AC Ripple and Power Factor

AC Ripple and Noise on the DC Outputs

The amount of AC ripple and noise present on the DC outputs was checked using an oscilloscope. This AC component may be present in the KHz range where most switching power supplies operate or it may be more prevalent at the 60 Hz line frequency. I adjusted the O-scope time base to look for AC ripple at both low and high frequencies.

The new ATX12V V2.2 specification for DC output noise/ripple is defined in the ATX12V Power Supply Design Guide.

Ideally we would like to see no AC ripple (repetitive) or noise (random) on the DC outputs – the cleaner the better! But in reality there will always be some present. I measured the amplitude of the AC signal (in millivolts, peak-to-peak) to see how well the power supply complied with the ATX standard. The following table lists the ripple/noise results during all of the load tests for the main output voltages of interest.

We were curious to see how the Nightjar 500W Fanless power supply would perform during this test. Several other hardware review sites that tested the ST50NF last fall found the AC ripple component on the DC outputs to be on the high side. SilverStone stated they have addressed this issue and tweaked the design so the AC ripple is well within spec on all current shipping units. SilverStone further told us that if a customer has an ST50NF that is out of spec, they will be happy to exchange the unit for a new one. While the AC ripple is still a little higher at max load than we would like to see, all of the AC ripple numbers for the ST50NF are in fact well within the recommended ATX guidelines.

Power Factor (PF)

Power factor is defined as the ratio of true power (measured in watts) to apparent power (measured in Volt Amps). It measures how effectively AC power is being used by a device. The difference between true power and apparent power is expressed as the power factor and results from the way true power and apparent power are measured. Ideally we would like to have true power and apparent power equal to one another, which would result in a PF of 1.00 or 100% effective power utilization.

AC Volts x AC Amps = VA (Volt Amp)

Purely Resistive AC Load: VA = Watts (same as DC circuits)
Inductive/Reactive AC Load: VA x PF = Watts

AC Volts x AC Amps x PF = Watts

I measured the AC Power Factor with an Extech power analyzer at both 115 VAC and 240 VAC input voltages. The Nightjar 500W Fanless power supply uses Active PFC circuits so as expected; the majority of readings were at or close to 1.0 at the higher loads.

Note: A power supply with active PFC is more environmentally friendly (doesn’t pollute the AC transmission grid with harmonics) and will draw less current, but it will not save you money on your monthly electric bill unless you are a commercial user whose bill is based on PF and usage.

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