4 pin Molex powered GPUs - 9800 Pro, 6800 GT etc \ VOGONS

4 pin Molex powered GPUs - 9800 Pro, 6800 GT etc

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First post, by Lostdotfish

Posted on 2025-03-30, 18:16

Lostdotfish Offline

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Does anyone happen to know if these cards pull 5v from the Molex as well as 12v?

I'm guessing the bulk of the power delivered via Molex is the 12v rail.

I'm wondering if I can power these cards from 12v only by using the PCIE output in my modular PSU (4x12v and 4xgnd per port).

If possible it would free up my SFX PSU Molex/sata connections for other things. It would be fairly trivial to make a cable that breaks out 2 of the 12v lines and gnds to 2 4 pin Molex connectors.

Reply 1 of 2, by The Serpent Rider

Posted on 2025-03-31, 05:15

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I believe someone posted here on Vogons a link to old article about how various GPUs distribute their power. Was it THG article? Radeon 9800 is mixed between 5v and 12v. Not sure about 6800GT, but judging by how Mac version was designed (and all PCIe 6800), it's pure 12v.

I must be some kind of standard: the anonymous gangbanger of the 21st century.

Reply 2 of 2, by momaka

Posted on 2025-04-01, 13:11

momaka Offline

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Radeon 9800 Pro most definitely uses the 5V rail very heavily - both from the Molex connector and from the AGP connector, along with 3.3V from the AGP connector. I think 12V rail is very lightly used on that card.

The 6800 GT AGP is a completely different animal and uses mostly the 12V rail from the 4-pin connector. Not sure if the 5V rail is used or not, though - I think that may be card-specific. For example, on a XFX 6800 XT AGP I have, that card uses the 12V rail from the 4-pin connector for the GPU V_core rail and the 3.3V rail from the AGP connector for generating the VRAM V_ddq rail (IIRC.) I don't recall if the 5V rail from the 4-pin connector was used or not, though.

Either way, don't just plug in stuff randomly to try it, unless you don't mind frying your hardware.
The better approach would be to take a multimeter and trace out where the 5V and 12V rails go from the 4-pin connector. There's a chance the 5V rail could be used for VRAM V_ddq instead of the 12V rail or 3.3V rail or for something else.

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