# Output Voltage vs. Firing Rate (fwd)

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From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Sunday, August 16, 1998 4:27 PM
To:  Tesla List
Subject:  Re: Output Voltage vs. Firing Rate (fwd)

Hi all,
There is a more complex and convoluted way to get an accurate
power reading than a standard power meter if you don't mind a bit of
math. Alternatively, you can simply get an indication of what kind of
waveforms you are attempting to measure. Use an oscilloscope to
simultaneously watch the input current and voltage waveforms and
their relative phases. If they are supermiposed, you can
mathematically derive a true peak, RMS and average power measure for
You wish to multiply and sum the common area enclosed by the two
waveforms.
I was shocked to see the current waveform of a single microwave
transformer when I put the scope across a current sensing resistor
for the first time. It was rich in harmonics and peaked at around 20A
off a 230VAC supply despite being rated for a load of 900W or so.
Highly non-sinusoidal.
NB - either the scope or the Tesla power transformer must be
isolated from the mains. I prefer to isolate the power transformer to
keep the scope at ground potential. One leg of the transformer has
both channel earth clips connected, one signal lead goes to the other
side of a resistor (say 0.1 Ohm) connected from that point to one
side of the power transformer primary (gives the current waveform),
and the other signal lead to the other side of the power transformer
to get the voltage waveform.

If you are not sure what you are doing, please don't try this.

Malcolm

> From:  Harri Suomalainen [SMTP:haba-at-cc.hut.fi]
> Sent:  Saturday, August 15, 1998 7:29 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Re: Output Voltage vs. Firing Rate (fwd)
>
> >I hadn't realized about the rms problem.  This wattmeter says it's
> >accurate to 1/4 of 1 % up to 125 Hz.  I wonder how severe the errors
> >might be?  Has anyone compared such wattmeters with true (electronic)
> >reading wattmeters in TC circuits to get a ball park figure of possible
> >errors?
>
> That depends a lot on meter. I just ran into a very affordable kWh/wattage
> meter. Some investigation proved it would be practically useless with
> any waveform disortion. Yet, it was specified to have 3% accuracy!
>
> With high-frequency signals meter bandwidth may also be limitation
> with many meters, including some electronic meters which might
> be slow sample-multiply-indicate systems.
>
> >Also any clue as to how inaccurate the rms technique might be
> >(ignoring power factor issues for now)?
>
> One easy way to do a power meter is to measure current and voltage
> (by transformers, resistive dividers or whatever) and then multiply
> those. Multipliers like MC1495 should give you pretty nice bandwidth
> (about 1MHz or so). Making such a meter would not be too difficult.
>
> Such a meter would be able to show true VA or wattage depending
> on the output section. I'd go for that method if power was interesting.
> Lots of examples are available in the literature.
>
>
> --
> Harri Suomalainen     mailto:haba-at-cc.hut.fi
>
> We have phone numbers, why'd we need IP-numbers? - a person in a bus

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