# PSU ATX version



## MeltingPoint (Apr 24, 2013)

I have this cheapy generic PSU lying around but I can't seem to find out if this and old ATX version or a newer one. According to Wikipedia, ATX12V 1.1 and older have a -5V rail while in ATX12V1.2 tthe −5 V rail was no longer required (it became optional).

My PSU' specs are as follows:


+12V - 30A
+5V - 15A
+3.3V - 22A
-12V - 0.3A
+5VSB - 2.5A

Its pretty obvious that this generic psu cant put out that much amperage.

When I took the PCB out I saw a -5V rail that is unused. This confuses me. Can anyone tell me if this is an older version or a new one? 

I need to know this stuff because I will be converting this into a lab PSU and I need to solder a dummy load to the correct rail.

If pictures will help, please tell me.

THANKS!


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## McSteel (Apr 24, 2013)

I think this is probably an old ATX 1.2/1.3 PSU, probably based on a (possibly copied) Deer/Sunpro/Leadman/Golden Tiger/etc design... Pictures are mandatory if we're to be of assistance, so please post some.


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## MeltingPoint (Apr 24, 2013)

I take crappy pics







Orange wire is PS-ON connected to GND






























The PSU has 4-pin molex, 4-pin atx, floppy drive and a 20+4 pin connector. There is no -5V on the 24 pin as far as Im aware


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## Jstn7477 (Apr 24, 2013)

Are you sure you really want to use that PSU as a lab PSU? It probably can't even handle 200w and the PCB is all burnt up where the resistors got too hot. A $50 PSU today would likely have 10-20% higher efficiency, double the current capability and less output ripple. Don't know if you want to spend anything or not, but that's just my 2 cents.


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## McSteel (Apr 24, 2013)

The pics are just fine 

I'm assuming you want to use the 12V rail.

First of all, as Jstn7477 aptly noted, this is a crap PSU. If you need a stable DC output, and especially if you're aiming for 200+W, you might be better off buying something a little less crappy. I don't think you should invest the whole $50 in it, as there are often good deals to be found for half that money. I've even seen Corsair CX430s for $25 after MIR on the 'Egg. Or, you could snatch a nice FSP/Delta/LiteOn/Hipro/AcBel built unit from an old HP/Dell/IBM/etc desktop/workstation... Those can routinely be found on eBay for $15-30.

If you do insist on using this one, I suggest you first beef it up.

As it is right now, it has almost no input/EMI filter, it's primary caps are a horror show, the 13007s (I guess, could be some other transistors) on the primary have been overheating for a while, the main transformer is nowhere near an ERL-35, it's not even an EI-31, the secondary silicon can't deliver what the label promises, and especially not the 12V parts (which are probably some (ultra)fast recovery diode pair with ~12A (maybe 16) current limit, it's Pi-filters are a disaster, with only a single crappy capacitor tasked with cleaning up ripple/noise and holding down vdroop.

In case you were wondering, those big-ish gray (and one pink) resistors on the secondary side are the minimum load resistors, so that the PSU enters regulation at power-up. Without them, the ATX2005 controller would have a blast trying to figure out what the hell is wrong with output voltages... BTW, the PSU is a product of Deer/L&C. I hope you held on to the voltage selector (doubler) if you're on 110VAC mains.

Here's what you'd need to do to get a ~300W @ 12V lab power source:

- "Enrich" the input/EMI filter. The PSU only has two very skinny Y-caps for this task. You need to add at least two more at the input receptacle (put one at phase-ground and the other between neutral and ground), and put at least two X-caps there. There is a silkscreen for one on the PCB, I can't see if there is room for another there, or if it would have to go to the input receptacle as well. It should go across phase and neutral. You also need to common-mode chokes, there's a silkscreen for one on the PCB (can't see if there's room for another there). If you need to put the other one on the receptacle, it should go in line with phase and neutral, such that one winding is in series with phase, and the other winding with neutral.

- Change the primary capacitors. Those CS/SC (12 Kuang Jin) caps are a delight, you never know when they're gonna pop. The PCB is wired so that the caps are in series, meaning their working voltage is added together, but their capacitance is reduced (in case of identical caps, their max voltage is doubled and their capacitance halved). I'd go with at least 200V/470uF per cap, but probably would go to 680uF or more if possible. As for the manufacturer, go with something at least remotely reputable. Top-tier caps (Rubycon, Nichicon, Nippon Chemi-Con, Panasonic/Matsushita, etc) are probably overkill, but can't hurt; OST, Samxon, Teapo, and even Su'scon, Elite or CapXon would all be fine for this PSU. 200V might be a bit close, as it means 400V total and the half-wave rectified current is at ~380V when it exits the rectifier bridge... You could go for 220V parts, I suppose, but they're not as abundantly available. This is why you need a good EMI filter. Don't want those pesky spikes to overvolt your caps.

