If it did this, it would under estimate rather than 3 times over estimating. That's why I said the form of the error was puzzling.
Yes, although if you don't sample fast enough to see the general form of a rapidly changing value, you will get a completely misleading impression (lookup Nyquist's Theorem).
No it wouldn't. It could have lots of amps flowing at 240 volts but with (say) 10% power factor the Volts*Amps would be way more than the power.
I was thinking of 'averaging' as in a moving coil meter and rectifier, that doesn't sample in the sense you mean at all, it genuinely averages the current (or voltage).
The example was a SMPSU, not a capacitive or inductive load. Averaging a SMPSU current pulse or failing to take into account it only happens when mains voltage is around 340V would give an under reading. I can't imagine what mistaken assumption they've made which results in such a load 3 times over reading.
To quote from an earlier post I made on this. "
A quick patch involving a 1.8 ohm resistor in the neutral lead and resistive dividers protected by zeners indicates that the current has a really odd shape.
The couple of switchers I checked exhibited the expected short half-sine looking pulses at maximum voltage. The power indicated by the meter was within 20% of my eyeballing the scope.
On trying it on my PC, I found a rather odder shape sort of sharks-fin like shape. The current rises to a peak of 2.2A in .4ms, then decays over the next couple of milliseconds more or less linearly to 0.6A, then drops in 0.5ms to 0.
That's a total of 2.25Ams per half-cycle, at 320V, that's .721Ws per half-cycle, or 72W when idle.
The meter however reads 138W.
And in suspend, it's around 30W, around 10W 'off'.
"It may well be assuming that the current will be either a 'normal' sine wave, with a power factor of whatever or a sharp chopped-sine pulse near the wave peak.
This worked fine, until various regulations about harmonic content on power supplies came in. Then the PSU makers started working out the cheapest way to meet these directives. This ended up as very odd waveshapes.
As to why. Well, let's assume that it samples at the peak of this pulse, 2.2A.
Let's try 2.2A * 2ms = 4.4mAs.
4.4mAs * 320V = 1.4JSurprisingly this works quite well.
However, I suspect the real answer is that it's trying to make a nice smooth wave out of it. To get an accurate number, you're going to need to sample at better than
1Khz.Alternatively, if it's sampling at 500Hz or so, and trying to assume that the waveshapes may be symmetrical (be it intentionally, or through filters) then it may get vastly misleading answers.
Especially as we're not talking about a 2000 quid scope that's had hundreds of man-years spent on the design, and verifying it's correct, but maybe a few hundred hours and checking on a few loads around the office.
Anyway, hypothesising on how it fails is kind of pointless, in the face of the fact that it can't be repaired (practically)
With my observations, it diddn't, but was consistent within a range of voltages (about +-2%) and with a 1.8 ohm resistor wired into the mains lead.
Hi again, Yesterday I recieved one of the Maplin Power meters in the post. It is made in Taiwan, by Prodigit Elecronics Co Ltd. Its of reasonlably good quality build, given the price.
I've tried it out on stuff around the house. After much bending over plugging and unplugging things, I've acquired a list of results and a backache. If anybody is curious, here is what I found out:
Appliance: Reading: Kettle rated at 1800-2200W 1980W Desklamp with 60W bulb 58W PIII Dell Laptop 20W idle PentiumPro 200MHz, 1HDD 40W idle Athlon 1.4GHz, 1HDD 90W idle, 105W runing sims2 Pentium IV 1.8GHz, 2HDDs 75W idle, 100W running hl2
22" Iiyama CRT 5W standby, 70-100W on Lower back Light throbbingI can see that the device has correctly measured the kettle and desklamp, because the reading I get matches their power rating. However I don't know how accurately it is measuring the various computers and the CRT monitor. I hope it is accurate, because I was pleasantly surprised to see that the monitor consumes about 75W when displaying my windows desktop, I thought it would be higher.
However:
So it seems I can't trust the measurements from my device (yet). Would anyone know where I can learn more about switched-mode PSUs?
Aniseed
Yes, I've got one now, thanks to all the plugs (sorry) here. I think it's absolutely fantastic value for money, compared to any professional instrument (c.f. the price of RS stock no. 292-1274). I've not done any detailed checks on it yet, but all the readings on stuff around the house (like yours) seem about right. I've yet to try it though on any SMPS loads with a high current crest factor and harmonic content though...
One clear limitation is a lack of resolution and the presence of offsets at the low current end of the range. Mine tends to read 10 mA and 2 W with no load connected, so the overall accuracy for powers under 10 W is going to be pretty poor. There's a table of accuracy figures on the instruction sheet, to which I'd suggest adding plus or minus 2 counts on the LSD. If I find the time I'll experiment with adding an outboard
10:1 (or even 100:1) current transformer, which should dramatically improve its performance at low power levels.All those figures seem quite plausible, IMO [*].
Display readability is not its strong point is it? But for £10.63 + VAT you can't complain!
[*] Below, FYI, is a re-post of some figures I took a while ago and posted here in Sept '03.. Power Current Earth Equipment /W /A VA PF /uA
-------------------------------- ----- ------- ----- ---- ----- Old sys unit (P75) idle 30 0.185 44.4 0.68 30 Old sys unit (P75) CPU 100% 39 0.233 55.9 0.70 30 Iiyama 17in. CRT monitor 90 0.566 135.8 0.66 60
New sys unit (P4/2.8GHz) idle 100 0.505 121.2 0.83 30 New sys unit (P4/2.8GHz) CPU 100% 155 0.800 192.0 0.81 30 Dell 18 in. TFT monitor 42 0.268 64.3 0.65 40
PSU for computer speakers 4 0.048 11.5 0.35 -
HP LaserJet 4 printer (warm up) 790 3.300 792.0 1.00 140 HP LaserJet 4 printer (standby) 35 0.239 57.4 0.61 140
Notes
----- Power measurements taken with a Feedback Instruments EW604 wattmeter. Current figures are true RMS, taken with a Fluke 87.
VA figures are apparent power, calculated assuming Uo = 240 V (i.e. the approximate actual supply voltage, rather than the nominal 230 V).
'PF' column is power factor (W / VA).
'Earth' column shows protective conductor current (in microamps) taken on a Fluke 77 (mean responding). Note that the measured figures are far less than is commonly assumed in threads about spurious RCD trips.
Figures for the monitors were with 'typical screens', not max. brightness & contrast.
'CPU 100%' figures taken with the SETI at home client running.
There's obviously been a run on them since the initial post - I've ordered mine but there's a 28 day delay!!
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