Need some help with ATX psu repair! - Page 1
Forum section: electronics / repair

author: baitz
author_rank: Newbie
timestamp: February 03, 2012, 01:23:07 pm
content: Greetings!So I plan to build a cloud chamber using a peltier cooler and an atx psu. My problem is that the psu I have available is, as of recently, dead. I now need some help with repair. All the outputs are at 0V, standby and power_on are at around 5V and when I short power_on to gnd nothing changes. I'm very much a beginner so is repair even viable for me, since I don't know where to start.Before we go further I want to be sure the psu is even usable for my project. My cooler operates at 24V, so I was planing to use the two 12V rails on my psu in series. Would that work? They both provide enough current.Another unrelated question:Doing some initial testing of my psu I got a really nasty shock just from touching the plug. I measured the voltage on it and it was around 230V (mains) and dropping. This was all with the switch off. I then noticed a cap that was wired directly on the power cord connector. Is this standard, safe?Thank you for any help!

author: Bored@Work
author_rank: Super Contributor
timestamp: February 03, 2012, 01:32:25 pm
content: The cap is standard and it is reasonable save - if the PSU is build according to regulations. However many PSU's aren't build according to regulations. Our Chinese friends give a fucking fart if they kill someone or not.These PSUs work by rectifying mains. If you happen to live in a 240V area the result of the rectification is a 400V DC, charging a cap. If the cap has no bleeding resistor or it is broken the deadly 400V DC can be around for a long time. This is why it is in general recommended that laymen should not open these PSUs. They can kill you.Second, these PSUs are shit for everything but powering PCs. They are just capable of doing this, and are build with the idea in mind to save every cent possible. Used in other ways they are great fire starters.My recommendation: Throw it away and get a reasonable PSU for your project.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: February 03, 2012, 01:34:25 pm
content: Trash it. Obviously, you don't have reasonable knowledge on electronics, and poking around in such power supplies can get you killed...- no, it's not normal to get nasty bite from an unplugged device,- no, you cannot connect the two 12V ouputs in series, you'd short circuit one of the two.

author: Rerouter
author_rank: Super Contributor
timestamp: February 03, 2012, 01:40:19 pm
content: the capacitor on the output is a normal thing yes, however it should rapidly discharge itselfas for the power supply, was it working previously?, for unknown reasons some models of power supplier will not wake up with shorting those 2 pins, (maybe uses the 3.3V sense line as a safety check) though rare in my own experiences,as for the 12V rails, mainly its just a marketing term and they actually end up tied together on the same regulator 90% of the time, so no for series, so yeah, like the others have said, this wont do, there is a way to make an ATX supply float, but really, you wouldnt want to,

author: baitz
author_rank: Newbie
timestamp: February 03, 2012, 01:55:05 pm
content: Yeah the shock I got was before I opened the thing. China I guess....The psu was in my pc up until a week ago when it died and this opportunity arose.Are there cheap alternatives for my requirements?

author: Bored@Work
author_rank: Super Contributor
timestamp: February 03, 2012, 02:19:46 pm
content: Quote from: baitz on February 03, 2012, 01:55:05 pmYou haven't really stated your requirements. So, for now just google "Meanwell 24V" for a start.

author: baitz
author_rank: Newbie
timestamp: February 03, 2012, 02:50:56 pm
content: Yeah true sorry!Datasheet for my coolerI need to power one of those. So I guess that means 24V with 13A output current. I checked your recomendation and it's a bit out of my price range at the moment.

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2012, 03:16:41 pm
content: If you are running a 24V cooler and really only need it to run for an hour at most then try 2 12V 7Ah SLA batteries in series. When flat you just recharge them. The batteries are cheap, and a charger is as well. If you grab a old burglar alarm control panel you will find a charger inside it, and then you can recharge the 2 batteries by putting them in parallel across the charger. Another source is a old UPS which probably has one battery inside already. Just extend the battery leads to outside the case amd put 2 sets of spade connectors on then to charge the batteries overnight. Then a simple jumper lead to run the cooler. Hopefully the heatsink on the hot side is both big and has a fan to help cool it.

author: baitz
author_rank: Newbie
timestamp: February 05, 2012, 12:20:32 am
content: Thanks for the idea Sean but the whole idea of this project was "ease of use". Otherwise I would just cool the chamber with dry ice.What about a linear supply. Easy build for beginner? I see there's an active thread about that.Thanks guys!

author: Unknown
author_rank: Unknown
timestamp: February 05, 2012, 01:14:16 am
content: Quote from: baitz on February 05, 2012, 12:20:32 amA constant voltage linear supply is fairly easy to build, though less so for 13A. It's going to be more expensive than a commercial power supply, especially a switching one. Transformer, buffer capacitance, power transistors and heat sink are going to be expensive.

author: Randall W. Lott
author_rank: Regular Contributor
timestamp: March 05, 2012, 08:18:42 am
content: Only certain capacitors are rated to be placed across mains safely.The datasheet states that for reliability, it should operate at <.7 IMax. That's about 9A or less. It's best to not play with dangerous circuits until you gain enough knowledge. We've all gotten zapped in our time, but we don't want you to get hurt when it could be prevented. Just get a proper, commercial power supply.Sadly, these Chinese companies would rather save a cent on a bleeder resistor, than to create a safer product. Be careful.

author: nukie
author_rank: Frequent Contributor
timestamp: March 05, 2012, 11:53:47 pm
content: Quote from: baitz on February 03, 2012, 02:50:56 pmHow much is your budget? Meanwell prices varies across regions and there are a lot of fakes.http://www.sureelectronics.net/goods.php?id=1401If that's above your budget, hop on ebay, look for industrial power supply brands such as Omron, Lambda, TDK, Mitsubishi, Allen Bradley, Idec, Cosel. Hopefully you will find a used item that will fit your budget.

author: T4P
author_rank: Super Contributor
timestamp: March 06, 2012, 08:43:02 am
content: Quote from: nukie on March 05, 2012, 11:53:47 pmTDK and Lambda ... Is TDK Lambda ...

author: nukie
author_rank: Frequent Contributor
timestamp: March 06, 2012, 11:58:52 am
content: I am talking about older Lambda PSUs before TDK bought Lambda, so it will suit the budget. Buying a TDK-Lambda would cost him an arm and leg.

author: T4P
author_rank: Super Contributor
timestamp: March 06, 2012, 05:48:10 pm
content: Quote from: nukie on March 06, 2012, 11:58:52 amWell yeah , always thought they were together , lambda was much cheaper before TDK ( just did my research ) then it suddenly now it looks like i will have to sell both livers to get them *if you know what i mean*