Questions about ATX bench power supply build - Page 1
Forum section: electronics / beginners

author: dennisyuki
author_rank: Contributor
timestamp: January 04, 2013, 02:06:24 pm
content: I am in the process of building a bench power supply out of an ATX power supply. I am basing the build from this video.http://youtu.be/-zRFwJdTBdwI really like how he made it detachable and used the light bulb as the dummy load.I have 2 questions.1. why do atx power supplies need some type of dummy load in it when making a bench top power supply?2. What happens if no dummy load is added to the system?3. What is the purpose of -3.3 VDC, -5VDC and -12VDC? How would I use those rails on a bench power supply with electrical projects? or would I?Cheers!

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2013, 02:24:46 pm
content: Quote from: dennisyuki on January 04, 2013, 02:06:24 pmThey don't all. It depends on the design, and modern supplies just don't need it.QuoteRegulation will be shockingly out of whack (high +5V, low +12V).Quote-12V is used for things like RS232. -5V hasn't existed since 2003, and -3.3V never existed. Neither -12V nor -5V if you have it provides significant current (usually specced to 500mA, many supplies, especially the type you're likely to use, wouldn't be safe for 50mA).Butchering an ATX supply really isn't the way to go.

author: Kibi
author_rank: Unknown
timestamp: January 04, 2013, 02:49:13 pm
content: I don't like these ATX PSU mods, but if you are going to do it, Ian decribes a good way of doing it which doesn't involve butchering or mucking about inside the ATX PSU. His method also employs resettable fuses to avoid cremating your projects, something which can easily happen when you have 20+ amps available to pump into an otherwise harmless short circuit on a MCU board.

author: dennisyuki
author_rank: Contributor
timestamp: January 04, 2013, 03:31:06 pm
content: I am using an ATX that has 20 pin motherboard connector. I think this power supply is late 90s-early 00. I will have to look at the production date when I go home.I think I will leave the -5v off.The video I posted is a homemade version of the dangerous proto types board. I like the Dangerous prototype board. thanks for the recommendation.How do I know if my power supply needs the dummy load on the 5volts? Ian did not mention a dummy on the 12 volts.I like the resettable fuse option on the Dangerous board. Anyone have recommendations for a through hole version of that resettable fuse?the load in the video I posted used a car taillight as the load resistor. that is ingenuous, but produces a lot of heat. would using a load resistor, as in ian's video (7v, 10ohm resistor), be better. Will if give off alot of heat too?

author: mariush
author_rank: Super Contributor
timestamp: January 04, 2013, 05:05:30 pm
content: Only the old power supplies need some load on 5v to regulate properly.You can usually tell there's not enough load on 5v when the voltage on 12v oscillates or is pretty low, at around 11.5-11.7v, or in some cases it's way above 12v.Most power supplies that need some minimum loads already have some resistors near the cables which put a small load on the 5v and 12v and 3.3v, usually around 0.1-0.3A. But some old designs need just a bit more on 5v to work right, usually about 1-3 A of load.Like everyone else said already, the -5v and -12v are very low current rails, usually below 0.5 A. -12v is still used for rs232 (Serial ports) and it was still present in the PCI slot (but few cards actually used it). I've seen modern quality power supplies that can provide as low as 0.1 A on -12v according to their label.-5v was last used in ISA slots... I don't know what else was using it in computers. It was removed from the power supply standard years ago.So the only rails you should rely on are the 3.3v, 5v and 12v.

author: ve7xen
author_rank: Super Contributor
timestamp: January 04, 2013, 06:55:46 pm
content: Quote from: dennisyuki on January 04, 2013, 03:31:06 pmYes. The whole point of the load is to waste enough energy that the PSU regulates properly. The only cheap, easy and safe way to do that is to turn it into heat (which is going to be the end result anyway...). Tail light bulbs are typically ~5W, which is about double what you'd get with a 10R resistor on the 5V line (2.5W). The resistor is probably cheaper and easier to hook up too, and doesn't give off a lot of useless light either.I don't like these builds either, I'd avoid using a supply without current limiting for a project. These ATX supplies are great for what they are, but they are capable of far too much power/current output for general lab use. If you want a 'zero budget' supply for hobby projects I'd rather see you use some old wall warts in a case as they're not capable of this sort of current output which can easily destroy boards and explode parts.

author: dennisyuki
author_rank: Contributor
timestamp: January 04, 2013, 08:18:02 pm
content: That is all some great information.I am just creating basic electrical designs and nothing too technical. Therefore my power specs are not demanding yet.So I think I will even leave off the -12V line, use a resistor instead of the light bulb, and figure a way to incorporate a resettable fuse to the 5v and 12 v rails.I do understand the negatives to converting atx PS to benchtop supplies, but this is hands on learning for a newbie project.

author: ptricks
author_rank: Frequent Contributor
timestamp: January 04, 2013, 09:31:57 pm
content: If you are just starting out an ATX power supply is about the worst thing you could use because of all the issues it brings with it.First there is the safety issue, unless you are working with high power equipment, you don't need any voltages over 3A most of the time. Connecting a 20A+ supply to something like a micro powered project using 300ma can be a costly mistake. The other reason not to do it is switching supplies can produce a fair bit of noise in the power itself, and that isn't something you want to have when starting out because it can cause things like an MCU to reset itself and you will spend hours trying to figure out why. For prototyping stuff and projects you really should try to use linear power supplies , the output is nearly perfect DC power and the cost isn't anything huge.Batteries. Yes , the best possible DC power source and often overlooked by many. Yes they can run down, and the alkaline type are throw away items, but lithium rechargeable are really affordable and chargers are too. Powering a circuit from a lithium battery pack is a lot safer, easier, and a cleaner form of power than anything you can power off the AC mains. You can power things like arduino off a lithium pack for hours and hours without needing a charge.The only thing I recommend ATX supplies for is people doing things like car audio repairs, just about everything else it is better to go with small linear DC supplies.