Re: Experimental Help - Terry?

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

> >         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).

	The two-light bulb method with "eyeball brightness match" seems by far
the cheapest and easiest way to me.  All it takes is a pair of bulbs
with appropriate current rating and a variac or rheostat to adjust the
current in the reference lamp.  Nothing to buy but the bulbs.
 
> 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).

	I have a system which works a little differently.  Intended for
measuring the output power of low-power UHF oscillators.  Two identical
bulbs are used, one matched to the load and the other fed from a DC
source.  Each goes to one CdS photocell of a matched pair, and a simple
feedback amplifier is used to adjust the current through the reference
bulb to keep the PEC resistances the same.  Could be calibrated in terms
of current but instead I measured the input power and calibrated the
output meter against it.  Result is almost a linear scale.  The bulb
resistance changes with current, of course, but that falls out in the
measurement.

		The second is standard for bolometer bridges, but seems a little more
complicated to me.

> 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.

	Woops.  For 100 watt, 120 volt bulbs the operating resistance is
(120)^2/100, or 144 ohms.  Did I misunderstand what you are saying?

Ed