running motherboard off uncertain 12V supply - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 12:27:42 pm
content: I have an old PC I've setup. Now i put a laptop hard drive in it, so in effect there is nothing needing 12V - except for the motherboard that is putting it through a SMPS to power the CPU. So could run the motherboard off a non too strict 12V supply, like a car battery ?

author: Psi
author_rank: Super Contributor
timestamp: January 15, 2012, 01:23:22 pm
content: Depends if its in a car at the time?If it's in a car then no, a car battery changes voltage too much. (8-15v)It's possible that it would work for a little while, but i suspect the motherboard won't last very long.It would probably crash often from the noise / spikes / over and undervoltageAlso the motherboard is probably using the +12 for other things.The soundcard maybe using the -12 and +12 for opamps/speakeramp etcOn the other hand, if you mean just connect it to a car battery on its own then it might work.The +12v rail in pc powersupplies can be as low as 10v without it worrying the pc.Its overvoltage that may cause problems.When i encounted this problem i bought a voltage stabialiser.Its basically a SMPS that takes a wide input voltege range and can stepup or stepdown the input to get a regulated output.This is probably overkil for your application but this is the one i used.http://store.mp3car.com/DSX12VD_140W_DC_DC_Regulator_p/pwr-025.htmHowever they do sell other ones on that site. So might be worth a look.EDIT: if your trying to run the PC off nothing but 12V dont forget about the -12v and -5v rails.Some of them will definitly be used for things. soundcard/serial port etc.One of them is optional on newer powersuppliers but i cant remember which it is.

author: im_a_human
author_rank: Regular Contributor
timestamp: January 15, 2012, 01:23:53 pm
content: I wouldnt try it because a fully charged car battery can be much high than 12 volts, it can be as much as 13 or 14 volts even.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 01:34:55 pm
content: well it won't be in a vehicle, if anything it will run off solar panels and the battery will be a buffer.I'm using it to run boinc and was thinking of running it off the panels or a battery charged by the panels. I was thinking of a buck/boost converter. not sure just ideas running through my head

author: sonicj
author_rank: Frequent Contributor
timestamp: January 15, 2012, 02:45:43 pm
content: Google runs their servers directly from 12V batteries. Not sure if those boards are available to the public...-sj

author: ipman
author_rank: Regular Contributor
timestamp: January 15, 2012, 02:52:13 pm
content: It seems that the PS and the battery are connected, making a sort of UPS together.Even solar systems will behave as the electrical system in a car, so will have a voltage between 10.5V (lower cut-off voltage) and 15V as VRLA solid-plated batteries need. That will exceed the ATX specs by far, so do not expect to have a happy motherboard.

author: RJSC
author_rank: Regular Contributor
timestamp: January 15, 2012, 03:05:59 pm
content: I don't think sound cards use negative supplies.In every one I have seen, the is an electrolytic capacitor at the outputs/inputs and when it outputs 0V to the speakers, the DAC is actually outputting half the supply voltage.The capacitor blocks the VCC/2 DC offset and so the signal on the output seems to swing between positive and negative values.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 04:00:56 pm
content: well I could regulate the panels down to say 12.5-13V with the battery in parallel and power the PC. Alternatively, charge the battery by day and run the pc off it by night, with the battery under load it will not be anything more than 12.5V

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 15, 2012, 04:02:42 pm
content: I have a project planned that powers a modern ATX motherboard from a 12V lead acid battery. The low voltage limit (10.5V) is unlikely to be a problem for the motherboard, but could be a problem for the hard drive, which means there will be a boost converter running from the motherboard 12V supplying the 12V rail for the drives. The high voltage limit (14.4V) is definitely a problem, so I will use a buck converter to solve that.

