ATX power supply issue - Page 1
Forum section: electronics / repair

author: GeorgeTsak
author_rank: Contributor
timestamp: November 15, 2015, 11:30:52 am
content: Hello my name is GeorgeTsak and this is my first topin in EEVblog. Before some months I got an old ATX power supply from an old PC and it was working perfectly. However in the last 5 days I was pushing the power supply to it's limits.(I was pulling out 24A at 5V when it is rated at 26A 5V). Anyway the problem is that it won't turn it on, when I hit the switch It will provide power for milliseconds and it will turn off again.(The exact same behavior when sorting cables, e.g. the 5V cable with the COM cable.). I opened it up to see if there is any physical damage but I could not see anything wrong with the board. Accidentally I discovered that one of the two big heatsinks was live at 170VDC, which seemed strange to me. I did some research and I rode that many devices use live heatsinks in order to have common base in the transistors. However when looking at the transistors I can see 3 pins going to the board and this is my question: If indeed the transistors share the same base shouldn't be two pins going to the board?(the Emitter and the collector). So the reason my power supply is turning off is because there is a leakage? Thanks in advance!If it matters here is the power supply brand/model: Enhance ATX-1125BTA

author: Aodhan145
author_rank: Frequent Contributor
timestamp: November 15, 2015, 12:53:04 pm
content: If you can get a new one they are so cheap. Scrap that one take the heatsinks, etc.

author: max666
author_rank: Frequent Contributor
timestamp: November 15, 2015, 01:22:36 pm
content: Yes, be careful. Heatsinks in SMPS can be live, not only by accident, but by design. And keep in mind that the main filter cap can store hundreds of volts for some time after you disconnected the supply, discharge it before you touch anything on the pcb.In an ATX power supply and the majority of other SMPS's you will find MOSFET's as switching transistors not bipolar transistors and MOSFET's usually have source and body terminals connected together internally (sometimes drain and body):Quote from:https://en.wikipedia.org/wiki/MOSFETNow why would you ever need that third pin then? Well, often you can't have a live heatsink, maybe for safety reasons, maybe you have other devices mounted on the same heatsink that have a different potential on their tab. In that case you have to isolate the tab from the heatsink, by using a sil pad for example, or you can buy transistors with isolated tab, but now you obviously need a third pin to connect to the source.And for why your power supply isn't working any more, I'm no expert, but your power supply may have several MOSFET's paralleled up to increase the current handling capability and one of those transistors might have died when you pushed it, because cheap ass ATX supply probably don't implement paralleling very well. Or your electrolytic cap's have gone bad. At least those two things come to my mind.

author: amyk
author_rank: Super Contributor
timestamp: November 15, 2015, 03:36:25 pm
content: It is probably a standard half-bridge TL494 type PSU, see schematics here:http://danyk.cz/s_atx_en.htmlThe heatsinks are live because it reduces the need for insulation and you're not supposed to touch them anyway.

author: wraper
author_rank: Unknown
timestamp: November 15, 2015, 04:09:55 pm
content: Quote from: AcHmed99 on November 15, 2015, 03:59:50 pmThis one actually should be quality stuff holding the rated load because this is good manufacturer. Most likely dual diode on the 5V rail became short. Actually measure resistance from 5V rail to GND, if it is less than 10 ohm, the fault should be there.

author: GeorgeTsak
author_rank: Contributor
timestamp: November 15, 2015, 05:50:12 pm
content: Quote from: wraper on November 15, 2015, 04:09:55 pmI measured the resistance and I got around 1?. So it seems I burned the diodes. Thanks everyone for their response!!!