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RE: [TCML] RF Ground and Brass
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of bartb
> Sent: Thursday, March 06, 2008 9:38 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] RF Ground and Brass
> Hi Gary,
> Lau, Gary wrote:
> > Hi Bart,
> > I don't believe that I have misinterpreted Terry's measurement. His measurement
> > is consistent with mine, based on the current through my light bulb. His post is at
> > http://www.pupman.com/listarchives/2000/July/msg00478.html and is very clear:
> It is not saying that this is base current. Terry says "secondary rms
> current". I can't tell which currents he's discussing (can you?).
In that post, I had asked Terry specifically what the RMS secondary base current of a 15/60-powered coil was, so I'm pretty sure when he says "The secondary RMS current is around 350mA for a 15/60 coil", he is indeed referring to the base current. I can't imagine what else he could be referring to.
> I'm talking about both actually. 10A peak, so 7A rms. Either way, not
> 350mA unless this figures in the off time between transfers (which makes
> no sense to me). Even with off times, there are still 7A rms bursts
> occurring. Say you pulse 7A rms at some duty cycle across a thin wire.
> Does this equate to some continuous low current value? If the current is
> large enough compared to the wire size, the wire size will heat quickly
> but cool slowly. The bursts over time raise the wire temps up until some
> equilibrium is reached. It's easy to assume no big deal, but is that
> true? If true on one coil, is it true on others? I would error on the
> side of a larger than assumed needed wire size.
> This whole topic came up due to the ribbon. I agreed that "no, it's
> overkill" and stated what "I" used. I really don't care what anyone uses
> (that's up to them). But then I read 350mA base current and I know
> better (so I had to speak up). This is an area where Terry Fritz is
> probably the most experienced at.
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.
Regards, Gary Lau
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