Providing 12V Power to RV-Based Hardware? 44
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!"
Re:RV power (Score:3, Insightful)
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.
Re:RV power (Score:1)
I am bulding a 12vdc POS system and have meet most of my needs with COTS (Commercial Off The Shelf) equipment. I have assembled two of my three stations using VIA EPIA-5000 mini-ITX motherboards. They have 500mhz very low power processors. The neat thing is that the cases I am using is by Morex (imported by casetronic) have a built in 55w switching PS that is feed by a 12vdc wall wart.
My aim is to power all computers, displays, network switcher and receipt printer from a couple of large 12 volt gell cell batteries. The batteries will be charged by a solar panel. I plan on trying for a 20hr runtime on battery alone. The only display I have so far for this system is a KDS Rad-5c which I just purchased. It also uses a 12vdc wall wart.
Re:RV power (Score:2)
Appliances are the problem (Score:3, Informative)
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 (Score:3, Informative)
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 [skylab.org], but not much else. (at least for the minimal searches I've done.)
Engines are the problem (Score:2)
Helpful information on inverters & laptop adap (Score:3, Informative)
http://www.techtv.com/screensavers/wirelessandm
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!
Re:Helpful information on inverters & laptop a (Score:2)
Tread carefully here. You can go to any truckstop and buy a 2000W inverter for about $300. If you were to buy a 2000W inverter to power your home with all its expensive electric appliances from batteries (PV, wind, etc.), you're gonna pay $2-$5k for the same output, especially if you go true sinewave. Power quality is one of the main things you buy from these systems.
Yes, I know, there are many other things in those fancy inverters that also push the price: grid intertie, overload/temperature monitoring, battery charging/conditioning, data loggin, etc.. However, the old saying "you get what you pay for" usually holds true for inverters.
That said, I frequently run a laptop off a $30 400W inverter I got at Wal Mart and I haven't had any issues yet. But I disagree with the parent post's implication that an inverter would provide "quality" power.
Same as in-car mp3 solutions? (Score:4, Informative)
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)
The URL I promised (Score:1)
12VDC-1.5 to 12VDC cigarette lighter adapters (Score:2, Informative)
related stories (Score:2)
Why not use a generator? (Score:3, Interesting)
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 [google.com]).
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.
Re:Why not use a generator? (Score:2)
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.
My advice is: For something small and simple, that takes a single DC voltage at low current, buy one of the better 12 adaptors you see at Radio Shack or Wal-mart.
If it takes more than one voltage, or takes a lot of current, just use an inverter. Designing a DC-DC converter to be a) low noise, b) efficent, c) well-regulated, and d) reliable is a tricky task - there are whole companies that make a very good living selling this sort of thing. The efficency of a modern inverter is very high, and the total loss of going 12VDC -> 120VAC -> 17VDC or whatever your laptop wants is not much - you would save more power by picking the right computer than by trying to fink around with a power supply.
Re:Why not use a generator? (Score:3, Insightful)
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
mp3car.com (Score:2, Informative)
Re:mp3car.com (Score:2)
Look for DC-DC powersupplies. I know Keypower makes one. They are also used in 1U servers.
I have seen serveral Mp3car systems using both.
You can also build your own "sproggy" supply. Look on the forum on that website for help.
It can cost under 50$ to build a supply to meet your needs.
The Boaters have lots of documentation... (Score:3, Insightful)
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.
Re:The Boaters have lots of documentation... (Score:2)
Re:The Boaters have lots of documentation... (Score:2)
I would add a "generally" to that. If your voltage regulator is starting to go you could easily see 10-18VDC. I've seen it happen, and it's pretty freaky, especially when your battery starts inflating like a balloon as the acid starts to boil!
Anyway, one of the boards I worked on at my previous job had a really sweet 12V to 5V convertor+regulator that involved an Op Amp. The board took 12V in and had to supply 5.000VDC +0.003/-0.000 to pin 1 of the PLC. You dialed in this Op Amp circuit to the voltage you wanted (I usually put it at 5.001V) with a trim pot attached to one of the inputs, and it held that voltage within +/-0.0001V no matter what happened on 12V (which ranged from 13.6-11.5V during normal operation).
In all honesty, I never fully understood how the circuit worked, but it was incredibly simple; just an Op Amp and a few resistors (and an assload of filter caps, since it had to sit about 5 inches away from an RF transmitter).
Yes, car power is dirty, but cleaning it up isn't that difficult.
Re:The Boaters have lots of documentation... (Score:2)
Switchers (Score:4, Insightful)
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.
Re:Switchers (Score:2)
But inverters are switching power supplies. And good ones are 80 to 90% efficient.
I do agree it may seem silly to run two switchers in series, but it's a very economical way to do the job with commonly-available (read: you can get the stuff at Wal-Mart) components.
Try amateur radio sites... (Score:4, Insightful)
Suggested keywords: "communications van", "emergency communications", "stormchaser", "RACES", "ARES"
EZ (Score:2, Informative)
Using that IC is a _very_ easy way to get started in an electronics hobby. Try it...
Re:EZ (Score:1)
Re:EZ (Score:1)
Yeah, I know that, but I also know these things are virtually indestructible and are really easy to build. Anyone who can build a switching power supply wouldn't be asking how to change 12V to 5V on slashdot, and I wouldn't reccomend building a switching power supply as a "starter" electronics project.
>The drop-out voltage for this part is always above 1.0V and can even reach 2.5V in some cases. (See the PDF).
From my experience the parts don't actually completely stop supplying voltage. They just stop regulating, and if that means a 1 volt increase to your stuff (its a battery, so there's not going to be a lot of noise on the line), I doubt you need to worry about it.
