Blizzard Ppc and mediator and ATX vs AT psu

[rjd324]

I had an amiga 1200 with the original 1200 mediator. I installed a blizzard PPC which would just cause a black screen with ATX psu. The ATX went into one of those ATX to AT converters (p8/p9) into the mediator.

This worked for my blizzard 1260 but not for the PPC.

I was told to use an AT power supply for the PPC. So I did just that and now it boots up fine. P8 and p9 directly into the mediator.

I just wanted to know why.

Thanks.

[grelbfarlk]

Probably because the ATX power supply you were using did not regulate on the 5V rail properly.
Try an ATX power supply which people with heavily loaded A4000s are using and I will guess it would work.

[jbenam]

AT power supplies had more emphasis on 5V, while modern ATX power supplies are focused on 12V.

Amigas use 5V for almost everything. 5V on modern ATX supplies is kinda anemic and might not be sufficient for your heavily expanded Amiga.

On top of that, IIRC, modern ATX power supplies expect a certain kind of load as a minimum on all lines. Without that, you might have regulation issues.

[Daedalus]

Indeed, most PSUs regulate a single rail and base all the other rails on that one. So if the primary rail is out, all the others are too. In modern ATX PSUs, that rail is the 12V rail, so if that's not able to regulate properly and drifts too low for example, all the other rails will drift low too.

Almost all PSUs have a minimum draw requirement on the primary output rail to ensure correct regulation. This spec is rarely published because they're intended for PCs and every PC will meet the minimum spec, but an Amiga's 12V consumption is usually below this minimum, meaning all outputs are at risk of being out of spec. A good PSU will shut itself down in that situation, whereas a cheap PSU will happily output out-of-spec (and potentially dangerous to equipment) voltages on all rails.

You need to measure the 5V rail, ideally at the accelerator - it's likely too low. Adding a significant 12V load to the PSU might be enough to bring it within spec, and using a PSU with a lower capacity might help too since a lower output capacity often means it has a lower minimum draw too. For example, an 800W PSU might need 5A minimum on the 12V rail, whereas a 300W PSU might only need 1A.

[Toni Wilen]

That is partially obsolete information. I wouldn't call group regulated ATX PSU "modern" today..

All high end ATX PSUs have separate regulation for each rail (big main 12V, 5v and 3v use DC-DC converters) and don't have "old style" minimum load limits, at least since Intel introduced "Haswell" PSU requirements.

There are some minimum load tests in some PSU reviews that go down to about 20w or so with almost no change in voltages.

In other words, cheap ATX PSUs probably are bad idea, more expensive ones should work fine

[Jope]

And Haswell was launched back in 2013. It should be trivial to find a suitable PSU these days.

[Daedalus]

Fair enough. I've tested a fair few PSUs that have come across my desk that still show poor regulation without significant loads on the 12V rail, but they are indeed typically the cheap ones.

[rjd324]

Perhaps just connected the floppy drive would have made it work then.

The ATX PSU was a cheap Tesla 500TEBR.

I decided to get a brand-name older style AT PSU, Dell. It is 150W and I am hoping that max rating of 18A for 5V is enough to supply all the devices.

It will look something like:
A1200, 1200 PCI Mediator (original 4 slot) with Radeon 9250, Sound Blaster, Ehternet, Indi MK3, Fast ATA (if I can fit the damn thing in), 1 IDE, 1 floppy, 1 CD Drive.

Do you think that will guzzle up too much juice?

The voltage readings under simple load ("simple": A1200 with mediator with PPC) were bang on the right levels.

Looking at Ian Stedman's table of various systems and the wattage each consumed - I think I recall that there was an A1200 with a few SCSI drives and a 1260 that used a total of 22W. Does the mediator and Radeon guzzle the juice?

[grelbfarlk]

The most I ever measured was 95W at the wall in a towerized A4000 running Quake 2 with:
CSPPC (PPC not in use)
Mediator
Voodoo 3-3000
100MB NIC
600MHz PCI G4 PowerPC board (This PPC in use)
Spider USB
FM801 Sound card
10k RPM SCSI Drive
DVD Drive
SSD with SATA to IDE and Acard IDE to SCSI
USB Media reader
IDE CF adapter
With a couple fans (one on CSPPC PPC, one on PPC card, one case fan)

This is with a Corsair CS550M ATX PSU