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Is -5V needed on a Pentium board?

N

NHVintage

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Oct 1, 2023
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Hi - I'm working on a Gateway PC with a Pentium 1st-gen board on it (LX chipset). I would like to try a newer power supply, but new AT ones are difficult to find, and the few I've seen have power cables too short for this tower, so I was thinking of using an ATX PS with the adapter you can find that bridge the needed 20 port ATX voltages to the AT connection. However, most ATX power supplies (and all the ones I could find) lack -5V, which is on Pin 9 of the AT connector. I've been seeing mixed responses on whether it's needed, so I figured I'd ask the experts here. My board has PCI and ISA slots, it's the 'Batman' Intel board. Thanks as always for your help.
 
You need the -5V rail for some ISA cards, but not all of them need it. If you are interested in using specific ISA cards, you should check whether you need it or not. Alternatively, you can use something like a "Voltage Blaster" (https://github.com/necroware/voltage-blaster) to provide -5V from the -12V rail at the cost of an ISA slot in case your PSU does not provide it.
 
Great, thanks! The way this board is situated in the case there is an ISA slot you can't actually use if you need access to the back slots, which would be good for a Voltage Blaster.
 
Given that the original 5150 supplied -5 at perhaps <100 ma, you could get by with a 7905 voltage regulator and build it anywhere that's convenient. I suspect a -5.6V zener and a resistor would be adequate. On the 5150 16-64K, the -5 was used to provide the 4116 DRAM substrate bias. There aren't a lot of cards that use -5.
 
Whether or not you have an ISA card that needs -5v, there are some motherboards out there that check for the rail being present and won't POST without it. I've had a number of Pentium era boards with that "feature".
 
Wow, thats interesting. I may have to stock up on Voltage Blaster boards. At least I'll get one to test these theories.
 
A cheaper solution would be to just get an LM7905 regulator and hang it off the -12v rail with a small capacitor in the 10-100uF range. If you have one of those cheap ATX to AT wiring harnesses, you can splice it into the harness.
 
Probably for irrational reasons the "Voltage Blaster" gives me the willies. I mean, in *theory* most motherboards would just have a common web of traces connecting the power connector to the pins on the slots and it shouldn't matter where you inject the power, but... yeah. A small regulator on the wiring harness adapter seems like the inherently "correct" way to do it.
 
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