Hi Moose,
I have seen your arithmetics, and it makes sense (but the figures need
adapting for the watt/volt change, I suspect, because 6 watt is a
_very_ small power for a microscope light... 20-50W is a normal
wattage for these lights).
But the whole thing is largely dependant on the fact that the rheostat
has been built in the transformers secondary coil, directly in series
with the light (unless it is part of more advanced electronics, of
course). Am I right here?
I have built a contraption like this about twenty years ago, and it is
still working properly. I have tried it a few minutes ago... It was
meant for the Olympus LSD with 6V 30W bulb, befor I got my hands on
one of the original Olympus variable transformers TE.
Only a few parts were used: transformer, on/off switch, rheostat and
fuse.
Important thing here is, that I wired the rheostat (475 ohm) in series
with the primary coil of the transformer, i.e. the 220 volt mains one.
The secondary coil is 8V, to which the 6V lightbulb is attached
without further adjusting circuitry. The rheostat is a current limiter
here, if I am not mistaken.
I do not remember why I did this; probably I just copied the
electroniscs circuitry of an existing unit. The current in the primary
coil is much lower than in the secondary one, which is perhaps the
reason for doing it this way. As it is the rheostat is already fairly
heavy; about two inches in diameter and thickness and with the wire
wound on a porcelain donut shape core, so it may get rather hot. There
is no power or amp value on the body, just the resistance (475 ohm).
Hope this little snap of information will help you further, Charlie.
Frank van Lindert
Utrecht NL.
Wed, 19 Nov 2008 13:42:36 -0800, Moose <olymoose@xxxxxxxxx> wrote:
>None of those but the last one is even of the appropriate type. All the
>others use a wiper against a carbon layer on a substrate, and can't
>handle much power. As you may have noticed, the last one is labeled
>'rheostat', rather than 'potentiometer', typical labeling for a
>wirewound variable resistor for power control, which is what you need.
>
>Next, you need to know what the maximum resistance of the existing unit
>is. Easy enough to test with an ohmmeter. Resistance and power rating
>may possibly be marked on the case of the rheostat.
>
>Six watts @ 20v gives a current of 300ma. From that, the effective
>resistance of the bulb at full power is about 67 ohms. Add 25 ohms to
>that and assume bulb resistance will go down when running cooler at
>lower voltage, and the rheostat at maximum resistance will reduce
>voltage drop across the bulb by about 1/3, I'd guess enough to reduce
>light output to a dull glow. Then with 1/3 of 20v across 25 ohms, the
>rheostat is dissipating about 1.8 watts, about 60% of rated capacity.
>
>So, if your numbers are correct, the 25 ohm, 3 watt rheostat will
>probably work. I'd guess that the brightness range will be greater than
>with the existing unit, with the useful range narrower.
>
>However, if Frank's assumption is correct, and it's a 20 watt, 6 v lamp,
>the calculations are different and that rheostat won't work.
>
>Moose
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|