Electronic – PSU: Is the DC load current the same measurement as the combined amperage output

power supply

In selecting the right Power supply unit for high performance system, I have learned that besides the watt all the amperage is an important measurement. The following site provided information or advice to look for a combined amperage output while selecting the Power supply unit.

http://www.newegg.com/product/CategoryIntelligenceArticle.aspx?articleId=199

Now, I have been looking for specs for PSU specifications for the PSU in question which is 825W, 80 plus from Dell (see link below)

http://www.itcreations.com/view_product.asp?product_id=37043

In here, the only figure that relates to ampere is the DC load current.

DC Terminal Voltage (V)/ DC Load Current (A)
12V (cumulative of 12V1, 12V2, etc.)
12.1/33.4 (in a typical usage scenario, drawing 457W and providing 416.41W)

So, to my understanding, the combined average amperage would amount to 33.4 in this case.

Is the correct, or do I misunderstand the figures / and if yes, how do I get the real combined amperage output?

Best Answer

The specified Dell power supply is rated for 12 Amperes supply current (from the AC mains) for 110 Volt AC mains, and 6 Amperes supply current for 220 Volt AC mains.

This translates roughly to 1320 Watts of maximum power drawn from the mains (typically at start-up).

Efficiency rating at maximum load is stated as 87%, hence peak output power is no more than 0.87 x 1320 = 1148 Watts instantaneous / start-up power.

All of this has little or nothing to do with the 825 Watts continuous power rating of the power supply, as we will see.

How does the 825 Watt value fit in with the current ratings for the various output lines of the SMPS? Actually, it never does.

The data sheet states that:

  • Each +12 Volt line supports 18 Amperes (7 lines) = 1512 Watts!
  • The +5 Volt line supports 4 Amperes = 20 Watts
  • The -12 Volt line supports 0.5 Amperes = 6 Watts

While it is tempting to simply add those values up to get an output wattage, that isn't how a PC power supply is expected to work. In reality, individual supply lines will have differing loads at various instants, with the transients being fulfilled by reservoir capacitors at these outputs. So long as each individual output is loaded within its specification, and the combination of actual loads does not exceed 825 Watts, everything runs smoothly.

Beyond that point, regardless of which specific outputs are most heavily loaded, the on board processor of the SMPS will throw an overload alert, and attempt to gracefully shut down the system.

For less sophisticated PC SMPS units, the "graceful shutdown" is a myth, but with server grade power supplies, there is typically control logic in there to notify the operating system of a power supply problem, and then downgrade or cut off specified outputs (look for the technical reference) until the load returns to the safe operating area. The server OS is in the meanwhile expected to raise appropriate alert flags, save machine state to disk, and shut down until manual intervention.

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