To measure the hard disk drives power consumption as precisely as possible we put together a simple electronic circuitry that allows us to register random currents varying at high frequency. The main problem is that they normally use an oscilloscope for measurements like that, however, it should receive voltage, but not current. So, it means we need a converter to transform current into voltage.
The latter uses two shunts with 0.05Ohm resistance that are connected to the power cable of the tested hard disk drive. As a result, the voltage on the shunt drops by 0.05V per each Ampere of the HDD current. The operational amplifier (LM324N) multiplies the shunt signal by a little less than 20, so that in the end we get the voltage proportional to the current consumed by the HDD scaled as 0.96V per 1A. Moreover, 0 HDD power consumption corresponds to 1.525V output voltage in our circuitry that is why the obtained signal is translated from volts U into amperes A using the following formula:
I = (0.96*(U-1.525)) А
We use Velleman PCSU-1000 oscilloscope that registers the voltage output by the above described circuitry to measure the exact current varying at high speed. The oscilloscope time base setting or sweep speed is set at 0.5msec/div (digitization frequency equals 250kHz, which is enough to register the signal with up to 125kHz frequency), the sensitivity is set at 0.5V/div. Oscilloscope works in auto trigger mode and the taken oscillograms are then submitted to special software for further processing. This software translates the volts from oscilloscope into amperes using the above provided formula and calculates the average and maximum values. At each stage we take 180 oscillograms to ensure precision (each measurement takes 60 seconds, each second the program requests three oscillograms from the oscilloscope). Each oscillogram is 4000 points long, i.e. the end result is calculated based on 720 thousand measurements of immediate consumption current. You may increase the number of measurements if necessary. Since the oscilloscope we are talking about is a dual-channel one, we can simultaneously measure the HDD power consumption along the +5V and +12V power rails using two I/U converters.
The program processes our measurements and reports the average current along +12V and +5V power rails in amperes (and the corresponding power in watts) as well as maximum registered current values.
The above described circuitry is connected to the hard drive right inside the computer – into the feed circuit. As a result, we can measure the HDD power consumption under any type of workload emulated in such tests as Intel IOMeter, for instance.