Aniseed wrote:

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.

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.

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:

monospace font please
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

All those figures seem quite plausible, IMO[*].

Lower back Light throbbing

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.

--
Andy