Custom 12V-only power supply for an old PC? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: August 08, 2014, 07:58:00 pm
content: Hi.I got an old PC (Celeron 366MHz, Win98) motherboard that i want to put into a neat box (DOS games just don't feel good in an emulator).I also have a similar vintage PC PSU, the size of half the motherboard.And i want to get rid of it, to make the whole system run off a 12V supply/battery .The MB got a P8/P9 power connector, fairly straightforward pinout.Question is, what are the specs needed for it to run?-Does it need some special low-ripple supply, or would your typical off-the-shelf 1% ripple DC-DC converters work?-How much current should i expect/design for?I tried to measure it, with uncertain results:+12V: 0.22A-12V: 0.02A, boots without it, but i suppose something would not work?+5V: 3.7A, would not boot with ammetre in there. That is the uncertain part-5V: 0A, boots without it.Disk (CF-to-IDE converter):5V: 0.2A max, mostly near 0.12V: 0ATotal: 18W on 5V, 3.6W 12V, 1W disk20-30W, mostly 5V?So - 14.4V battery, LDO for 12V, a 30W DC-DC converter for 5V, isolated 12V-to-12V 1W converter to get -12V, and ignore the -5V?

author: madires
author_rank: Super Contributor
timestamp: August 08, 2014, 08:06:43 pm
content: Quote from: Artlav on August 08, 2014, 07:58:00 pmThe -5V are part of the ISA specification, but if you got no ISA card requiring -5V simply ignore them.

author: daveshah
author_rank: Unknown
timestamp: August 08, 2014, 08:10:26 pm
content: Chances are, -12V is only for the RS232 serial port, so you might be able to get away without it if you don't need the serial port.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 08, 2014, 08:11:17 pm
content: Quote from: madires on August 08, 2014, 08:06:43 pmThe sound card is an ISA one.Does that count?I suppose better safe than rebuild.How much current can i expect on it?Can't google up any numbers...

author: madires
author_rank: Super Contributor
timestamp: August 08, 2014, 08:21:15 pm
content: Quote from: Artlav on August 08, 2014, 08:11:17 pmPlease seehttp://arstech.com/install/cms-display/ste_pwrsupplyneeds.htmland check your sound card.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 08, 2014, 08:32:32 pm
content: One more thing:The +5V on the P8 side (called VSB?) appears to be isolated from the +5V on the P9 side, both on the supply and on the board.The VSB one draws a little bit of current, and the system won't start without it.Should i provide isolated +5V to that pin?Quote from: madires on August 08, 2014, 08:21:15 pmAh, nice. Thank you.Nope, no -5V pin on the sound card.And no floppy drive present or expected.So, safe to ignore.

author: jeremy
author_rank: Super Contributor
timestamp: August 08, 2014, 10:17:49 pm
content: There are lots of these on the market:http://www.aliexpress.com/item/DC-12V-Pico-ATX-switch-PSU-Car-Auto-MINI-ITX-ATX-Power-Supply-120W/836368113.html12V in -> everything out. Perhaps this might be useful for you?Also: VSB is 5V for standby. Technically it should always be on as long as the power supply is plugged in, it doesn't matter if the computer is on or not.

author: madires
author_rank: Super Contributor
timestamp: August 09, 2014, 03:28:24 pm
content: Quote from: Artlav on August 08, 2014, 08:32:32 pmAT PSUs don't provide a standby voltage and +5V should be the same rail for both connectors. But there's an orange wire for the "power good" signal which could be isolated from the +5V rail. Cheap PSUs simply wire that signal to the +5V rail.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 09, 2014, 06:13:35 pm
content: You can use a charge pump to get the -12V, driven off the switching node on the buck converter.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 09, 2014, 07:20:23 pm
content: One more snag i ran into.The IDE.I have a CF-to-IDE converter in there, that takes a floppy power connector.It only needs 5V.So, then i wired everything up today for a proof-of-concept with a couple of universal DCDC converters everything worked except for the card.Whatever i tired, the thing won't initialize - it showed SM223En-aa as the device name, and was 33MB (it's a 16Gb card).Isolated 5V? Nope.Smoothed and filtered to the limit? Nope.A wire coming from the original PSU Bingo!What is the difference.... Huh? None. +5V, no ripple, no nothing.Eventually it developed that the IDE converter should be poweredbeforeorafterthe rest of the system, even half-a-second delay either way suffices.With wire from original PSU i invoked that rule accidentally, being unable to turn both switches quite at the same time.So, the question is- is there some sort of a power-up sequence required from an AT power supply?Does the disk connectors get a delay start normally (or other way around)?Final data on the max power consumption i measured, across all use modes:+12V: 0.29A, with monitor - 0.66A-12V: 0.02A+ 5V: 5.5A- 5V: 0ATotal: 8W on 12V, 28W on the 5V.So, 40W converter for 5V.Quote from: jeremy on August 08, 2014, 10:17:49 pmNice, exactly the thing i'm making.Nope, no use for me in this case - it would just kill all the fun.But i'll keep it in mind for future projects, thanks.Quote from: madires on August 09, 2014, 03:28:24 pmI connected the VSB wire to the rest of the +5V, it works fine like this.The power good is connected over a 1k resistor (exactly what i measured between it and the VSB on the original PSU), also not a problem.Quote from: NiHaoMike on August 09, 2014, 06:13:35 pmI had a couple of P6LU isolated 12V to 12V converters (of which existence as a class i'm aware thanks to EEVblog #324), and it fit the task perfectly.I doubt i can make a smaller charge pump inverter myself.

author: miguelvp
author_rank: Super Contributor
timestamp: August 09, 2014, 10:05:32 pm
content: While looking at what this ebay seller has in his store. Tons of things btw, I came up with this DC 12V Pico ATX switch PSU Car Auto MINI ITX ATX Power Supply 160W$20 with shipping.http://www.ebay.com/itm/DC-12V-Pico-ATX-switch-PSU-Car-Auto-MINI-ITX-ATX-Power-Supply-160W-/181161040523Input voltage : +12 V / DC (11.4V-12.6V)Input Current: Max. 17AOutput ATX: 160WOutput Current +12V: 8AOutput Current +5V: 8AOutput Current +3.3V: 8AOutput Current +5VSB: 1.5AOutput Current -12V: 0.1A