Re: Experimental Help - Terry?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<evp-at-pacbell-dot-net>
> 
> Tesla list wrote:
> >
> > Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <jimlux-at-earthlink-dot-net>
> >
> > very carefully <grin>
> >
> > If what you want to do is measure RF current, (rather than power), then, a
> > bolometric scheme in the base of the secondary might be a practical scheme.
> > I'd use the brightness of a filament lamp, calibrated with DC.
> 
>         Not sure what you meant by calibration.  Calibration of the
reading of
> a light meter vs current? 

Exactly... you could use a calibrated (DC) current source and generate a
"current vs light" curve.  Say you used a piece of fiber optic cable and a
photodetector hooked up to a computer connected DVM.... (Radio Shack has
Rs232 DMMs on sale occasionally in the $30 area).


A common technique was to use two lamps of
> the same type placed close to each other. One was connected to the
> circuit where the current was to be measured and the other was fed from
> a variable DC (or powerline AC) source in series with the appropriate
> ammeter.  The current through the reference lamp was adjusted until the
> brightness of the two filaments was equal (a pretty accurate thing to
> judge, and used all the time in optical pyrometers).


Or, do the replacement current technique to servo the DC current through
the sensor to keep the brightness at a constant, or the resistance the same
(in a bridge).

All manner of thermal RMS measurements exist...

  About the time of
> WW2 Sylvania made a special line of current-measuring gadgets consisting
> of two matched filaments, sealed in a vacuum (or maybe gas filled, I'm
> not sure) tube.  The couple I have were mounted in a loctal tube
> envelope, but ones for higher currents may also have been made.
> 
>         Of course, the easiest way to go is to get thermocouple RF ammeters,
> which show up from time to time on surplus or at swap meets.  Think Fair
> Radio Sales may have some at times.  Full scale currents from a quarter
> amp to 100 amps are common.  Handy gadgets, but their use requires care
> as they are prone to burnout with rather small overload.  I have quite a
> few here which I use for all sorts of RF current measurement.  The
> advantage of these over the light bulb method is that, for a given
> current, they have a much lower resistance and voltage drop.


high wattage bulbs have pretty low filament resistance.  E.g. 100W
lightbulb has about 1 ohm resistance (hot) at 1 amp (RMS).. probably a
tenth that cold.