Dell PSU Questions about Rails

clutchc

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I have noticed that most Dell Optiplex PCs' stock PSUs usually have their +12v supply split into 2 rails; A and B. Any idea how they are split up? CPU (A) and all the rest (B)? CPU/peripherals (A) and PCIe slot (B)? Vice-versa? Is there even such a standard to be assumed when trying to calculate loads?
 
Solution
What looks like two rails is really a UL split that is, in theory for safety reasons.
Do not worry about it at all.
If the psu has the watts/amps, it can deliver power to the supplied power leads.

Virtually all power supplies really have only a single power generating rail.
What looks like two rails is really a UL split that is, in theory for safety reasons.
Do not worry about it at all.
If the psu has the watts/amps, it can deliver power to the supplied power leads.

Virtually all power supplies really have only a single power generating rail.
 
Solution
Look on the label on the PSU. It should show the 2 rails. Usually 2x 18A. 12V. rails for a 300W PSU. The wiring will be yellow for one 12V. rail and white for the other. You can tell what's being powered by each rail by looking at the harness. Realistically Dell only supplies connectors on the PSU that stay well within those limits. Dell just keeps adding rails for extra items. A 425W will have 3 rails with blue wring for the third 12V rail and added GPU connectors.. 1000W has 5 rails. They used that PSU for multi CPU workstations and multi GPU Alienware system by supplying different wiring harnesses to split the rails differently.
 

clutchc

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@ William p
I know what you mean about the colored coded +12v rails on most Dells, but these I'm working with now don't list colors on the nameplate.

@geofelt
In that case, since the total capacity of the 2 x +12v rails is 240W, I should be able to assume that the max load of the +12V rail is 20A, yes? (240 ÷ 12)
 

Not if overcurrent protection kicks in. Though if multirails are designed properly then it shouldn't be an issue.


For 12V that is, since multirail is nothing more than protection circuit traces. But on certain units the 3.3V and 5V rails are still regulated independently of the 12V rail.

clutchc you'll probably be perfectly fine.
 
I know what you mean about the colored coded +12v rails on most Dells, but these I'm working with now don't list colors on the

They don't list the color codes, you just have to look at known 12V connectors, or 12V.pins on connectors to see which is which.
If you do the math you'll find that each 18A.x 12V. rail is 216W. That's why the 375W PSU from the T3400 has the same 2x18A rails as the Optiplex 305W. but adds a 75W GPU cable ( on the white rail). The optional 425W has a 3rd 18A. rail and adds another 75W GPU cable. The 525W. from the T35000 adds an 8 pin CPU connector but just 1 GPU cable on the same 3x18A rails. That's also where they start with replaceable harnesses.
https://www.hardwareinsights.com/dell-h305p-01-power-supply-review/