Computer power supply.

[Dave Plowman (News)]

I've had a PS failure on one of my old Acorn RPCs. I do have a spare but it's the low powered alternative and it would struggle with the extras I have on that machine. I can probably repair the original since I have a good one to get the values of the burnt out resistors etc on, but of course a standard PC one is so cheap it's not worth bothering. However, they are the wrong size and shape to fit the case. But looking at the CPC site I found one that would. Plenty beefy enough too.

The Acorn PS only supplies at a max:- + 5.1v @ 11A +12v @ 3.6A -12v @ 0.25A

So the one that would fit is fine - but has in addition a +3.3v and a +5v marked SB.

Do these extra outputs need some load to allow the thing to switch on? And just how do you power it up? The Acorn has just a simple mains switch.

The cover says the 'on' lead is the grey one - but doesn't say what it expects to see.

[Dave Liquorice]

I expect some of the silicon bits will also be fried, like the mains switching transistors and possibly the controller chip, also check the smoothing capacitors but how you do that for something that is smoothing bridge rectified raw mains I'm not sure. Peak voltage being >300 odd...

I don't think so, try it and see. The +5V SB is the limited supply for running a LAN card to enable the "wake on LAN" function of PCs to operate.

You can find it on the web, lots of information about ATX power supplies and using them for other things. Try google with "ATX bench supply" or similar...

[Andrew Gabriel]

Probably not. There are PSU's where the 12V regulation doesn't work well unless you draw a reasonable load on the 5V rail too. (People find this out when using them just to power disk drives.)

The +5v SB is permanently on (Stand By), but only provides a few 100mA or so. You switch the PSU on by connecting the (usually green) PS-ON to one of the black ground wires.

The grey one is POWER-OK I think, which is an output from the PSU to tell the PC it has all rails in correct voltage range and can come out of reset.

(Warning, it's a long time since I played with one of these, and this is from memory, and a quick google check on the colours.)

If you can find one, an AT power supply (rather than an ATX one) would be a perfect match for what you want.

[Alex]

Usually they will switch on when the green motherboard wire (or grey in your case?) is grounded, with load or not. YMMV :)

Alex

[John Rumm]

Not on the standby or 3.3V - some load on the 12 and or 5V will usually be enough to stop the PSU shutting down.

Pin 14 of the ATX connector (green wire) - you need to ground it.

They grey one is usually the "power good" indication - so its a tellback and not a control.

[chris French]

In message , Andrew Gabriel writes

Freecycle is a good place to find such things, assuming an old AT supply is big enough for what Dave wants to power

[Andrew Gabriel]

I've got one in a 486 system here which I bought in 1990. I think it's a 200W PSU which was typical of the time. It was in use as my firewall until about 6 months ago. I should probably chuck it out, but it still works fine, and it will probably take a couple of years of non-use before the emotional attachment to that first 486 which I bought for £3300* can be chucked.

• Included extra I paid for 8Mb memory and a SCSI disk;-) It's since been upgraded to the motherboard limit of 32Mb memory and a 486DX2/66 (BIOS was too old for a 486DX4/100). With my ADSL line running flat out, I couldn't get it quite up to 10% CPU usage though.

[geoff]

In message , chris French writes

I'm sure I've got one lying around

email me an address

[Dave Plowman (News)]

I'm confused. On this CPC one pin 14 is grey pin 8 orange. ;-) And some of the other colours don't conform either. The 5 and 12 volt mains ones do so I'm ok. But all the plugs look from memory the same as the ones in my PC.

The snag is the shape and size - if they're roughly square like an ATX. They're too wide to fit the space in the Acorn case. The Acorn one is long and narrow rather than square. But the one that's just arrived from CPC after my first post is fine physically to fit. I'm not quite sure what it's made for - the model starts with MPT. I've actually put the PCB in the original PS case.

[Andrew Gabriel]

Well, it's the pin positions in the connector which count.

The square ones are the standard ones used in tower and full height desktop cases. As you move to smaller cases and larger manufacturers, you tend to find less standard PSU's used. I have a shuttle cube which has a very small PSU which would be a swine to replace no doubt if it failed. I was playing with a Sun X4600 last week (which has 8 dual core Xeon CPU's in it), and that has 4 very long thin hot-swap PSU's in it, 850W each!

[Andy Wade]

There are two standards: original ATX with a 20-pin connector and the current ATX V2.2 with 24 pins. The /PS_ON (!enable) line is pin 16 in the newer standard. All may become clear if you study the two hardwarebook links that John Rumm posted.

[Dave Plowman (News)]

Yes - but this one is 24 pin with basically an extra 4 pin one which clips to the 20 pin. Same as the one fitted to my PC. And the 20 pin part conforms to the ATX standard (apart from colours) according to this site :-

says the ATX2 has the added 4 pins. It would seem strange to alter the connections to the 20 part for no good reason. I assumed you could use a suitable ATX2 to replace an ATX1. Now I'm confused. ;-)

[John Rumm]

You can - the pin assignments (in terms of their position) remains the same in both versions for the big block of 20 pins.

However, the numbering changes since the pins are numbered up one side and then up the next. So pin 13 on the bigger connector is in the same position as pin 10 on the smaller one. If you read the ATX2.2 standard you will see that 13 - 22 correspond to 10 - 20 on the old standard.

(in fact on many PSUs with the longer connector, you can snap off the section at the end with the extra four pins, and the big bit is then a functional replacement for older V1 designs.

[Man at B&Q]

For some motherboards you *have to* snap off the extra section as the

4 way socket is physically seperate on the motherboard. In fact,the last power supply I bought had separate 20 way and 4 way connectors for this very reason.

MBQ

[John Rumm]

One caution - some makers (notably Dell) used to do some PSU layouts that looked standard, but had wires in the wrong places to match their custom motherboards. It was a crude lock in that made upgrading or replacing bits rather expensive (i.e. buy dell bits at inflated prices, or but extra standard bits)

[Dave Plowman (News)]

That's what I thought. On my 'proper' PC the 4 pin Molex is just keyed onto the 20.

Yes. Andy was trying to confuse us all. ;-)

That is logical for replacement types at least.