Reconditioning a 50+ year old microscope - part 3 - LED conversion

The microscope came with what I believe it must have been the original illumination system:

• a 30 Watt / 110 Volts tungsten bulb (in this case a GE branded one):

• a light dimmer circuit apparently made up of just a Triac, some capacitors and the potentiometer to control at which threshold the waveform is chopped (therefore causing the light to dim proportionally to how much of the wave is chopped):

• and interestingly, a Hitachi branded mains plug:

However for current use, this system would be impractical several fold:

• the specific type of tungsten bulb it requires is very difficult to obtain today, and those available (mostly new old stock) are not cheap;
• as listed in the specs of these light bulbs, the lifespan is stupidly small, in the range of 50 to 60 hours.

• incandescent illumination is not ideal because of reddish spectrum of light in lower power levels. A blue filter is normally required to adjust to a more even spectrum:

• given the low efficiency of tungsten bulbs, heat dissipation and IR radiation are high, affecting some types of specimen;
• in my location the mains voltage is 220 Volts instead of the 110 Volts required. This would imply adding a transformer to convert the voltage;

As such I studied the conversion to an LED light source, which I knew it would have multiple advantages:

• LEDs are widely available and cheap;
• lifespan is high, in the order of tens of thousands of hours;
• light spectrum is consistent across all levels of intensity. Also (depending on the selected LED), a nearly pure white source is possible without having to add any filter;

Nevertheless I had to take into consideration a few aspects:

• make sure a good thermal dissipation would be allowed. LEDs don't like too much heat;
• ensure the maximum current delivered to the LED is actively limited, ideally by some regulator circuit;
• Like in the original system, provide a means for the LED intensity to be varied through a potentiometer installed in the same location of the original one.

As such I put my hands to work. Started by removing the original illumination gear:

Ordered from banggood:

• a bunch of 3 Watt white LEDs (rated at about 210 lumens each);
• a power LED tailored heatsink:

• a step-down (buck) converter rated at a maximum of 3 Amps, with controls for constant voltage and constant current applications:

The work started with the preparation of the heatsink to fit the area originally occuped by the bulb. Some cutting had to be done in order to make it a good fit. Tested the LED mounted on the heatsink for proper operation:

Made some tests with the step-down converter, limiting the current to 500 mA (below the rated 700 mA but more than enough for the lighting requirements), and replacing the on-board multiturn pot by a 500 Ohm panel potentiometer and a 220 Ohm resistor in series (and as such obtaining only the safe voltage range to be used for controlling the light):

• machined an aluminium disc to provide the necessary width for fitting a DC connector in the hole where originally the thick mains cable would pass through:

Well these specs won't remain true though (in particular the 20 Watts of consumption) :) 

While possibly not making much difference, preserved the original black mask:

Put it all together, inside the microscope base (mounted the LED + heatsink, the step-down converter PCB, the DC connector, and the potentiometer, in the place where the original one would go).

Fired it up and..Bob's your uncle:

See also:

Part 1 - Fine Focus recovery- http://creationfactory.blogspot.pt/2016/08/reconditioning-50-year-old-microscope.html

Part 2 - XY specimen stage mechanism lubrication - http://creationfactory.blogspot.pt/2016/09/reconditioning-50-year-old-microscope.html