VOGONS


First post, by phosgene

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Hello all,

I'm sorry if this question has been asked before; but how do you go about delivering -5V to an ATX motherboard that has ISA slots?

A little bit of background...

I have an Intel E139761 Slot 1 ATX motherboard, that I'm planning a new Pentium II build around. I would like to use a sound card in one of the the ISA slots, but here's my problem: The ISA specification has -5V on pin 5, but the -5V rail was removed in the ATX12V 1.3 revision in 2003, so there are no modern ATX power supplies that seem suitable.

I do have an old AT/ATX power supply, that has the -5V rail, but when I measured the outputs the 12V rail is not within the +/-5% spec (it measures too low).

So I'm not sure what to do, should I try and rejuvenate the old power supply, or is there another way of getting -5V with a modern PSU?

Cheers, and thanks for reading.

Reply 1 of 11, by Gamecollector

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1) Check your soundcard, if there is no the B5 pin (the back side, the 5th pin from the bracket) - you not need -5V.
2) Use the +5V -> -5V invertor.

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Reply 3 of 11, by phosgene

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Thanks for the quick response,

Gamecollector wrote:

2) Use the +5V -> -5V invertor.

Do you know where I could buy an inverter like this? I spent some time googling for one, but the only thing I could find was an ATX to AT converter. Although...

kikenovic wrote:

What sound card will you be installing? Only three sound cards need the -5v rail. The Sound Blaster 2, the Roland LAPC-1 and the Pro Audio Spectrum.

I was thinking of using a Sound Blaster 16, so maybe I don't need to worry about the -5V?

Reply 5 of 11, by SSTV2

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If you really wish to modify a newer ATX PSU, examine this circuit, it shows what is needed to form a -5 rail: http://danyk.cz/s_atx01d.png.
Basically all you need to do is to add a reversed polarity center tapped bridge rectifier with filtering components, you can assemble such circuit on separate PCB, parts can be bought anywhere or salvaged from other PSUs.

Reply 6 of 11, by QBiN

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My suggestion would be to try to find a ATX v1.2 or earlier power supply on eBay or similar site. They usually still supply -5V. Otherwise, an inline converter like what SSTV mentioned would be required.

Reply 7 of 11, by PARUS

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You shouldn't to be slave of exact PSU in reality. A great way is already shown here:
http://i.imgur.com/PDTpEeo.jpg
It is necessary to plug this blade in any of present ISA slots and before sold these elements on it:
http://i.imgur.com/JKXi3Z3.jpg
This method was given by jwt27.

Reply 8 of 11, by phosgene

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Thank you all for your input, I really appreciate it. If the SB16 doesn't need -5V, then that reduces the complexity a lot.

But I would like the flexibility of using other cards in the future, so purely for academic purposes...

PARUS wrote:
You shouldn't to be slave of exact PSU in reality. A great way is already shown here: http://i.imgur.com/PDTpEeo.jpg It is neces […]
Show full quote

You shouldn't to be slave of exact PSU in reality. A great way is already shown here:
http://i.imgur.com/PDTpEeo.jpg
It is necessary to plug this blade in any of present ISA slots and before sold these elements on it:
http://i.imgur.com/JKXi3Z3.jpg
This method was given by jwt27.

This looks like it might be the easiest thing for an electronics noob like me to implement. So in looking at jwt27's original post, it looks like that TO-220 package is a L7905ACV. I found the data sheet for the chip:

http://www.st.com/content/ccc/resource/techni … .CD00000451.pdf

...And it looks like all I need to do is take -12V from B7 for the input, and output the -5V back into B5. With a 2.2uF cap across the input, and a 1uF cap across the output, both tied to ground. Essentially building a -12V to -5V loop-back card?

Reply 9 of 11, by QBiN

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Easiest thing would be this bad boy:

http://www.ebay.com/itm/CONVERT-20-PIN-NO-5V- … W-/252708495849

Basically exactly what we're talking about.

Reply 10 of 11, by phosgene

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QBiN wrote:

Easiest thing would be this bad boy:

http://www.ebay.com/itm/CONVERT-20-PIN-NO-5V- … W-/252708495849

Basically exactly what we're talking about.

Thank you! I found a different Ebay seller who will ship to Australia by searching "20 to 20 pin 5v atx", I might not have been looking hard enough before. 😀

Reply 11 of 11, by Jepael

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phosgene wrote:

This looks like it might be the easiest thing for an electronics noob like me to implement. So in looking at jwt27's original post, it looks like that TO-220 package is a L7905ACV. I found the data sheet for the chip:

http://www.st.com/content/ccc/resource/techni … .CD00000451.pdf

...And it looks like all I need to do is take -12V from B7 for the input, and output the -5V back into B5. With a 2.2uF cap across the input, and a 1uF cap across the output, both tied to ground. Essentially building a -12V to -5V loop-back card?

Well I'd just take -12V from the blue wire between PSU and ATX connector and connect the regulator there (it needs black wire for ground too). Then another ATX connector from a broken PSU can be sacrificed to get the metal pin and wire (preferably white) so you can connect the -5V from regulator to the ATX connector.

Yes the 7905 needs capacitors at input and output, and without a load it might go out of regulation, so maybe at least 5mA dummy load (like 1kohm resistor) to stabilize it would be nice.