author: Unknown
author_rank: Unknown
timestamp: January 15, 2012, 04:04:27 pm
content: People building PCs in cars have long been running computers without negative voltages, I think RS-232 was the last major user of -12V, and I don't think -5V has been used since the disappearance of the ISA bus from most systems.You may run into trouble running the 12V rail far out of spec, since the DC-DC converter was likely not designed for 10V or 15V. I would search the Car PC forums for experiences. Most modern computers draw the majority of power from 12V, and only use 5V and 3.3V for the PCI bus and for some small consumers (eg. legacy I/O). Older (especially pre-P4, when the extra 12V connector was introduced) tend to draw more from 5V and 3.3V.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 04:24:06 pm
content: this is a motherboard with an athlonXP 1600 running PC133 ram I'm not sure what the voltage distribution is, I'll probably put my clamp meter on each rail and see what is being drawn

author: mariush
author_rank: Super Contributor
timestamp: January 15, 2012, 04:31:11 pm
content: You can buy one of those mini power supplies that plug into the connector - some of those can accept anything between 8 and 30v to generate the 3.3, 5 and 12v for the PC.There's a review of several models here :http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=207Just pay attention to the fact that the ones with variable voltage input are a bit on the low side on 12v, they only do 3-6A on it... and they need the voltage to be around 13-15v to get the max amps - anything below or over starts decreasing the amps.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 04:35:48 pm
content: sounds like a plan, I can easily produce 5 and 3.3V with off the shelf parts the 12V is trickier as it's a buck/boost situation

author: mariush
author_rank: Super Contributor
timestamp: January 15, 2012, 05:05:06 pm
content: That one sounds like it's heavy on the 5v, less on 12.. it probably doesn't even have that 4pin cpu connector. You'll still have to regulate the 12v line somehow, for the hard drive's motor.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 05:29:58 pm
content: no there is no 4 pin cpu power connector and it is a 20 pin motherbord connector. As said before I'm using a laptop hard drive so it runs on 5V, I'm not really sure if the 12V rail is doing much. looks like I need to check each rail.Te one thing that I don't like about those pico PSU's is that they don't give the amperages on each rail. then again if not a lot of 12V is needed i can get away with my own. I must admit I'm suspicious of the 96% efficiency claim

author: mariush
author_rank: Super Contributor
timestamp: January 15, 2012, 06:09:31 pm
content: They do give it, just have to look harder for them .. the review I linked to lists the specs ... just look between the pictures:http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=207It's about 6 amps on each rail.If it's a 20pin connector, that system is basically guaranteed not to use more than 80-100 watts. Just not sure 6 amps on 5v will be enough, you'll have to check.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 06:11:47 pm
content: yes well i went through the whole range on their website and their product details did not show the amperages. They just list the type of computer that can be powered. Not much help to me.

author: LaurenceW
author_rank: Frequent Contributor
timestamp: January 15, 2012, 06:38:53 pm
content: Be aware that if you have an accident with a PC SMPS power supply and short the 12V rail, it may drive 20-30A for a fraction of a second, and then the power supply will shut down.Short out a car battery, and you are looking at 100-300 amps, a new wiring harness, a possibly new motherboard, and almost certainly a fresh pair of underwear.Newer motherboards tend to generate the 1-2V CPU voltages on board via SMPS suppliesfrom the 12V line, not 5V,so the 12V current might be greater than you think. My own experience of (one of) the PICO power supplies is that they only pass through the 12V, being no more than 5V/3.3V SMPS units, themselves.

author: mariush
author_rank: Super Contributor
timestamp: January 15, 2012, 08:02:18 pm
content: QuoteHis system is one with PC133 ram and a single 20pin power connector - the ATX standard allows a maximum of about 75w (maybe less, i'm not sure) on the 12v through such connector because it's a single 12v wire. The standard does allow up to 24A on 5v through at least 3 wires in the connector, which is probably what that board concentrates on.QuoteThat's correct, if you read the review I already linked to a couple of times, the power supplies with fixed input voltage only pass through the 12v. There are however such picoPSUs with variable input (also in that review) which will filter and regulate (by the nature of the psu design) the 12v line as well.Honestly now that I thought more about it, I really don't think it would be worth going this route, with picoPSUs. They're expensive at about 60$ and they're most likely not going to be enough...I guess they would work with these mini-itx motherboards and processors like the new AMD Brazos and Intel Atoms - they have a tdp of 45w and on average the whole system uses less than 40 watts:http://www.anandtech.com/show/4134/the-brazos-review-amds-e350-supplants-ion-for-miniitx/7It might be cheaper recycling an old 200-250w power supply and buy a small inverter:http://www.amazon.co.uk/Vehicle-Caravan-Inverter-Output-Adapter/dp/B0035VZH9A/ref=sr_1_1?ie=UTF8&qid=1326657387&sr=8-1http://www.amazon.co.uk/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=12v+inverter&x=0&y=0And you get the benefit of having usb ports for phones built into the inverter and maybe a second plug for something else... but I guess you can open it up and change the socket or add an extension or several sockets...Could use it with a car battery or a row of batteries that you charge separately using solar power.But if you really want to have as much life from the batteries, you should get a psu with high efficiency - old 200-250w are barely 75% efficient.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2012, 08:57:13 pm
content: If you want to build your own regulated 12V supply you need a SEPIC converter or one which can either work in buck or boost mode. Various ICs are available from National Semiconductir, Maxim, Linear Technology etc.

