The literature I've read indicates that the 12V switchers are a little small, so don't stress it too much. Also, some of the cap selection is sub par, so those should be checked for their condition and replaced as necessary.

One big plus, though, is it's not missing vital parts like input protection or output filters.

Quite a long read this thread but I have learnt much; thank you for your insights.

• Modern PSUs can be used with vintage hardware.
• The overall wattage is less important than the specific ampage of voltage rails (or rather, the overall wattage is a misnomer).
• Modern PSUs have lower ampage +3.3V / +5V rails as contemporary components demand +12V.
• If you go over these or need a -5V rail you may be in trouble with a modern PSU.

I have some questions though:
1) What is a negative voltage and what is it's use?
2) Why was it phased out with ISA?
3) What is the -12V used for as this still in modern PSUs?

😕

Recently I decided to buy a modern PSU (Seasonic P 400) for my P3 / Voodoo 3 build and it is doing a good job so far.

I have one of those Chieftec PSUs that died a few years ago due to bad caps. I keep meaning to get around to re-capping it (seems a shame to bin it) but yeah, the caps aren't great.

CU-133A+ wrote:

I have some questions though: 1) What is a negative voltage and what is it's use? 2) Why was it phased out with ISA? 3) What is […]
Show full quote

I have some questions though:
1) What is a negative voltage and what is it's use?
2) Why was it phased out with ISA?
3) What is the -12V used for as this still in modern PSUs?

1 - negative voltage is good for many things, rs232 serial goes from -15 to +15! its good for sound/opamps too. (this is the answer to #3).

2 - when the ATX spec was written, motherboards still had isa slots as well as pci etc. ISA used -5v for dram chips way back. -5 was also a negative for old pmos voltage levels, and the xt casette chip apparently.

3 - -12v is used for rs232 but most rs232 chips these days do their own voltage conversions.

with going away of isa, the -5v pin changed to not connected, but -12 is still in use.

CU-133A+ wrote:

1) What is a negative voltage and what is it's use?

Think of it as 'relative to ground'. Ground is arbitrarily assigned '0V' by convention. Thus ground is '5V above -5V', and '12V below +12V'.

If you probe with a multimeter on +12V and ground, it'll display 12V. The potential difference between +12V and ground wires is 12V.

If you probe with a multimeter on +12V and -5V, it'll display 17V. The potential difference between +12V and -5V wires is 17V.

If you probe with a multimeter on +12V and -12V, it'll display 24V. The potential difference between +12V and -12V wires is 24V.

In absolute terms it just means that the potential difference between ground and -12V wires is 12V, and between ground and -5V wires is 5V.

CU-133A+ wrote:

2) Why was it phased out with ISA?

For simplicity and cost reduction, as it was no longer used anymore, -5V was officially pronounced over a decade ago (ATX12V 1.2/1.3).

CU-133A+ wrote:

3) What is the -12V used for as this still in modern PSUs?

For backwards compatibility with some vintage RS232, PCI, LAN, etc. stuff.

In fact with 2016 hardware chances are everything will work OK even if you cut the -12V wire off (but don't try this at home 🤣 ).

BloodyCactus & TELVM, thanks for the info. 😀

Im rebuilding my capture system.

The old PSU had an 20pin ATX connector and I needed a PSU with a 24pin connector for the "new" motherboard. I found a FSP 250W PSU and as the capture system wont use more than 150W I thought it was a good candidate.

A missing screw and full of dust, great signs.

AAAAAAAAAHHHHHH! Take it away! Take it away! I will have horrid nightmares involving this PSU.

On to candidate number two, a ~10 year old Chieftec 400W PSU.

The Chieftec (Delta) GPS-400AA-101 A.

The Chieftec (Delta) GPS-400AA-101 A inside.

This looks like a better choice! The system has been running for about 2 hours now without even smelling funny. 😉

The Chieftec is better but the FSP doesn't look too bad (save for oozing crappy CrapXons 😵 ).

