Good power supply for a P2 with ISA?

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Good power supply for a P2 with ISA?

Postby och » 2017-10-22 @ 23:19

Thinking of building a Pentium 2 system, but it seems that all modern power supplied lack -5 rail on the 20pin connector (essentially making them 19pin)? I really don't want to resort to using an older used power supply. What are my options?
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Re: Good power supply for a P2 with ISA?

Postby BitWrangler » 2017-10-22 @ 23:44

There is very, very few uses for the -5V, unless you've got a 286 or below or you're very sure you need it for something, then don't worry about it. Unless you've got a nasty board that actually looks at all the voltages and throws a wobbly about the one it will never use.

It is fairly easy to rig a -5V supply off the -12V though.
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Re: Good power supply for a P2 with ISA?

Postby gdjacobs » 2017-10-23 @ 00:14

You can find new Startech ATX 1.3 supplies with appropriate rail allocations for Athlon 3200+ type machines and -5V rails. There should also be some availability of FSP/SPI supplies of the same design era as they were still in production until fairly recently. If your P2 board uses AT, Startech makes those new as well.
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Re: Good power supply for a P2 with ISA?

Postby och » 2017-10-23 @ 09:53

So I should be ok running ISA Awe64 gold using a power supply without -5v?
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Re: Good power supply for a P2 with ISA?

Postby Koltoroc » 2017-10-23 @ 12:51

Yes. The AWE 64 does not use -5v. actually most creative labs soundblaster cards don't use it AFAIR the last to do so was the Soundblaster 2.0.
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Re: Good power supply for a P2 with ISA?

Postby gdjacobs » 2017-10-23 @ 16:22

och wrote:So I should be ok running ISA Awe64 gold using a power supply without -5v?


There's a thread for that.
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Re: Good power supply for a P2 with ISA?

Postby .legaCy » 2017-10-24 @ 13:34

If i recall correctly modern psu has -12v , modding the power supply atx connector and using one 7905 regulator you can create a weak -5v
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Re: Good power supply for a P2 with ISA?

Postby BitWrangler » 2017-10-24 @ 15:33

.legaCy wrote:If i recall correctly modern psu has -12v , modding the power supply atx connector and using one 7905 regulator you can create a weak -5v


-5 is kinda weak anyway, and 7905 is good for an amp, depends how crappy the -12V is, I've seen as bad as 300mA so you won't get much out of that, if it's 1A that's nice because average for each was half an amp.
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Re: Good power supply for a P2 with ISA?

Postby gdjacobs » 2017-10-24 @ 15:59

There are switching regulators to invert the polarity as well. See charge pump and buck-boost configurations.
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Re: Good power supply for a P2 with ISA?

Postby .legaCy » 2017-10-24 @ 18:24

gdjacobs wrote:There are switching regulators to invert the polarity as well. See charge pump and buck-boost configurations.

If you look for premade modules is kind of okay and way more efficient than a linear regulator(7905), but for diy its kind of way harder to design a buck-boost converter
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Re: Good power supply for a P2 with ISA?

Postby gdjacobs » 2017-10-25 @ 03:24

Premade modules are definitely the easiest, but most VRM application notes have a baseline design that you can roll with.
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