Is this sort of plug-in power meter suitable for measuring a desktop PC with a switched mode power supply?
I bought something similar a long time ago but found it was giving misleading readings. Are these better now?
Switchmode power supplies don't seem to adversely affect the supply electricity meter, but I am sure a large inductive load with a directly chopped supply would affect everyone up the road.
I don't think they are particularly sophisticated (and I have a couple that look exactly like that) and may have difficulty reading much other than a near resistive load with any real / absolute accuracy.
However, I find them fine as a general indicator of power consumption, especially when comparing like with like (SMPSU powering a Raspberry Pi or a PC etc).
I can't think if I've ever measured something that was actually rated that didn't also have variables / additions that could have impacted the result or if I have, I didn't note that it was far away from what was stated (suggesting they were accurate and the meter also reasonably so).
I think the last things I measured / compared were a RPi NAS, a slimline Shuttle PC running as a NAS, a Synology NAS and a Buffalo Terastation NAS and the results were about what I expected. As mentioned though, you often had to take several readings with the HDD idle, unit sleeping etc.
Cheers, T i m
I have a meter (from Maplin) which measures power in both W and VA, one assuming that current and voltage are in phase and the other taking into account the fact that they are not. Real power [W] = apparent power [VA] * power factor.
I presume it is the real power that an electricity meter registers and that you want a plug-in meter to read.
I can remember when I worked at a factory which made tvs back in the valve days, they used an autotransformer, so chassis was live and a half wave rectifier. They ran soak tests on the sets in huge banks, and got a visit from the Electric company about them squashing the half phase they wre on they had to have a three phase supply fitted and run some off of each phase. What I never quite got was that most makers at the time did the same thing so out in homes these sets were all doing this, surely? Brian
Yup. I'm guessing about half the sets used one polarity, half the other, at least for HT. The heaters used both.
NT
Big Clive review
According to Big Clive it has a dedicated power analyser chip with a 100 page data sheet.
Cool.
I guess what I meant was 'I don't think they are particularly sophisticated' ... compared with professional and more sophisticated kit that costs many times more and probably comes with a 100 page instruction book and a certificate of calibration ...'. ;-)
If they were as good for that price that would be nice to know.
Because the 'cheap stuff' is made down to a price rather than up to a spec (or a much higher price ), they often fail / become less accurate at the extremes, or when measuring the more awkward stuff. All down to the laws of diminishing returns on the cheaper gear.
Cheers, T i m
In an home environment, they would be randomly spread across the three phases. There would still be the issues of drawing current on one half of the cycle though.
You would like the meter to measure W and VA, so that when picking a UPS (uninterruptable power supply), you measure both quantities and pick a UPS which deals with the more taxing one.
Sometimes the loads have more W, sometimes more VA (relative to the ratio of W to VA the UPS offers).
They typically use dual sigma-delta converters, with maths in firmware. And the maths in firmware, have been slightly tweaked over the years for better accuracy. (This is what happens when your privately held maths are exposed to public scrutiny.)
Yes, the meter is good, and fit for intended purpose. With better quality analog components used inside, the accuracy might be improved slightly. But not in a way that affects the gross improvement these meters bring (7W reading on the above meter, versus 100W approximate reading with separate regular meters).
Using an ordinary digital meter to do volts, a digital meter to do amps, the calculation of VA resulting, the 100W, is dead wrong. You can validate the 7W number, against the known loads inside the (sleeping) PC at the time.
Vetting a PC in S0 run state, that's harder to do. That's why I recommend checking PC in sleep state, then compare to any other method you want to cook up for sleep state, to do the same task.
The refresh power for DRAM chips is known and on the datasheet. You can compare that load, to what the meter indicates.
You can also apply DC loads to USB ports, then figure out roughly what kind of efficiency exists in the +5VSB converter (a separate converter, separated from the main converter). In the old days, +5VSB was only 50% efficient (terrible).
Paul
Very useful. Thank you.
Clive says at the end: "It's actually really well made. It's accurate and works surprisingly well". (25 min) That's all I need to know.
Mind you I did once follow Clive recipe for porridge which wasn't a total success. Using raisins was okay but his suggestion to add Coffee Mate didn't work for me.
No. Power is Watts, and VA is not Watts.
Power is the rate of delivering energy, and one Watt is needed to accelerate one kilogram by one metre per second per second.
Electrically, average Power W is the average of instantaneous (Voltage x Current); and VA is Root-Mean-Square Voltage x Root-Mean-Square Current. Power Factor is the ratio of those two, W / VA
That is NOT what it registers. It registers real Energy, in real kilowatt hours.
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