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[TCML] Re: Dummy load for optimum cap size Experimentation

OK, back from vacation, time to start the experiments!  Igor???

I tried monitoring the lamp brightness using a photocell.  Previously I just
applied Variac'ed AC to the bulbs to test the photocell response.  Today I
hooked up the actual spark gap and capacitor to an NST.  Just as I feared,
the chaotic gap behavior resulted in extremely erratic resistance readings.
Useless.  I tried placing a big cap across the photocell terminals, but that
didn't work.  Back to monitoring the bulb temperature with a thermocouple.

Previously I was using a fairly heavy-gauge thermocouple, something like 20
gauge.  I think this may have contributed to the long time needed to reach
thermal equilibrium.  I found another thermocouple made of very fine wire,
and I used a couple turns of thin copper wire to strap the junction to the

The thermocouple went to a battery-powered Fluke model 52 digital K/J
Thermometer, same size as most Fluke DMM's.  But as soon as the gap started
firing, the meter display went nuts.  The thermocouple wire is as far from
the lamp wires as possible, definitely not making electrical contact, but
inherently close.  The display returned to correct operation after cycling
power, but the temperature was already in decline.  I tried looping the
slack thermocouple wire 6 times through a big ferrite toroid, hoping to
reduce the interference, but no luck.  Maybe a grounded metal sheath on the

I also tried reading the thermocouple directly into a DMM on the 200mV
scale.  Yes this lacks cold junction compensation, but could be useful just
for relative readings.  But the readings were similarly flakey as long as
the gap was firing, though at least didn't send the meter into a

I also tried an analog wattmeter.  Someone suggested using a Kill-a-watt
meter to just monitor real power consumed.  But even the analog meter shows
wide fluctuations with the gap's chaotic firing - not useful for any but the
most coarse reading.

I'm stuck.  Any ideas, either for filtering, or an alternate readout
device?  I'm starting to think about finding an analog pyrometer meter on
eBay, but I was really hoping for digital resolution.  Dang - what seemed
like such a simple method is turning into real project!

Regards, Gary Lau

On Thu, Jul 29, 2010 at 9:36 PM, Gary Lau <glau1024@xxxxxxxxx> wrote:

> Time to eat some words.
> I tried various amateurish ways to couple the thermocouple to the halogen
> lamp.  The kapton tape made a drippy stinky mess as soon as the bulb got
> maybe 30% power.  I tried pressing the thermocouple to the bulb with a
> spring, and put a layer of fiberglass drapery between the spring and
> thermocouple.  The drapery pigments smoked and made a mess.  Perhaps some
> better quality fiberglass may have worked better, but when I was using the
> kapton tape, the time to reach equilibrium seemed excessive - several
> minutes, and given that I'll be taking data on MANY data points, I'm just
> not that patient.  So I'll be using a photocell and Ohmmeter.  Maybe I can
> just throw a cap across it if there are fluctuations.  At the power levels
> (~50%) I was trying just now with simple line AC, the brilliance of the
> halogen lamp swamped any room illumination, so shielding probably is not an
> issue.
> On Thu, Jul 29, 2010 at 10:14 AM, Gary Lau <glau1024@xxxxxxxxx> wrote:
>> I'm gathering materials and plans to perform this experiment - determining
>> the cap size that will extract maximum power from an NST, at 120 and 140VAC
>> input.  The power indication will be via monitoring the surface temperature
>> of the halogen lamp dummy load that replaces the primary coil.  I plan to
>> test a 15/60 NST, and two 15/30 NST's, all unmodified.
>> <snip>
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