Load Tests

We made several tests with this power supply as described in the article Hardware Secrets Power Supply Test Methodology. All the tests described below were taken with a room temperature between 45º C and 49º C. During our tests the power supply temperature was between 48º C and 52º C.

First we tested this power supply with five different loads patterns, trying to pull around 20%, 40%, 60%, 80% and 100% of its labeled maximum capacity (under “% Max Load” we list the actual percentage that was used), watching how the reviewed unit behaved under each load. On the table below we list the load patterns we used and the results for each load.

+12V2 is the second +12V input of our load tester and on this test it was connected to the power supply ATX12V connector. Since this connector was the only one connected to the power supply +12V2 virtual rail the +12V1 and +12V2 inputs from our load tester were really connected to the +12V1 and +12V2 virtual rails from the reviewed power supply.

If you add all the powers listed for each test you may find a value different from what posted under “Total” below. Since each output can have a slight variation (e.g. +5 V output working at 5.10 V) the actual total amount of power being delivered is slightly different from the calculated value. On “Total” row we are using the real amount of power being delivered, as measured by our load tester.

We must say that ST-420BKV surpassed our expectations. We were expecting a low-end power supply with a lousy efficiency and not being able to deliver its rated power. Even though efficiency was below 80%, it wasn’t below 70% as we were expecting, so this unit isn’t that bad, especially when we think about the design that was used. And it could deliver its nominal power with a room temperature of 49.5º C, which is impressive.

Voltage regulation was also one of the highlights during our tests. All outputs were within 3% of the nominal voltage during all tests, which is outstanding, as ATX spec states that regulation should be within 5%. Translation: the voltages were closer to their nominal values than what is stated by the ATX standard. The only exception was -12 V output during tests one and two, which was at -10.96 V and -11.40 V, respectively. Even though these values are still inside the ATX specification (-12 V has a 10% tolerance, while all other outputs have a 5% tolerance) we wanted to see values closer to -12 V, especially on the first test.

Electrical noise was also at a very low level, always below 29 mV on +12 V, below 26 mV on +5 V and below 22 mV on +3.3 V – ATX spec states a maximum noise level of 120 mV for +12 V and 50 mV for both +5 V and +3.3 V outputs. We wanted to see lower noise levels on +5 V and +3.3 V, even though they are within specs – very good power supplies have a noise level below 10 mV on these outputs.

Below we show the noise level we found on the power supply outputs while the unit was operating at its full load (test number five): +12V1 rail was at 28.6 mV, +12V2 rail was at 26.8 mV, +5 V rail was at 25.6 mV and +3.3 V rail was at 22 mV.

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Figure 15: Noise level at +12V1 input of the load tester.

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Figure 16: Noise level at +12V2 input of the load tester.

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Figure 17: Noise level at +5V input of the load tester.

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Figure 18: Noise level at +3.3V input of the load tester.

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