Can I convert an ATX PSU to run directly off of 12v? - Page 1
Forum section: electronics / projects

author: MarkS
author_rank: Unknown
timestamp: October 31, 2022, 11:04:46 am
content: I'm a truck driver. I want to build a computer for my truck. While I do have an inverter, it's only a 2000 watt and my company will not allow it to be upgraded or a second one to be added. As I have other things running off of the inverter, and as I've had issues with digital electronics connected to an inverter, I'd like to bypass it entirely, if possible. The truck has an auxiliary power unit (APU), so even when the main engine is off, the batteries are being charged and there is never a drop in voltage unless something goes wrong.What I'm thinking is to bypass the mains circuitry, remove the rectifier, and connect the 12-14.1v battery voltage directly into what would normally be the rectified output. Is this even a possibility? Do modern PSUs even use a rectifier as such? I haven't opened a PSU in many, many years, and something tells me gone are the days of a big transformer and solid state rectifier in place of something more complex and clever for efficiency's sake.Is this even worth the trouble to investigate?

author: ledtester
author_rank: Super Contributor
timestamp: October 31, 2022, 11:15:38 am
content: You might look into this product:https://www.mini-box.com/M4-ATXand their other DC-DC supplies:https://www.mini-box.com/DC-DCThey also sell complete "car pc" systems.

author: MarkS
author_rank: Unknown
timestamp: October 31, 2022, 11:27:27 am
content: Quote from: ledtester on October 31, 2022, 11:15:38 amI've looked at those PSUs, but they are always very low power. Also, the system I want to build is a Ryzen 9 with a RX 6950 XT GPU. No "car computer" will come close and part of the fun is in the building of the system.

author: Ian.M
author_rank: Super Contributor
timestamp: October 31, 2022, 11:37:14 am
content: Quote from: MarkS on October 31, 2022, 11:04:46 amNo. The PC ATX PSU's multiple output voltages come from different secondaries on a high frequency switched mode transformer, and the primary side and its associated switched mode controller and output stage is designed to work from a HV DC bus, typically up around 350V for a universal input PSU with PFC. You'd have to replace the whole primary side circuit, rewind the transformer and probably make substantial changes to the secondary side supervisory circuits. Get anything wrong and the most probable consequence is letting the magic smoke out of your motherboard and *everything* connected to it.OTOH you don't need to as 12V input ATX PSUs are available to buy 'off the shelf', e.g. the minibox.com ones Ledtester suggested, orhttps://www.powerstream.com/DC-PC-12V-1000.htm(the first half-way reputable one I googled)There are also a whole bunch on Amazon:https://www.amazon.com/s?k=12v+DC+input+ATX+PSUranging from simple non-isolated multi-output DC-DC converter mini-boards that mount by plugging straight into the motherboard, to full featured industrial grade DC input ATX PSUs with conventional enclosure and mountings. I wouldn't recommend the mini-board style in a vehicle application with an APU as they provide very little protection against any spikes on the vehicle 12V supply, so should only be connected to it while the engine and APU are both off.On the gripping hand, if the company is that restrictive about how you can customize your rig, you may well be better off with a laptop + a 12V PSU (charger) for it. Many laptops take 19V DC in (or near that voltage) and 12V input PSUs for them are readily available.

author: MarkS
author_rank: Unknown
timestamp: October 31, 2022, 11:49:28 am
content: Quote from: Ian.M on October 31, 2022, 11:37:14 amThank you! That's what I suspected.

author: magic
author_rank: Super Contributor
timestamp: October 31, 2022, 05:00:44 pm
content: Actually, modern computers consume very little of 5V and 3.3V and there are ATX PSUs which only have a 12V secondary and all other voltages are derived from that by buck converters. Unfortunately, I haven't been looking for a new PSU in several years so I'm completely out of the loop wrt what's available on the market today.I anticipate a bigger problem: electric installations in cars are infamously poorly regulated and full of nasty transients (from switching off of inductive loads like solenoids and motors, for example). A computer probably couldn't be fed that filth directly, with no filtering and regulation.IIRC, the ATX spec says: 12V ±0.6V and maybe 100mV peak to peak ripple.

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2022, 05:11:30 pm
content: Quote from: MarkS on October 31, 2022, 11:27:27 amYou are going to need abigger boat, but it is going to cost.As said in a previous post, internally the power supply runs off of a roughly 340 volts DC which is produced by the rectifier stage or power factor correction stage. Stepping 12 volts up to 340 volts is certainly possible, but not trivial.

author: MarkS
author_rank: Unknown
timestamp: November 02, 2022, 03:21:16 pm
content: Thank you for the replies. I'm glad I asked and you guys probably saved my life. I was expecting a 120v to xx step down, not a HV step up. I would have avoided the caps while probing around and 120v AC isn't lethal, but I would never have guessed HV.

author: Monkeh
author_rank: Super Contributor
timestamp: November 02, 2022, 03:29:12 pm
content: Quote from: MarkS on November 02, 2022, 03:21:16 pm...Yes. Yes it is.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 03, 2022, 08:23:23 pm
content: Well, I'm not a gamer nor have ever built a gaming pc, but thehttps://www.amd.com/pt/products/graphics/amd-radeon-rx-6950-xtwebsite suggests a 850W PSU, which have this as output:https://m.media-amazon.com/images/I/91lotulM6rL._AC_SL1500_.jpg3V3 @ 25A5V @ 25A12V @ 70.8AYou say it would run on a truck, I believe its 28V (well, 24V, no engine running) could use off the shelf buck, brick, converters and make your on ATX.CUI, GE Critical powerI'd take a look for FPGA buck converters, they'd have similar requirements IMHO.https://www.digikey.com/en/products/filter/dc-dc-converters/922?s=N4IgjCBcpmBMAGKoDGUBmBDANgZwKYA0IA9lANrgJwCssIxAnABxg0AsDVA7AGzvMuvRrzABmMVzAJ2jOLykI6MqfDCiu7Pu16TisGs2ad94uCNWN24qe0SypYxmGYL9Ttm-AfeNW8xE9cC0JLzAQ8M1WYRNwOhoxblsWGj99DnZEqUMxXWzGRgQIfVFGbiSS0XMpZjFZIJdYbkF9I3UW8EK4bjga53Y08ADxYs6wPgbGMWZEKREtDrhwp1ilml9V8ebGLiXC1N3EZgQd4jhE7kzd62Nes8yN694Z05A4HXNX2l5RUfkBlRnbi5ehA5ocXbMbhKQQAXWIAAcAC5QEAAZSRACcAJYAOwA5iAAL7pWjIEBoSBYPBEUgUcCXAzXZzcUZQ1lwxEoyDorF4wkkt5LCDQCkYHAEYhkSCUCDwkDI1EASVxSPw%2BPwmOJxAAtL1RZSsQBXWnSyiDHawomCvyihFQMCI%2B2QWiChSi7EAE1ROukdwV3JAXCRAE8EfhUZ7cGhrUAhttps://www.digikey.com/en/products/detail/cui-inc/VHB350-D24-S12/2714032https://www.digikey.com/en/products/detail/cui-inc/VHB350-D48-S12/2714037

author: Haenk
author_rank: Super Contributor
timestamp: November 04, 2022, 10:46:42 am
content: May I suggest just using one of those "fat" powerbanks (including mains output)?Charge it up while driving, and maybe use a slightly lower PC configuration, so the powerbank lasts for a couple of hours.Unfortunately, those huge powerbricks are not cheap.Another idea: Use a remote computer/gaming service - that will only require a mediocre device basically displaying streamed graphics output, however a good and stable internet connect is a must.