Re: Wattmeters

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Original poster: father dest <dest@xxxxxxxxxxx>

Tl> Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Tl> I just got hold of an old-style moving-coil wattmeter which I intend
Tl> to use to measure my coils' power draw from the mains. It's totally
Tl> electromechanical with air-cored coils- no electronics to overload or
Tl> iron to saturate- so I expect it to read accurately even at high
Tl> crest factors. I've got it hooked up right now and it says the PC I'm
Tl> typing this on is burning 70w. I checked it against a true RMS DMM
Tl> using a resistive load and it seems good within 5%.

i`ve got an ferrodynamic wattmeter (50 hz) - it has "iron to saturate",
nevertheless it doesn`t harm to work properly.  the experiment was the
following:
i`ve connected an electric heater (220v 500w) throught the variac to
the mains. then, i bring a variac to  220v - wattmeter showed 460w.
after this, i took a bridge rectifier and electrolytic caps - total
capacitance 2000 mF, connected the heater after the bridge, and the
wattmeter - beetween the bridge & the variac, move the variac to
make dc 220v on the heater, ampermeter in series with the heater
showed 2.08a.
wattmeter showed me the same 460w. i don`t remember the equation for
calculating peak current charging the caps,  but i guess it was more
than 5a, but 5a - it`s limit to my wattmeter.

in generally - how wide is the spectrum of the current drawing by real
TCs - without mains filters, power correctors? i`m speaking about
classic and transformerless coils - could we use usual 50 hz
wattmeters? coz even in your wattmeter the coils inductances could
harm the measures begin from some frequency.

Tl> I would go out and buy a Kill-A-Watt, but they don't sell them over
Tl> here in the UK. I can get other brands of electronic wattmeter but
Tl> they're not the ones you all use ;)

why don`t you take a cheap four-quadrant multiplier (ex. AD633) and
make a simple wattmeter scheme, so  that even a beginner could make
it?  there`s a "Terry filter", but you could make a "Steve meter" -
then you  could know exactly, what "we all use" ;-)