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Kill-A-Watt vs. analog wattmeter

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi all,

A while back I wrote about how I thought
Kill-A-Watt-type wattmeters could give inaccurate
readings when used with SSTCs. I recently found a
Kill-A-Watt type thing for sale in the UK, so I bought
one to test. (Maplin part no. L61AQ, costs 9.99 GBP)


It's made in China by the Prodigit Co. which if I
remember right is the same company responsible for the
original Kill-A-Watt. I opened it and inside was a
microcontroller plus some chip that I guess is an A/D

I set up the DRSSTC, and ran the mains feed to it
through the Prodigit meter, in series with my old
Crompton Parkinson 2-coil wattmeter. This is totally
air-cored so shouldn't be affected by high crest


I then fired up the coil to a reasonable power and the results were:

Analog meter read "150" which is 750w since you
multiply the reading by 5.

Prodigit meter read 746w, 1140VA, 6A RMS, pf=0.62

The low power factor means that the DRSSTC was drawing
awful peak currents, but the two meters still agreed
well. I conclude that the Prodigit wattmeter sold by
Maplin works fine for Tesla coiling. It also seems to
be fairly resistant to RFI, although I didn't let the
coil arc to ground heavily.

Steve Conner