GPU power from molex

[lemming9]

Would like some confirmation on these points:

1. Does a 2 molex to 8-pin PCIe connection provide the full 150W/12.5A just like a standard 8-pin PCIe connection would?

2. Also, are the GPU, CPU, molex/sata drives, MB all powered from the +12V rail?

3. If so, what do the +5V +3.3V -12V +5VSB power? The fans, USB ports and RAM?

Thanks

 

[Pinhedd]

Would like some confirmation on these points:

1. Does a 2 molex to 8-pin PCIe connection provide the full 150W/12.5A just like a standard 8-pin PCIe connection would?

2. Also, are the GPU, CPU, molex/sata drives, MB all powered from the +12V rail?

3. If so, what do the +5V +3.3V -12V +5VSB power? The fans, USB ports and RAM?

Thanks

1. No it does not. Each Molex connection is rated at 40 watts. 6 pin PCIe are rated at 75 watts. 8 pin PCIe are rated at 150 watts. A 2x Molex to 6 Pin PCIe is safeish as it provides 80 watts (meaning that it should work, but a PSU that is deficient in PCIe connectors may not be up to the task), but a 2x Molex to 8 Pin PCIe is not. There's a real fire hazard there.

2. The CPU, IGP (if present), GPU, memory, and chipset are all powered through programmable voltage regulators. These take in 12 volts and spit out a programmable value, typically in the 0.5 to 2.0 volt range.

Fans and pumps use straight 12 volts, USB and other powered connectors use 5 volts. Peripherals such as hard disk drives and optical drives typically use a combination of 5 volt and 3.3 volt supplies.

The motherboard uses an assortment of voltage levels for varying components. Many integrated circuits still use the 3.3 volt LVCMOS standard. LEDs and other bells and whistles may use either 3.3 volts or 5 volts depending on design.

 

[lemming9]

Thanks for that but I'm still looking for a bit more clarification.

1. I thought it was a lot more than that. If not why else would GPU manufacturers even provide a 2 molex to 8-pin PCIe connector if it's not even close to providing the 150W that a direct 8-pin PCIe would.

2. I noticed the drives draw power from the +12V rail as does the CPU/GPU/MB. Roughly how much power does a MB use?

 

[Pinhedd]

Thanks for that but I'm still looking for a bit more clarification.

1. I thought it was a lot more than that. If not why else would GPU manufacturers even provide a 2 molex to 8-pin PCIe connector if it's not even close to providing the 150W that a direct 8-pin PCIe would.

2. I noticed the drives draw power from the +12V rail as does the CPU/GPU/MB. Roughly how much power does a MB use?

1. Some manufacturers design their PSUs very, very well. They use thick cables, solid internals, and good construction. These PSUs may very well deliver the requisite 150 watts through a pair of 4 pin Molex connectors, but these same PSU manufacturers also include plenty of PCIe connectors so such workarounds aren't necessary. Many PSU manufacturers cut corners so heavily that their products aren't even allowed to bear the ATX trademark. Some put it on their anyway. It's just not something that you want to leave to chance, trust me.

2. The motherboard itself typically only uses around 10 watts or so. Some older motherboards from NVidia were enormous power hogs that drew upwards of 50 watts, and some high end Intel ones draw around 15-20 watts right now. Most mainstream motherboards are 10 watts or below though.

 

[lemming9]

Thanks again. I'm pretty sure the PSU is of high quality and has the ATX12V v2.3 standard. I'd still however need to use a 2 molex to 8-pin PCIe connector. Is there any specifications I could check to confirm that it would be ok? i.e what AWG would be best? does single/dual rail distribution make a difference?

As for the power output on the +12V, does single vs dual make a difference to the final output? e.g. a Single rail might be a certain wattage, so if this was divided between two rails that also add up to the same amperage would it still provide the same wattage as single rail?

 

[Pinhedd]

Thanks again. I'm pretty sure the PSU is of high quality and has the ATX12V v2.3 standard. I'd still however need to use a 2 molex to 8-pin PCIe connector. Is there any specifications I could check to confirm that it would be ok? i.e what AWG would be best? does single/dual rail distribution make a difference?

As for the power output on the +12V, does single vs dual make a difference to the final output? e.g. a Single rail might be a certain wattage, so if this was divided between two rails that also add up to the same amperage would it still provide the same wattage as single rail?

The power rail specifications and arrangement should be plastered on the PSU. If you can tell me what the make and model is I'll be able to answer it better

 

[lemming9]

Thanks. I've already done a lot of the calculations but am still looking for confirmation on if single/dual rail makes a difference:

Antec Earthwatts
+12V1 17A
+12V2 15A (at stock CPU uses just under 8A)
+12V OCP 25A (per rail)

Would rather not upgrade right now if the numbers work out to be adequate.

 

[Pinhedd]

Thanks. I've already done a lot of the calculations but am still looking for confirmation on if single/dual rail makes a difference:

Antec Earthwatts
+12V1 17A
+12V2 15A (at stock CPU uses just under 8A)
+12V OCP 25A (per rail)

Would rather not upgrade right now if the numbers work out to be adequate.

Antec Earthwatts is definitely a good brand.

Yes rail arrangement can make a difference.

The Molex/SATA power connectors are typically connected to the same rail as the CPU and mainboard. The rest of the rails are typically either 6+6 PCIe or 6+8 PCIe as these keep the current with the ATX recommended maximum of 240VA on any one output wire. If you draw 150 watts from that connector on top of whatever it is that your CPU and mainboard are chewing up you may end up tripping the OCP.

 

[lemming9]

In the case of this PSU only the CPU 4+4 pin connector is on +12V2.
Everything else, like the 24-pin board/PCIe, Molex/SATA is on +12V1

If the CPU uses only 8A of 15A then that leaves 7A free on +12V2. Is this unused amount from +12V2 directly added to the amount from +12V1 to power the MB, HDD and GPU? If so it seems like it should be enough.

I don't mind if it trips the OCP. What I don't want is for it to damage the GPU. Can PSU using the 2 molex to 8-pin PCIe connector cause damage to a GPU or is the OCP sufficient protection to prevent that?

 

[Pinhedd]

In the case of this PSU only the CPU 4+4 pin connector is on +12V2.
Everything else, like the 24-pin board/PCIe, Molex/SATA is on +12V1

If the CPU uses only 8A of 15A then that leaves 7A free on +12V2. Is this unused amount from +12V2 directly added to the amount from +12V1 to power the MB, HDD and GPU? If so it seems like it should be enough.

I don't mind if it trips the OCP. What I don't want is for it to damage the GPU. Can PSU using the 2 molex to 8-pin PCIe connector cause damage to a GPU or is the OCP sufficient protection to prevent that?

No the unused current on +12V2 will not transfer to +12V1, that would violate the whole purpose of having multiple OCPs.

A failing GPU can cause damage to pretty much anything. With that said, OCP is designed to prevent that, and Antec power supplies are rather reliable. I think that you're really pushing this a bit too far, but if you insist on playing with fire, you can try and find a way to hook a molex up to each of the rails if there's connectors for it.

 

[lemming9]

I was really looking to confirm some things before I made any choices, like how much power a molex provides and if dual rail is really separate or just a split single rail. Some differing info out there but your replies have been helpful. Thanks.