Not to mention that a Lead-Acid car/marine battery is fatally discharged at about 12.8 Volts.
Re:EZ (Score:2)
True enough. I guess I overlooked that part.
From my experience the parts don't actually completely stop supplying voltage. They just stop regulating, and if that means a 1 volt increase to your stuff (its a battery, so there's not going to be a lot of noise on the line), I doubt you need to worry about it.
True enough, thinking back (ok, way back) I think my first project was a power supply based around 78XX parts and a selector switch. That is a long way from a switching supply.
Replace my wall warts, please!!! (Score:4, Interesting)
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:Replace my wall warts, please!!! (Score:2)
I guess the bottom line would be:
-start on your cheapest equipment and slowly go over to your more high end stuff. And look out for any new/strange behavior.
Re:Replace my wall warts, please!!! (Score:1)
Well doing that would work, but it won't be all that efficient. If you had something that needed 5V at 500mA and you achieve that with a linear regulator you'd be dissipating (24-5)(.5) = 9.5W which is way more than most linear regulators can handle without a huge heat sink and/or fan.
The important thing is to determine what is really required of your devices. The output of a typical wall wart can vary substantially: unregulated AC, unregulated DC, regulated DC. It makes no sense at all to take a regulated bench supply, then step that down with another linear regulator, and feed that to a device which likely has its own voltage regulator (since its wall-wart was just a cheap transformer and unregulated rectifier.)
If your primary goal is to reduce the clutter, then what you mentioned would probably work well, but try to make the main supply voltage as low as possible to still cover everything you need. And don't worry about being a few volts "off" of what is printed on the wall wart. Most electronics step that voltage down anyway. Also, don't measure the open circuit voltage of the wall-wart's output, it will not be a reliable number. They're so cheap that the voltage will drop substantially when loaded.
Anoter option is to get one large power supply with several outputs. It doesn't have to have super regulation. The idea is to just connect whatever output is closest to what the device needs, and don't worry so much about exact voltages. A switching supply might work here, but watch out for poorly filtered outputs, and a lot of cheaper ones have a minimum load below which they either won't operate or will operate with poor regulation. It's easy to get a cheap $20 switcher with gobs of output power available, but you have to be aware of the caveats.
Re:Replace my wall warts, please!!! (Score:2)
Don't bother using linear regulators. They are cheap and all but they perform their down-conversion by basically burning off the extra voltage, i.e., no saving of power. You typically need heat sinks. If you stuffed it into a film canister it would probably slag over time/catch fire. The equation from onomatomania seems reasonably correct, I've never looked at what it really is. I would suspect there's an additional diode voltage drop to account for too.
Any talk of wasting power due to line loss associated with DC is crap because you're only going to be running about 6ft of wire. Cheap 12 gauge automotive wire is more than adequate for most devices.
You would not want to use a "vampire" tap whatever that is. A power distribution bus is much easier and more efficient. The quality of the connection is important. Avoid crimping connectors. Solder everything.
leak power back into the regulator? I'm not sure what that means. you should have diodes protecting from connecting the juice backwards. And some hefty surge suppression since a car's voltage swings a lot.
It's bascially impossible to tell you how to do everything correctly. You need either a book detailing everything or someone to model your system after. Maybe a RV setup or better yet, check out the solar rigs since those off-the-grid people are understandably fanatical about power loss. You won't find your answer here.
t.
What I did... (Score:3, Informative)
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...
Probably not worth the trouble (Score:2, Informative)
In automotive applications 12V is really 13.8VDC when the engine is running and anywhere from 11 to 13VDC when running off the battery.
I have an RV I use as a mobile office with computers and such, too. I just use regular inverters and the generator and have all the equipment running off standard off 120VAC UPS units.
Be aware that some inverters and generators produce a square wave AC output that is bad for some equipment (especially laser printers, I hear).
There are many RVer sites on the web with more info. Check out these sites and their message forums to get started: irv2.com, rvnetwork.com, rv.net.
My Car (Score:1)
The biggest problem will be with monitors. plasma and flat screen monitors are easy. just get a power converter that converts 12v to 9v or whatever the displays use. CRTs need the 120 volts, because they have that huge electro-magnet tucked in there. so, if you can afford it, use flat screens
The cheapest ATX DC to DC Powersupply (Score:2, Informative)
http://www.pcpowercooling.com/products/sleeklin
it is $129. refer to this thread if you need help:
http://www.mp3car.com/vbulletin/showthread.php?
I'm going to assume you're green. (Score:2)
OK, I topped out my own irony-o-meter on that one. But I get pretty tired of the
Anyway, if you want efficiency, and you're handy, check out Home Power Magazine [homepower.com]. The current edition has an article on choosing a laptop for low power consumption.
In your RV, you can use a standard laptop car power adapter, or you can build your own. Both my laptops run on DC (they have transformers and converters built into a pod in the middle of the power cord) and although they use odd voltages (14.5 VDC or something like that) I was able to get a DC/DC converter at the local ham radio store.
I have heard that some laptops do the AC/DC conversion internally, so if you have one of those you'll either have to crack the case open and add a connector or you'll need to sacrifice a battery pack to make a "fake battery" containing a simple voltage regulator circuit or else just a jack that you can connect an external PS to.
If you go the "fake battery" route don't be fazed by all those crazy connection points on your LIon battery pack - all but two of them are usually for charging the battery (to equally distribute the charge to all the cells) so you don't have to hook those up - just the two off by themselves, and you can use a cheap $20 multimeter to find polarity off a good pack.
Oh, and be careful if you dissect a battery pack, and dispose of the cells properly. Supposedly some of the laptop cells can blow up if you short 'em out (not sure if I believe that, but better safe than sorry).
Good luck!
Its really quite easy (Score:1)