Charles Geilfuss wrote:
> Frank,
> Thanks for your help. I'm trying to replace the built-in potentiometer
> with one that can be wired into the microscope body in a fashion similar to
> the original. My microscope table is small so I'm trying to avoid the
> separate control box.
> At this link are multiple potentiometers but I don't know haw to determine
> with would be appropriate:
>
>
> http://www.radioshack.com/search/index.jsp?kwCatId=&kw=potentiometer&origkw=potentiometer&sr=1
>
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
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