RE: [TCML] RF Ground and Brass

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Bart,

Sorry, but a peak secondary base current of 5A on a disruptive coil does not at all equate to an RMS current of 3.85A.  It does not help to say that the current is time dependant, even though it is.  The definition and common use of "RMS" does not allow for that.  Only if the secondary current was a flat, CW 5A sinusoid could one say that it has an RMS current of 3.85A.

One way of looking at RMS current is that if the highly irregular base current is in fact 350mA RMS, then that current flowing through a light bulb or resistor would heat it to the same degree as a steady DC current of 350mA.  A 5 Amp sinusoid flowing through a light bulb would heat the filament to the same degree as a 3.85A DC current.  That's why RMS is used - it provides a DC or steady-state equivalent.  If my base current was actually several Amps RMS, then I would have blown out my flashlight bulb, so there is no doubt that that measurement is in the ballpark.

Unless you have a digital scope that can render a true RMS value, a bolometric metering solution (i.e. light bulb with time-averaging light sensor, or resistor with thermistor), or an accurate waveform simulator that can calculate the RMS value, the best that you can do is to just state the peak current.  Proclaiming an RMS value based only on the peak value of an irregular waveform can be nothing more than wrong and misleading.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of bartb
> Sent: Friday, March 07, 2008 12:38 AM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] RF Ground and Brass
>
> Gary,
>
> Yes, Terry's value is 10Ap-p, 5Ap, 3.85A rms. Is that better? I simply
> grabbed the page to link in the post (simply to show several amps of
> base current) [I hope I still understand base current otherwise I will
> go back to school and shoot my the teachers and burn my books.]. I guess
> I should have deduced detail so I could have said "3.85A rms" on Terry's
> coil!
>
> I don't know how many times now I have said "time dependent". At least 3
> or 4 emails now. I've said all I can about that.
>
> An rms value in the mA range: Is that Ambient temp dependent? Current
> dependent? BPS dependent? Energy dependent? Base current dependent? Lead
> length dependent? Wire size dependent?
>
> Answer = All of the above.
>
> Are you "so" sure mA range or is this just a number from Terry's email
> you listed?
>
> Now your going to have to measure for yourself and deduce all of the
> above in the process.
>
> Bart
>
>
> > OK, looking at Terry's paper, I see the waveform you refer to.  The Peak
> secondary base current is 5 Amps, not 10.  But I think you may have an incorrect
> understanding of RMS current.  If one has a _continuous_ sine wave with a peak
> current of 10 Amps, the RMS current is 7.07Amps.  But as you know, the
> secondary current in a disruptive coil is not continuous.  It has a low duty cycle of
> something roughly like 1%, and even during the bang-time, the amplitude envelope
> is very complex.  There is no simple conversion between the 5A peak current in
> Terry's waveform, and the RMS current.  The BPS, coupling, and quench time all
> factor heavily into the RMS current. There are ways to measure RMS current, the
> best being with a digital scope that does a lot of number-crunching over a time
> interval that includes both the bang on and off times.  Measuring the brightness of
> an incandescent bulb also works, because it has a sufficiently long thermal time
> constant to kind of average the current over the bang on and off times.  But the key
> is that you need to look at a waveform over a time interval where it repeats.  That's
> how a waveform with a peak current of many Amps can have an RMS value in the
> mA range.  You can't just look at the peak value and deduce the RMS value.  It's
> the RMS current that correlates to how much wire heating will occur.  If you're
> writing tools that purport to give RMS results, it's important to understand what that
> means.
> >
> >
>
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