Help deciphering/modding PSI with multiple12V rails

[wickit]

I made a workstation, will mount the PSU to the underside. The PSU is an 240 watt HP DPS-240MB-3A 460974-001.

For my purposes I need to draw 100-110watts of 12V power (96 watt thermocoupler and 4 fans)

The specs show is having a 7.5A +12V Rail and a 11.5A on "+12VCPU" ...its too cheap to have 2 seperate 12V rails.

First Question how do I tap into the 11.5amps.
(I assume by the way the specs are listed the molex lines or pcie lines can only supply 7.5amps or....?)

Second question, this came out some crap SFF HP case, that low rpm 80mm fan is both the PSU cooler and the only case cooler. Since Im not using it in a case, do I still need a fan? (Without the fan I can mod it to be an unobtrusive rectangle, with the fan the foot print is bigger than a standard ATX PSU

 

[DR4G00N]

I would assume it means exactly what it says on the label, cpu 4-Pin has one rail and everything else uses the other. It's a Delta unit so it wouldn't surprise me if it does actually have two 12V+ rails.

 

[Bill_Bright]

Not sure why the fact it is made by Delta would suggest it is a multi-rail supply. For the general consumer market, Delta has never been known for making anything other than low cost PSUs, and HP has never been known to spend an extra penny it does not need too. "True" multi-rail PSUs require a more sophisticated design and use more parts, therefore cost more.

Sadly, the term "multi-rail", when used in power supplies, is often a misused term, distorted by marketing weenies. In effect, the +5V and +3.3V voltages are often supplied on separate rails, separate from the +12V voltage. And so in the marketing hype, the 3 output voltages are some times worded to suggest it is a multi-rail PSU for the 12V side too.

You rarely see "true" multi-rail PSUs in low-power supplies either. Note in "true" multi-rail PSUs, the rails are isolated - again adding to the complexity, thus parts, thus costs. And typically, if it really was a multi-rail PSU, the marketing propaganda would say so. But as seen here, it doesn't.

So I agree with the OP and it is two cheap to have truly separate rails.

This means the current supported by each connection is simply being limited to ensure there will always be enough current available on the other power connection - even when that connection is unused! That's a waste. And that's why you don't normally see true multi-rail PSUs in today's quality PSUs either. It was a passing "marketing" fad. A big single rail is better because it means the full capacity of the supply is always available.

First Question how do I tap into the 11.5amps.

What do you mean by "tap"? That suggests to me cutting wires or PCB traces and splicing in new connections. Or "tapping" off the output side of the transformer windings. I would just use the existing connectors.

econd question, this came out some crap SFF HP case, that low rpm 80mm fan is both the PSU cooler and the only case cooler. Since Im not using it in a case, do I still need a fan?

Probably. Fans are used in power supplies typically to save space and money. "Passive" (no fan) cooling requires massive heat sinks and unrestricted ventilation to allow the heat to escape from the heatsink and rise up and out of the case. Fans are cheap - especially cheap fans. Big hunks of aluminum are more expensive for the raw materials, and to manufacture into a heatsink. Then big heatsinks take up more space than a flat fan.

If you can get some, even a little, air flow over the heatsinks from another fan source, you might be able to go without the PSU's fan. But frankly, another thing budget PSUs tend to lack is quality and efficient thermal protection circuitry. You don't want this unit to over-heat. I'd keep the fan.

 

[wickit]

...So I agree with the OP and it is two cheap to have truly separate rails.


What do you mean by "tap"? That suggests to me cutting wires or PCB traces and splicing in new connections. Or "tapping" off the output side of the transformer windings. I would just use the existing connectors.


Probably. Fans are used in power supplies typically to save space and money. "Passive" (no fan) cooling requires massive heat sinks and unrestricted ventilation to allow the heat to escape from the heatsink and rise up and out of the case. Fans are cheap - especially cheap fans. Big hunks of aluminum are more expensive for the raw materials, and to manufacture into a heatsink. Then big heatsinks take up more space than a flat fan.

If you can get some, even a little, air flow over the heatsinks from another fan source, you might be able to go without the PSU's fan. But frankly, another thing budget PSUs tend to lack is quality and efficient thermal protection circuitry. You don't want this unit to over-heat. I'd keep the fan.

Z It's not going to power a computer, so I will be cutting and capping the unused lines. By "tap in" I mean which wire/wires are needed to get the max 11.5. At face value the spec "12V = 7.5A" "+12VCPU=11.5A" implies the 12v on the molex line maxes at 7.5A And I guess the 26pin atx power connector can supply 11.5amps(id that right?). The Atx connect has 26wires, two are 12v. does that mean I could get 11.5 Amps our off one of those atx pin, or do I have to solder those 2 together....?

2) Since it will be on a desk I think a 30mm fan from a basic vga card will fit in the fan compartment and let me cut the power supply top flat. I can still have 1.2 cfm flow and always running under 45% max load, "I THINK" its sufficient.

 

[Bill_Bright]

I would not dare to venture a guess on the pin-out of any connector of any PSU brandishing the HP (or Dell) logo. Both companies are infamous for using proprietary configurations in noncompliance with ATX Form Factor standards.

The fact it is a 26-pin and not a standard 24-pin already smells proprietary.

That said, note that graphics cards are often the most power hungry devices in our computers. And PCIe connectors support a maximum of 75W through the connector and associated motherboard traces. This is exactly why the ATX Form Factor standard for power supplies require they provide a separate power connection for power hungry graphics cards - so motherboard circuit traces are not melted (if lucky, melted is the only damage) due to the large amount of current demanded by the graphics card.

If me, I would not try to get this power through the 24/26 pin main power connector. Instead, I would look to use the 6-pin or 8-pin (6+2) PCIe graphics card connector, or the 4+4 pin ATX 12V connector - assuming the supply has one of those and they are wired to industry standards - not a given.

 

[wickit]

If me, I would not try to get this power through the 24/26 pin main power connector. Instead, I would look to use the 6-pin or 8-pin (6+2) PCIe graphics card connector, or the 4+4 pin ATX 12V connector - assuming the supply has one of those and they are wired to industry standards - not a given.

yup I'm an idiot. you're defiantly right.

I haven't seen a budget PSU in years, It doesn't have PCIe connector, so I assumed the ATX connector was PCI-e. If any one connector can out put over the full 12v load it's that one.

 

[Bill_Bright]

When I was referring to PCIe connectors above, I meant the PCIe "slots" on motherboards - as they get their power via traces on the motherboard and through the main 24-pin power connector.

The PCIe "power" connector coming directly from the PSU should indeed, as you noted, support significantly more current.