Re: TDC Brightness (500 vs 750w.)

[P3D Tom White <tomwhite@xxxxxxx>]

George Themelis said:
>It is possible...but does not explain the entire picture.  Some 
>voltage drop is witnessed when the light output through one lens
>is measured with the other lens ON and then OFF.  When the other
>bulb is turned off, the light increases by 0.2 EV which shows that
>running both bulbs causes some voltage drop, but not significant.
>
>The amps drawn BTW are about 10A for 500W bulbs and 15A for 750W.
>
>I once tried to measure the current drawn during the operation of
>the projector.  I emailed the results to Steve and do not have my
>notes now... but I remember that the calculated Wattages (amps x
>voltage) were significantly lower than expected (1000W+ with 500W
>bulbs and 1500W+ with 750W bulbs).  Out of memory I'll say that
>it was like 800W for the 500W bulbs and 1200W for the 750W bulbs.
>I am not sure if these bulbs consume the rated wattage.  Will 
>repeat the experiments when I get a TDC 116 and 716 and two pairs
>of bulbs lined up.

I have some experience in measurments like this. You need to get your hands
on a decent volt meter with a current transformer attachment. Such as one
made by Fluke perhaps.

Measure the voltage at the bulb *base* or as close to it as you can.
Measuring the voltage with the bulb on vs bulb off will tell you what the
voltage drop is due to the IR loss in the wiring. If it is more than a
couple of volts then I would rewire or check out some of the obvious loss
potentials such as the switch or the plug.

Measure the current by clamping the current transformer around one of the
wires (not the pair) anywhere. At the wall, or mid way or at the lamp base.
It doesn't matter. One convenient way is to use a short but heavy duty
extention cord made from zip cord, the kind you can split apart. Split a
couple of inches and clamp around one wire. And of course, current times
volts is watts.

If you know the current throught the system and you know the voltage
actually getting to the bulb, then there is only one place the for the watts
to be. And those will be expressed as heat and light.

If you can't measure the difference in brightness between 500W and 750W with
your light meter, then I think there must be something about the
construction of this kind of bulb that governs the light output that is not
obvious. But the difference is on half an f stop, right? That's not much.
Certainly would be hard to see with your eyes.

Later this spring perhaps I will be able to afford TDC 117 or 716 and make
these measurements myself.


Tom White
Driftwood, Texas


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