Also I can't see an MOV or Zener on the FSP. But, yeah, doesn't look too bad -- just a downsize of another platform to a smaller power envelope.

Anyone has bad experience with Tagan or Enermax ?

Enermax can be quality.

gdjacobs wrote:

Enermax can be quality.

I haver one Enermax EG425P-VE 420W an never let me down, I bought it from new, it is 3rd year, I think of open it an recap it, just in case. Warranty gone so I do not have nothing to loose (like Max Payne).
If you could say something about different models.

Old PSUs about 120-200W, AT and early ATX, usually have 30A or even 40A by +5V line and only one +12V line with 11-15A. Modern PSUs about 650-700W usually have one +5V line with 10-20A and one 12V line up to 60A or two 12V lines both are about 30A.

When I was choosing a new PSU for my retro system I looked up for powerful +5V line. FSP 800-80-GLN is one of them:
+5 30A (!) - very important
+12 first 20A
+12 second 20A - summary 40A is enough
+3,3 30A

My load:
Core2 up to 3GHz
i865 board with 24pin+4pin
AGP card 256mb with MOLEX
PCI Voodoo4
PCI X-Fi
PCI Live! or AU8830
ISA EWS64XL
ISA AV310
ISA AWE64
ISA GUS Ace
Roland SCB-55
NEC 385XR
2xDVD IDE
Floppy
2x250GB SATA drives

gdjacobs wrote:

Enermax can be quality.

They are great units of high build quality but the caps are shit like almost everything else from the period, they often have a very strong +5v rail while still have good +12v. Nice heavy units with good cooling.

These are quite hard to beat unless you really need the -5v rail:
http://www.ebay.com/itm/SEASONIC-550W-ATX12V- … ssAAOSw0UdXripI
http://www.ebay.com/itm/SEASONIC-550W-ATX-Act … N4AAOSwmtJXYanS

Active PFC, 80 Plus certified efficiency...

+5V = 30A
+12v = 18A x4 (four rails)

I recently bought 10 of them for $135 shipped. They are fantastic power supplies for nearly any system one could build... as long as -5v isn't required.

For older, less power hungry systems that aren't likely to need 30A on the +5v, I like to use these:
http://www.ebay.com/itm/SEASONIC-350W-ATX12V- … poAAOSwal5YG0no

Active PFC, 80 Plus (sometimes Bronze)
+5v = 20A
+12v = 17A x2

Recently bought 10 of these for $80 shipped.

So, for $215 I have stocked 20 power supplies that are well suited to very old ATX or very new ATX computers (either low powered or high powered), with good efficiency and one of the best brand names on the market. I use an 80 Plus Gold XFX 550W (made by Seasonic) in my main system, but these units are plenty good enough for any other system I have or will likely build\upgrade for many years. I can replace every OEM Bestec unit I come across without hesitation now. 😉

nforce4max wrote:

gdjacobs wrote:

Enermax can be quality.

They are great units of high build quality but the caps are shit like almost everything else from the period, they often have a very strong +5v rail while still have good +12v. Nice heavy units with good cooling.

Every model is different. Enermax does have some models with decent topologies and component selection, but not all are very good.

PARUS wrote:

Old PSUs about 120-200W, AT and early ATX, usually have 30A or even 40A by +5V line and only one +12V line with 11-15A. Modern PSUs about 650-700W usually have one +5V line with 10-20A and one 12V line up to 60A or two 12V lines both are about 30A.

Vintage PSUs usually hit those current levels on the 5V rail at the 300-450W power range. 200W supplies usually were specified for 20A or so on 5V.

Yes, I'm wrong, sorry. 120-200W can't have 40A. But one of my AT PSUs had +5V-40A. It was only 230W. It is rarity.

That would allow 30W for the 12V, 3.3V, and negative polarity rails. I wonder if the load table was overstating things a bit.