author: Simon
author_rank: Unknown
timestamp: January 15, 2012, 08:59:18 pm
content: Quote from: Hero999 on January 15, 2012, 08:57:13 pmI think that is best, I don't really want to use an inverter and psu, it will be horribly innefficient, I'd thought of that already

author: Psi
author_rank: Super Contributor
timestamp: January 15, 2012, 11:24:39 pm
content: Quote from: Simon on January 15, 2012, 05:29:58 pmTo get that efficiency they use synchronous rectification on top of a very efficient SMPS core.But you can expect their value to be for optimal conditions. I'd expect 90%+ for real world usage.My DSX12VD is pretty damn close to its rated >95%Even at the full 12A its still pretty good.

author: mobbarley
author_rank: Regular Contributor
timestamp: January 16, 2012, 12:03:58 am
content: if you are talking about this picopsu:I'm pretty sure they only pass-through 12v and regulate lower voltages as they are designed to be run from a regulated 12v supply.

author: mobbarley
author_rank: Regular Contributor
timestamp: January 16, 2012, 12:05:09 am
content: crap didn't notice the second page

author: Rufus
author_rank: Super Contributor
timestamp: January 16, 2012, 01:28:16 am
content: Quote from: Simon on January 15, 2012, 04:00:56 pmOr you could just power it from mains which will cost less than the battery.

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 16, 2012, 02:39:15 am
content: Back when your CPU had a small/passive/no heatsink due its low power consumption, most boards used the 5v or 3.3v rails. Newer PCs which require far more power use 12V and switching circuitry on the board to generate the required voltages so the current through the power wires is greatly reduced.As for whether your board needs negative voltage, most if not all <5yro ones do not, but some older stuff does. Simply remove these wires from a normal PSU and see if it fires uphttp://www.goodluckbuy.com/24pin-car-dc-dc-atx-mini-itx-pc-power-supply-12v-150w.html<--- this one lists current ability and also mentions +/-5% regulation on the 12v implying is does regulate the 12v input? Also specifically mentions its for Car use.There's also wide voltage range input boards that look a lot more beefier ($50+ though):http://www.goodluckbuy.com/160w-8v-28v-mini-itx-m2-car-pc-atx-psu-power-with-itps.htmlhttp://www.goodluckbuy.com/210w-8v-28v-mini-itx-car-pc-atx-psu-power-with-itps.htmlhttp://www.goodluckbuy.com/8-28v-160w-dc-dc-dc-atx-atx-itx-car-pc-power-supply-module.htmlhttp://www.goodluckbuy.com/8-28v-200w-dc-dc-psu-itx-atx-dip-car-pc-power-supply-module.html

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2012, 02:52:26 am
content: The current needed at -5V/-12V is probably pretty small.You can get small dc/dc modules for $5-15 from digikey that have isolated outputs.So producing -5V/-12V at low current from a positive supply is simple/cheap.Quote from: metalphreak on January 16, 2012, 02:39:15 amYeah, i blew up one of the driver transistors on my 10year old diamond monster sound MX300 by doing that.I think without the negative supply the driver switched hard on in one direction and overheated.Fixed it though, new transistor and it was all good.

