same as in-car mp3 solutions?

[TinheadNed]

If all you want to do is power a computer from a car battery then just visit the DIY in-car mp3 player websites. There's pages on how to make a DC-DC inverter (not DC-AC-DC) that is designed to power ATX motherboards (so it provides 12V, -12V, +5V etc.)

I hate to be the first person in an Ask Slashdot thread to say search on google, but that's where you'll find them. Unfortunately right now I can't connect to google for some reason, but I'll post the URL to the supply I'm thinking of (and attempting to build incidentally)

Why not use a generator?

[north.coaster]

Will the computers be used when the RV is moving? If not, then why not simply buy a portable generator and use it to supply the AC power? Portable generators are very popular among the RV crowd, so I'm sure that you can find lot's of information on their use (such as here).

There are probably solutions for mounting and wiring the generator so that it can be used while the RV is in motion, but that sounds like a much more difficult problem and may be more than what you need.

/Don

Re:Why not use a generator?

[pete-classic]

Also, the efficency of a generator powering a small load is TERRIBLE! If you are running loads totaling a couple of KW gens are bad, but for a 200 watt computer the generator will not run well at all.

I've only ever worked with gens in the Army, but you should always run a gen loaded. If you don't have a sufficent load you add what we just called a load, which is nothing but a heating coil AKA a big ass resistor.

Then it comes down to what you mean by efficiency. Pound-for-pound and dollar-for-dollar gens and gas are pretty hard to beat, and you can piss a lot of gas away in heat before batteries start looking good.

-Peter

The Boaters have lots of documentation...

[Ocelot Wreak]

I echo the comment on boats - there are a LOT of "How To" manuals complete with schematics for designing your own 12V systems in boats, right down to the size of wire to use. [I will post a title and ISBN when I get home to check the bookshelf.] Same principles apply to cars.

I would be very afraid of the 12V coming out of the cigarette lighter adapter - I heard a story of someone working away on a laptop just fine using the car's 12V battery system and the car ignition turned off. However, they had their laptop fried when they turned the car ignition switch on. It send some crufty power through the cig lighter and into the equipment.

Switchers

[Matt_Bennett]

If battery conservation is important, stay away from the inverter- a large portion of the power will go away as heat- look into switching power supplies, which can run up to 80-90% efficiency. Look through surplus catalogs and so forth, buying them factory new will probably be prohibitively expensive, but they can be quite reasonable in surplus.

Also, try to stay away from linear power supplies- they also will convert a lot of your precious power to heat.

Running a desktop computer off a 12V system looks to be pretty easy at first, especially when you look at the common voltages (+/- 12, 5, 3.3V) But the problem starts when you look at how much current each one of those voltage rails use, it's pretty amazing- 30, 40 amps is not unusual.

With lower voltage DC supplies, you have high currents for the same amount of power delivered, so you're also going to want to use pretty thick wire to distribute the power around. This will minimize the loss of power to the resistance of the wire.

Try amateur radio sites...

[MaggieL]

...because this is a project that a lot of amateur radio folk undertake in building emergency communications or stormchaser vans. Most amateur radio equipment runs on 12v DC, and these vehicles also usually pack a number of computers for digital text communications modes, processing weather info and satellite tracking.

Suggested keywords: "communications van", "emergency communications", "stormchaser", "RACES", "ARES"

Replace my wall warts, please!!!

[no_such_user]

This would be far more useful to me as a way to replace the vast array of wall warts and chaotic tangle of black wires which warm my feet.

I'd chop off the devices' wall warts and replace them with some sort of quick-disconnect plug on both sides of the cut wire; this way I could use the wall wart again if I was out of this environment.

EE's, help me here. Let's say I have a single 24VDC bench supply (I'm starting at 110v at home)at 15-20A. I'm thinking that for any item drawing under, say, 500ma, I could build mini step-down regulators (in something like 35mm film canisters) using something in the 78xx series of regulators (i.e. 7805 for 5v). I'd put one of those quick disconnect plugs (mentioned above) on the output, so it would easily and quickly interface with the device.

As for the input to these regulators, I'd like to be able to run one 6-ft. wire around my equipment and let the regulators somehow "vampire" tap (I *think* that what it's called) this one wire (perhaps 12 gauge stranded? thicker? help?!), so the number of wires around my setup are drastically cut down. Is this realistic?

I definitely need input here, so please -- someone in the know reply and correct my errors here. I'm specifically concerned with my lack of knowledge in DC power distribution and power loss, not to mention if anything would be likely to leak power back into the regulator somehow.

Re:RV power

[t]

This is not informative. The poster is asking specifically if there are power savings to be had by avoiding the 12VDC to 120VAC to 5,9,12,20VDC process and instead using a DC-DC transformer to go directly from 12VDC to 5,9,12,20VDC as needed.

All the above "Informative" post says is "Duh like dude yeah, plug that baby into the cigarette lighter dude. and yeah like dude, plug it into the AC plug too. Like noooo problem."

I would think some benefits would be that by not using the typical AC power bricks you'll avoid the continuous drain that they cause. For some devices it can be a considerable quantity. Sloppy design I guess. I'm not sure how some of the latest tech in DC-DC converters works but I would expect there to be less electrical intereference generated too. Helpful if you're parked near a wifi point, just don't run the microwave.

Another issue is that some devices want low power AC for odd reasons. In those cases it may be possible to find a 12VDC to 9VAC converter that would run cooler (waste less).

t.

What I did...

[cr0sh]

I got tired of this as well - my solution didn't replace all of my wall warts, but it did replace most of them.

First off, realize that most electronic equipment that uses a wall wart typically has a regulator in it anyway - ie, it may use a 9 volt wall-wart, but it has a 5V regulator in the device. So, basically if you can get a power supply with a 12 VDC line, and a 5VDC line, you are set. You then would have your pick of three voltages - the two already mentioned, plus 7VDC by using the 5VDC line as "ground" with the 12VDC line (this really isn't a great thing, not electrically sound, but I have yet to see anything fail because of it - but if you know of any problems using this kind of system, let me know). So, what power supply to use?

You want a PSU with the two voltage lines and ground. The power supply should provide relatively high current capacity on the lines. As a bonus, it would be nice to have extra lines for minor low-amperage things like fans and an LED indicator light. So, what did I use?

I used a switching powersupply I found used at a local electronics junk yard - it was a Sun power supply (not sure for what model, maybe and IPX/IPC). I mounted the PSU on the bottom of my desk (a cheap folding table), and ran wires (12 or 14 gauge auto wire, I think) from the power output lines to several "bus" terminal strips on the underside, in parallel. From the bus terminal strips I could tap into the power where I needed it. The power supply also had a couple of smaller connectors - one provided 12VDC and the other 5VDC. I hooked the 12VDC one to a fan I have mounted on my monitor (it gets hella hot without it), and the 5VDC went to an LED I have mounted in a hole in metal edge of the folding table, to show that the PSU is on.

While this isn't as clean as what you are proposing, it was all mostly off the shelf, and was easy to set up. The terminal strips used screws, so it is easy to hook into, plus the switching PSU should have a cleaner output than most wall warts...