Re: I am such a dick!

The message <part1of1.1.v74PtK3RzbivDw@xxxxx>
from Dominic <news@xxxxxxxxxxxxxxxxxxx> contains these words:

Johnny B Good <jcs.computersbutt@xxxxxxxxxxxxxxxx> wrote:
The message <part1of1.1.05IjHeAfVLBAsA@xxxxx>
from Dominic <news@xxxxxxxxxxxxxxxxxxx> contains these words:
I also use energy saving lightbulbs, and bugger the
fact that they have a horrible power factor

Only the first generation CFLs limited to 20 watts max to take
advantage of the regs pertaining to domestic use permitting such loads
to be without a PF correction capacitor would present such a problem.

The reason you never saw such CFLs rated any higher than 18 watts was
to guarantee that it would keep below the twenty watt limit even at the
highest permitted supply voltage and worst case manufacturing
tolerances.

The more modern electronically ballasted lightweight CFLs have a PF of
nearly unity even if the commutating effect of the full wave rectifier
on the current is to produce narrow (but still in-phase) conduction
angles. The conduction angle would be even narrower except for the fact
that our so called 50Hz 'Sinewave' mains supply is quite grossly
distorted with the peaks being rather flatter than a real sinewave.


Fair enough then. I still wonder if the manufacturing costs of CFLs
outweighs the power savings; assuming that you replace them
when they get annoyingly dim which I suspect is rather sooner
than the '8 years' life quoted on the box...

I suppose with the number of non-linear loads on the mains
(transformers/bridge rectifiers and switch mode supplies) it
might be surprising if the mains stayed a perfect sine wave. I've
not recently connected a suitable scope input to the mains...
always assumed the nasty mains cycle shape you see (by
touching a scope probe, for example) was due to poor coupling
of the waveform through my body into the scope.

I used to assume the same for the 50Hz 'calibration source' on my scope
(about 2 or 3v off a seperate dedicated winding on the PSU mains
transformer) but I got rather suspicious when the waveform looked
exactly the same using an independent mains stepdown transformer.

This led me to examine the 'sinewave output' of a secondhand Upsonic
UPS600 unit I had handy (I'd previously witnessed the 'horror of quasi
sinewave' output on an even more elderly Emerson UPS, still in service
today, so wasn't expecting much by way of sinewave purity).

The result, to my astonishment, was the purest looking 50Hz sinetone
I'd ever witnessed on any scope, even when using the scope's own 50Hz
calibration source. IOW, a true sine output UPS gives purer mains
voltage than the mains supply itself when running off its battery.

Must try a toy energy meter (like
http://cpc.farnell.com/jsp/Electrical,+Lighting+&+Security/Mains+Plugs+&+Adaptors/PRO+ELEC/2000MU/displayProduct.jsp?sku=PL09564
) and check out some of the appliances (and light bulbs!) at home.

"Toy energy meter" is a very apt description. They're not too bad for
non reactive loads above 10 watts such as lamp and heater filaments, but
can show anomalous readings for reactive (and narrow conduction angle
current) loads below 50 watts. The main problem with the cheaper digital
power meter is the low sampling frequency and resolution of the ADC
components used in these designs.

Trouble is, to get a decent digital powermeter, you need to spend
hundreds of pounds rather ten or so (or else buy a seconhand analogue
wattmeter for around 30 to 50 quid if you only need +/- 3 % (but
consistent) accuracy.

The major weakness with the analogue type is that loads with extremely
narrow conduction angles can cause under-reading errors due to
saturation of the magnetic material used in the field part of the meter
by the extremely high current spikes that can result.

Fortunately, this effect usually manifests itself as an audible buzz
from the meter itself and, accepting a reduced accuracy of resolution
due to the lower scale deflection, can be overcome by increasing the
current range to the next higher one than would usually be used for a
less extreme load. IOW, even this limitation can be compensated for (if
in doubt, try the next amps range setting and compare results).

About ten years ago, I invested £35 in a secondhand german made
'Metrawatt' (a radio ham rally purchase). It did seem, at the time,
rather a lot of money to spend on a meter that I reckoned would only
find occasional use but it turned out to be a very useful diagnostic
tool for investigating a surprisingly wide range of PC problems. I don't
know how I'd manage without my trusty Metrawatt now!

I got hold of a cheap (£9.99) meter a few months back (the Wetekom
PM-30-UK model) similar to that cpc offering (basically the same spec).
BTW, I was rather amused by the 'power consumption' for the cpc one
being given as 20VA (Don't they know the unit of power is the watt? :-).
That alone should shatter any illusions over the accuracy of such a
product.

TBH, _this_ was more of a 'novelty purchase' to satisfy my curiousity
over the 'accuracy' issues that had occasionally been mentioned of
similar cheap 'n' cheerful 'energy monitors' in various news groups.

Anybody using such a meter without the luxury of witnessing the
discrepencies between it and a real wattmeter with low power devices
would end up making some rather strange conclusions regarding the power
consumed by plugtop powered and similarly low powered electronic
devices. As a test meter for such kit, these meters are pretty well
worse than useless and certainly no substitute for 'The Real Thing'.

If you just want to monitor total daily or weekly power consumption of
your electrical white goods appliances such as washing machine, kettle,
toaster, fridge or freezer (or even your "George Formby" lean 'n' mean
grilling machine :-), then it will do a reasonable job. Any other use,
though, is likely to produce untrustworthy results, so beware!

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

.

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