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RE: A new tuning idea?
Thanks Jim for the info on calibrating the lamps. It has been several years
since I have made these tests. One thing I might add is that I used several
different size lamps depending on the size of the Tesla coil. This was
necessary because the lamp brightness could be used only over a very limited
range for good results.
John Couture
-------------------------------
-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Friday, July 21, 2000 4:32 PM
To: tesla-at-pupman-dot-com
Subject: Re: A new tuning idea?
Original poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
-----Original Message-----
From: Tesla list <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Friday, July 21, 2000 12:53 PM
Subject: Re: A new tuning idea?
>Original poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi Gary,
>
> The secondary RMS current is around 350mA for a 15/60 coil. At least it
>is for my big LTR coil but say 100-400mA should cover them all...
>
>John Courture's Tesla Coil Construction Guide describes this exact
>technique on pages 14-5 and 14-6. He suggests a number 1891 lamp. The
>resistance of the lamp is non-linear in this high current, low duty cycle
>use so calibration would be tricky but for basic qualitative tuning it
>works. Perhpas a way could be found to calibrate it with some thought...
>
>I seem to remeber the "first" person to think of this was that guy from
>Croatia :-))
>
>Cheers,
>
> Terry
>
>At 08:56 AM 7/21/00 -0400, you wrote:
>>Several months ago I raised the possibility of metering the light output
>>from a fluorescent tube as a means of quantifying the output of a coil for
>>tuning purposes. I never actually tried this as further thought made me
>>think that varying streamer length and attachment would result in variable
>>E-fields and unstable readings.
>>
>>Now I'm wondering if metering the secondary base current might provide a
>>more stable and indicative reading of "in-tune"-ness. This might be
>>accomplished simply by inserting a small incandescent bulb in series with
>>the secondary base connection, and again using a photocell and analog ohm
>>meter to indicate the relative base current. It would probably work best
to
>>provide a close target for the streamers so that they are essentially
>>continuous and the same length.
>>
>>Does anyone know what the RMS base current for a medium-sized (15/60 NST
>>powered) coil might be?
>>
>>Regards, Gary Lau
>>Waltham, MA USA
>>
>
>
>
Calibrating is actually pretty easy. Do it like you do for thermistor and
bolometric RF power meters.. the replacement method. Observe the brightness
of the lamp with a light meter. Now, pass a DC current through the filament
to light it to exactly the same brightness (doesn't matter what it actually
is, just so that it is the same). Measure the current and .. You're done...
you now know the RMS current (averaged over the time constant of the lamp).
You can also compare the brightness of the filament against a reference
filament with a DC current in it, but your eye is only able to detect
brightness changes around 1 dB
One can actually get more fancy by having an isolated adjustable DC supply
in parallel with the lamp (with lots of RF chokes so the RF doesn't go
through the supply). Set the lamp to a given brightness. Turn on the coil.
The lamp will get brighter. Reduce the power from the DC until the lamp
returns to the original brightness. The difference in DC currents gives you
the RF current. This has the advantage that the resistance of the sensor
isn't changing as your RF current changes (because it's always at the same
temperature, etc.) The adjusting process can be done with a photocell and
an amplifier automatically, so you can directly read out RF current (or
power, or whatever...)
I have a LOT more information on this area, and you can make very precise
measurements (traceable to primary standards, if you like).