Modification of an ATX Power supply... - Page 1
Forum section: electronics / general

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 16, 2013, 12:15:43 pm
content: Hi,To cut a long story short I need a 'roughly' regulated 3V3 supply, this is then buck converted down to various voltages:0V951V52V4I need about 60A for a custom project (Two or 3 ATX supplies should be fine)Each Custom PCB takes 5-6A max and they work fine for hours at a time on a bench PSU set to 3V3, I have thrown in a Multimeter as I add a PCB to the setup, and the PCB does pull about 5A. (10-11 PCB's)I thought if I went out and purchased some good quality ATX power supplies , that state 3V3 at 30A, I should be set...However as I add PCB's to the setup, the smell of Hairy crab starts to emanate from these ATX supplies, seems not a single brand can come anywhere near the 'rated' 3V3 spec.I cannot seem to get more than two of these PCB's running off an ATX supply, one other brand makes a "rattling sound" and the rail drops below 3vRather than try to pull the Amperage via the 3V3 rail, how easy is it to modify the 5V or 12V rails of an ATX supply to 3V3?

author: mariush
author_rank: Super Contributor
timestamp: April 16, 2013, 12:33:26 pm
content: The cheaper ATX power supplies are group regulated, having the 5v (usually) as the control rail. If you have nothing attached to 5v, the output won't be regulated properly, so the 3.3v would oscillate or otherwise be unstable.Pretty much all cheap power supplies LIE about the output capability, check the labels carefully and assume about 60% of the label is actually possible.Also, check the label carefully, some say 3.3v 30 A, 5v 20 A , but both added up can do only 100 watts.The more modern power supplies are 12v-centric , they do 12v only and then use dc-dc converters to get the 3.3v and 5v. These 3.3v and 5v are relatively weak, sometimes as low as 15A on each, but 100-120w both. These modern psus however should give you no problems even if you use only 3.3v for a long time.You said you bought some good quality atx power supply, could you tell the brand name and model, maybe a link to it? I could possibly tell you more about it if I know the exact model.You could possibly lower the 5v rail closer to 3.3v depending on the psu, but it would lower then 3.3v and 12v as well (depends on design of psu). I don't think you could lower the 12v rail as low as 3.3v, it would probably be possible to get it to about 6-8 volts.

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 16, 2013, 12:57:59 pm
content: It seems like it should be fairly good quality, but I cannot see any buck converters from 12VAnd I don't see any limitation on the 3V3 or 5V lines as regards wattage.This sort of lie on products really get's my backup, maybe I should just burn them out and return them, after all it clearly states they have 'overload' protection

author: mariush
author_rank: Super Contributor
timestamp: April 16, 2013, 01:46:30 pm
content: I guess it's this one :http://www.enhance.com.hk/products/3rr400xg.htmlYeah, it's a 250-300w power supply and it's group regulated. It looks decent, not like the super cheap models, but it's still an atx v1.3 design, somewhat obsolete these days. But the good thing is that those designs were heavy on 5v and 3.3v so it would suit your needs.If you want it to be stable on 3.3v up to about 10-20A, it needs some load on the 5v and possibly the 12v as well. Something like about 2-3 A on 5v and 12v should do. A car light bulb or something similar would work.If it works for you, you could use several linear regulators LT1084 :http://www.digikey.com/product-detail/en/LT1084CT%23PBF/LT1084CT%23PBF-ND/891835This LT1084 can output up to 5A of current at 3.3v from 5v, provided you give it a good heatsink. If you want something cheaper, there's LT1085 that does 3A.If you want a more realistic value for the maximum current it could give on 3.3v, write down what it says on the chips on the second heatsink (the one closer to the cables coming out from the psu). That heatsink has some diodes which are rated for a maximum current, the output on 3.3v would be smaller than that rating (usually).

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 16, 2013, 06:21:04 pm
content: You could grab something like thishttp://www.aliexpress.com/item/20pcs-lot-KIM-3R35L-DC-DC-Converter-12V-Wide-I-P-9-35V-Step-Down-to/819860451.htmland give it a try, stick one on each board, run it all from 12v

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 17, 2013, 07:36:26 am
content: Quote from: mariush on April 16, 2013, 01:46:30 pmAhh..... maybe it is because the 12V is floating and the 5V is sitting at under 1 A load....Yep I had spent the moring looking on 'ALiXpress' at the 'JennYear' modules and speaking to thier technical staff.http://www.aliexpress.com/item/DC-Buck-Converters-10-14V-to-2-5-5V-15A-Peak-20A-High-Current-Power-Supply/553910150.htmlThey claim that EVEN their 15A 20A Peak units cannot hit 15A at 3V3 no matter what the input voltage.so I asked them what could they offer from ANY of their modules at 3V3 vrs current, and now everytime I connect to their tech staff ..they cut the connection!!!!!!Why can they not just reply:"unit XYZ can supply 'ABC' current at 3V3 with an input of 'MNO' voltage'", why must everything be so god damned difficult.....

author: Balaur
author_rank: Unknown
timestamp: April 17, 2013, 07:50:21 am
content: Have you look at the POL modules from TI?Theirconfiguratorpage can show you the devices that fit your criteria.There are 6 devices with an output capability greater than 30A that can provide the voltages you need from 12V.Thepth08t210wseems to fit. If you like the price or not it is a different question.

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 17, 2013, 07:53:59 am
content: Hi,OKMarked on 3 devices strapped to the Heat sinkS2045F12c20cS2045There are some 'small' power diodes which look like flyback devices, but they cannot be more than a couple of amps.So really if f12C20C is rated at 16A max.. it is a crock to mark the output as 30A

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 17, 2013, 08:54:01 am
content: Quote from: Balaur on April 17, 2013, 07:50:21 amyep they are fine but too damned expensive........