author: benemorius
author_rank: Regular Contributor
timestamp: January 16, 2012, 03:27:12 am
content: Years ago I ran an athlon 1700 system in a car with the 12v rail unregulated. I was retiring it and I wanted to find out whether I had been going to a lot of trouble regulating the 12v rail for nothing. It was a short test since I still didn't really want to lose anything (it was not old hardware back then) but it did work. I think it even survived cranking voltage which was probably under 9v.If I were you I'd give it a try. Everyone knows what the outcome will be if you go with a regulated solution, but there is something to be learned if you attempt it without regulation.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 16, 2012, 04:44:32 am
content: Quote from: Psi on January 16, 2012, 02:52:26 amEven easier is to run a discrete charge pump from one of the main buck converters.

author: Simon
author_rank: Unknown
timestamp: January 16, 2012, 06:58:08 am
content: Quote from: Rufus on January 16, 2012, 01:28:16 amMy idea was to use it for Boinc crunching but make it eco friendly as I have the panels but not yet done anything with them

author: Rufus
author_rank: Super Contributor
timestamp: January 16, 2012, 09:50:32 am
content: Quote from: Simon on January 16, 2012, 06:58:08 amHow eco friendly is manufacturing a big lead acid battery which gets shipped halfway round the world and will be shipped back to some third world shithole for recycling in a couple of years when it is knackered?

author: Rerouter
author_rank: Super Contributor
timestamp: January 16, 2012, 10:12:14 am
content: alternativly you could abuse a charge controller, and just run the computer direcly off the panels and a large capacitor, when the voltage exceeds a level charge controller snaps on, and computer can power up, when it gets too low, you can make something to shut it down early, e.g. one shot comparitor, and just have the capacitors serve to smooth out things like planes and etc,or if using one of those wide input supplies, you could probably run straight off the panels, and use a similar one shot comparitor thing to turn it on and off,just thoughts that i myself have motioned through in the past

author: mobbarley
author_rank: Regular Contributor
timestamp: January 16, 2012, 11:13:16 am
content: Quote from: Rerouter on January 16, 2012, 10:12:14 amEven several large 1F 12v capacitors for car audio would only buy you a couple of seconds before the voltage sagged.

author: im_a_human
author_rank: Regular Contributor
timestamp: January 16, 2012, 11:46:03 am
content: Quote from: Simon on January 15, 2012, 12:27:42 pmI could be wrong but something like this might work...http://uk.rs-online.com/web/p/linear-regulator/5455685/

author: Simon
author_rank: Unknown
timestamp: January 16, 2012, 12:39:26 pm
content: Quote from: Rufus on January 16, 2012, 09:50:32 ambut i already have the battery, would be a shame for it to sit there and go to waste

author: Rufus
author_rank: Super Contributor
timestamp: January 16, 2012, 01:04:54 pm
content: Quote from: Simon on January 16, 2012, 12:39:26 pmThen sell it to someone who can do something useful with it.

author: Simon
author_rank: Unknown
timestamp: January 16, 2012, 01:48:25 pm
content: Quote from: Rufus on January 16, 2012, 01:04:54 pmi use it as a general battery to have handy. like when i need to run 12V devices when i'm away

author: Rufus
author_rank: Super Contributor
timestamp: January 16, 2012, 02:52:14 pm
content: Quote from: Simon on January 16, 2012, 01:48:25 pmAnd if you deep cycle it daily to run this thing it will be knackered in 18 months and you will have to buy a new one costing more than the mains electricity it saved.

author: Simon
author_rank: Unknown
timestamp: January 16, 2012, 05:43:43 pm
content: this is getting booooorrring.OK forget the battery, that is secondary, the PSU is more important. As I said previously the other option is to charge the battery by day and then use it by night

author: Tooms
author_rank: Unknown
timestamp: January 16, 2012, 05:46:47 pm
content: I find the PicoPSU to be very good and i am in the process of building a home server setup where i am trying to get the complet watt very low and still have very good performance on the system.Part1:http://www.tooms.dk/Tblog/Showblog1.asp?BlogID=201110091828348325Part2:http://www.tooms.dk/Tblog/Showblog1.asp?BlogID=201110252324515723Part3:http://www.tooms.dk/Tblog/Showblog1.asp?BlogID=201111082030033196Tooms