Re: Input power measurement

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


 > I don't know how well they stand up the the punishment of a TC, but
 > surely with all the coilers on the list here, there must be a few
 > who are already runnings coils off a domestic supply which is fitted
 > with solid state metering.  Has anyone ever blown their electric
 > meter?

I should think that the "industrial" watt hour meters used by the utilities
(even for residential use) need to be substantially more "bulletproof" than
the $40 Kill-A-Watt units.  To get a sophisticated meter from the utility
here that does time of use metering costs a regulated (hence close to
actual) hardware (not including installation) cost of around $500 (they only
charge it if you go back and forth between TOU and non TOU metering.. the
first install is free).  Given the wide variety of stuff that one can
perfectly legally attach to your power at home, the meters would have to be
fairly robust.


 >
 > Another approach would be to use an analogue multiplier IC fed from
 > a current transformer and a voltage transformer.  Maybe one of the
 > circuit design whiz kids could invent a standard model in kit form.
 > That would give you a cheap, self contained unit which would be easy
 > to calibrate, and might be better than fiddling about with modified
 > electric meters and frequency counters.
 >
 > This issue of input power measurement is the first stumbling block
 > on the road to comparative measurements on different coils, and
 > really should be made to succumb to a determined effort.

A very elegant solution is one described recently on this list: a pair of
transformer isolated sensors connected to a PC sound card.  Low parts cost,
and should be able to be made "PC death resistant" by a combination of
careful design and resistive isolation (i.e. a 10:1 resistive pad or
something)...

600:600 ohm low capacity high isolation transformers are readily available
surplus, very cheap (they're used in every modem for isolation from phone
line)...

A 0.1 ohm series shunt would produce 100 mV per amp, plenty of signal to
detect with a sound card.
The voltage divider for the potential might actually be more of a challenge,
especially driving 600 ohms.. Say you want 1000:1 (i.e. 120 mV for 120VAC)..
That's roughly a 600K series resistor for 0.2 mA.. I don't know about
weirdness with phase shifts from inductance of transformers though...

Another alternative for the potential side is to get a small power
transformer (i.e. one of those 2-5 Watt output things with 5-10V
secondaries). They don't distort the waveform too much.