- You need a stronger rectifier on the 12V rail. As far as I can see, the 5V rectifier is the strongest, it's probably an SBR3045 (schottky barrier rectifier, 30A/45V max), which you can swap with the 12V part. Alternatively, you could put a rectifier of your choice in there, but that's unnecessary additional expense at this point. 30A is plenty (360W).

- The secondary output filter needs an overhaul. Again, I think top-tier caps would be wasted on this, so look for something readily available. Output quality on the rails you won't be using is inconsequential, but you need to put in two good, at 2200-3300 uF caps on the rail you will be using, and a good filter coil. You could salvage one from another PSU, ot if you're buying, it should probably be at least 22 mH.

- Avoiding crossload. In order to keep the regulation in check, you need to load the 5V rail with around 1/5th of the load on the 12V rail. You can use more (no need for it, though), but you should probably avoid using less if you can. It's probably a good idea to use a power adjustable rheostat (potentiometer), so that you can fine tune the 5V loading. But, using a constant 6A load would only be a problem if you're drawing less than ~1.5A from the 12V rail, so if you're not going that low, it's safe to use a fixed load. You'd need a 820 mOhm, 30+W resistor (ideally), or, just get a standard 1 Ohm 25W resistor for a ~5A load on the 5V rail. The resistor that's already present in the PSU is 1.3 Ohm, if I'm reading the colors correctly, but you can't tell how well it's holding up after all the work it's done. You could measure it with a DMM and see if it's still usable.

That's about it. If you enjoy tinkering, good luck and enjoy your project!
I still strongly recommend going for a better quality PSU as a basis, though...


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## MeltingPoint (Apr 25, 2013)

WOAH! 


I will be using this to power a 40W audio amp so a stable voltage is not really necessary. I will be adding a filter capacitor at the output to reduce the ripple.

So is this an ATX12V 1.2 or higher PSU?


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## fretrvxd (Apr 25, 2013)

1.0 Feb, 2000 • Public release
1.1 Aug, 2000 • Increase 3.3 V current; add more explanation for power sharing; do
minor edits and format fixes
1.2 Jan, 2002 • Typical Power Distribution. Change +5V loading on all power
supplies distribution tables defined in DG to 0.3A
• PS_ON# Add text “The power supply should not latch into a
shutdown state when PS_ON# is driven active by pulses between
10ms to 100ms during the decay of the power rails.”
• Remove –5V from all power distribution tables.
1.3 April, 2003 • Update Power and Current guidance
• Added efficiency guidance at typical and light load
• Increased min efficiency at full load from 68% to 70%
• Serial ATA* connector definition added
• Acoustic levels added for low noise power supply design
• Reformat and update revision table
• Update Disclaimers
• Remove guidelines for ATX
• Remove guidance for –5V rail
• Updated guidance for Energy Star and stand by efficiency
2.0 February, 2003 • Added Terminology section
• Updated power and current guidance
• Includes 250W, 300W, 350W, and 400W guidance
• Updated cross regulation graphs
• Updated load tables
• Updated required efficiency targets. Added recommended
efficiency targets.
• Increased required minimum efficiency at typical and light load.
• Main Power Connector changes to 2x12.
• Aux power connector removed.
• Required Serial ATA Connector.
• Isolated current limit on 2x2 connector for 12V2 rail.
2.01 June, 2004 • Updated 3.3 V remote sense pin # on the main power connector
• Updated 12V2 DC Output Noise/Ripple information
• Removed -5V reference
• Updated 5Vsb maximum current step


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## McSteel (Apr 25, 2013)

This is an ATX1.3 compliant design from circa 2003


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## MeltingPoint (Apr 25, 2013)

*edit*

Wait, this still kinda confuses me. Based on the design of the PSU, where should the dummy load be connected? Should it be between GND an +5 or GND and +12?


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## MeltingPoint (Apr 26, 2013)

Bump.

Anybody know the answer to my question above? ^^^

I need to finish this asap. Thanks all!


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## McSteel (Apr 26, 2013)

5V and 12V are group regulated, meaning they're routed through the same regulation coil. When one of the rails is loaded down (causing it's voltage to drop), the other rail's voltage goes high as a result.

Since you plan on using the 12V output exclusively, you'll be crossloading the PSU so that 12V would sag more than it normally would, and 5V would go sky high. In order not to trigger OVP (overvoltage protection) on the 5V rail, you should load it down with a power resistor. I believe 5A is plenty for your needs, so you'd need a 1 Ohm, 25+W resistor across 5V and GND. Remember to remove the minimum-load resistor that's already present on the 5V rail.


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